Shure AMS4000, AMS8000 User Manual

The Shure Automatic Microphone System [AMS] turns
microphones on and off [with automatic gating], greatly
reducing the reverberant sound quality and feedback prob­lems often associated with the use of multiple microphones.
The special AMS microphones are gated on only by sounds
arriving from the front within their acceptance angle of

20°. Other sounds outside the 120° angle, including

1
background noise, will not gate the microphones on, regard­less of level. In addition, the AMS adjusts gain automatically
to prevent feedback as the number of "on" microphones
increases.

The resulting sound is clearer than that of conventional
multiple microphone speech reinforcement and recording
systems-and free of the clipped and missed words, clicks
and pops, and noise pumping often associated with other

"automatic mixer" systems. Besides its major advantages
of simple setup and unmanned operation, the Automatic
Microphone System operates over an extremely wide
dynamic range without the possibility of threshold-setting
misadjustments.

AMS Mixers are supplied in 4- and

tions [Models

3lh-inch rack-mount package. Both contain logic ter-

single

AMS4000 and AMS80001, each housed in a

Schannel configura-

minals [for channel muting, override functions, and gating in­dications], and link circuitry for expansion to as many as 200
linked channels.

AMS

Features:

w

Reliable, quick-acting, noise-free gating-virtually insens­itive to changes in sound source loudness or distance

w

Smooth pleasant-sounding turnon and turnoff
characteristics

Shure Brothers lncorporated
222 Hartrey Avenue
Evanston,

IL

60202-3696 U.S.A.

AMS4000 AND AMS8000 Installer's Manual

w

No threshold settings to misadjust

w

Front-panel microphone channel gain controls and
Master control operate as in conventional mixers

w

Selectable hold time keeps microphones on during short
pauses

w

Preset or adjustable Off-Attenuation control for unob­trusive gating

Automatic gain adjustment as additional microphones
gate on

w

Wide, flat frequency response and low distortion up to

+

18

dBm output

w

Logic inputs and outputs enhance system versatility

w

Linking capability for systems of as many as 200
microphones and 25 mixers

LED indication of gating operation and output level

w

Automatic muting prevents annoying thumps and
loudspeaker damage when unit is turned on and off

w

Four or eight special microphone inputs [for use
AMS microphones] use standard two-conductor shielded
cables and three-pin connectors

w

Balanced output switchable to line or microphone level

w

Front- and rear-panel unbalanced Aux inputs and outputs

w

Front-panel headphone monitor jack

w

Direct [non-gated] outputs available from individual
microphones

w

Underwriters Laboratories Listed and Canadian Stan­dards Association listed as Certified

only

with

.....

-

......

Shure AMS4000 and AMS8000 Mixers are designed for use only with Shure AMS Condenser Microphones. Conventional condenser or

-

OCopyr~ght 1997, Shure Brothers lncorporated
27A8273(QC)

other microphones will not operate properly with the

-.

AMS4000 and AMS8000.

U.S. Patent 4,489,442; other patents pending

Printed in U.S.A.

TABLE OF CONTENTS

General
Specifications
Connections. Controls and Indicators
MicrophonePlacement 6
Typical Applications 6

LogicFunctions

OperatingHints

Internal Wiring Modifications
Troubleshooting
Appendix I: Effects of Acoustic Environment on Gating

Appendix

Appendix

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ConferenceRoom 6
Church

..........................................................

Courtroom
Legislature

.......................................................

.......................................................

CoughButton
Chairperson-Controlled Muting
Disabling the Gating Function [Bypass]
Remote Channel-On Indication
LoudspeakerMuting

"Filibuster"Mode
Preventing Room Noise Modulation
MicrophoneLock-On
Diode Isolation of Logic Controls

External Logic Devices

15-VoltCMOS

Digital Controls or Microcomputers
WirelessMicrophones

Direct Out Gating

Controlling Non-AMS Microphones

Phasing

........................................................

MicrophoneMuting
Grounding

......................................................

RoomNoise

Reverberation
TalkerDistanceandAngle

Reflectivesurfaces

II: Theory of the Shure Automatic Microphone System
Ill: AMS Mixers and Conventional Microphones

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4

6
6
6

8
8

8

8

8

8

9
9

9
9

9

10

1

0

10

10

11
11

I1

11

11
11

15

16
16
16
16
16
16

1

7

1

WARNING

Voltages in this equipment are hazardous to life . Refer all internal
wiring modifications and servicing to qualified service personnel

.

SPECIFICATIONS

Output Level

Off-Atten at -1 5,
terrnlnatlons: Llne 6000,
Phones 20001

INPUT

Mtcrophone

lnput

Senstttv~ty

172 dB

SPL

--

Aux

Input

Senstttvlty

1-22 dBV ~n]

--

'Depend~ng

Frequency Response

Aux lnput to Outputs: 30 to 20,000 Hz, *2 dB
Mic

In to Outputs: 70 to 20,000 Hz, * 2 dB [controll-

ed low-frequency

Aux lnput Impedance

70k or greater, unbalanced [designed for use with less
than

10k source impedance]

Outputs

[at full galn, 1 kHz, one channel gated On,

wlth AMS26 probe mtcrophone, output

MIC

1500. Aux 50k. Olrect 50k,

OUTPUT

tn]

on

Lina

+

15 8 dBV

[+

18 dBrn]

+

158 dBV

Aux

Mic

34

dBV

34

dBV

-

control settlng

Aux

+

17

dBV

+I7
dBV

Direct

56 dBV

--

Phonaa

-4

dBV

-4 dBV

rolloff below 50 Hz]

Input Clipping

Leva1 at I kHz

128 dB

+7 to

+

20 dBV'

SPL

Off-Attenuation

F~xed: -1 5 dB
Varlable

-cx,

to -8.5 dB
[Slngle mtxer; attenuation Increases as addltlonal mixers
are l~nked]

Overload and Shorting Protection

Shortlng the lnputs or outputs, even for prolonged
perlods,
damaged by
damaged by

Logic Terminals

will

cause no damage; mc lnputs wlll not be

slgnals up to 3V; aux lnputs

will

slgnals up to 10V

[all specifications referenced to Logic

not be

Ground terminals]

lnputs [Override, Mute]

High [inactive]: Greater than

1.9V [5.OV typical] [no
input current with 5.0 to 20V applied]
Low [active]: Less than

1.9V [OV typical] [sources

80 when grounded]
MinlMax Applied Voltage: + 20V

Outputs [Gate]

High [inactive]:
[sources 0.2

voltage: -0.5 to
Low [active]: Less than 0.5V sinking

5.OV in series with 10k resistance

mA with 3.OV output] [minlmax applied

+

1

5V]

[I

00

mA max]

OUTPUT

Mtc

Llne

Aux

atrect

Phones

150R balanced

ltnes
600fl balanced

lhnes
10k or greater
10-50k unbalanced

rno

IMPEDANCE

Deaigned for Actual

Uw

With [Internal]

clrcutt

200n

Output

Clipping

bval

Hum and Noise

Equivalent lnput IYoise: 27 dB SPL, A-weighted, with
AMS26 probe microphone

Output Noise: -62 dBV [master up], -88 dBV [master

[300-20,000 Hz, input controls down, Off-Atten

down]

51

at -1
Output Hum and Noise: -58 dBV [master up],

-79

[master down] [20-20,000 Hz, input controls down, Off­Atten at -1 51

Distortion

THD 0.35% or less, 30 to 20,000 Hz at + 15 dBm
output; IMD 0.5% or less up to + 15 darn output

Phase

Positive pressure on AMS microphone diaphragm pro­duces positive voltage on pin 2 of

LinelMic balanced out­put with respect to pin 3, tip of Aux output, and tip and
ring of Headphones output, and negative voltage on tip of
Direct output. Aux output is in phase with Aux input.

