Sparta AS-30B User manual

SPARTA AS-3OB
AUDIO CONSOLE
TECHNICAL MANUAL

AS-30B CONSOLE
SPECIFICATIONS
MIXERS:
Five
INPUTS:
OUTPUTS:
Nine stereo pairs; One each for Mixers 1 through 4 and five
for Mixer 5, plus external AIR input.
Low-level; -55 db nominal from 150/250 ohms (50 ohm selected
by jumper), standard on Mixer 1 (optional for Mixer 2).
Hi-level; -10 dbm nominal from 600 ohm source, standard on
Mixers 2 through 5. See note 1.
AIR; Depends upon external monitor-amp.
Program; 8 dbm into balanced 600 ohms at 0 vu. 22 dbm max.
Audition; 8 dbm nominal into 600 ohms single-ended, (from 60 ohm
source. See note
Monitor; Iv nominal into hi-Z load.
Cue; 0. lv nominal into hi-Z load.
Phones; 1 mw nominal into 10K ohms.
FREQ. RESPONSE: All outputs less than 2db down, 20 Hz to 20 kHz.
NOISE:
DISTORTION:
CROSSTALK:
Below 65 db at 0 vu out with -55 db low level input
Below 75 db at 0 vu out with -10 dbm hi-level input
All outputs less than 0.5% THD at normal operating levels.
Less than 1%THD at max outputs.
Below noise in all channels and modes
POWER:
SIZE:
SHIPPING WT:
MUTING:
NOTE ;1:
117/234 vac, 50/60 Hz.
W, 15 1/2"; H, 6 5/8"; D, 10".
24 lbs (incl. pwr. supply)
Switch closure to rear-panel terminals from Mixers 1 to 3, closed
in Aud. & Prog, modes for control of external DC relays.
Hi-level inputs are balanced-bridging to allow individual termination
of input lines at rear panel, if required. Permits constant line load
plus use of high-impedance sources.
NOTE 2:
Identical line amplifiers for Program and Audition outputs.

SPARTA ELECTRONIC CORPORATION
Dear Customer,
Careful attention to Quality Control is another important element in our daily effort to provide you
with excellence of product and service. At SPARTA each piece of equipment and sub-assembly
receives numerous inspections and tests in the process of production. The final results must measure
within our exacting requirements before it is shipped to you. Listed below are just a few of the major
check points and tests this particular piece of equipment has received before being prepared for
shipment. Should you note any discrepancy in the appearance or operation of your SPARTA Products
or if you have any general comments as to how we might be of greater service, your suggestions
will be greatly appreciated.
AS-3 OB
Customer \ SERIAL NO. 2-f/ DATE: /^ "
r
^
PWR. SUP. SERIAL NO: (p / CAL. BY
No< 0F
PREAMPS
OUTPUT DISTORTION SIGNAL/NOISE PHASE
CH-1L: + ^dbm r / b db
CH-1R: + dbm , / y £ / db <
CH-1L Freq. Resp. -2db at Hz -2db at KHz
CH-IR Freq. Resp. -2dfa at Hz -2db at " KHz
MONO-STEREO MIC.R & L MONO-STEREO OUT CROSSTALK
CH-2L + ^ dbm * / % /f db
CH-2R + 9,~ dbm , / >-• ~ "
Freq. Resp. CH-2L / CH2~R /bf Level ?
CH-3 L + dbm , JL
CH-3 R + % ~dbm , / % - / y ~db
Freq. Resp. CH-3L z'.Q ~ 2
cf
K
CH-4 L + - dbm _ , / Z. db f
CH-4R + dbm
A
Freq. Resp. CH-4L If — % \
CH-5 L A Th. E + f dbm , )•? % - db ^
CH-5 R A Th. E -t- V" dbm
Freq. Resp. CH-5L y^Q -^.71 f
Audition L + "f>dbm
t
f db
Output R + yjdbm / Q
%
nr
db
CH2-R
f
%
db
%
? v
db
CH3-R
%
? f
db
%
7
db
CH4-R / <2
% 9~-
db
%
& db
CH5-R /
%
db
%
7/
db
Monitor Air R & L Prog R & L Aud R & L
Pl"I ONE
u
CUE ^ MUTING
ASC-305B
TEP-3S Wow & Flutter RTT LTT
Calib by Cart.
3/29/71

