GREG HANKS DESIGN
Issue
1
Operation and Service Manual
BA-660
CONSCIENTIOUS AUDIO DESIGN AND MANUFACTURE
Use and Care of your Mic Pre/limiter
Greg Hanks Design
35 Hardscrabble Hill Rd.
Chappaqua, New York 10514-3009
(914) 238-4172 FAX (914) 242-0282
Table of Contents
TABLE OF CONTENTS .....................................I
SUGGESTIONS FOR SAFETY ......................... 2
SPECIFICATIONS.............................................. 3
Gain:.............................................................. 3
Input Impedance (20 Hz - 20 Khz) ................. 3
Frequency Response...................................... 3
S/N Ratio ....................................................... 3
Output Impedance ......................................... 3
Phantom Voltage nom. +48VDC (Internally
adjustable to +300 VDC @ 5 ma.).................. 3
Attack Time - 200µs to 150 ms ....................... 3
Release Time - 15 ms - 6 sec.......................... 3
Ratio range..................................................... 3
Range of limiting........................................... 3
Maximum Input Level................................... 3
Maximum output level................................... 3
Harmonic Distortion...................................... 3
IM Distortion................................................. 3
Pin conventions:............................................ 3
OVERVIEW I/O CONNECTIONS.................... 6
MIC In............................................................ 6
Line In ............................................................ 6
Insert Send ..................................................... 6
Insert Return .................................................. 8
Side chain input.............................................. 8
Side Chain Couple.......................................... 8
Line Output .................................................... 8
Chassis Gnd (Green Post) ............................... 8
Signal Gnd ..................................................... 8
Power Input.................................................... 9
Input meter select..........................................11
Gain Reduction bypass..................................11
Preset/Manual...............................................11
Threshold ......................................................11
Gain Cell Clip...............................................12
Attack Time ...................................................12
Release Time.................................................12
Ratio .............................................................12
Output gain ...................................................12
Power switch .................................................13
OVERVIEW OF OPERATION .........................13
INSTALLATION CONSIDERATIONS............15
Shields and signal grounding ..........................15
Rear Panel Ground connections......................16
Input and Output balance................................16
THEORY OF OPERATION..............................17
SUMMARY OF HOW TO FOLLOW THE
DIAGRAMS. .............................................18
Basic tube operation......................................18
Gain Settings.................................................18
System Gain Structure Control ......................19
Insert Sends and Returns ...............................19
Level Meter ...................................................19
Gain Cell.......................................................20
Gain Cell Control ..........................................21
PRESETS.......................................................21
Output Stage..................................................21
WARRANTY......................................................22
CONTROLS: FRONT PANEL........................ 9
+48 V ............................................................10
Input Selector ...............................................10
Input overload LED......................................10
Insert Bypass.................................................10
Input trim......................................................10
Phase reverse................................................10
Meter zero.....................................................11
Limiter Tip In...............................................11
BA660.doc 01/01/97 i of 25
Suggestions for Safety
WATER AND MOISTURE - - Do not use
the unit near any source of water or in
excessively moist environments
O
BJECT AND LIQUID ENTRY - - Care
should be taken so that objects do not
fall and liquids are not spilled into the
enclosure through openings
V
ENTILATION - - When installing the unit
in a rack or any other location, be sure
there is adequate ventilation..
H
EAT - - The unit should be situated away
from heat sources such as radiators,
heat registers, stoves or other units
that produce heat.
P
OWER SOURCES - - Care must be taken
to ensure that the unit is only
connected for voltage that is selected
via the internal selector(s).
P
OWER CORD PROTECTION - - AC
power supply cords should be routed
so that they are not likely to be walked
on or pinched by items placed upon
or against them, paying particular
attention to cords at plugs,
convenience receptacles and the point
where they exit from the unit. Never
take hold of the plug or cord of your
hand is wet and always grasp the plug
body when connecting or
disconnecting it.
G
ROUNDING OF THE PLUG - - This unit
is equipped with a 3-wire grounding
type plug, a plug having a third
(grounding) pin. This plug will only
fit into a grounding type power
outlet.
This is a safety feature. If you are
unable to insert the plug into the
outlet, contact your electrician to
replace your 2 pin receptacle. Do not
defeat the purpose of this groundingtype plug by removing the grounding
pin.