Gating

Attack Time: 4 rnsec:
Hold Time: 0.5 or 1 .O sec [switchable]
Decay Time: 0.3

sec after Hold interval

dBV

LOGIC EQUIVALENT CIRCUIT DIAGRAM

FIGURE

1

Operating Voltage

105-1 32 Vac, 50160 Hz, 20W [fused internally]. Can
be rewired for 210-264

Vac operation [see 240V

Operation]

Temperature Range

Operating: -29" to 57°C [-20" to 135"FI
Storage: -29" to 71 "C [-20" to 1 60°F]

Dimensions

See Figure 2

OVERALL DIMENSIONS

FIGURE

2

Weight

AMS8000: 6.6 kg
AMS4000: 5.8 kg
AMSB000 [packaged]:
AMS4000 [packaged]: 7.1 kg

[I

[I

4

Ib 8 oz]

2 Ib

7.8

1 3 oz]

kg

[I 7 Ib 4 oz]
[I

5 Ib 9 oz]

Certifications

Listed by Underwriters Laboratories, Inc.; listed by

Canadian Standards Association as Certified

CONNECTIONS, CONTROLS, AND INDICATORS

[Paragraph numbers that follow refer to pictorial views on
inside back cover .]

1.

Microphones:

Microphones are designed specifically for use with Shure
Automat~c Microphone Systems, AMS4000 or
AMS8000 mixers

The Shure AMS Condenser

Do not attempt to connect AMS

microphones to standard phantom-powered or
standard non-phantom-powered inputs; they will

not function properly.

The AMS microphones, in conjunction with the special cir­cuitry of the AMS mixers, uniquely discriminate between
desired sounds that originate within
ceptance angle and all other sounds. The desired sounds
from the front of a microphone are detected and cause
the microphone to be gated on, transmitting its signal to
the mixer output. Sounds outside the acceptance angle
will not gate the microphone on. When a microphone is

"on" [accepting signals], it operates like a cardioid

microphone [or like a

profile microphones]. Each AMS microphone operates

completely independently in analyzing its own sound field

and deciding whether a sound source is within the front

acceptance angle.

Among available microphones are: a low-profile

mount model [AMS22], a gooseneck model for perma-

nent mounting

front pop-filter grille, and a
to the reinforcement of sound waves from the adjacent

boundary surface, the sensitivity of the surface-mount

model is twice as high

model. The gooseneck model is supplied with a cable, but

less connector.
The microphone connector is a standard 3-pin profes-

sional audio type [XLR]. Under most circumstances,
lengths of 1 50 meters

2-conductor shielded microphone cable can be used as
extensions between the microphone and the AMS mixer
microphone input. The same conductor must be wired to
the same numbered pin at both ends of the cable to en­sure proper functioning of the units. The shield should be
connected to pin 1 at both ends of extension cables. Good
practice dictates that microphones and extension cables
be grounded only to the AMS mixer chassis ground.

2.

Microphone Inputs:

either four or eight microphone inputs per unit. The
microphone inputs are designed for use only with Shure
AMS microphones; the Mixer will not operate with other
microphones; and Shure AMS microphones will operate
properly only with AMS mixers. The microphone input
connector is a female XLR type.

[AMS24], a probe model [AMS26] with a

hemi-cardioid in the case of low-

lavalier model [AMS28]. Due

[6 dB more] as that of the probe

[500

ft]

The AMS mixer is supplied with

their 1 20" front ac-

surface-

or greater of good quality

4

Direct Output:

microphone-level

behaves
mlcrophone output It can be converted to balanced low
Impedance using a line matching transformer [Shure
A95UI An unbalanced low Impedance mlcrophone level
can be
nected

The
the output
div~dual Channel control or the Master control, even when
the Channel control
["O"] position

Each channel can be
on the input module board [see section on Internal
Mod~fications for deta~ls] Thls change causes the level at
the
control

Channel LED Indicator:

5.
the Channel control
on. The status of the Direct output is not related to the

LED.

6.

Hold Time:

microphone stays on after the user stops
delayed turnoff br~dges pauses In speech and reduces un­necessary
minimizes microphone on-time, while the I .O-second
position bridges longer pauses.

For special applications, the

increased to as long as 2 seconds by inserting a

[see Internal

7.

Off-Attenuation:

tenuatlon of microphones that are not gated on. It is un­necessary for "off" microphones to be totally off to
the benef~ts of automatic mix~ng. Keeping them slightly on
at all times contributes to smooth
tlon.

The -15 setting of the Off-Attenuation switch is

recommended for most applications. The Variable set­ting allows continuous adjustment of the Off­Attenuat~on from minus infinity to -8.5 dB [-8 setting]

by the screwdriver-slot adjustment directly above the
switch. [See sections on Link Jacks and on Theory of

Operation for further

8.

Power Cord:

designed for

only. To modify the

on Internal

ltke a conventtonal cardloid h~gh-impedance

obtalned by loadlng wlth a 100-ohm reslstor [con-

t~p to sleeve]

'/4-1nch phone jack 1s supplled w~red pre-fader so that

1s not affected by the posttion of ether the in-

Dlrect Output to follow the settlng of the Channel

This swltch determines how long the

gatlng action. The 0.5-second posltion

Modifications

connection

Modifications.

This provldes a non gated

s~gnal from the mtcrophone Thls output

IS

In the counterclockwise detent

wlred post-fader by moving a jumper

A yellow LED indicator above

l~ghts when the mlcrophone is gated

talklng. Thls

1 .O-second positron can be

reslstor

section for details].

This switch determines the at-

unobtrus~ve gatlng ac-

Information on Off-Attenuation.]

A 3-conductor cord and grounded plug

to 120 Vac, 50160

unlt for 240V, refer to the section

Hz

outlets

galn

3.

Microphone Channel Gain Control:

the

"0"

posltlon [detented counterclockwise], the

mlcrophone is not

clockwise

trol
to gate on
or by a connection to the Override
descr~pt~on of Logic Terminals below].

The Microphone Channel Gain control does not affect the

Dlrect Output level unless a jumper change has been
made for post-fader output as
below.

ether for sounds within ~ts acceptance angle

permitted

from this position permits the microphone

to gate on. Turning the con-

described

When set to

log~c terminal [see

in Paragraph 4

9.

On-Off Switch:

panel turns the power to the AMS mixer on and off.

Power-On LED:

10.
lights when the power

Normal LED:

11.
the Power-On LED
aux output levels are above approximately -20

[I

00

mV].

12.

Overload LED:

Push-buttorr switch on the front

A green LED on the front panel

is on.

A yellow LED on the front panel above

beg~ns to turn on when the line and

A red LED flashes when the

dBV

LineIMic, Aux, and Phones outputs approach clipping. If
this light flashes on, turn down the Microphone Channel
controls of the channels in use or turn down the
Master control until the light stays out.

1

3.

Aux Input:

A

l/4

-inch unbalanced phone jack input on
both the front and rear panel of the AMS mixer,
suitable for Aux level and line level sources, such as
tape recorders or players or conventional

mrxers. This
non-gated input is mixed with the combined microphone
signals to appear at the

LineIMic, Aux, and Phones out-

puts.

14.

Aux Control:

The position of this control deter-

mines the level of the Aux Input.

15.

Master Control:

The position of this control deter­mines the level of the combined microphone and aux
signals at the

16.

LineIMic Output:

LineIMic, Aux, and Phones outputs.

This male XLR audio connector
has switch-selectable levels: either low-impedance
balanced microphone level or 600-ohm balanced line
level. This output provides the combined gated
microphone and non-gated aux input signals.

Aux Output:

17.

