PAiA 4710 Balanced Modulator/VCA

The 4710 Balanced Modulator/VCA is one of my favorite PAiA modules and one of the reasons that the 4700 system is so fun. Here's a description of how it works via PAiA




















DESIGN ANALYSIS

The 4710 Balanced Modulator is built around a type 1496
balanced modulator integrated circuit. The internal workings
of this I.C. package are not pertinent to this discussion but
certain general points should be made.

The 1496 chip has differential inputs for both carrier and
modulation ports. Biasing for the carrier inputs (pins 8 and
10) are supplied by the voltage divider consisting of Rll and
R12 with this biasing voltage coupled to the IC through R14
and Rl5. Input signals applied to the carrier input jack J4
are capacitively coupled by C2 to input isolating resistor
R26 and finally appear across R14. Fixed resistors R13 sad
R16 along with trim pot R25 are used to supply adjustable
current into the carrier input port to balance out variations
that occur in the integrated circuit during manufacture.
With no signal applied to either of the inputs there should
be 7v +/-20% present at pins 8 and 10 sad at the junction of
Rll and Rl2.

The modulation inputs (pins 1 and 4) are similar to the
carrier inputs except that they are tied to ground through R8
sad R9 with R7, R10 and trimmer R24 supplying tolerance
compensating bias currents. Pins 1 and 4 should read
0V +/-0.1V.

R19 and R20 serve as load resistors for IC-2 with the in-
phase output coupled to output jack J5 by way of C3.
quiescent voltages at pins 6 and 12 should be 8v. +/- 20%.

The modulation input of the 47I0 module is buffered by the
748 type operational amplifier IC-1. Control voltages are
direct coupled to the inverting input of that IC by R2 and R3
while audio signals are capacitively coupled by R1 and C1.
With the modulation LEVEL control fully counter-clockwise and
no signals present at the inputs, pin 6 of IC-l should be
0V. +/- 0.1V.

Rotating the LEVEL control in a Clockwise direction causes
less of the output of the operational amplifier to appear as
feed-back at the inverting input which in turn causes the
voltage gain of this amplifier to go from approximately 4.5
to 90 (13 - 40 db.) for signals applied to the audio input
and 0.3 to 6. 3 (-10 to 16 db.) for signals applied to the
control inputs. The voltages at pins 2 and 3 of IC-l should
be 0V. +/- 0.1V and pins 7 and 4 are positive and negative
supply respectively.

The overload Light Emitting Diode becomes forward biased and
begins to conduct on negative excursions of the output of IC-
1. Because of the forward voltage drops of D1 and D2, as
well as the LED itself, conduction begins to occur at about -
2v. The output of the buffer amplifier is coupled to IC-2
through R6.

Power supply decoupling is provided by the combinations R2l,
C6; R22, C7 and R23, C8. The voltages across C6, C7 and C8
respectively should be 13V., 7V. and -8V. +/- 20%.