Audio AD256 User Manual

CONTENTS

SCENARIO 1

INTRODUCTION 3
UNPACKING 3
VISUAL INSPECTION 3
SPECIFIC POINTS 3

IMPORTANT SAFETY INSTRUCTIONS 5

POWERING 6

TECHNICAL SPECIFICATION 8

EQUALISATION CURVES 11

BLOCK DIAGRAM 12

CONNECTOR PANEL 13
CONNECTIONS 14

INPUT MODULE 16

OUTPUT MODULE 19

ADJUSTMENTS AND CALIBRATIONS
23

AD100-015 POWER SUPPLY UNIT 25

CUSTOMER NOTES AND FACTORY MODIFICATIONS 26

TECHNICAL LIBRARY 27

SCENARIO

Time was when it became necessary to update the ubiquitous AD145 PICO mixer.
Our survey showed two main requirements: as well as Audio Developments' build
and audio qualities, narrower and shorter modules were required, and the simple but
effective equaliser from PICO had to be retained. By this time, 4-track recording
equipment had begun to trickle into the marketplace (Nagra D etc), so we decided to
give the new mixer four outputs. Because of the lower noise-floor of modern digital
recorders, the level of input signals may be reduced. We have taken advantage of
this fact and lowered the slope ratio of the limiters to 7:1. By so doing, not only is
signal distortion greatly reduced, but also the artefacts associated with limiters
become much less noticeable or objectionable. Also, sixty years after the event,
sound engineers have come to realise the potential of Blumlein's M-S techniques; to
this end, facilities have been incorporated to take advantage of these
techniques. The result is AD146 - a four-output mixer.

This was followed by AD148 - edit mixer. Based on AD146, two comprehensive left
and right monitor modules have been added for editing purposes. If a mic/line
module is included for commentary or voice-over purposes, its input-gain switch can
be changed to a potentiometer, and its gain structure changed to ensure consistent
level matching with a fixed, mechanical point of reference. Unfortunately, this way of
working does reduce headroom and also compromises a mixer's noise performance.

After AD146/AD148 it was back to the drawing board. Our customers were still
demanding a two-output mixer - as a true replacement for the PICO, and with the
PICO's simplicity. Despite past assurances to the contrary, T powering is still
required. And could we incorporate auxiliaries? And could we possibly bring it to
market at 'entry level'? We have, and we have and that's MERCURY (AD147).

AD149 completes the 140 series of mixers, and has a repertoire of party tricks not to
be found in any other mixer - not even for 'ready money'. The design team took, as
its starting point, our list of all the ideas and suggestions presented to us over the past
few years. Many of these requests came from film-sound recordists - a sub-set of
recordists we have unintentionally neglected in the past.

1

Being latter-day converts, we have included circuitry to take full advantage of all M-S
techniques - even shuffling - in both production and post-production.

AD149 is Audio Developments' tribute to, and celebration of the genius of Alan
Blumlein.

Well - we HAD every intention that AD149 would complete the 140 series.
We'd reckoned, however, without the persistence - nay, insistence - of our customers
for a mains-operated version of the AD146 with four auxiliaries ...
we've called it AD144.

Radical change and PICO have proved to be uneasy bedfellows: never a matinée
idol and denied its dulce et decorum death, AD145 has now been repackaged in the
140 series metalwork - thereby reducing its size and weight. Facilities remain
largely unchanged: internally, the microphone amplifier has been replaced with
the one designed for AD146 and externally, the mono return is now in stereo form.
Now designated AD245, shall we have PICO - like the poor - always with us?

With the advent of multi-track recording and its general acceptance as a useful tool in
drama and film location; our market research has indicated a need for a sound mixer
with facilities similar to AD245 but with multi-track features, this giving birth to the
AD255 and AD256 Mixers. The addition of direct outputs from each input and two
auxiliary busses make up the main extra features over those of AD 245. The analogue
meters have been replaced by two LCD displays to enable the signal levels of the two
main outputs, two auxiliaries and an input module to be monitored simultaneously.