The '/4-inch phone jack outputs on the
front and back panels are intended for unbalanced Aux
or line level loads. This output also provides the combin­ed gated microphone and non-gated aux input signals.

gates the channel off. The channel output drops to the
level set by the Off-Attenuation switch. The Mute func­tion dominates Override when both are simultaneously
activated unless a jumper change is made so that
Overr~de dominates [see Internal Modification section
for details].

OVERRIDE IN:

Applying a logic "low" to this terminal

[from a logic gate or a switch closure to ground] forces
the channei on. The mixer is supplied so that when both
Mute [described above] and Override of a channel are
activated, Mute dominates.

lnhibit Function: The Mute function can be altered

lnh~bit by an internal jumper change for each

to
channel [see Internal Modifications section for
details]. After the change, a logic "low" at the Mute

In terminal prevents the channel from gating on if it

is off, but allows it to remain on if it is already on.
After the Inhibit modification, for certain specialized

applications a logic connection can be made between
a channel's Gate Out and its Mute In terminal.

IMPORTANT:

To prevent high-frequency oscrlla­tion, never connect a channel's Gate Out to its
Mute In unless the lnhibit modification has been
made.

Phones Output:

18.

This %-inch phone jack is suitable

for most stereo or mono headphones with

3-circuit phone plugs. The signal at this output is iden-

tical to that at the

19.

Logic Terminals:

LineIMic output.

These four screw terminals are
built into a female barrier block module that plugs into
a male connector on the rear panel below the Direct

31.

Output jacks (Figure

The barrier block's plug-in

design ensures proper placement and simplifies ter-

minal wiring by eliminating the need for soldering. Con­nections to these terminals are not necessary for basic

AMS operation, but allow the AMS mixer to perform
additional functions. Unshielded wire or

multi-conduc-

tor cable is adequate for the connectors.

The logic levels are

nomtnally 0 volts [low] and
[htgh]; they are directly compatible with standard
5-volt logic families [including CMOS and

TTL].

GATE OUT: This terminal follows the channel gating
and goes to logic "low'' when the

mrcrophone is gated
on. Sufficient current-sinking capability is provided to
l~ght externally powered LED'S [see Logic Functions
sectlon for example].

LOGIC GROUND:

The Logic Ground terminals of all
channels are connected together internally and are
dist~nct from the AMS aud~o ground. All logic ground
connections should be made to these terminals. The
power supply ground of external logic circuitry should
be connected to the

8

[or Channel 4 In 4-channel AMS mixers]. To avoid

L.ogic Ground terminal of Channel

sw~tch~ng clicks, do not Interconnect the Log~c Ground
with the audio,

chassrs, or rack grounds.

MUTE IN: Applying a logic "low" to this terminal
[from a logic gate or a switch closure to Logic Ground]

5.0

2-

volts

or

LOGIC

GND

MUTE IN

AMS

MIC

IN

3

REAR-PANEL CHANNEL PLATE

3

AMS mixers together to pro-

20.

Link Jacks:

are used to link up to

FIGURE

These rear-panel %-inch phone jacks

25

vide an input capability of as many as
microphones. To link mixers, use short I -conductor
sh~elded cables with -inch phone plugs on both ends.
Plug a cable between the Link A Out jack of one mixer
and the Link A In jack of the next mixer;
cable between the

L~nk B Out jack of one mixer and the

AND

Link B In jack of the next. Leave open the Link In jacks of
the first
last mixer

mlxer in the chain and the Link Out jacks of the

In the chain. Both the Link A and Link B jacks

of each mixer must be connected: A Outs to A Ins

q

Outs to B Ins. Use the Link jacks for l~nking only,

and
not for audio inputs or outputs.

IMPORTANT:

When uslng the logic term~nals on

200

plug a

linked mlxers, connect the Channel 8 Logic Ground ter­minals of each unit together.
result if

thts is not done.

Switching

clicks may

When mixers are linked, the combined signals of all the
mlcrophones appear at the outputs [MicILine, Aux, and

Phones] of ALL the linked mixers. Thus, you can take

outputs from several mixers for a multiple feed.

However, an aux source connected to an AMS mixer is
heard at the outputs of that mixer only, NOT at the
outputs of linked mixers. For this reason, plug the aux
source into the mixer providing the audio output. If
multiple feeds are being used from linked mixers and it
is desired that the aux source appear in all, parallel the
aux source [using Y-adapter cables] to the Aux inputs
of all the mixers used for the multiple feeds.

The Master Gain control on each linked mixer controls

overail level at its own outputs of

the

mlcrophones and of its

own

Aux inputs. The Off-

all

the linked

Attenuation controls and the Normal and Overload

LED'S operate in the same way-they control and show
the status of the outputs from the mixer on which they
appear. The Hold

Tlme switch on each mixer affects

only the microphones connected to that mixer.

Use as few microphones as necessary to pick up everyone.
The closer the microphones are to their sound sources, the
greater the loudness of the sound system before feedback

1

occurs. Each microphone should be at least
from the wall behind it, and at least 0.3 meters

meter [3 ft]

[I

ft] from
objects behind it, such as large ashtrays or briefcases.
When using

AMS28 Microphones, avoid acoustic feedback

from loudspeakers near the microphones.

MICROPHONE ACCEPTANCE ANGLE

FIGURE

5

TO

AMPL,

TAPE

REC,

ETC

LINKING MIXERS

FIGURE

4

MICROPHONE PLACEMENT

Use Low-Profile AMS22 Microphones on tables and desks;
use permanently mounted Gooseneck
on tables, desks or lecterns; use Probe

AMS24 Microphones

AMS26
Microphones on floor or desk stands or goosenecks; use
Lavalier

AMS28 Microphones where the talker must have

freedom of movement.

Locate the microphones so that intended sources are within
60° of either side of the front of the microphone; that is,

1

within the

20° acceptance angle [see Figure

undesired sound should be located outside the

51.

Sources of

1

20° accep-

tance angle.

TYPICAL APPLICATIONS
Conference Room

Refer to the AMS Operators Manual for

microphone

place­ment at conference tables. Connect extension cables to the
microphones and to the microphone input connectors on the
rear panel of the mixer. Connect the

LineIMic Output to the
line input of the PA amplifier. To record the meeting, con­nect the Aux Output to the aux

Input of a tape recorder.

Church

Connect extension cables to the microphones and to the
microphone input connectors on the rear panel of the mixer.

Connect the

LineIMic Output to the line input of the PA

amplifter. To record the service, connect the Aux Output to

the aux input of a tape recorder [see

Flgure 61.

Note that the cholrs are inslde the acceptance angles of the

pulpit and lectern microphones. Thus, some choir members

may gate on the pulpit and lectern

mlcrophones occasionally.
This will not seriously degrade the performance of the
system.

Courtroom

Connect equipment as described for the church setup. Also
connect each Direct Output to a separate microphone input
channel of a multitrack tape recorder for easy

identification

of talkers during transcription. The Aux Output containing a
mix of all the microphones connects to another channel of

mult~track recorder. Th~s channel can be monitored to

the
hear the entire

proceedings.

[See Figure 71.

Often a tape playback 1s used for evidence or for transcript

ver~f~cation. Connect the tape player's aux output to the Aux
lnput of the mixer.

Legislature

A typical legislature setup is shown in Figure 8.
[Chairperson-Controlled Muting-see LOGIC FUNC­TIONS-is also shown.] Two or more mixers are linked via

L~nk A and Link B jacks. The Channel 8 Logic Ground ter-

the

ALTAR

ol

MUTING SWITCH

0

CHAIRPERSON*^

CHANNEL

RECORDER

r*i

PLAYER

$GK-~

ma

CHURCH SETUP

FIGURE

6

LEGISLATURE SETUP

FIGURE

LOGIC FUNCTIONS
Cough Button

The user can turn off his

Off-Attenuation level] during coughing or private conversa­tions. To establish this function, wire an SPST pushbutton
switch between the Mute In and Logic Ground terminals on
each channel requiring a cough button. See Figure

or

0

8

her microphone [to the selected

9.