ARTS AND STANDS - - The unit is heavy,
C
and made with delicate components
and as such should only be used with
racks and stands recommended by the
manufacturer. The unit and cart
combination should be moved with
care. Quick stops, excessive force and
uneven surfaces may cause the unit
and cart combination to overturn.
W
ALL OR CEILING MOUNTING. - - The
unit should be mounted to a wall or
ceiling only as recommended by the
manual.
C
LEANING - - The unit should be cleaned
only as recommended by the
manufacturer.
N
ON USE PERIODS - - The AC power
supply cord should be removed from
the unit when left unused for a long
period of time.
D
AMAGE REQUIRING SERVICE - - The unit
should be serviced by qualified service
personnel when;
A) The AC Power supply cord or the
plug has been damaged; or
B) Objects have fallen into the unit;
or
C) The unit has been exposed to rain
or liquids have been spilled into
the unit or
D) The unit has been exposed to
thermonuclear explosion; or
BA660.doc 01/01/97 2 of 25
E) The unit does not appear to
operate normally or exhibits
marked change in performance; or
A
Specifications
F) The unit has been dropped or
the enclosure exhibits
significant damage.
• Gain:
Mic In
>+97 db max.
<-20 db min.
Line In
> +34 db max.
< -10 db min.
• Input Impedance (20 Hz - 20 Khz)
Mic = 1k3
Line > 20K Ω
Insert Return > 100 K Ω
Side Chain Insert > 220KΩ
Ω
• Frequency Response
Mic In - 6 Hz - 60 Khz +0, -3
Line In -6 Hz ->60 Khz+0/-1
• Attack Time - 200µs to 150 ms
• Release Time - 15 ms - 6 sec
• Ratio range
1.2:1 - > 20:1
• Range of limiting
≥ 18db
• Maximum Input Level
Mic In
@ +14 > + 45 dbm
@ -15 > +25 dbm
Line In > +40 dbm
• Maximum output level
> +34 dbv ref. 600
150
Ω
Ω into
• Harmonic Distortion
< .6% at all levels up to +34
dbm
• • S/N Ratio
> 120 db
• • Output Impedance
Line Out (20-20Khz)
100
50
Insert Send < 100
Ω balanced
Ω unbalanced
Ω
• Phantom Voltage nom. +48VDC
(Internally adjustable to +300 VDC @
5 ma.)
BA660.doc 01/01/97 3 of 25
• IM Distortion
< .1% at all levels up to +34
dbm
• Pin conventions:
XLR connectors are wired as
follows;
Pin 1 = Shield
Pin 2 = Signal Plus +
Pin 3 = Signal Minus -
All 1/4" connections are TRS,
and are wired as follows;
Tip = Signal Plus +
Ring = Signal Minus -
…CUT TO THE CHASE…!!…CUT TO THE CHASE…!!
Microphone use
• Plug the mic into the unit
• Set the input selector to -45
• Monitor the input, (Push monitor select button "IN")
• Set the input selector and trim so that the average signal is around the yellow
led.
• Set the Threshold control for desired limiting.
• Set the attack, release and ratio controls to 12:00
• Monitor the output signal, (Put the monitor select button in the "Out"
position)
Sleeve = Shield/Chassis
• Set the output gain makeup control for around the yellow LED.
Line level use
• Plug the Line level signal into the input of the BA-660.
• Set the monitor selector switch to the input (in) position
• Set the input selector and trim controls so that the signal is lighting the the
Yellow on the peaks.
• Set the input selector and trim so that the average signal is around the yellow
led.
• Set the Threshold control for desired limiting.
• Set the attack, release and ratio controls to 12:00
• Monitor the output signal, (Put the monitor select button in the "Out"
position)
• Set the output gain makeup control for around the yellow LED.
BA660.doc 01/01/97 4 of 25
This page intentionally left blank--
BA660.doc 01/01/97 5 of 25
Overview I/O Connections
SIGNAL
GND
AC IN LINE OUT
CHASSIS
GND
PRESET
FLAVORS
OUTPUT UNBALANCE
• MIC In
Accepts low impedance balanced
signals between -70 and +14
Nominal level. Phantom power is
provided via the front panel +48V
switch. Internal provisions are
available for phantom voltages up
to +300 VDC. Shield is carried to
Pin 1 via the power ground
internally for phantom purposes.
The shield connection of the XLR
is made via the chassis connection
internally.