2

INTRODUCTION

Unpacking

If there are any signs of damage to the outside of the carton, please notify us or your
supplier immediately, regardless of the unit's apparent physical condition. This is in
case a claim has to be made at a later date because of previously undetected transit
damage. The packaging material should not be discarded until the mixer has been
acceptance tested and a suitable transit/storage case is available for secure, safe
storage.

Visual Inspection

Identification - please make a separate note of the serial number for your own capital
equipment records. Ensure that it agrees with the number on the invoice/packing
note. The serial number label is on the back cover, adjacent to the battery
compartment.

Temperature - check the meter glasses for condensation. If the package has been in
transit during cold weather, leave the mixer for at least 12 hours to allow it to return to
normal room temperature. Any measurements or subjective tests then made, will be
to a known temperature reference.

Specific Points

Battery compartment - the mixer has an integral battery compartment to accept
8 size-C cells, and is formed as part of the bottom transversal extrusion. This helps
to lower the centre of gravity, as well as adding to the rigidity of the frame.

DC-DC converter - is mounted on the top side of the battery housing and is
underneath the fader section of the modules.

In order to facilitate the testing and calibration of all modules, a set of two extender
modules is available from the factory.

Connectors - for convenience, the connector panel is labelled for reading from above.
All connectors are in line with their corresponding module.

3

Fuses - to protect the mixer and internal power supply are mounted on the power
supply/converter board. Access is gained by removing the output module. 20mm, 1A
ANTI-SURGE HRC fuses are used - one for the internal battery power supply and one
for the external DC supply.

Output Limiter threshold - is set at the factory at +8dBu, but an internal preset
potentiometer allows adjustment to any other level above 0dBu. The limiters are to
be found on the output module printed circuit board.

Input Limiter threshold – is set at the factor at +10dBu, with the input and output
faders at their calibration points. An internal preset potentiometer allows adjustment to
any other level above 0dBu and is mounted on the main input printed circuit board.

Microphone powering - the mixer will remain unconditionally stable if the powering on
unterminated input channels is switched off - this also improves the noise
performance and crosstalk. Powering - 48V phantom or 12V tonader - may be
selected before or after the microphone is connected to the module.

Module fix screws - Hexagon head screws are used to fix the modules and back
cover. The size being 1.5mm HEX A/F with a 2.5M thread.
If it becomes necessary to remove modules or back cover it is strongly advised using
a good quality hexagonal head driver.
Use of a screwdriver, however desperate, is not recommended.

We wish you many trouble-free hours of use from your mixer. As a company, we are
fully committed to BS EN ISO 9001. Should you have any problems or require any
further information, please do not hesitate to contact us on:- tel [phone] or by
fax on 01922 457008 or by e-mail [e-mail].

4

WARNING

IMPORTANT SAFETY INSTRUCTIONS

The user of electrical products must be familiar with their potential dangers, and
fundamental precautions must always be taken. Please read the following text
carefully.

Power supply units manufactured or supplied by Audio Developments Ltd are not user
serviceable. There are no user-serviceable parts associated with any such power
supply unit.

THE OUTER COVERS MUST NOT BE REMOVED

Such a power supply unit is solely for use with audio mixers and sound processors -
hereafter called the equipment - manufactured by Audio Developments Ltd.
Always use a cord set accepted by a National Approved Body.

EARTHING/GROUNDING: When using an external power supply unit that is
connected to the mains supply to drive the mixer it must be CONNECTED TO
EARTH.

In certain types of malfunction or breakdown, earthing provides a path of least
resistance for electric current and considerably reduces the risk of electric shock.