AS-3OB CONSOLE
DESCRIPTION
GENERAL:
The AS-30B is a desk-type Stereo Audio Console featuring five mixing channels with
push-button-selected multiple inputs for Mixer 5. As normally supplied, the first
mixer includes low-level preamplifiers for 50 to 150 ohm microphones and the
remaining four are supplied with high-level balanced input transformers. The input
transformers for Mixer 1 are located on cards to allow replacement with another pair
of low-level preamplifiers.
The high-level inputs are balanced bridging, which avoids the source loading of the
typical 600 ohm input and greatly increases the flexibility of the console. In the very-
rare instance where a 600 ohm termination is required, it is a simple matter to add a
pair of 620 ohm resistors across the line at either the source or load end. Some
sources, such as Ampex and TEAC, provide for this internally. When operating from
sources intended for single-ended, high-impedance loads,the minus input terminals of
the consoles are connected to the shield and the console now appears as a single-ended
high-impedance (10K) load.
The Audition and Program Amplifiers in the AS-30B are identical and interchangeable.
In the Audition side, the line-amplifier gain is determined by internal resistors and is
set to provide approximately the same level output as the Program side with normal
gain control settings. The audition output terminals are fed directly from the audition
line amplifiers from a single-ended source impedance of 60 ohms. The audition out-
put is then capable of driving multiple loads with little, if any attenuation.
The input-source selection for Mixer 1 is via a push-button assembly which allows
more than one source to be selected simultaneously. This feature, if used judiciously,
will permit more tiian one source to be mixed simultaneously on the one mixer pot.
If this is attempted, the individual source levels must be adjusted externally but
the console will not load either source. It must be considered, however, that each

AS-30B Description (cont'd)
of two sources will then be loaded by die other, and suitable isolation must be provided,
if required.
INSTALLATION: Input and Output Connections, with the exception of die Mixer 1 XLR
connectors, are made via rear-panel barrier strips. Spade lugs or fanning-strips are
not required since the barrier strips are designed with captive plates to easily
accommodate several stripped wires per terminal, either solid or stranded. Rear
panel connections are clearly identified to facilitate installation without resorting to die
manual or to a numbered diagram.
Audio connections are normally made with twisted-pair, shielded cable such as Belden
8737 (stranded) or 8739 (solid). Either may be used, although it is generally more
satisfactory to use solid wire in a permanent installation where flexibility is not
required since solid wire is easier to handle and less apt to inadvertently short to an
adjacent terminal. Single-ended outputs and inputs can use die same type cable by
simply clipping off die unused conductor. The input terminals include a ground
connection for the cable shield and die transformer input windings are ungrounded or
"floating". This prevents the possibility of setting up a severe ground-loop and also
permits one side of the transformer input to be grounded for single-ended operation
if desired. In like manner, die program line outputs are also isolated.
A central ground lug is provided on the rear panel for connection to the system
master ground, and as heavy a guage strap or braid as practical should be used.

CIRCUIT DESCRIPTION, SPARTA AUDIO CONSOLES
GENERAL . The description to follow is generally applicable to all
SPARTA consoles, mono or stereo,and outlined the basic system of
selecting, mixing and amplifying the usual program-sources plus the
methods used to process auxilliary functions such as cue and monitor
signals. Features applicable to specific consoles are then discussed
elsewhere in the appropriate manual sections.
Microphone Preamplifiers. With a nominal gain of 55db. the output
level from the preamp to the mixer potentiometer is typically -10
to 0dbm. depending, of course, on the sound level and microphone
used. In a normal situation, then, the preamplifier output is at
essentially the same level as the high-level input signals, resulting
in approximately the same mixer-level settings for normal operation.
Preamp gain can be changed to accommodate unusual situations by
referring to the preamplifier circuit description.
As normally supplied by SPARTA, the'input transformer is wired for use
with 150/250 ohm microphones. A transformer tap is provided for
easy conversion to use with 30/50 ohm microphones.
All SPARTA Audio Consoles are supplied as standard with a microphone
preamplifier in the first mixer position. The preamplifiers are interchange
able with high-level input cards, however, so additional microphone
1.