• Line In
Nominal level is between -10
and +14. No provisions are
SELECTOR
SIDE CHAIN
COUPLE
INSERT
RETURN
made for phantom power and
system will accept balanced or
single ended signals. Shield is
expected to be served by the
driving source, and is internally
lifted. If desired, there is a
jumper available to provide
shield connection to Pin 1.
• Insert Send
The nominal signal level is -10.
The signal is balanced, but may
be used as single ended. A TRS
plug should always be used. Tip
is hot, Ring is minus and the
shield is served on the sleeve.
SIDE CHAIN
INPUT
INSERT
SEND
LINE IN
MIC IN
BA660.doc 01/01/97 6 of 25
NOTE- The circuitry associated with the
insert send and return is
bypassed when not in use,
(the bypass circuitry senses
either the presence of a
jack in the insert return
connector or the position of
the front panel switch.
When active, this circuitry is
the limiting factor in the
system headroom.
• Insert Return
Nominal signal level of +4. Located
at the input of the gain cell, this is the
input of the limiter, bypassing the
mic preamp and associated circuitry.
Input is balanced. See above note
regarding headroom. This signal is
normalled to the side chain input
• Side chain input
Nominal signal level of +4, balanced
input, normalled source from the
insert return circuitry. Provides access
to the control circuitry for use in Deessing or Ducking.
• Side Chain Couple
ground without degradation of either
headroom, gain, distortion or noise.
Care must be take however to select the
correct polarity to be grounded with
the Un-Balance switch on the rear. The
ground will be taken at the output of
the unit, and should not be take at the
input to a single ended device. Rather,
wire as balanced and have the single
ended input pick up it's reference from
the output of the BA-660. The shield
is source’d at pin 1 of the XLR. Pin 2 is
wired as plus (+) and Pin 3 is wired as
minus (-)Chassis
• Chassis Gnd (Green)
The chassis ground connection
provides the ohmic contact to the
enclosure of the BA-660. This signal is
also the conductor present at pin 1 of
both the line in and the line out.
Inside, this line is tied directly to the
3rd pin of the IEC Power entry
connector. When the unit is operated
with the 3rd pin of the AC cord
connected, The link that is supplied
should be used to connect the signal
ground and the chassis ground
together. In this configuration, no
other studio grounds should be used.
Provision for coupling multiple units.
Both source and input are carried via
this port, and the signal at this port is
the active DC control line. The
signal(s) are summed on this port and
it should only be used with other
BA-660 limiters. The protocol for
direct operation of the gain cell is
available upon request from the
manufacturer.
• Line Output
The line output is a balanced and
floating solid state class A type power
stage that can have either leg taken to
BA660.doc 01/01/97 7 of 25
• Signal Gnd (Black)
This is the ohmic contact to the
technical earth of the BA-660 and ties
via a #12 cable to this point. If any
studio grounds, aside from the 3rd pin
of the power cord are supplied, they
should be referenced to this binding
post. The link should be removed
when the 3rd pin is attached as well as
the studio ground wire. If the 3rd pin is
not attached, and the link is lifted,
severe hum is likely. There is a small
degree of safety provided by 2 cross
shunting diodes and a .047
internally ties these two points together.
µ cap that
• AC Input
The IEC power input
connector ties to a small PC
board internally that carries a
2 Amp Slo Blo 3AG fuse and
voltage selection to
accommodate an input
voltage range of 90 to 250
VAC. Refer to the schematic
diagram for the appropriate
switch settings outside of the
US.
Controls:
100 VAC 120 VAC (Default)
220 VAC 240 VAC
INPUT
SELECTOR
+48V
PHANTOM
INPUT
TRIM
OVERLOAD
INDICATOR
INPUT
METER
ZERO
Reduction
INSERT
BYPASS
Gain
Meter
PHASE
REVERSE
Front panel
LIMIT “TIP-IN”
CAL
METER
SOURCE
SELECT
AUDIO
LEVEL
METER
GAIN
REDUCTION
BYPASS
THRESHOLD GAIN
CELL
CLIP
PRESET
SELECTOR
ATTACK
TIME
OUTPUT
LVL TRIM
RELEASE
TIME
POWER
SWITCH
RATIO
BA660.doc 01/01/97 8 of 25
• +48 V
Switches phantom power on
and off at the Mic input
connector. Internal provision
is made for other voltages by
changing a zener diode.
Contact the manufacturer
for details. Always switch the
phantom power off when it
is not needed as it can
damage single ended signal
sources. The internal build
out resistors of 6k8 provide
for the IEC recommended
current limiting, but do not
provide adequate protection
against idiocy.