DANGER: Incorrect connection of the equipment grounding/earthing conductor can
result in the risk of electric shock. Where possible obtain a pre-wired mains lead from
a reputable supplier with the correctly fitted mains connector for the type of mains
outlet in use; otherwise, one correctly wired and checked by a qualified electrical
engineer. If your mains lead is not suitable for the mains outlet, have the correct plug
fitted by qualified personnel.

The MAINS PLUG of this equipment is the primary disconnect device. Therefore, in
the final application, ensure it remains close to the equipment and easily accessible.

5

POWERING

The mixer may be powered from either internal cells or an external DC power source.

The integral battery compartment requires a total of 8 size-C cells. Access is gained
via a captive lid which is retained by two, 90-degree-turn buckles. The lid hinges
outwards 45 degrees from the back panel. When installing new cells, the row
nearest the hinge should be fitted first.

Either conventional dry or rechargeable nickel-cadmium cells may be used. NI-CADs
may be recharged in situ through the 4-pin POWER IN connector. (The circuit for
recharging is already incorporated within the mixer.) A voltage in the range +15V to
+24V DC @ 250mA is required on PIN 2 of the POWER IN XLR. A suitable unit is
supplied by Audio Developments. (AD Part No 94-100-016)

When driving the mixer from an external power source, PIN 1 is the 0V connection
and a voltage in the range +12V to +15V DC should be supplied to PIN 4. The power
source should be capable of delivering approximately 1.0A - allowing some capacity
for phantom powering.

If an external power supply unit (PSU) is to drive the mixer and simultaneously charge
a set of NI-CADs, a current capability of at least 1.0A is required. It is poor practice to
run a PSU at its limit, therefore we recommend a minimum of 1.25A.
A suitable unit is supplied by Audio Developments.(AD Part No 94-100-015)

WARNING: When NOT using the PSU (AD 94-100-015) supplied for the mixer,
ensure your 4-pin XLR is correctly wired to match the POWER IN connector. Failure
to do so may result in the breakdown of the internal DC-DC converter. Make this
check even if using a PSU which may have been supplied to you in the past.

AD100-05 and AD100-06 PSU are NOT suitable for use with an AD256 mixer and
must not be used.

6

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7

TECHNICAL SPECIFICATION - ELECTRICAL

REFERENCE 0dB=775mV at 1kHz unless otherwise stated.

MAX GAIN MIC 75dB
LINE 45dB

MAX INPUT LEVEL MIC @ MAX GAIN -46dB @ MIN GAIN -1dB
LINE @ MAX GAIN -15dB @ MIN GAIN +20dB
STEREO-RETURN +18dB
TB RETURN +18dB

INPUT IMPEDANCE MIC >2k2R
LINE >6k5R
STEREO-RETURN >100kR
TB RETURN >20kR

MIC POWERING 48V PHANTOM & 12V TONADER

MAX OUTPUT +23dBm L-R TRANSFORMER BAL (600R LO

MONO, AUXILIARY 1 & 2

TRANSFORMER BAL (600R LOAD)
+18dBm DIRECT OUT UNBALANCED (600R LOAD)
H’PHONE UNBALANCED (600R LOAD)
TB SEND UNBALANCED (600R LOAD)

OUTPUT IMPEDANCE <60R L-R & MONO
<20R DIRECT OUT & MONITOR
<20R TB SEND

FREQUENCY RESPONSE 0: -1dB 20Hz to 20kHz L-R & MONO
0: -1.5dB 20Hz to 20kHz MONITOR PATHS

HARMONIC DISTORTION <0.05% @ 1kHz @ 0dBm OUTPUT
<0.15% @ 40Hz to 15kHz @ +15dBm OUTPUT

OVERLOAD INDICATOR ILLUMINATES @ +15dBu at the PRE-FADER POINT

CROSSTALK <-70dB 40Hz to 15kHz
INTERGROUP & INTERCHANNEL

NOISE MIC <-125dB EIN 20Hz to 20kHz; 200R SOURCE

LINE <80dB SNR 20Hz to 20kHz 0dBu IN & OUT

8

EQUALISATION HF: ±10dB @ 10kHz

LF: ±10dB @ 100Hz

MF: ±15dB
CENTRE FREQUENCY 2k5Hz Q=1.2

HPF:

SWITCHED PRE-TRANSFORMER -3dB @ 70Hz

VARIABLE POST-TRANSFORMER 20Hz to 250HZ

INPUT LIMITER THRESHOLD +10dB FADERS AT CAL POINT
FACTORY SETTING:­ ATTACK 2mS
RELEASE 200mS

OUTPUT LIMITER THRESHOLD +8dB
RATIO 7:1
ATTACK 2 SETTINGS INTERNALLY SWITCHABLE
1: 4mS 2: 0.8mS
RELEASE 2 SETTINGS INTERNALLY SWITCHABLE
1: 250mS 2: 80mS
FACTORY SETTING: SLOW

CURRENT CONSUMPTION 350mA (6 INPUT)
12V DC SUPPLY

9

TECHNICAL SPECIFICATION - MECHANICAL

SIZE

6 I/P 8 I/P 10 I/P 12 I/P
A 320 381 442 503
B 295 356 417 478

ALL DIMENSIONS IN MILLIMETRES

WEIGHT

6 I/P 8 I/P 10 I/P 12 I/P

6 7 8 9

ALL WEIGHTS IN KILOGRAMS

10

EQUALISATION CURVES

AD256 HIGH AND LOW EQ

AD 245 HIGH AND LOW EQ

20

15

10

5

dB

0

-5

-10

-15

-20
10 100 1000 10000 100000

FREQUENCY

AD255 MID EQ AND HPF

20

15

10

5

0

dB

-5

-10

-15

-20
10 100 1000 10000 100000

AD 245 MID EQ AND HPF

FREQUENCY

11

BLOCK DIAGRAM

12

CONNECTOR PANEL

13

All input and output impedances and levels are to be found in the TECHNICAL
SPECIFICATION.

All inputs to, and outputs from AD256 are to be found on the connector panel.

14

Module connector (4) accepts balanced microphones and balanced line-level inputs.

XLR (input & output) Pin 1 Shield
Pin 2 Signal +
Pin 3 Signal -

In the case of unbalanced line-inputs and outputs, pins 1 & 3 should be connected.
This will not lead to a loss of level.

Channel direct outputs (12) are at line level and unbalanced

Main stereo (2), mono (1) and auxiliary outputs (10) are transformer balanced and are
at line level.

The auxiliary outputs appear on two ‘A’ type stereo jacks.

The electronically-balanced stereo tape-return enters the mixer on two standard, ‘A’
type stereo jacks (9); left and right. Adjacent is the input calibration preset. For a
0dBu return signal, the system is calibrated when the preset is fully counter-clockwise.
From that point 20dB gain is available for lower level signals.

Unbalanced headphone monitor output appears on an ‘A’ type stereo jack (5). This
output is capable of driving 25R at 0dBu.

STEREO BALANCED JACK STEREO UNBALANCED JACK

Tip Signal + Tip Left signal
Ring Signal - Ring Right signal
Sleeve Shield Sleeve Shield

The 6-pin XLR connector, SUB (8), carries all signals to and from an outstation.
Reverse-talkback level may be set by REV TB preset (3).

XLR Pin 1 Shield Pin 4 Ret +
Pin 2 Send + Pin 5 Ret ­ Pin 3 Shield Pin 6 Control

The unbalanced send is low impedance with a capability of driving headphones of 25
ohms impedance or greater.

The balanced (or unbalanced) signal from the outstation may be at mic level or line
level. The mixer leaves the factory set for a line-level return, but the gain of the return
amplifier may be increased by 20dB via the DIL switch on the sub board attached to
the connector panel. Preset (9) controls the level of the return - which feeds on to the
PFL mixing buss and is routed by grounding the control line (Pin 6) at the external
source.

