CGS Serge quad VCA

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CGS101 the CGS Serge quad VCA is a variation on the Serge Quad Voltage Controlled Amplifier (QCA) module, and can be configured to work for several other Serge designs such as the Serge Universal Equal-Power Audio Processor (UPAP).

The module will work on +/-12 volts or +/-15 volts.


The quad voltage controlled amplifier (QCA) employs four high quality VCAs. Each VCA has one input suitable for DC control voltages to control the overall amplitude of the audio input signal. Envelope generators or sequencers are normally patched to this input. A second control voltage input is ideal for amplitude modulating signals. This input has less dynamic range than the other input and normally oscillators or other control voltages are patched to this input for simultaneous modulation and gating functions. A mixed output of all four VCAs is also included, so that the QCA can be used as a four-in/one-out voltage controlled mixer with independent level control for each input. A unity gain input is used to bus other QCAs together to form mixers of eight, twelve or more input channels.[1]

In addition to this, there are extra mixer have been added to the design, giving alternate mixed combinations which are useful in some of the other configurations.

A little on how it works

The schematic for Serge Quad Voltage Controlled Amplifier module.


The component overlay for the VER1.0 PCB. Click through for an enlarged, printable version. Print at 300dpi.

The CGS101 QVCA uses four CGS108 CGS Serge voltage controlled gain cells as it's voltage controlled elements. Refer to the CGS108 page for construction details. The boards are built exactly as per that page. They should be the last things you install on the CGS101 PCB.

Before you start assembly, check the board for etching faults. Look for any shorts between tracks, or open circuits due to over etching. Take this opportunity to sand the edges of the board if needed, removing any splinters or rough edges.

When you are happy with the printed circuit board, construction can proceed as normal, starting with low profile components such as resistors and diodes first, followed by successively taller components.

Take particular care with the orientation of the polarized components, such as ICs, electrolytics, diodes, and transistors.

When inserting the ICs in their sockets, if used, take care not to accidentally bend any of the pins under the chip. Also, make sure the notch on the chip is aligned with the notch marked on the PCB overlay.

The unit will run on either +/-12 volts or +/-15 volts.

The first time you power it up, I would suggest you do so with 22 ohm resistors in series with the positive and negative power rails. This should save the chips if you have made a blunder.

On the CGS101 VER1.0 PCB there are some corrections required.

  • Four 5k6 resistors need to be installed on the rear of the PCB. Each resistor goes between pins 7 (CV) and 8 (0V) of one of the CGS108 submodules.
  • If you are installing CGS102 Equal Power Panner submodules, two connections to each CGS102B submodule need to be reversed. Fortunately this can be done entirely on the rear of the PCB by cutting the tracks in the areas shown in light green on the diagram below. After some solder mask is scraped from the remaining track stubs, (shown in copper below) six links can be added. The four links shown in blue can actually be replaced with two short lengths of component wire offcuts. The two links shown in purple are best if done with insulated wire as they pass over other pads.
  • If you are installing CGS102 Equal Power Panner submodules, two wires need to be connected to pads directly below the submodules. See the diagram for details.
Modifications required on the rear of the CGS101 VER1.0 PCB. Click through for an enlarged version.
Heat shrink tube was wrapped around the transistors to assist with temperature stability.
Pad identification
Pad ID Function
W 0V power connection
X +12V power connection
Z -12V power connection

Set Up

There should be no setup required, other than to set the trimmers on the CGS108 PCBs as described on that page.

Parts list

This is a guide only. Parts needed will vary with individual constructor's needs.

Part Quantity
Resistors (1% metal film)
330Ω 8
5.6k 4
82k 4
100k 37
RPD. Use 100k 6
220k 4
100k lin pot 4
82pF ceramic 4
220nF (=0.22uF) 4
10uF 25V electrolytic 2
TL074 1
Ferrite Bead 2
4 pin MTA-156 connector 1
4 pin MTA-156 header 1
CGS108 submodule 4
CGS101 VER1.0 PCB 1


  • 330R refers to 330 ohms. 100n = 0.1 uF.
  • PCB is 6" x 2" with 3mm mounting holes 0.15" in from the edges.


Readers are permitted to construct these circuits for their own personal use only. Ken Stone retains all rights to his work. Serge Tcherepnin retains all rights to his work.

See also


  1. ^ Voltage controlled amplifier functions, Serge Modular Music Systems, 1982

External links