• Input Selector
Switches between the mic
and line inputs as well as
establishing the gain
structure for the unit. The
extremely wide dynamic
range is in part made possible
by the judicious use of gain
setting relays that allow us to
manipulate the gain structure
by modification of the gain
stages and internal
attenuation.
• Input overload LED
This LED lights when the
output of either the first or
second stage exceed +34 dbv.
Occasional lighting still
allows an additional 6 db of
headroom!
These signals isolated from
the main signal path with
high voltage op-amps. When
the insert points are not
being used, we do not want
to degrade the purity of the
audio path with any solid
state devices so we sense the
insertion of a jack in either
the insert send or return
connector. When the system
is hard wired into an
installation, such as a rack in
a control room, we wanted
to offer the provision of
bypassing the insert silicon
from the front panel. When
there is no jack inserted in
the inserts, the signal is
automatically bypassed, and
the front panel switch
provides no function.
• Input trim
This control provides
of trim for the front end
gain. Used in conjunction
with the input selector, you
set the input level to read
approximately "0" on the
Audio Meter, and then
establish your threshold
setting.
• Phase reverse
Provides for 180
both the Mic and Line
Inputs. The Phase Reverse
switch point is post insertion
and Pre gain cell.
± 5 db
"
shift of
• Insert Bypass
Provided at the rear panel is
the output of the preamp
and the input to the gain cell.
BA660.doc 01/01/97 9 of 25
• Meter zero
Trim pot for setting the "0"
Gain Reduction meter
position. This is done by
removing all signals from the
unit, and adjusting until the
meter is at "0"
• • Limiter Tip In
is accomplished, and when it
is out the meter is looking at
the output of the unit as it
feeds the outside world. "0"
is equal to +4 dbu
Because the Mu of the
tubes employed in the
manufacture of the device
vary significantly, we must
provide for adjustment of
the gain cell transconductance point. Adjustment of
the unit is accomplished as
follows;
1. Feed in a steady tone
of any amplitude.
2. Adjust the input sel-
ector and trim so the
input meter reads "0"
3. Depress the "Gain re-
duction Bypass" switch
4. Select Output metering
on the Audio Meter
5. Adjust the output trim
so that the Audio LED
meter reads "0"(Yellow
LED is lit)
6. Set the threshold con-
trol to fully Clockwise.
7. Insert the gain re-
duction by depressing
the gain reduction bypass switch.
• Gain Reduction bypass
This switch hard bypasses
the gain cell by completely
cutting it off. This is
accomplished by driving the
cathode to >6 volts above the
grid, (Clamped by zener
diode).
• Preset/Manual
5 presets are provided for
your ease of use and listening
enjoyment. The Presets on
the top of the selector switch
provide an alternate setting
to each of them to allow you
to trim the values of attack,
release and ratio. The bottom
settings bypass the front
panel adjustment. (The
threshold is always available
for your amusement.)
Additional presets are available from the manufacturer
on plug in cards in the
PCMCIA size format. Custom presets are available as
well. Contact your dealer for
details.
8. Adjust the Gain Cell
Tip in so that the audio
remains at "0" and
does not change level
• Input meter select
This switch determines the
source for the Audio Metering. When depressed, metering of the input gain stage
BA660.doc 01/01/97 10 of 25
• Threshold
The point the limiting is
initiated is set with this large
and sexy control. The range
of limiting is adjustable from
-40 to +20 ref. +4
• Gain Cell Clip
The clip LED indicates that
maximum attenuation of the
gain cell has been reached.
This indicates when the
amount of requested gain
control requires that the
transconductance cell act as a
diode. This sounds really
shitty, so we warn you when
it is about to happen. If this
light is blinking, you're a
dumb fuck and shouldn't be
allowed near a tool as
powerful as this in the first
place. [ALT WORDING
ED]Operating the unit with
this indicator lit will result in
distorted sound. So avoid
operating at these signal
levels unless colored sound is
desired.
• Attack Time
The attack time is variable
from 200
µs to 500 msec. In
FAST mode the unit can
sound pretty strange. The
best audio result usually
occur around the center
position. The grayed out
regions are to be avoided
unless unusual care and time
for experimentation are
available.
• Release Time
The release time is variable
from 15 msec to 10 seconds.