Thus, a two-way conversation can take place between mixer and boom operator with
the boom operator being able to listen to programme when no communication is
taking place. The programme is selected using a bank of five push-switches. Either a
main output signal or the boom signal (If the boom microphone is in input 1) can be

15

fed to the outstation. Selecting more than one source creates a mix of the chosen
signals.

A three-position rocker switch BATT/EXT (7) selects either internal batteries or an
external DC source. Correct power to the mixer is confirmed by the BATT indicator on
the lower LCD meter. The first bar of the indicator flashes when the internal voltage
falls below the safe operating level of 9V.

External powering of the mixer is via a 4-pin XLR (6).

XLR Pin 1 OV Pin 3 NC
Pin 2 Charge Pin 4 12-15V DC

A suitable external power supply unit can be supplied by Audio Developments Ltd.
(Part No 94-100-015), but any external DC source must be capable of delivering

1.25A at 12V.

MICROPHONE/LINE INPUT MODULE

16

17

The Microphone/Line input module functions are as follows: Switch (12) selects 48V
phantom and 12V tonader power for condenser microphones. Powering may be
selected before or after connecting the microphone, but switch off all powering on
unterminated modules to ensure unconditional stability of the mixer. For complete
safety of external equipment, switch off microphone power before connecting a line­level signal.

Phase change (15) is pre transformer and operates on microphone and line inputs.
∅1 is the normal position.

The switched high-pass filter (14) is pre transformer and, similarly, operates on
microphone and line inputs. In this position the filter protects the transformer from low­frequency saturation caused by wind, traffic, air conditioning etc. Operating
frequencies are to be found in the TECHNICAL SPECIFICATION. The high-pass filter
operates independently of the equaliser.

A continuously variable high-pass filter (2) is also incorporated and follows the
microphone pre-amplifier stage. This enables other troublesome low frequencies to be
attenuated and being variable can be adjusted to the optimum frequency and thus
minimising the effect on the required audio signal. This high-pass filter also acts
independently of the equaliser.

The input gain of the microphone amplifier is set by the rotary potentiometer (1) The
range of this control is 45dB. For line level signals, an input attenuator (30dB) is
inserted via switch (13); the MIC/LINE change-over switch.

To control the gain of the input amplifier a limiter may be introduced into circuit using
toggle switch (22) – limiting action being indicated by an LED (10). Factory settings of
the limiter can be found in the TECHNICAL SPECIFICATION. An internal preset
potentiometer allows a different threshold level to be set, but should be done with
great care. (An extender module is required if this adjustment is needed). Despite the
presence of the limiter around the input amplifier overload can still occur in later
stages. The limiter operates on both microphone and line level signals.

Following the microphone amplifier is the simple, but effective, equaliser which has
been retained from the original PICO - selector switch (19). Because of the inherent
inaccuracy of the centre-detent mechanism, it is not Audio Developments’ practice to
fit such devices to equaliser amplitude controls - HF (16), MF (17), LF (18). The three
equaliser controls have a mechanical push mechanism enabling them to be locked
away after setting, thus avoiding accidental adjustment.

The auxiliary section is incorporated below the equaliser controls. The two auxiliaries
are independent of each other; each has its own off/pre/post switch (20) and (21). The
off position disconnects the channel signal from the auxiliary busses and the pre/post
position routes the signal to the busses from either the pre or post fader position of
the signal path. The auxiliary routeing switches are independent of the channel L/R
routeing.

18

Auxiliary 1 is a fixed level and auxiliary 2 has a level control (6), which acts purely as
an attenuator - when set at 10 and source post-fader the output tracks with, and is at
the same level as the L/R outputs from the module. This also applies to the post-fader
position of auxiliary 1.