CIRCUIT DESCRIPTION, GENERAL
preamplifiers may be incorporated, or alternately, all inputs may be high-
level.
High-Level Inputs are brought to the primaries of input-isolation
transformers which are suitably terminated and connected directly to
the proper Mixer potentiometers. The primaries are floating (ungrounded)
to allow single-ended connection and also to prevent the possibility
of setting up undesired ground-loops or common-mode signals via the
input cables. •
Mixing takes place by feeding the output of each mixer potentiometer
through a high-value series resistor (10K or 27K) to a common mixing
bus, which in turn leads to a mixing amplifier. The mixing amplifier,
which is usually located with the line amplifier, is specifically designed
to have a very low input impedance, typically 100 ohms.
Since the Mixer feed resistors are so much higher in value, the mixer
amp becomes a "current sink" and responds to the current in the feed
resistors, which in turn is determined by the voltage available from the
Mixer pots. The output of the mixer amp, then, is proportional to
the sum of the currents at its input.

CIRCUIT DESCRIFHON. GENERAL
Mixer isolation is excellent with this system since the current from
each feed resistor will follow the easiest path — obviously into the
mixer amp rather than to another Mixer pot. Changing one mixer-pot,
then, will have relatively little effect upon the program level coming
from another mixer-pot.
There is obviously a significant loss of program power in the mixer
feed resistors, but the power levels are so low that it is a simple matter to
recover the loss in the mixer amplifier. Mixer amp. gain is set to restore
the program to substantially the same level as originally fed to the
mixer pots , with the mixer-pots at their usual operating positions,
about one o'clock.
Since a "current" mixer operates from a relatively high source impedance,
it allows the use of high impedance mixer-pots. And since it is not
critical of the precise source impedance, there is no need to resort to
the bulk and cost of precision ladder attenuators.
Output of the mixer system is then fed to the Master gain control at
essentially the same level as the original high-level program material.
To test the entire mixer system, a -10 or dbm signal is provided at
a high-level input, the appropriate mixer pot is set at the normal
operating position and the signal is viewed or measured across the
Master gain control.
3.

CIRCUIT DESCRIPTION, GENERAL
The Line Amplifier provides the final amplification between the Master gain
control and the line output terminals. It must therefore recover the signal
loss of the Master gain control, compensate for any losses in matching
or isolation pads, and provide power-gain for driving the output line at
rated level.
For a rated output of 8dbm into a 600 ohm line at zero VU, several factors
must be considered: A minimum of 10 dbm of additional gain, without
distortion, must be available to handle program peaks (headroom). Also,
the output transformer must be isolated from the line with a pad, typically
4 db, to prevent interaction with line reactances. Finally, the line
amplifier output-impedance must properly match the line transformer primary
so the output terminals are a true 600 ohm source .
The line transformer must then handle a nominal level of 12dbm, and 22dbm
on program peaks. The line amplifier must be capable of supplying 18dbm
to the line plus the power-losses in the line pad, insertion loss in the
transformer and internal loss necessary to obtain a proper impedance match.
This is considered in detail in the line-amplifier section of the manual.
VU Meter. The standard VU meter contains rectifier diodes and consequently
appears as a non-linear impedance to its signal source. To avoid introducing

CIRCUIT DESCRIPTION, GENERAL
distortion on the program-line, it is necessary to provide isolation between
the meter and the line.
It is also necessary to attenuate the level to the meter so it can indicate
0vu when the line is at 8dbm and finally, the attenuator must be so
designed as to appear to the meter as a 3.9K ohm source to preserve the
linearity of the meter indication.
This latter requirement accounts for the meter pad being a "T" configuration
rather than a simple series resistor. In SPARTA consoles, the metering source
is the line transformer output, ahead of the line pad, which allows an added
4 db of attenuation in the meter pad for increased isolation. A balanced
"H" pad is not required since the meter need not be balanced to ground.
Cue. Common to all SPARTA consoles is a "Cue" position on each Mixer
potentiometer. In the simplest case, program material from each mixer,
through the cue switches and isolation resistor's, is delivered to a rear-panel
termainal for feed to an external cue amplifier. In more elaborate consoles, a
cue level control, internal amplifier and optional-use internal speaker are
provided.
Monitor. Although provisions for program-line monitoring vary in different
SPARTA consoles, in all cases the source of program material is the output

CIRCUIT DESCRIPTION, GENERAL
stage of the program line amplifier. Since there are no active or non-linear
components to follow, the monitor provides constant assurance of both level
and quality, whereas the VU meter alone only indicates level.