GATE OUT

LOGIC

GROUND

MUTE IN

COURTROOM SETUP

FIGURE

minals of the linked mixers are connected. One mixer feeds a

PA system, and another mixer feeds a tape recorder and

equipment for a television broadcast.

Note that the Off-Attenuation can be set differently for the

TV

PA mixer than for the
may sound more natural if the Off-Attenuation is set at
the room noise is sufficiently low. On the other hand, for
highest gain before feedback in the PA amplifier, the Off­Attenuation should be set at
infinity. The Master Gain control is set differently on each
mixer to obtain proper levels feeding each system. The aux
source feeds both mixers via a Y-adapter cable but the Aux
control on each mixer may also need to be set differently.

mixer. The

7

Ti/

feed and recorder

-1

5

or, in some cases, at minus

-8

COUGH BUTTONS

FIGURE

Chairperson-Controlled Muting

By activating a switch, the chairperson can silence all the
other microphones and be heard without interruption. To
establish this function, connect together all the Mute In ter­minals of all the mixers, but make no connection to the logic
terminals of the chairperson's microphone channel. Wire an

SPST pushbutton or toggle switch between

if

Logic Ground terminals of any one channel except the

chairperson's channel. See Figure
The Chairperson-Controlled Muting feature is also illustrated

81

[see Figure

Note that the Logic Ground terminals of both mixers are

connected, and that the chairperson's logic channels are un-

connected.

as part of a large system for a legislature.

9

the Mute In and

10.

0

/

GATE OUT

0

01

CcHAIRPERsoN7.s

LOGIC GROUND
MUTE IN
OVERRIDE IN

CHANNEL

Loudspeaker Muting

In some applications, a loudspeaker is located near each
talker to provide audio reinforcement or to allow monitoring
of a telephone conversation or conference. Each
loudspeaker can cause feedback unless it is automatically
switched off whenever the talker near it speaks. To provide

this function, connect the Gate Out terminal of each channel
to a separate loudspeaker muting relay as shown in Figure

3.

Recommended relays are Guardian 1345-1 51 20 or

1

&

1475-1 C-120, or Potter
R1 0-El -Y2-V185, or equivalent.

Brumfield R50-E2-YI-l2V or

CHAIRPERSON-CONTROLLED MUTING

FIGURE

Disabling the Gating Function [Bypass)

To keep all the microphones on, wire all the Override In ter­minals together to a Logic Ground terminal [see Figure 1 I].
This is useful if it is desired to bypass the automatic function­ing and use the AMS mixer as an ordinary mixer.

GATING BYPASS

FIGURE

Remote Channel-On Indication

The LED's above each Channel control light when that chan­nel is gated on. These indicators can be used to identify the
talker or to indicate to each talker when his or her
microphone is on. To provide Channel-On indicators at loca­tions remote from the mixer, connect
power supply to the Gate Out terminals as shown in Figure

12. NOTE: To avoid switching clicks in the audio outputs,
do not ground the power supply negative terminal to the
audio system ground or rack ground.

IMPORTANT:

phone audio signal
shielded pairs
on a shielded pair may result in audible clicking due to
capacitive coupling between the dc power lines and the
AMS microphone lines.

If a single cable

and

the dc power for the

must

be used. Failure to carry the dc power

10

GATE OUT

LOGlC GROUND
MUTE IN
OVERRIDE IN

11

LED's and a 5-volt

is

to carry the AMS micro-

LED,

separate

GATE OUT
LOGlC GROUND
MUTE IN
0VERRIDE.IN

i

R= 150-300n

SUPPLY

Each loudspeaker should be placed behind its associated
microphone to prevent the loudspeaker from gating on the

microphone. The loudspeaker volume should be low;
wise the microphone may not gate on reliably when the
talker speaks.

If the existing sound system uses 24-volt relays, they can be
driven with the AMS Gate Out using internal wiring

tions described in the Internal Wiring Modifications section.

POWER

OVERRIDE

"Filibuster" Mode

In normal operation, when several people talk, all their
microphones gate on so that no speech is missed. But with
the mixer wired for "Filibuster" action, a microphone that is
gated on prevents other microphones from gating on. Once

a microphone is on, other microphones cannot gate on until
the talker has paused long enough so that his or her

microphone has gated off. Thus, the person talking has the

floor and cannot be interrrupted.

To establish this function, perform the internal Mute to In-

hibit jumper change [see Internal Modifications section for

details]. Then connect all the Mute In terminals of the

modified channels together; connect all the Gate Out ter-

minals of the modified channels together, and connect the

Gate Out terminal of one modified channel to the Mute In

terminal of another modified channel [see Figure

IN^

LOUDSPEAKER MUTING

FIGURE

13

GATE OUT
LOGlC GROUND

MUTE IN
OVERRIDE IN

other-

modifica-

F:gR

AMPLIFIER

141.

REMOTE CHANNEL-ON INDICATORS

FIGURE

12

FILIBUSTER"

FIGURE

MODE

14

NOTE: To prevent high-frequency oscillation, do not wire a
channel's Gate Out terminal to its own Mute In terminal until
the Mute to Inhibit change has been made.

Preventing Room Noise Modulation

This connection keeps at least one microphone on to
eliminate varying background noise or "pumping" [see Figure

151. The channel to which the transistor collector is con­nected will be turned on whenever all other microphones are

gated off [see AMS Theory section].

GATE OUT
LOGlC GROUND
MUTE

IN

OVERRIDE

IN

ROOM NOISE MODULATION PREVENTION

FIGURE

15

Microphone Lock-On

The circuit described in the preceding paragraph can be

expanded using diode isolation to perform a new function.

Even with the advantages offered by the AMS, there may be
installations where it is desirable that the last microphone
gated on should remain on until another microphone turns
on. For instance, the sound reinforcement requirements of a
church may dictate that an altar microphone remain on as
the minister moves outside the acceptance angle. Normally
the AMS would not remain gated on after the initial hold time
elapsed if the minister continued to speak outside the accep­tance angle.

With the circuit shown in Figure 16, the last microphone

to gate on remains on indefinitely. When a new microphone

gates on, it will release the lock-on for the previous

microphone, and the new microphone will lock on. The result

is the ultimate in automatic mixing: each logic-wired
microphone is capable of remaining on until no longer need­ed. Note that if two or more AMS microphones are

simultaneously gated on, normal AMS action will take place.

Since at least one microphone is always on, this circuit also

prevents room noise modulation. Bath the Mute In and

Override In remain usable for additional functions.

For each microphone to be given lock-on capability, the cir­cuit uses a
sistor [Motorola], a
number of 1

2N2222 general-purpose amplifier NPN tran-

I-megohm, %-watt resistor, and a

N4148 diodes [GE] equal to the total number of
AMS channels involved. For instance, if the installation has
five AMS channels to be wired, a circuit containing one tran­sistor, one resistor, and five diodes must be constructed for
each channel [a total of five transistors, five resistors, and

25

diodes]. The circuit in Figure 16 shows only

circuit; similar circuits must be constructed for all

one

lock-on
lockan

channels.

GATE OUT
LOGlC GROUND

MUTE

IN

OVERRIDE

IN

MICROPHONE LOCK-ON

[ONE CHANNEL SHOWN]

FIGURE

16

Diode Isolation of Logic Controls

Two or more control functions using the same logic ter­minals can be isolated with diodes. Here a channel can be
muted by an overall group mute switch, or by its own cough
button [see Figure 1

GROUP

MUTE

f

71.