The unbalanced channel output is available on a XLR on the connector panel in line
with the input connector. The level of the direct out signal is controlled by preset (3).
The channel direct output signal can be from either the pre-fader or post-fader point
and is selected by toggle switch (4). A six way rotary switch (5) enables different
signals, other than the direct output signal, to be fed to the channel output. These are
the L and R mixes, a mono of the main L and R mixes (MIX) and AUX 1 and AUX 2.

A panoramic potentiometer - panpot - (7) routes the module’s signal proportionately
between the left and right mix busses.

The MUTE switch (8) follows the panoramic control. When selected it prevents the
input signal reaching the main mix busses. An internal DIL switch controls whether the
auxiliaries appear pre or post the mute switch. The factory setting is that the
auxiliaries are not affected by the mute switch. The channel output signal is not
affected by the mute function.

Pre-fader listen (9) with its LED indicator routes the module’s signal to the
monitor/PFL mix buss for auditioning on headphones.

An overload LED (11) illuminates 3dB before clipping at the input to the fader.

The Penny & Giles fader (23) is calibrated 10dB from its fully open position,
allowing the operator to work with 10dB of gain in reserve. Faders on adjacent
modules can be coupled for stereo operation by the use of standard ganging clips.

19

OUTPUT MODULE

20

This module controls the level of main and monitor output signals and metering of the
output and monitor signals (including PFL). Calibration is achieved when the output
faders (7) are at maximum.

The two LCD meters read the various outputs and battery status. The upper meter (1)
reads left and right outputs; the upper bar reading left output and the lower right
output. The lower meter (2) reads the auxiliary outputs, channel output and battery
status. The upper bar reads auxiliary 1 output, the middle bar auxiliary 2 output and
the lower bar the channel signal level at the pre-fader point. This is activated when
PFL is selected on an input module. Battery status is read on the six segment bar in
the lower left corner. Beginning with the sixth bar, each bar goes out in turn as the
battery level falls.

When the internal or external voltage drops below a safe operating level, the first
segment of the battery level indicator will flash. Meters may be illuminated by ILL (8).
If a greater level of back-light is required then a facility exists on each meter, via a DIL
switch, to achieve this. A second DIL switch changes the meter specification between
PPM and VU. Refer ADJUSTMENTS & CALIBRATION section.

The main output signal is metered and monitored after the output faders and limiters.

A pair of limiters may be switched in to the main output - LIM (12) - and linked for
stereo operation - LINK (9). LEDs (10) indicate when limiting is taking place. A choice
of fast or slow attack and release times may be made via the two DIL switches on the
output printed circuit board. When the limiters are being used as a stereo pair, they
should have their attack times and release times set identically. The factory setting for
the DIL switches is SLOW.

OUTPUT MODULE PRINTED CIRCUIT BOARD

21

Line-up tone, when selected (11), replaces the normal signals on the main output.
The frequency of the line-up tone may be selected between 1kHz and 10kHz (13).
Line-up tone is calibrated on the (PPM-scaled) meters to ‘0’, giving a line-output level
of 0dBu with the output faders set to maximum.

The auxiliary output signal levels are controlled by two potentiometers (3).

Separate Penny & Giles faders (7) are employed for maximum flexibility and control of
the main left and right output signals.

The remaining controls are associated with headphone-monitoring of signals from the
mixer - internal signals (DIR) or return (RET) may be selected (14): these signals may
be in the L-R domain or the M-S domain. When switches (15), (16) and (14) remain
unselected the main-output signal from the mixer appears on the headphones. If
L (15) is selected, the left-output signal appears on both ear-pieces; if R (16) is
selected, the right-output signal appears on both ear-pieces. If switches (15) + (16)
are selected, a matrix will enable mono compatibility to be assessed on the left ear­piece and out-of-phase components to be checked on the right ear-piece. When
working entirely in the M-S domain, the matrix will transform the mixer output into the
L-R domain for monitoring purposes. Selection of RET (14) will allow these functions
to be performed on a return signal. Selection of AUX MON (4) allows these functions
to be performed on the auxiliary output signals accordingly. For example if L (14) is
selected, the auxiliary 1 signal appears on both ear-pieces. The level of the monitor
signal is controlled by potentiometer (17).