\
CIRCUIT DESCRIPTION/MAINTENANCE
PREAMPLIFIER, 1018
The 1018 Preamplifier is supplied standard for operation with 150/250 ohm mic-
rophones and provides a nominal gain of 55db into a 600 ohm or higher load. A
jumper is provided on the circuit board, either a wire or a low-value resistor, to
accommodate 30/50 ohm microphones.
The 1018 Preamplifier differs from its predecessor, the 1008A, in several respects.
The microphone transformer is a miniaturized PC-mounting type with extended high
and low frequency response. The 1018 circuitry takes full-advantage of the trans-
former's 80K secondary impedance to provide improved noise performance, expand-
ed frequency response and excellent overload characteristics.
With normal microphone input levels, the output level to the mixer will be approx-
0 dbm. The maximum output level of the 1018 before clipping is in excess of 14
dbm, which assures more than adequate head-room in normal operation. In the
event that microphones must be used, which provide unusually high output, a pad
may be added in series with the microphone or a resistor in the 1018 may be
changed in value to lower the preamp gain, as described later.
The input signal is amplified approximately 20db by the input transformer and then
amplified a further 35db by transistors Q1 and Q2. The output signal from Q2 then
passes through emitter-follower Q3 to'provide the necessary low output impedance.
The emitter of Q2 is heavily bypassed to ground by capacitor C8; therefore the
base impedance of Q2 is quite low, being essentially a forward-biased diode.
I.

Most of the signal current from Q1 passes to the base of Q2, then, rather than
through the much higher impedance of the collector resistor R3. This assures
maximum current gain from Q1, although the voltage gain is relatively small.
The emitter of Q3 is not bypassed, however, so the:base impedance of Q3 is
relatively high compared to Q2's collector resistor R4. The signal current from
Q2 must therefore cause a significant signal voltage across R4. This assures a
maximum of voltage gain from Q2, and since Q3 is an emitter-follower this signal
appears at the low impedance emitter of Q3 .
A portion of the output signal determined by feedback attenuator R8/R9 appears at
the emitter of Ql. Since Qland Q2 both invert the signals through them, the
feed-back to the emitter of Ql is in-phase with the signal input to the base of Ql.
This consitutes negative feed-back, since the feed-back attempts to cancel the
input signal. The open-loop gain (without feed-back) is extremely high, so the
normal closed-loop gain is determined by the turns- ratio of the input transformer
plus the ratio of R8 to R9. It can now be seen that changing the value of either
R8 or R9 will change the gain of the amplifier proportionally. For example, if R9
were to be increased to 360 ohms, the voltage gain of the amplifier would decrease
by 6db, since the output of the amplifier would only need to swing half as far to
provide the same feed-back voltage to the emitter of Ql. Consequently, reason-
able selection of gain can be obtained simply by selecting the value of R9.
The use of negative feedback to control gain via the input emitter also has the
effect of raising the input impedance to Ql. This permits the use of a high-ratio

input transformer for added voltage-gain without added noise. Distortion is
also lowered with negative feed-back since any difference between the input and
output wave-forms appears as an error signal. Distortion in the 1018 is held well
below 0.1%.
A stable operating-point , or "Q", is one of the most important characteristics of
an amplifier because the maximum signal output without clipping and therefore
the headroom is determined by the average DC voltage at the emitter of Q3 . The
emitter can move no higher than the supply voltage and no lower than ground, so
the ideal operating-point, permitting maximum dynamic range, is logically some-
where near to one-half the supply voltage.
The emitter voltage of Q3 is set by the collector voltage of Q2 and since the
collector current of Q2 is also the emitter current of Q2, any change in the collect-
or voltage of Q2 also appears as an inverted change at the emitter of Q2 . The bias
current for Q1 is obtained through R6 from the emitter of Q2; therefore, any change
of emitter voltage at Q2 results in a corrective change of bias current to Q1. This
not only provides normal operating bias for Ql, but any change at Q2 is also fed-
back to Q1, a s an error signal to maintain the proper operating-point.
Maintenance: From the foregoing circuit description, it can be surmised that
failure of any component involved in the DC biasing of the circuit, including
leaky capacitors, will cause a shift in the operating-point of Q3 . Stated in a
more useful manner, if the emitter of Q3 is at a reasonable voltage, then all of
the components involved must be functioning normally.