For thumping rock and roll
faster release times allow for
"pumpier" sound. For
minimum signal degradation,
setting around 12 O'clock
work best for most music.
Again, the grayed out regions
should be avoided by all but
the stout of heart, and
experimentation will provide
you with the best
information regarding your
preferred settings.
• Ratio
The ratio settings of 1.2:1 to
20:1 provide a range of
compression sounds from
easy limiting to a brick wall.
(NOTE: There is only 17-25
db of limiting available from
the gain cell, then it looks
like a fixed attenuator!) This
means that only the first 25
db of dynamic range is
dynamically modified.
Beyond this level the signal is
no longer controlled,
resulting in a compressed
"squashed" sound quality
which still can increase in
amplitude.
• Output gain
Provides for up to 20 db of
gain make up for the gain
cell. Fundamental operation
of the unit is as follows;
∆ Set the input selector to
get the signal within 5 db
of "0"
∆ Set the input trim to get
the signal close to "0"
∆ Set the Threshold control
for the desired amount of
limiting.
∆ Set the Preset / attack
time / release time / ratio
- controls for the desired
sound and feel
BA660.doc 01/01/97 11 of 25
∆ Set the output trim so
that when metering
output, approximately "0"
is obtained
.
Provides balanced switching
of the incoming power line,
pre-fuse.
• Power switch
Overview of Operation
Setting the input level
The input selector provides gain in 10 dB stepped increments, from -
65 to +14. At the lower extremes, the unit has 2 gain stages and an
input transformer. With higher gains the 2nd gain stage is switched
out. At even higher gains the input transformer is switched out, and at
the highest gains a pad is inserted in the signal path to further lower its
amplitude and along with that the 2nd stage gain is set to unity. In the
lower gain configurations, the equivalent input noise goes up so the
signal to noise ratio suffers. The signal should be close to "0" but with
>30 dB of headroom, this is not really an issue nor is it that important.
Our only real concern is being able to control the gain with the
dynamics that we find audibly pleasant as well as having the output
level be within reason. (It is real easy to start pumping out a nominal
+26 signal level, and not even notice that you're SMOKING the next
piece of gear in line!.
Be careful, for you can develop in
excess of 100 volts peak to peak signal energy .
Monitoring levels.
The LED's around the threshold knob are two audio meters in one.
The bright dot is indicating "VU" levels with ANSI ballistics. The solid
bar is indicating peak level at 10 dB per division. The input monitor
position of this meter looks at the signal level at the output of the 2nd
stage as it appears at the gain cell input. (This is the same point as the
insert return). The output monitor position is just prior to the mute
relay on the output. "0" indication on the meter is calibrated to +4 dbv
ref. 600
and the minimum is -56.
BA660.doc 01/01/97 12 of 25
Ω, or 1.23V. The maximum indicated signal level is +34 dbv,
Set threshold
The threshold control is designed to provide a range of -40 to +20,
with the "0" point occurring at the "0" LED. Threshold should be set
with an exaggerated ratio control, because the gain reduction meter
indicates how much limiting is going to occur, not where or when.
After the threshold setting has been achieved, then the ratio control
should be backed off to the point of auditory satisfaction.
Attack and Release considerations
In order to achieve pleasant results in heavy limiting for program
material, the attack and release times must be
sustained signal material in the face of heavy background, it is often
necessary to resort to very short attack and release times. Between the
two is where the majority of the operating life of these controls exist.
Stay between 9 O'clock and 3 O'clock in order to keep any concept of
fidelity.
VERY LONG. To recover
Ratio and the concept of open-ness.
The robust characteristics of the gain cell make it easy to squash the
crap out of a good sounding dynamically alive signal and make
everything sound like an AM radio mix. This is not a desirable
objective. The design goal of the limiter is to be able to control the
dynamics of acoustically originated signals without sucking the life out
of them. The higher the ratio, the less dynamic range, and the more
control over the signal dynamics you maintain. The lower the ratio, the
more dynamic range, and the less control you exercise, but the better it
sounds. In the beginning, use the presets, determine which ones you
like the sound of the most. The curious can contact their GREG
HANKS DESIGN dealer or the factory for technical information
about attack and release times along with the ratio for each preset.
Presets.
A PCMCIA format plug-in programming card is provided on the rear
of the limiter. With this device you can tailor the characteristics of the
unit to your satisfaction. The offerings in the "Original" preset card are
my impressions of;
1. Fairchild 660
2. LA-2
3. Neve Console Bus limiter
BA660.doc 01/01/97 13 of 25
4. SSL Quad limiter
5. RCA BA-6
Manual Control.