COUGH BUTTONS

GATE OUT
LOGlC GROUND

MUTE

IN

OVERRIDE IN

D=

IN4148

OR

EQUIVALENT

DIODE ISOLATION OF LOGIC CONTROLS

FIGURE

'I7

External Logic Devices

The AMS logic levels are directly compatible with TTL and
5-volt CMOS logic families. The following example [Figure

181 uses logic gates to perform the same function as the
diode isolation of logic controls. Suggested
CMOS NAND gates in Figure 1 8 are

TL, LSTTL, or

I/2-7420,74LS20 and
74C20. Suggested AND gates are 7408, 74LS08 and
74C08. In the example, the output of the Channel-On in­dicator goes to logic High if any channel gates on. [For infor­mation on logic gate use, see the

CMOS Cookbook,

both by D. Lancaster, Howard Sams

TTL Cookbook

and

Publishing Co.]

--

GATE

OUT

+3

c3

0 0

-

MUTE

IN

0

EXTERNAL LOGIC DEVICES

FIGURE

18

NAND

GATE

15-Volt CMOS

The logic terminals can be used with 15-volt CMOS if a pull­up resistor is used with each Gate output [see Figure

LOGIC

GROUND

MUTE

IN

OVERRIDE

IN

I

GI'

191.

Direct Out Gating

The AMS mixer Direct Out jacks can be converted to direct
gated outputs so that the level and equalization of each
microphone can be controlled by a studio mixing console.
Automatic gain adjustment [based on the number of

gated­on microphones] is out of the circuit-the decay time is
slightly decreased below the normal 0.3-second interval.

The logic circuit for direct out gating is shown in Figure

21
All resistors are 114 watt, and the opto-isolator can be a
Vactec
50k to

VTL5C2 or VTL2C2, or a Shure 86A8900. The
100k resistor is optional; it will provide a finite Off-

Attenuation capability.

.

15-VOLT CMOS

FIGURE

19

Digital Controls or Microcomputers

The AMS mixer logic terminals can interface with custom­designed digital control circuitry or even a microcomputer
for unlimited possibilities of system control functions.

Wireless Microphones

A wireless microphone can be used with the AMS [without
automatic gating], but its connection may be made in several
ways. If the microphone has a line-level output, connect that
output to the AMS mixer auxiliary input. An alternate
method is to connect a short jumper between the Logic

Ground and Override In logic terminals of the channel that

will contain the wireless microphone. With this channel per-

manently gated on, the balanced microphone-level output of

the wireless microphone receiver can be connected to the

channel input. [If the receiver has only a balanced line-level
output, the same connections can be made, but a line

at-

tenuator such as Shure's A15LA should be used in the

receiver-mixer line.]
The wireless microphone can be switched on and off re-

motely by grounding both the Mute In and Override In logic
terminals, and putting a switch in the circuit from the Mute
In terminal to ground. Since the Mute In circuit has
precedence over the Override In circuit, the microphone can
be controlled by this in-line switch.

If the wireless microphone receiver has a balanced line-level
output, a circuit can be constructed to use the wireless
microphone and still retain the automatic gating function. In
the diagram shown below [Figure

201, the value of resistor

R can be determined as follows. Start with a 2-megohm
resistor; at this value, the channel will not gate on until it
receives a signal of at least 0.01 6 volts [-36

dBV]. All AMS
logic terminals operate normally with this circuit. The gating
threshold can be varied by using a different resistor value ac­cording to the needs of the installation. Note that the
threshold is raised by a higher resistor value [more voltage is
required to overcome the threshold].

FROM

WIRELESS

LINE LEVEL

RECEIVER

OUTPUT

TOAMS

MIC

INPUT

MIXER

Note that if a gated mixed output is not needed for the
modified channel, that channel's internal circuitry can be
modified rather than using the logic terminals. In this man­ner, the level control and decay characteristics can be re­tained, and the need for a separate power supply eliminated.

[IMPORTANT: This conversion removes each modified
channel from the AMS main mix bus, so that it operates in­dependently of the unmodified channels. In other words, the
automatic gating function is retained, but the automatic mix-

ing is not.] Refer to the Internal Wiring Modifications section
for further information.

DIRECT

OUT

300

+5v

POWER

SUPPLY

GATE

LOGIC

MUTE IN

OVERRIDE

OUT

GROUND

(AUX/HIZ

MIC

IN

GATED

OUT

LEVEL)

DIRECT OUT GATING USING LOGIC

FIGURE

21

Controlling Non-AMS Microphones

If it becomes necessary to use a conventional [non-AMS]

microphone in an AMS system and the non-gated

microphone must be turned off whenever an AMS

22

microphone is gated on, the wiring shown in Figure

ac­complishes this easily. Connect the non-AMS microphone to
the desired input, and other AMS microphones to the other
AMS mixer inputs.

Connect the selected channel Logic Ground to its Override
In. Wire the Gate Out terminals of the remaining channels
together and connect them to the Mute In of the selected
channel. Now, whenever an AMS microphone is activated,
the Override In and Mute In on the non-AMS microphone
channel are grounded, and the Mute In takes precedence.

MICROPHONE INPUT TO GATING LlNE INPUT

FIGURE

20

An external, or "outboard," circuit can also be constructed
to use a conventional [non-AMS] microphone with on and off

not

gating. However, it will

have some of the primary AMS

microphone features: it will
gating will
adjustment
quired circuit. Note that

effective gating threshold are adjusted by potentiometer

[level adjustment-but not gating adjustment-is available
using the Channel Gain control]; [2] circuit power is supplied
by the mixer; [3] a metal enclosure must be used for

shielding, and

-

0

not

be direction-sensitive, and gating threshold

will

be necessary. Figure 23 describes the re-

[4]

transformer lead P2 is not used.

-

n

0

a-

-

-

CONTROLLING NON-GATED MICROPHONES

OPERATING HINTS

Phasing

Proper microphone cable phasing is essential to AMS opera­tion. If pins 2 and 3 are reversed in a cable for a conventional
balanced, low-impedance, microphone-mixer setup, the
microphone will still function properly [although with revers­ed polarity]. But if pins 2 and
cable, sounds from the
on. The AMS mixer perceives the microphone's rear as the
front and vice versa, and the microphone gates on only if the
rear becomes the voice entry. In
conductors that start out as pins 2 and 3 should end up as
pins 2 and

0
0

-

3,

respectively.

not

sense ambient room noise,

[I ] microphone-on sensitivity and

-

A

0

0

0

FIGURE

rear

-

3

0

0

0

-

22

3

are reversed in an AMS

of the microphone will gate it

all

GATE OUT
LOGIC GROUND

MUTE

IN

OVERRIDE IN

AMS installations, cable

R5

undesirable switching clicks. The switch must be a snap­action type, such as a toggle switch.

The circuit must be housed in a metal enclosure for shielding
purposes. The enclosure is grounded to pin 1 of the
microphone cable, but must be insulated from accidental se­cond grounding through the enclosure mounting surface.
This avoids the formation of a ground loop in the microphone
circuit.

Grounding

In AMS wiring, avoidance of ground loops is of the utmost
importance. Good grounding practices must be followed
when using extension cables, junction boxes, and cable
snakes.

Each microphone cable shield must be connected to ground

only

at the mixer. For instance, if the shielding shell of an
XLR connector is connected to pin 1, and the connector is
plugged into a grounded junction box, the cable shield
becomes tied to ground at the junction box
The result may be a considerable amount of hum and rf in­terference. Good noise rejection is virtually guaranteed by
elimination of ground loops.

MIC MlXE

510fl,5%

f

and

R

the mixer.