A mono mixdown of the main, stereo output from the mixer is available and its level is
adjusted by MONO (18).

Various signals can also be sent to a separate outstation – External 1. The signal
selection is facilitated using the bank of five push-button switches (5) mounted on the
left side of the module. Push-switches Left, Right, Aux 1 and Aux 2 enable the main
output or auxiliary signals to be sent to the outstation. If more than one of the four
switches is selected a mix of the selected signals will appear at the outstation. If the
Boom push switch is selected the signal from input channel 1 is fed to the outstation.
This assumes input 1 is used for the boom microphone. The selection of the boom
signal over-rides the selection of the first four switches.

The signal level sent to EXT 1 is set by screw-driver operated potentiometer (20)

The signal from the internal microphone (19) replaces any other signal appearing at
the outstation when routed via EXT (21). This enables the mixer operator to
communicate with the outstation.

SLATE (22) allows the mixer operator to ADD a voice signal to the main stereo output
together with a low-frequency identification tone (100Hz).

AUX 1 and AUX 2 talk switches allow the mixer operator to communicate with the
auxiliary outputs. The talk signal replaces any other signal.

22

Communications from external 1 outstation to the mixer operator is initiated at the
outstation and feeds the PFL mix busses. This is achieved by grounding pin 6 of the 6
pin XLR. Like other PFL signals it is under logic control and replaces any other signal
on the monitor output.

When PFL is selected on an input channel or when the outstation wishes to
communicate with the mixer operator (by grounding pin 6 on the multiway), logic
changes the signal on monitor output accordingly.

23

ADJUSTMENTS AND CALIBRATIONS

Meters - are to broadcast specification and either a VU or a PPM may be selected by
an internal DIL switch mounted on each meter driver printed circuit board.

Line-up-tone oscillator - the preset (VR6) that adjusts its level at the output of the
mixer is mounted on the output module printed circuit board.

Low-battery indicator - the preset (BATT) associated with this facility is also mounted
on the meter module printed circuit board. The low reading, when the first battery
indicator bar flashes, is set at the factory and corresponds to a set of batteries having
discharged to 9V.

Left
BATT
Right
IC1
PPM/VU Input
ON
ON
ILL

Meter Calibration

Each meter drive card has one preset potentiometer calibration control per indicator
bar; they control the adjustment of the indicator bar reading in relation to the signal
level being measured. All other aspects of the meter specification are taken care of
within the software and fixed values within the circuit components.

Access is gained to the meter drive card by removal of the output module; the drive
card being mounted on the rear of its LCD indicator. Extender cables are required to
enable the output module to be powered and operated outside of the chassis.

Meter adjustment - Set an input module for a line input and introduce a 1kHz tone
from an audio signal generator. Adjust levels to achieve a reading of 0dBu, if setting a
PPM scale, or +4dBu if setting a VU scale, at the appropriate main output, on an AC
millivoltmeter. Adjust the associated preset potentiometer to give the correct meter
reading for the signal level set.

Battery level adjustment - The low reading is set at the factory and corresponds to a
set of batteries having discharged to 9V. If a different calibration is required this can
be set by the preset potentiometer marked BATT on the above diagram.

24

PPM or VU setting - A DIL switch controls this function and is shown on the above
diagram. When the switch is in its off position the meter follows a PPM characteristic
and in the on position a VU characteristic.

Meter illumination - A DIL switch enables the intensity of the meter back-light to be
altered. The factory setting is with the switch in the off position. It is recommended
that this setting is retained, because the higher intensity setting obviously increases
battery drain.

Main-output limiters - calibration involves two presets per output.
Using the 'L' output as the example …

Biasing and threshold are adjusted as follows: VR5 biases the limiter circuit to the
point of correct operation and VR4 sets the threshold. (Labelled SET 0 and THR
respectively.)