There are two positions for the presets, primary and trim. The primary
position (lower) allows you Trim control of the threshold, with no
control over the ratio, attack and release times. The “Trim” position
(Upper) allows complete control over the threshold and trim over the
ratio, attack and release. Manual control leaves the audio characteristics
at your complete mercy.
Output level
A trim is provided for output gain make-up that has a range of a few
dB of loss to +20 dB of gain, to compensate for whatever attenuation is
provided by the gain cell.
Summary
The operation is a little different from other limiters that you have
used. This unit provides for no gain in the gain control cell. There is
no provision for gain change as in most compressors, and this unit only
provides for program controlled attenuation of the signal path. As such,
when you increase the ratio, the attenuation increases and the signal
level decreases. This is to be made up manually. You have to play with
the controls to get where you want to go, but after walking around the
solo a couple of times with the unit, you should find it to respond as
you now expect it to.
Installation Considerations
• Shields and signal grounding
In the best of all possible
worlds, you plug in the
input, output and power and
everything sounds GREAT,
no problems, and everything
is right with the world. Such
is not usually the case when
BA660.doc 01/01/97 14 of 25
interfacing large quantities of
equipment in a cogent mess
such as a recording studio
control room.
⇒ The microphone must
be shielded. If it is not,
the phantom power
won't work and it will
probably hum badly. In
fixed installations, please
ensure that the only
connection to the shield
of the microphone is
though the signal line
that connects it to the
unit, and not touching
the building steel
through the mic box!
Tech Earth, (Audio Common) are brought together.
This connection must be
maintained either through
the link or via the studio
wiring. Your technical
consultant will be your best
guide to determining the
correct installation method
for your room.
⇒ The line input should
have the shield source’d
at the origination point
and the shield should
drop at the BA-660.
There are internal
provisions for supplying
a shield source from the
BA-660, but this is not
recommended.
⇒ The Insert send sources
the shield. The return
shield is dropped
internally. If this is a
problem, correct it by
tying the shield to the
chassis at the returning
signal source.
⇒ The side chain insert
return shield is also floating
and should be referenced at
the driving source.
⇒ Shields and the output. The
shield at the Line Output is
carried from pin 1. This is
the source of the shield for
the output wire, but should
not be carried through the
input of the driven load.
• Input and Output balance
Both the Mic, Line, insert
return, side chain insert and
the Line Output circuits are
balanced and floating. The
Insert send is balance
⇒ When taking a single
ended input, always
bring the grounded line
to ground at the signal
source. This will treat
the line as a balanced
line and reduce the
hum induction.
• Rear Panel Ground connections
There are two 5 way binding
posts on the rear of the unit.
The green one is attached
directly to the chassis, and
the Black one is tied directly
to Tech Earth internally. The
link that is provided external
to the chassis is the only
point that the chassis and
BA660.doc 01/01/97 15 of 25
⇒ The output of this
device must be brought
to ground at pin 1 of
the output connector if
it is to be unbalanced.
The output mode
switch must be switched
to the correct position.
In un-balanced
operation, if the switch
is in the wrong position
the unit will either have
no output or it will
oscillate.
⇒ When taking the insert
send to an unbalanced
input it is imperative
that the low side be
disconnected from the
ring and tied to the
sleeve at the BA-660.
The output drive is not
ground current sensing,
and should not drive
ground. This WILL
oscillate when driving
ground.
100 VAC 120 VAC (Default)
220 VAC 240 VAC
!DANGER!
Fuse location and Input
power voltage selection.
The fuse is located
directly behind the IEC
Power input connector.
There are two internal
switches with which the
input voltage range can
be set to accommodate
anything between 90
and 250 VAC. Please
leave this adjustment to
qualified personnel.
When in doubt, call the
factory!
BA660.doc 01/01/97 16 of 25
Theory of Operation
SUMMARY OF HOW TO FOLLOW THE DIAGRAMS.
The best way to understand the operation of the BA-660 is to study
the system drawing of the schematics. The boxed figures that
indicate sheets are hierarchical symbols that relate directly and
functionally. As an example, the first stage of gain is on sheet three
and all of the interconnections to the rest of the box are indicated
by the lines on sheet one. The input coupling is all contained on
sheet 3. The input coupling of the "Line In" is indicated
completely on the system diagram.