Microphone Muting

To install an at-the-microphone muting switch for an AMS

channel
cuit is slightly different from that of
microphone and mixer. Figure 24 shows the required
ponents. All resistors are 1/4-watt, and the capacitor is a
metallized polyester film non-polarized type [CDE
MMWA05W5-20, Sprague 431 P505X9R51. A low
leakage current capacitor like this is needed to avoid

without

PR

p13

1

P

P3

using the logic terminals, the required cir-

LO

z

NlC.

IN

I)

*

MARY

2

-

rRy

INPUT

s

2

DC

RESISTANCE

PI-P3

75

S1-S2

4300

TRANSFORMER

OHMS

OHMS

mas2

P

2

(f

a

R1

7%

20%)

conventional

corn-

R2

1

OOK

MICROPHONE MUTING

FIGURE

INTERNAL WIRING MODIFICATIONS

This section describes the internal wiring modifications
previously mentioned.
logic function for detailed information on the purpose of each
modification.

Refer to the specific application or

24

-

11

02

C3

154F

C

4

Parts

C1: .22uF 1OV
C2: 2.2uF 16V
C3: 15uF 16V
C4: 2.2uF 16V
J1: Female XLR
J2: Male XLR
(31: 2N5210 TI : Shure A95UF or 90A8032

R6

Q2: 2N5087
R1: 75K
R2:
R3: 33K watt

R4:

R5: 20K Pot. Audio Taper
R6: 4.7K

I14

100K

4.7K

watt

Ih

'1.1

'14

J2

watt
watt
watt

NON-AMS MICROPHONE GATING CIRCUIT

FIGURE

23

I

No user-serviceable parts inside. Refer all ser­vicing, including modifications, to qualified ser­vice personnel.

AMS mixers can be disassembled for modification as
follows:

1. Remove line cord from ac power source.

2. Remove screws securing top cover to chassis.

3.

Remove individual Channel boards by removing: [a]
Channel board bottom screw; [b] Channel Level con­trol knob and nut; [c] Channel-On LED leads; and
ribbon cable connector.
When replacing Channel boards, be sure to perform

4.
steps

3[a] through 3[d] in reverse order.

240-Volt Operation

To change the AMS mixer operating voltage from 120 Vac
to 240 Vac, follow these steps.

Locate the Power board [A5].

1.

2. Remove the transformer TI plug from connector P501
[marked 120 VAC], and carefully insert it in connector
P502, making sure all four pins are properly engaged.
Remove the 0.25N250V fuse from the fuseholder

3.

marked F502 and insert the
[packaged with the AMS mixer] in the fuseholder mark­ed

F501.

4. Replace the ac connector with one designed for the

240-volt source. If the mixer is to be used outside the

U.S. and Canada, local regulations may require replac­ing the line cord with one having wire insulation colors as
follows:

U.S.,

Canada

Europe

Mark the rear panel of the mixer with the new operating

5.
voltage.

WARNING

TI 25 mN250V fuse

"Live"

or Earth or

"Hot" Neutral Ground

Black White

Brown Blue

Green

Green/Yellow

[dl

I

MASTER BOARD A2: XR201

FIGURE 25

Mute Precedence to Override Precedence

As supplied, if a channel's Mute In and Override In logic ter­minals are both grounded, the mute function will take
precedence over the override function and the channel will
be muted. With the following modification [both logic ter­minals grounded], the override function will take precedence
over the mute function and the channel will be forced on.

Note that after modification the Channel Level control will

still turn the channel off at the full counterclockwise position

[independent of the Override IN terminal].
1

.

Remove the top cover and Channel board to be modified

as described above.

2. Locate jumper

XI04 and jumper holes XI03 on the

Hold Time Increase

The 1 .O Sec position of the rear-panel Hold Time switch can
be increased to 2 seconds as follows.

1. With the top cover removed, locate the Master board
[A21.

2. For AMS4000 mixers, no further disassembly is
necessary; for

8

board as described above.

3. Solder a
marked

4.

Reassemble the mixer and mark the Hold Time switch

position

Pre-Fader to Post-Fader

The Channel Level control can be rewired to

rear-panel Direct Output level as follows.

1

.

Remove the top cover and the Channel board to
be modified.

Locate jumper XI01 at the top center of the

2.

Channel board, and jumper holes
bottom center of the board [see Figure

Unsolder the jumper at XI

3.

new jumper in the holes of

Reassemble the Channel board and top cover as

4.

described above.

AMS8000 mixers, remove the Channel

30k,

lh

W resistor in the Master board holes

XR201 [see Figure 251.

"2.0" instead of "1 .Om.

also

XI02 at the

01

and solder it or a

XI 02.

control the

261.

CHANNEL BOARD A1: XI01 AND XI02

FIGURE

26

Channel board near integrated circuit U105 [see Figure

271.

3. ~nsolder the jumper at XI
jumper in the holes of

4.

Reassemble the Channel board and top cover as

described above.

Mute Action to Inhibit Action

As supplied, a channel will mute when the Mute In terminal is
grounded. After this modification, grounding the Mute In ter­minal will not affect the channel if it is already "on", but it

will

prevent the channel from gating "on" from the "off"
condition. The modification is required for the "Filibuster"
mode previously described, and for any logic
quiring connection of a channel's Gate Out terminal to its
Mute In terminal.

1 . Remove the top cover and Channel board to be modified

as described above.

2. Locate jumper

bottom front of the board [see Figure

3. Unsolder the jumper at XI05 and solder

jumper in the holes of

4. Reassemble the Channel board and top cover as

described above.

XI 05 and jumper holes XI 06 at the

04

XI

03.

XI 06.

and solder

terminal use re-

271.

it

or a new

it

or a new

riel

t

4.

5.

6. NOTE: With this modification, the Direct Out jack now

iuue resistor R101 [near the Direct Out jack] and
replace it with a
Reassemble the Channel board and top cover as
described above.

provides a gated signal for that channel only. The signal
is high impedance
nominal signal level of -24
with the Channel Gain control centered.

directly with the Channel Level control. This is equivalent

to a typical aux

balanced,
ing transformer [Shure
nominal level then becomes -48

5.6k, 5% resistor.

[2.8k to 5.6kl and unbalanced, with a

dBV [60 mV] for 74 dB SPL

It

will vary

level signal. To obtain a low-impedance

mic-level output, plug in an impedance match-

A95U or equivalent]. The

dBV [4 mV].

BEFORE AFTER

EHANNEL BOARD A1: X103-XI06

FIGURE

Gated Direct Out

In contrast to the external wiring additions described under
Direct Out Gating [see Logic Functions], this modification
applies the internal gated microphone signal to the Direct
Out Jack while retaining the optimized decay characteristics
and avoiding the need for a separate power supply. It does,
however, remove the modified channel's gating from the
mixed output, and thus does not provide automatic gain ad-

justment for the number of open microphones.

1. Remove the top cover and the Channel board to be
modified as described above.

2.

Locate and remove resistor R119 and the wire jumper
physically located between

[see Figure 281.

3.

Solder a wire jumper between the hole closest to the
printed marking
hole closest to it.

"R119" and the now-empty jumper

27

R119 and capacitor C118

DIRECT GATED OUTPUT CONVERSION

FIGURE

Direct Out to SendlReceive Jack

AMS Direct Out jacks can be modified to function as
sendlreceive jacks for use with equalizers, limiters, voltage­controlled amplifiers, or other external devices. With the
modification, the microphone output signal is present on the
tip terminal of
ring terminal. The modified output signal has a nominal level

dBV at 74 dB SPL [I 7 dB higher than the unmodified

of -37
Direct Out signal level], with a maximum level of

1 28 dB SPL. The patch point is before the Channel Gain con­trol [pre-fader] and before the microphone gating [ungated].
The external device should be a nominally unity gain circuit,
with an input impedance of
pedance it sees at the ring terminal will be 5k to 1
pending on the Channel Gain control setting. Consult the pro­cessing device manufacturer's literature for proper applica­tions within these

1. Remove the top cover and Channel board to be modified
as described above.

2. Locate,
R104 and jumper

Figure

3.