During trouble-shooting most components, including the transistors, can
be removed from question simply by making two DC voltage measurements:
First, the supply voltage at terminal 3 , and second, the voltage at the
a
emitter of Q3. When we consider that resistors may vary 5% or 10% and
individual transistor gains may vary by a factor of 2 or more, it is
reasonable to expect a possible variation of + 20% or so in the operating
point. This, then, would indicate a reasonable voltage range for the
emitter of Q3 of from approximately 10.5 to 15.5 volts. In the event of
catastrophic failure of any of the resistors or transistors, or of excessive
leakage or shorts in any of the capacitors, it will almost invariably
result in a gross shift of the operating-point towards the supply or towards
ground. If the operating-point is beyond the limit given but still capable
of moving further in either direction, the amplifier will still be operative; .
it simply will not have as much dynamic range or head room.
If the operating point is found to be correct, but the amplifier gain is
abnormally high or low, (which usually would be accompanied by high
distortion) it would most likely be due to either an open capacitor or a
defective input transformer. Transistor failure would normally not be a factor
simply because, at audio frequencies, a transistor cannot tell the difference
between AC and DC, so we would look for a component which could alter
the signal gain without affecting the DC operating-point. Excessive noise
can be due to almost any component. The most likely suspects would be
the input transistor, Ql, followed by the resistors and capacitors associated
with the first stage since noise generated there would be subject to the
most amplification.

CIRCUITRY DESCRIPTION/MAINTENANCE
Mixer & Line Amps 1020
The 1020 board consists of two independent amplifiers; a mixer amp and
a program line amp. The mixer amp is characterized by very low input
and moderate output impedances and the program line amp by high input
and low output impedances.
Mixer Amplifier: The input signals to the mixer amplifier are obtained
from a group of mixer potentiometers and each one must be capable of
being switched or adjusted in level without affecting the level coming
from the others. The mixer amplifier is specifically designed for very
low input 3 (100 ohms) and is supplied signal currents from the mixers
through high value resistors of 10K ohms or more. Each input current,
then, follows the path of least resistance into the mixer amplifier,
independent of the condition of the other mixing channels, thereby providing
excellent isolation between mixer channels.
With a very low input Z and high source Z, the input signals take the form
of a current which is a linear function of the source voltage. The unusually
low distortion characteristics accrue in part because small changes of an
already low input Zhave virtually no effect on the signal input currents.
Obviously, the voltage attenuation of the mixer system is quite high — it
is the input current which determines the output voltage of the mixer amplifier.
The output impedance is sufficiently low to serve as a voltage-source for
the Master gain control. The mixer amplifier therefore operates in the

Mixer Amplifier (cont'd)
"trans-impedance" mode; that is, the output voltage is a linear function
of the input current, not the input voltage. A measure of voltage gain is not
appropriate unless the input voltage is applied through a series feed
resistor and the measured gain
will then depend on the value of this
resistor as well as the amplifier gain.
Circuit Operation: The first stage of the mixer amplifier Ql, is a con-
ventional grounded-emitter amplifier with operating bias determined by
current from the collector via R3. A decrease of collector voltage will
cause a decrease of base current, resulting in a decrease of collector
current which tends to raise the collector voltage. R3 then, forms a
negative feedback path which assures that Ql's operating-point remains
within a reasonable range. Normal operation of Ql will be obtained at
any collector voltage between approximately 5 and. 20 volts.
Bias resistor R3 also performs a secondary function: Since signal voltage
from Ql's collector is also fed back to the base, it is a negative-feedback
path to the signal, as well. An input signal current to the base of Ql is met
by an opposing signal current from R3, with the result that R3 appears to the
input signal as a shunt impedance much lower than the schematic value.
This contributes towards lowering the input impedance of the amplifier.
The second stage Q2, is biased in the same manner as Ql by R5. Although
R5 does have the effect of lowering the input impedance of Q2, it is not
nearly so effective since the voltage gain of Q2 is limited by the un-bypassed
2.