All signal interconnections are done via signal names, such as
"MIC_OUT-". The interconnected names will be indicated
wherever possible on the left and right sides of the individual
schematic sheets.
The basic function is indicated by the name of the sheet.
The circuitry is unique as it fully balanced from input to output both
internally and externally. The signal paths for the "+" and the "-" are
cross-coupled and balanced. The circuit topology keeps them separate
and they are not combined at any point internally in the unit.
Basic tube operation.
The tube operates at a nominal quiescent voltage of +150 VDC. The
swing range is +300 to 0V. and the idling point is set by the servo
feedback op-amps that drive the bottom current source. The tubes are
operating pure "Class-A" at 8.6 ma. This operating point is
established by the resistor on the bottom current source that is
approximately 475
bypass capacitor have been replace by a triple cascode connected
constant current source.
Gain Settings
The gain of the three gain stages is determined by the Mu of the tube
minus the degeneration of the coupling resistor between the
Ω. The traditional cathode biasing resistor and
BA660.doc 01/01/97 17 of 25
cathodes. When the cathodes are shorted together the tube gain is
almost the idealized Mu converted to db!
When the unit runs out of range using degeneration, ( it is a somewhat
noisy gain setting method at the lower ranges), pads are inserted and
gain stages are eliminated to maximize signal to noise ratio and
minimize total harmonic distortion.
System Gain Structure Control
Because of the humongous gain available, and the response to near
DC, the gain switching, stage switching and transformer removal
are accomplished via gas filled gold plated fast relays. These relays,
along with special timing circuits, enables the unit to delay the
change, mute the output, make the change, allow the output to
settle, and then remove the mute. This logic and timing is all
accomplished with Q46, Q17, Q10 Q29, Q56 and Q23 on sheet
two. The left side of the drawing delays the switch action for
approximately 10 ms while the right side enables the mute for
approximately 500 ms. Because of this muting action, the
input selector nor the preset switch should be used
after initial setup when employing this unit for live
broadcast !
Insert Sends and Returns
The signal level appearing at the insert send is nominally -10 when the
meter reads "0". This is done because the 2nd stage output is
buffered to this point with 2 op-amps. The maximum level at
these devices is +26, the maximum level out of the 2nd stage is
+42. Therefore, in order to not make the insert send buffer the
limiting element of the headroom of the box, the level is lowered
at the send, so that +40dB can be accommodated internally. The
insert return is nominally +4 (this gain differential must be made
up via the insert loop) and goes directly into the gain cell through
2 small isolation resistors, and 2 100k dividing resistors. Any
unbalance that occurs will result in signal degradation, so care
must be exercised in this regard. The silicon is bypassed when
there is no insert connection. This is accomplished via relay K7.
The output of this relay is also the point that the side chain signal
is developed. The nominal signal for the side chain is also therefor
+4.
BA660.doc 01/01/97 18 of 25
Level Meter
The 10 segment level meter employs a unique combination of 2
separate audio level meter circuits. The output of a VU driver and
a Peak driver are hard wire Or'ed to obtain the dual metering
function. In order to distinguish between the ballistics of the two
metering circuits the visual intensity of each scale is varied
according to signal level. The peak scale is dim for the first 7
LED's and then is bright, whereas the Vu scale is Bright for the
first 7 LED's and dim for the last 3 segments. This is set so that a
wide dynamic range signals can be simultaneously displayed along
with subtle 1 dB changes in signal level. The signal is selected at
the insert return point and just prior to the mute relay at the
output. signal is attenuated by 18 db to obtain the dynamic range
necessary for operation. The signal is converted to single ended at
the metering detection circuitry. The signal conditioning is
performed on the gain cell control PCB, and the DC meter drive
is presented to the front panel board independently.
Gain Cell
The gain cell employs the same medium Mu twin triode as the gain
stages. The tube is used as a transconductance shunt element.
What this means in layman's terms is that the conductance of the
vacuum tube, (i.e. resistance) is a function of the ratio of the
Cathode current to the grid bias. If you will look at sheet 7, and
note R12 and R13, these are build out resistors for the tube acting
as a shunt resistor. The cathodes are shorted together, so the more
the tube conducts, the closer the two plates are to each other the
more attenuation through the circuit. The transistors above and
below are there to control the bias and no audio flows through
them! The plate circuit and the cathode circuit are forced by the
gain control line which appears on the right side of R6 to increase
the current through the tube, in equal and opposite directions.
