T902 Voltage Controlled Amplifier Bank

Introduction

The T902 VCA clone was next on my list to create a stand alone instrument. I don't think there's too much to say about it - I think most of you know what a VCA is and what a VCA does - although my favorite point to criticize is the name "VCA" itself, as the name contains the "(A)mplifier" term, which is confusing and irritating, especially in this case. Amplification means: weak signal input, amplification by a certain factor, and strong signal output. A synthesizer module called VCA does not do an (A)mplification, but an attenuation instead, and does this (V)oltage (C)ontrolled. But this attenuation is done in a reverse way, meaning zero control voltage / minimum control voltage = maximum attenuation, closes the VCA completely, and maximum control voltage means minimum attenuation, the VCA opens up totally, but there is no amplification, as "totally open" means amplification is done by factor 1.
So a better name for this module would be "Reverse Voltage Controlled Attenuator".

T902 Voltage Controlled Amplifier Bank

Differences to the original

Like for my T901 clone I decided to combine more than one module to a module bank behind one front panel. I decided to combine three VCAs to one single bank because in a lot of Moog modular systems three VCAs are configured, together with three ADSR (Envelope) modules, one trigger delay and one CV router, all as separate modules of course, to one functional unit. This makes sense, and the way how to use this unit is described in the Moog Modular User's Manual. So here is the first part of this functional unit - the VCA bank.

Complete Module

Click to enlarge

Frontend / Human Interface

VCA

On top you find a switch to select the so called "Control Mode" where the response of the VCA to the control voltages provided to the module can be selected. The (LIN.)ear behaviour means the attenuation of the module follows the control voltages quite linear, and the (EXP.)onential behaviour means an exponential response. The attenuation lowers quite fast at increasing control voltages, then the reaction gets slower when attenuation approaches "open" status.

The FIXED CONTROL VOLTAGE knob adds a voltage to the control voltage node within the module (see circuit description below), and if no CV is provided the VCA can be opened or closed manually. If control voltages range in negative volts they can be shifted up with this potentiometer.

Below the FIXED CONTROL VOLTAGE knob there are blocks with connectors for input and output signals. There are two of them for input and two for output. Each input connector is connected to one of two opposite sides of input differential amplifiers (see circuit description below), meaning the phase of one side ist inverted related to the other side. The same on the output side. The two output connectors belong to phase inverted outputs of the output differential amplifier (see circuit description below). This leads to the effect, that a signal on I+ can be obtained at O+ phase identical and O- phase inverted. This is interesting for LFO processing. On the other hand similar inputs at I+ and I- might result into phase cancellation, so be careful in mixing signals. In general: I+ <= O+, I+ <= /O- and vice versa.

Last but not least you find the three CONTROL INPUT connectors on the front panel. Here you can plug in the CVs of ADSRs, LFOs or whatever. The CVs are mixed and control the attenuation behaviour of the VCA.

Circuit

Schema: Attention: Clicking the schema means acceptance of disclaimer on page bottom!

Please refer to the original schematics for more details / component values.

Circuit description

The three control inputs are mixed by the standard moog CV mixer subcircuit which can be found in several Moog modules. Note the fourth CV input labeled as "993-Modul". This is an internal input of the CV router module described in the functional block description above.
CV mixer: after the high impedance inputs (100k) the CVs are amplified by a pair of differential amplifiers (NPN, PNP, Q1 - Q4) and filtered by the low pass filter R2 and C1. The CV mixture is decoupled by Q5. Note that the FIXED CONTROL VOLTAGE pot wiper output is included here. And there is an additional pseudo CV which is labeled "SILENCE LEVEL", which means the CV mixtures can be lowered with this trim potentiometer below zero to total silence of the VCA. This is necessary if ADSR CV inputs don't reach true zero voltage due to capacitor charges at the end of the release phase. Note that this is NOT part of the original circuit by Moog but is added by me.
The collector of Q5 works as a "sink" for the attenuation circuit consisting of Q8 - Q10. The Emitter of Q10 is connected to the collector of Q5 via the CONTROL MODE switch. If Q5 opens due to CV changes, the voltage level of this connection sinks, and the attenuation lowers, the output signal becomes stronger. If Q5 closes due to CV changes, the level rises and the attenuation rises also meaning the output signal becomes weaker. So here the audio volume control is done.
Back to the CONTROL MODE switch: it has two functions, so you need a dual switch. First of all depending on the switch position the amplification level (Q5 => Q10) is lowered by R16 in linear mode, and second the feedback loop of the CV mixer / differential amplifiers is manipulated also to obtain lower amplification in linear mode (R5, R24, R27).
The audio signal inputs are preamplified by the npn differential amplifier (Q6, Q7) and fed into the attenuator (see above). The output signal is created and decoupled by the differential amplifier (PNP, Q11 - Q13).