Circuit Operation: (cont'd)
emitter resistor R8; therefore, the relative feedback signal current is much
lower.
The voltage gain of Q2 is determined by the ratio of resistor R8 to the
collector load impedance R7. Consider Q2 as having a typical beta of 100,
and no external load on the collector. The emitter resistor R8 carries the
collector current plus the base current, but since the base current is
only 1% of the collector current, we can assume the emitter and base
currents to be the same for all practical purposes.
The emitter is not bypassed, so a change of voltage at the base results in an
equal change at the emitter. This in turn changes both emitter and collector
currents by the same amount. But the collector current is flowing through
a resistor, R7, which is ten times larger than R8, so it must cause ten
times the voltage-change. The voltage-gain of Q2, then, must simply be
the ratio of R7 to R8, or ten. In normal application, Q2 is loaded by the
Master gain control (5K ohms) so the AC collector load impedance is 1.75 K ohm
instead of 2.7K resulting in a normal stage gain of approximately 6.5,or 16db.
The unbypassed R8, then, forms a third negative-feedback path — this time
to fix the ac gain — because the collector current through R8 produces an
emitter voltage change which tends to cancel the input base voltage, thereby
limiting the gain.
The collector voltage of Q2, like Ql, is not critical. The nominal signal
level at the collector of Q2 is 0dbm, or about 1 volt rms. If we allow for
12db of headroom, or 4volts rms, the collector must be free to swing
12 volts peak-to-peak or 6 volts in either direction. So an operating-
point between approximately 8 and 18 volts will assure normal operation.
3.

Circuit Operation (cont'd)
The signal voltage at the emitter of Q2 is the same as at the base of
Q2 and so is a second source of shunt negative feedback to the base of
Ql. Only this time, the source is a low impedance (R8) permitting the
feedback resistor R6 to be low, also. R6, then, is a fourth negative-feedback
path and has the most significant effect in lowering the input 2 of Ql.
In addition, the source for R6 is a voltage which is a fixed portion of
the output voltage. Therefore, R6 also serves to set the gain of Ql.
When an input signal current is applied to Ql, the signal voltage at the
emitter of Q2 can only rise to the point where the combined feedback
currents from R3 and R6 approach cancellation of the input current, with
hthe current through R6 being dominant.
Very low distortion results, typically 0.035%, since any difference of
amplitude or phase between input and feedback currents appears as an
error signal. The output voltage is therefore a linear reflection of the input
current, which in turn is a linear function of voltage at the source end of the
m ixer feed resistors.
Gain of the mixer amplifier may be verified by applying an input signal
current of 10 microamps through a minimum resistance of 2K ohms. Signal
level at the collector of Ql should be about 130 mv, and at the collector
of Q2 nearly 850 mv with the output connected to the 5K Master gain pot.
With no external load, the output should be nearer to 1.3v.

Circuit Operation (cont'd)
Do not be concerned by 10 or 20% gain variations, since many 10% resistors
are used and are easily compensated for by the normal control settings.
Line Amplifier: . The input stage, Q3 and 04, is a differential comparator
which performs three separate functions: First is signal amplification wherein
Q3 operates in the transconductance mode; that is, the collector current is a
function of base voltage. Second, a large proportion of the average dc
voltage at the emitters of Q6 and Q7, which appears at the base of Q4, is
compared to the base voltage of Q3 to stabilize the operating-points of the
output transistors. And third, a small proportion of the output signal, which
is fed back to the base of Q4, is compared to the input signal at the base of
Q3, thereby setting the ac gain.
When power is first applied, C13 must be charged to one-half of the supply
voltage through Q6, and to protect Q6 the charging-rate must be limited.
QG cannot turn on until Q5 and hence Q3 begin to conduct. But Q3 cannot
conduct any faster than C6 can charge, so C6 not only filters the bias current
to 03, but also controls the charge-rate of C13.
The ideal dc operating-point for the output transistors Q6 and Q7 is one half
of the supply voltage, because this is the point at which the greatest peak-
to-peak output-voltage swing, or headroom, is available. When first turned
on, C6 charges and the base of Q3 is brought to 40% of the supply voltage.
As C13 charges, the voltage common to the emitters of Q6 and Q7 rises
towards the supply. Upon reaching one-half of the supply voltage, 80% of this (or
40% of the supply voltage) reaches the base of Q4 via dc divider R16, R15,
5.