The two tube sections are part of the same tube, therefore will
track one another. The transistors are part of a super-matched
array, so they will track one another, and U1A sets the quiescent
bias point with no limiting. The zener Z13 clamps the cathodes to
+6 v above the grid so that the tube doesn't become a diode.
Whenever the control line goes above .75V the gain cell clip
indicator comes on and shortly thereafter distortion ensues.
The gain reduction bypass is a relay that simply cuts the tube off by
putting a large negative bias on its grids. We also insert a 2.5db
loss via shunt resistor elements to accommodate the tip in
characteristic of the tube.
BA660.doc 01/01/97 19 of 25
The tip in adjustment is necessary because the point that the tube
conducts initially is both noisy and abrupt. This point varies
greatly from one tube to another, which is why the offset voltage
between the output of the gain control system and the point of
conduction is adjustable. This point is set so that with no limiting,
the tube conducts approximately 2 db. The reason for this is that
without it, the transition to limiting is sonically abrupt and
thumpy. More than 3 db of limiting doesn't really seem to help
and it comes right off the top of the maximum available limiting
which is tube bound to approximately 20 db.
Gain Cell Control
The audio signal is converted into a DC voltage that is an average of
the LOG of the input signal. This is then presented to a
logarithmic charge pump, whose charge time is adjustable linearly
in time/db. This results in an attack time that tracks the average
level of the input signal and is not affected by its crest factor. The
parameters of ratio, attack and threshold are set with linear dc
voltages. The release time is also determined via a log converted
charge pump, which resets the past memory of the average signal
level at a fixed recovery time. The objective was to provide
transparent gain control without removing the life from the
performance.
PRESETS
A linear DC voltage controls all of the parameters of the signal
dynamics, making remote and preset operation easy to implement.
The control elements are voltage dependent, not current, so that
they can have some variability in their switch elements. This
enables simple FET gates to provide the matrix selections. The
manner in which the preset voltages were obtained is a trade secret
but for some good PR we will tailor them for YOU; consult your
dealer or the factory for details.
Output Stage
The same circuitry from the previous gain stages is employed for the
output driver. The output stage itself is something else however.
The power supply winding that feeds the 60 VDC to the output
stage is balanced and floating in relation to ground. The circuit
can operate in balanced or unbalanced configuration like an
output transformer and allows either polarity of the output to be
grounded. An unbalancing switch is provided on the rear panel to
select the desired signal output to be grounded via an internal
relay.
BA660.doc 01/01/97 20 of 25
Warranty
GREG HANKS DESIGN will repair or replace this product with
new or rebuilt parts, free of charge in the USA or Puerto Rico
in the event of a defect in materials or workmanship as
follows:
1. Parts:
New or rebuilt parts in exchange for defective parts for the
duration of the warranty of the unit, excluding vacuum tubes
which carry a 6 month limited warranty.**
2. Labor:
Carry in or mail in service for three (3) years after the date of
original purchase. **
TERM:
The warranty term is to be for the 3 years to the original purchaser,
and is transferable.
LIMITS AND EXCLUSIONS
There are no express warranties except as listed above.
GREG HANKS DESIGN SHALL NOT BE LIABLE FOR SPECIAL,
INCIDENTAL, CONSEQUENTIAL OR PUNITIVE DAMAGES,
INCLUDING WITHOUT LIMITATION, DAMAGE TO TAPES
RECORDS, OR DISCS, LOSS OF GOODWILL, OR ANY
ASSOCIATED EQUIPMENT, DOWNTIME COSTS OR CLAIMS
OF ANY PARTY DEALING WITH PURCHASER FOR SUCH
DAMAGES, RESULTING FROM THE USE OF THIS PRODUCT
OR ARISING FROM BREACH OF WARRANTY OR CONTRACT,
NEGLIGENCE OR ANY OTHER LEGAL THEORY. ALL EXPRESS
AND IMPLIED WARRANTIES, INCLUDING THE WARRANTIES
OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
PURPOSE ARE LIMITED TO THE APPLICABLE WARRANTY
PERIOD SET FORTH ABOVE
We will not honor claims laid against the innocent, or for obvious
physical abuse. Pot damage, (looking for "11") meter glass and
cosmetic damage are not covered
BA660.doc 01/01/97 21 of 25
Loading...