Part replacements

I tried to build my module as close to the original as possible, but I had to do some changes due to actual components I use instead of the originals:

All NPN transistors have been replaced by the BC547C type in CV paths and the low noise type BC550C in audio paths.
All PNP transistors have been replaced by the BC557C type in CV paths and the low noise type BC560C in audio paths.
All onboard potentiometer have been replaced by multiturn versions ==> general recommendation: Don't use standart onboard trimmer, use multiturn versions and decrease the connection resistors if exist. This leads to much higher tuning ranges of the specific functions and more precision.

Component changes

Beside the functional changes concerning "silence level" and the general changes mentioned above I changed the following component values:

C3 was changed from 80uF to 100uf (easier to obtain)
C4 was changed from 400uF to 470uf (easier to obtain)
R36 was changed from 750R to 220R (necessary to increase INP. BIAS current to equalize exponential and linear output level)
R25 and R26 were changed from 15R to 18R (resistors in stock)
R37 and R38 were changed from 680R to 2k4 (necessary to increase output level to +6db)
R12 was changed from 820R to 390R (necessary to change OUT. BIAS range for zero output adjustment)
R22 was changed from 2k7 to 3k9 (necessary to change OUT. BIAS range for zero output adjustment)
Q8-Q9-BAL. potentiometer was changed from 100R to 50R multiturn (easier to obtain)
INP. BIAS potentiometer was changed from 2k5 to 1k multiturn (necessary to increase INP. BIAS current to equalize exponential and linear output level)
OUT.-BAL. potentiometer was changed from 2.5R to 50R multiturn (easier to obtain)
OUT.-BIAS. potentiometer was changed from 250R to 1k multiturn (necessary to change OUT. BIAS range for zero output adjustment)
I decided to use matched transistors for all differential amplifier pairs

A lot of changes, especially the new resistor values were necessary because of the usage of the newer transistor types which have higher betas as the originals. This is only an assumption, and as I don't have an original module to compare I can't prove it. But LTSpice simulation showed that these changes were necessary to let the module work as demanded in the setup and test procedure description of the service manual.

Board of the T902 clone

Board dimensions: 95 x 120 mm
Upper left: CV mixer
Upper middle and right: Attenuator
Lower left: Transistor matching circuit, power supply
In contrast to the original concept of the Moog modulars I supplied the board with a +/-15V power supply. Voltage regulators on the board convert it to +12V, -6V and -10V. Benefit of this is a higher stability of the power supply.
Lower right: In's and Out's

Click to enlarge

Sound examples

Simple patch consisting of T950 Keyboard Controller, T901 Oszillator Bank, T904A Voltage Controlled Low Pass Filter, T902 VCA Bank, Formant ADSRs

Please send questions or remarks to:
Carsten Toensmann

Home

Disclaimer:
No warranty for completeness or correctness of technical descriptions, schemas or any information provided on www.analog-monster.de/* pages.
Contents of linked pages are in responsibility of the page owners and analog-monster dissociates from them.
All tracks provided on www.analog-monster.de/* pages is in ownership of www.analog.monster.de (except especially marked) and for private use only. All rights reserved.