Solder a wire jumper between the XI07 solder hole

farthest from the edge of the board

solder holes
R104 resistors.

4. Locate point
J1 01
ing [board edge] solder hole of

5. For circuit continuity when a processing device is
connected to the modified Direct Out jack, wire the
jack's tip and ring switching terminals together. [These

J101, and the return input signal is on the

level and impedance limits.

unsolder and remove resistors Rl 01 and

XI07 on the Channel board [see

291.

between

Z

on the Channel board [ring terminal of

1,

and solder a jumper between it and the remain-

28

+

17 dBV at

10k or greater. The load im-

Ok, de-

and one of the

the now-removed Rl 01 and

XI 07.

not

are the two terminals on the opposite side of the jack body

RlOl and R104.] Wiring can be made at the top

from

after

[component] side of the board, or to the foil side
bottom
run
plate to assure proper bottom plate fitting. Replace the bot-

tom

by plugging a stereo phone plug with the tip and ring shorted
into the Direct Out jack; this must be done if the modified
Direct Out jack is to be used as a standard Direct Out jack.

plate is removed. Note that the jumper should be

around

plate after wiring. Note that a similar effect is obtained

the area that will be covered by the bottom

the

24-Volt Relays

To use the AMS Gate Out logic terminal to drive 24-volt

relays for loudspeaker muting, each channel must be
modified as follows:

.

Remove the top cover and Channel board to be modified

1

as described above.

2.

Unsolder and remove diode Dl 13.

3. Reassemble the
described above.

4. To prevent circuit damage caused by inductive

"kickback" when the relay is de-energized, make cer-

tain a diode is

Figure 13.

Channel board and top cover as

placed across the relay coil as shown in

AMS4000: Input Expansion

Model AMS4000 mixers, supplied with four input channels,
can be expanded to accommodate up to eight AMS
microphones through the

boards [RKCI

88;

order one for each additional channel] and

installation of additional Channel

a nine-connector cable
to eight inputs].

Remove the top cover as described above.

1.

2. Remove the five-connector
boards, and remove the blank rear

to be added.

Using the

3.

the front-panel nameplate holes for the LED [0.254"

&0.005"] and Channel Level control [0.296"

&0.005"] for each channel to be added. [NOTE: It is
not necessary to
locator pin.]

4. lnsert the new
sure the rectangular locator pin is properly centered in
its mounting hole.

5.

Use the supplied nut and washer to secure the level con-

trol to the front panel.

6. Secure the new Channel board bottom plate with the
screw previously removed.

7.

lnsert the new Channel-On LED through the front panel
[leads first] and use a 3/16" nut driver against the
bezel to secure the LED flush with the front

tach the white and red LED leads to the marked ter-

minals at the upper front of the new Channel board.

8.

Attach the new nine-connector cable to the Master and

all Channel boards [unused channels will have unused
connectors].

Attach the new Channel

9.
the cover.

template supplied with the Channel board, drill

[RKCI 89; one cable provides for up

cable between the Channel

plate of each channel

drill the front panel for the rectangular

Channel board from the rear, making

panel. At-

Level control knob and replace

CHANNEL BOARD Al: X107, R101, R104,

FIGURE 29

"2"

TROUBLESHOOTING

Symptom

No sound

Probable Cause

Channel control or Master
control turned too far down;
microphone not connected; mix­er not connected to power
source; Power switch not on.

Defective microphone, cable,
channel input module

Defective power amp, tape deck,
speakers, etc.

Defective AMS mixer

To Diagnose or Correct

Make indicated correction.

Listen to headphones plugged into the AMS front-panel Phones jack.
To check microphone: replace suspect microphone with another. If
there is sound, replace microphone cartridge and PC board
assemblies, or return the unit to the Shure Service Department for
repair; if there is still no sound, check cable.
To check cable: replace suspect cable with another. If there is
sound, repair or discard defective cable; if there is still no sound,
check input module.
To check input module: connect microphone and cable to another
channel and follow initial setup procedure. If there is sound, replace or
repair defective input module; consult Shure Service Department.

If there is sound on headphones but no sound from system, check
other components and cables.

If there is no sound on headphones, consult Shure Service

Depart-

ment.

Erratic micro-

phone gating

Distorted sound

Micmphone gates
on for unintended
sound

Excessive hum

from one or more

microphones

[proper gating

"

may be affected]

Reflective surf ace near
microphone (closer than 200 to
300 mm

-

8

to

1

2

in.] not in­cluding table top for surface­mount models); talker positioned
on edge of microphone accep­tance angle; excessive steady
room noise; defective
microphone, cable, or input

chan-

nel.
Master or Channel control set

too high; defective microphone,
cable, or channel; defective
amplifier, recorder or speaker

Source is within

1

20° accep-

tance angle

1

If source is outside

20° accep­tance angle: microphone cable
defective or wired out of phase;
input module defective

Microphone input ground loop

If there is a reflective surface near microphone, and neither micro­phone nor surface can be moved, cover surface with a
absorptive material at least 100 mm

[4

in.] thick. If talker is on edge

sound-

of acceptance angle, move microphone or talker. If loud, steady room
noise [typically from air-handling equipment] swamps out quiet
speech, reduce room noise to reasonable conference levels or reduce
talker-to-microphone distance. If none of above, check microphone,
cable, channel, and system as in NO SOUND section. Check

teleconference equipment for malfunction.

Check that red Overload LED is not on: if on, turn down Channel or

Master control until LED goes out. If overload LED is not on, check
whether one or all microphones sound distorted. If only one is
distorted, check microphone, cable, and channel as in NO SOUND
section. If all microphones are distorted, remove AMS mixer from

system, and connect undistorted signal source to system inputs. If

final sound is still distorted, check system components and recording

level. If final sound is

not

distorted, consult Shure Service Depart-

ment.

Move source or microphone if possible.

Repair or replace cable. If microphone still gates on, check micro­phone and channel as in NO SOUND section.

Make sure microphone cable shield and 3-pin connector shells are

not

connected to earth or grounded metallic objects. Shield ground

connection should only be made at AMS mixer input connectors.

15

EFFECTS OF ACOUSTIC ENVIRONMENT ON GATING
Room Noise:

talker's sound pressure level must be at the microphone for
it to gate on reliably. Generally this is not a problem because

people tend to talk louder in noisy environments. In addition,
the sensing circuitry has been equalized to reduce sensitivity
to room noise. If room noise is causing erratic gating, in­struct the talker to stand or sit closer to the microphone.
The quieter the room, the farther the talker can be from the
microphone before gating becomes erratic.

Gating action is degraded most by continuous noise [such
as from air-moving equipment]. Noises of a transient
nature, including outside-acceptance-angle talking, have little
interfering action.

Reverberation:

microphone so that the sound field at the microphone is dif­fuse, the microphone will not stay gated on. For example, in
highly reverberant environments the microphone may gate
on initially for a distant sound source, then gate off when the
reverberant sound field builds up.

The less reverberant the room, the farther the talker can
be from the microphone before gating becomes erratic.
Typically, a source will gate the microphone on reliably up to
2

to 6 meters away [6 to 20 ft], depending on the

reverberation time and noise level of the room.

Talker Distance and Angle:

from the microphone and his speech becomes weaker in
comparison to room noise and reverberation, the accep­tance angle for reliable gating narrows.

If the talker speaks just outside the acceptance angle, the

microphone will gate on occasionally. It will not gate on at all if
the talker is well outside the acceptance angle.