With no signal present, VR5 should be adjusted to give a reading in the
range

-1.5V to -2.5V at PIN 8 of IC 2. VR4 should be adjusted to give a reading in the
range -2.5V to -3.5V at PIN 12 of IC2.

Apply a signal at 1kHz to the mixer to give a level of 0dBu at 'L' output. Then select
the limiter function and adjust VR5 until the output signal starts to fall (typically

0.2dB). The DC voltage at PIN 8 of IC 2 should be approximately -2.0V.

After setting VR5, adjust the output signal to just greater than +8dBu with the limiter
deselected. Introduce the limiter and adjust VR4 until the output signal falls to
+8dBu. This is the THRESHOLD setting. (If a different threshold setting is required,
alter the signal levels accordingly.) The DC voltage at PIN 12 of IC 2 should be
approximately 0.6V different from that at PIN 8.

The right output can be set using the above procedure, but the preset and IC numbers
change to correspond with the output being calibrated.

The link function has no individual setting of its own. To ensure this works within
specification it is important that L and R outputs are set up as a pair. After following
the above procedure, the DC voltages at PINS 8 and PINS 12 of the ICs
should be identical. (Tolerance ±0.05V with typical figures at PIN 12 of -2.6V and
PIN 8 of -2.0V)

There are two DIL switches associated with the attack and release times: one for the
left output, the other for the right output.

25

POWER SUPPLY UNIT

TYPE AD100-015

The AD100-015 mains POWER SUPPLY UNIT is suitable for driving most of
AUDIO DEVELOPMENTS’ range of portable audio mixers. This PSU is a single-rail
device providing 1A of current at +15V DC potential and is used as a substitute for
battery power with mixers containing an internal DC-DC converter.

The AD100-015 may be powered from either a 110/120V AC source or a 220V/240V
AC source. Ensure that the AC voltage source is in the range 100V to 240V AC.
Operating the equipment at the wrong voltage could be hazardous. Care must be
taken to connect the LIVE, NEUTRAL and EARTH pins of the PSU’s IEC mains
connector to the corresponding terminals associated with the AC source. The PSU
contains an indicator that illuminates when the PSU is operational.

FOR SAFETY REASONS, AD100-015 POWER SUPPLY UNIT MUST BE
CONNECTED TO MAINS EARTH. Any maintenance to the PSU or its mains cable
assembly should be performed by a qualified engineer.

CHARGING: If nickel-cadmium cells are fitted in an AD250 series mixer, they may
be recharged in situ from an AD100-016 power supply - whether the mixer is in use or
not. (Maximum current is set at 250mA - in addition to the 500mA of current
supplying the audio electronics.). The charge circuit within the mixer is powered by
connecting to Pin 2 of the 4 pin DC Power XLR. DO NOT ACTIVATE THE CHARGE
CIRCUIT UNLESS THE MIXER IS FITTED WITH NICKEL-CADMIUM CELLS.
REMOVE CHARGE LINK FROM POWER-IN XLR IF DRY-CELLS ARE FITTED.

FUSE: A fuse link should be fitted to the mains supply to protect against fault
conditions

For continued safety the specified fuse link must be fitted in the mains fuse holder
when a replacement is required. Ensure it is of a type approved by a National
Approved Body.

DC-OUTPUT XLR PIN 1 OV PIN 3 NOT CONNECTED
PIN 2 CHARGE PIN 4 +15V DC

DO NOT REMOVE THE OUTER COVERS

NOTE: The power supply unit should be serviced by a suitably qualified engineer.
Only genuine spare parts with identical specification must be used.

It is DANGEROUS to change the specification or modify the product in any way.

250mA HRC TYPE T 240V AC

26

CUSTOMER NOTES AND FACTORY MODIFICATION

27

TECHNICAL LIBRARY

28