Reflective Surfaces:

face behind the microphone can hamper proper gating. The
microphone should be at least 1 meter
behind it, and at least 0.3 meters
it such as large ashtrays or briefcases. For this reason, do
not place the AMS mixer on the table near the rear of the
microphones.

The louder the room noise, the greater the

If the talker is far enough from the

As the talker moves away

Sound reflections from a hard sur-

[3

ft] from a wall

[I

ft] from objects behind

An idealized multi-microphone installation would have the
microphones spaced in a uniform, diffuse sound field, and
operated at identical effective gains. Under these condi­tions, the increase in system gain compared to a single
microphone is given by:

G[dB] = 10 log1 n

Where

In a sound reinforcement application, total system gain
must be adjusted below the feedback threshold with all
microphones operating. In a conventional mixing system,

each microphone would have G decibels less gain-before-.
feedback available than it would operating alone. G also

represents the increased pickup of room noise and
reverberation compared to a single microphone.

A solution to these problems is to gate on only the

microphone or microphones immediately in use. The Shure
AMS does this effectively and reliably by gating an individual
microphone on only in response to a sound source within its
acceptance angle. A microphone will not gate on for diffuse
room noise and reverberation, or for sound sources outside
its acceptance angle. While gated on, the AMS microphone
has a standard cardioid [unidirectional] pickup pattern
or hemi-cardioid in the surface-mount AMS221.

With multiple talkers, or a talker within the acceptance

angle of more than one microphone, a number of
microphones may be gated on simultaneously. In a
reinforcement application, if the system gain is set below the
feedback threshold with a single microphone gated on, the
increase in system gain with additional "on" microphones
could cause feedback. The AMS prevents
automatically reducing all "on" microphone gains by at least
G decibels as additional microphones are gated on. This
maintains constant total system gain, avoiding feedback and
permitting the maximum microphone gains at all times.

G is the system gain increase in dB over a single
microphone and n is the number of "on"
microphones.

[half-

sound-

this by

Early reflections from nearby walls may trigger occasional
microphone gating for sound sources outside the accep­tance angle. This is more likely to occur in smaller rooms ex­cited by narrow frequency range loudspeakers. This effect is
minor and should not cause any operational difficulty.

Operation on the verge of feedback [ringing], with sustain­ed feedback or with test tones, can be expected to cause
some gating due to standing wave patterns in the room.

APPENDIX

THEORY OF THE SHURE AUTOMATIC MICROPHONE
SYSTEM

Two problems associated with conventional multi­microphone installations result from unwanted sound pickup
from temporarily unused microphones. These microphones
contribute excess room noise and reverberation, reducing
clarity and intelligibility. They also increase the total gain of a
sound reinforcement system, pushing the system close to
feedback [howling] and reducing the gain-before-feedback
available to individual microphones.

II

This constancy of system gain means that pickup of room
noise and reverberation also remains constant at the level of
a single microphone. Except for transitions between zero
and one "on" microphone, gating action does not result in
audible modulation ["pumping" or "breathing"] of the room
noise. In applications where room-noise modulation caused
by the gating action of the first microphone is objectionable
[e.g., critical recording or broadcasting], the logic terminals
can be used to ensure that at least one microphone is always
gated on. An Override In terminal can be used to keep a
commonly used microphone gated on. Alternatively, the con­nection shown in Figure 16 can be used to force one of the
microphones on only when all the other microphones are
gated off.

As mentioned in the description of the Off-Attenuation
controls, partially rather than fully attenuating the "off"
microphone makes the gating action significantly less
noticeable. However, if insufficient attenuation is used, the
increased system gain from the unused but not fully off
microphones will result in room noise and feedback problems
approaching those experienced with all microphones fully on.

With a single microphone fully "on", the increase in system
gain due to incomplete attenuation in the "off microphones
is given by:

installer to check different combinations of "on"
microphones before declaring the system to be free of feed­back.

Where:

G'

is the system gain increase in dB compared to
full attenuation, Att is the Off-Attenuation in dB,
and m is the total number of microphones.

For an eight-microphone system:

-15

The recommended setting of

provides the benefit of
finite Off-Attenuation while degrading available gain-before­feedback by less than

dB. The Variable position set to

-8

1
[actually -8.5 dB, fully clockwise] provides very smooth
gating in applications where an increase in room noise and
degradation of gain-before-feedback of up to 3.0 dB is ac­ceptable. Automatic adjustment of the Off-Attenuation level
maintains these relationships when additional microphones

-00

are added through the linking of additional units. The

set-

ting is useful when a muted microphone should be fully off.

The above discussion assumed an idealized installation

with all the microphones operated under identical conditions.

In practice, of course, the microphones may not be operated
at identical gains, and the acoustical environment will vary at
different microphone locations. Because of its acoustical
location or the need for a higher gain setting, one
microphone will usually reach its feedback threshold before
the others and will establish the limit on system gain. The
AMS will act to prevent an increase in system gain beyond
that of the worst-case microphone operated alone.

APPENDIX

Ill

AMS MIXERS AND CONMNTIONAL MICROPHONES

If a conventional low-impedance microphone is connected

to the input of an AMS mixer, that channel will operate like a

normal mixer channel with the following exceptions:
1 . Normally no gating action will occur; the gate will be per-

manently off.

2.

The channel signal will be (compared to the properly
operating AMS channel] somewhat degraded through
loss of low-frequency signals [bass

6

dB/octave below 500 Hz], increase in noise, and loss

rolloff of about

in signal level.

will

However, the conventional microphone signal

be passed

if:

1 . That channel's Level control is sufficiently high.

2.

The Master Gain control is sufficiently high.

not

3. The Off-Attenuation control is

[-a].

set for minus infinity

Since the channel remains gated off, the Off-Attenuation
control determines the level at which the conventional
microphone signal will be passed.

A more desirable method of using an AMS mixer channel
with a conventional microphone is to use the Override In logic
terminal to force the channel on. In this mode, the

1

microphone signal is typically

5 dB higher than in the gated­off mode. Only the Channel Level and Master Gain controls
affect the microphone signal; the Off-Attenuation control
does not affect the signal because the channel is gated on.
Note that the use of a conventional microphone in one chan­nel will not affect other AMS mixer channels. They will con­tinue to operate properly with AMS microphones.

The formulas previously given in this section, which deter­mine the system gain controlling action, assume random
phase relationships among the sound fields at the
microphones. The approximation is valid for a large number
of microphones, but not for just a few. The sound fields at
two microphones will be in phase at some frequencies. The
combination of the two microphones will tend to increase
system gain by

dB at these frequencies, instead of the

3

6

dB for which the AMS compensates. Feedback can occur at
one of these frequencies with the gating on of the second

microphone, if the gain and phase criteria for feedback are
met. Although unlikely, this possibility should encourage the

.Many phantom powerable condenser microphones may
operate with the low dc voltage supplied by each AMS mixer
channel. For instance, Shure's SM85 will operate; however,
the microphone's clipping level is significantly reduced
because the powering voltage is lower than the minimum
rated voltage.

As this section indicates,

conventional microphone

can

a

be used in an AMS mixer channel. However, it must be
noted that signal degradation and loss of all automatic action
for that channel are the price paid for not using an AMS
microphone.

I

OFF-ATTEN

MODELS AMS4000AND AMS8000

FRONT PANEL

"oLoki;a

o

0.5

SEC

1.0

8

Om0

LINE MIC

OUTPUT

GATE OUT
LOGIC GND
MUTE IN
OVERRIDE IN

..

-

0

-.

MODELS AMS4000 AND AMS8000

REAR PANEL

The Sound of Professionals

Shure Brothers Incorporated

222 Hartrey Avenue

Evanston,

IL 60202-3696 U.S.A.

...

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