VCA/Mixer

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z 1975, 2010, 2011 Nyle Steiner
They may not be used for profit, sale, or reproduced in any way without the express consent of Nyle Steiner

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Function

This module combines a Voltage Controlled Amplifier (VCA) with a three input mixer. Typically, the output of an Envelope Generator (EG) is connected to a VC input with a signal connected to a signal input. The output amplitude of this signal is controlled by the voltage at the VC input.

This module also mixes three separate input signals into one output. With the input gain controls, this module can also function as a standard three input mixer.

From the Synthasystem manual:

Audio signals are fed into inputs one, two and/or three, and levels of relative mixing are set with the three input pots.

The overall gain can be controlled by adjusting the pot labeled GAIN and or by feeding control voltages into the V.C. input jacks.

For use in conjunction with the envelope generator, use the following steps.
  1. Plug the audio signal (from one of the oscillators) into one of the VCA Mixer inputs and turn up the pot for that channel about half way. Feed the out jack into an audio amplifier, guitar amplifier, or whatever sound system is being used.
  2. Turn up the master GAIN pot until a signal is heard.
  3. Turn the envelope generator output level pot for Env #1 all the way down.
  4. Turn the GAIN pot on the VCA Mixer counterclockwise until the audio signal just barely disappears.
  5. Turn the output level pot on the Env Gen #1 to about 12 o'clock.
  6. Now a signal should be heard in accordance with the setting of Env. Gen. #1 whenever a KBD trigger or other trigger is fed into the trigger jack of the envelope generator.

Envelope generators #2 and #3 can be used to control other modules while #1 is controlling the VCA Mixer. For example, to sweep the V.C. Filter and/or Ocs. Frequency.

On final note: Overdriving the VCA Mixer overrides control voltage rejection. You should keep the control voltage pot at about the 12 o'clock position, generally. Lack of control voltage rejection makes a "popping" sound.


Use

Inputs/Outputs

This module has 5 inputs and 1 output:
  1. Signal in - There are 3 separate signal inputs. They each have an attenuator and are then summed and fed into the amplifier section.
  2. VC In - There are 2 VC inputs. These inputs are summed with the master gain and control the output level of the summed signal inputs.
Knobs

This module has 4 knobs:
  1. Master Gain - This control sets a fixed gain for the output level. In normal use, this would be set to full CCW and the VC inputs would control the output level. This knob is more typically used when using this module as a 3 input fixed mixer.
  2. Input 1, 2, and 3 - These knobs are attenuators for the input signal and allow you to set the relative mix levels of the three inputs.
Switches

No switches

General Connections

Connect up to 3 signals to mix. Set the input levels as desired. Set a fixed master gain level. input up to 2 control voltages, typically from an EG, but can be any voltage source.

Component Selection

See the Component Notes page for more information.

Resistors

This module was originally built with carbon core, 5% resistors with one or two 1% metal film resistors. So, you have a wide range of options here. I recommend using 1% tolerance, metal film resistors everywhere, but the critical resistors are R5 and R8, input summing resistors. These should ideally be hand matched or purchased to 0.1% tolerance to insure consistent response between the inputs.

Capacitors

There are probably a billion different ceramic capacitors at a place like Mouser. Pick a capacitor that can fit the hole easily, typically 0.1 inch on centers.

Pick good quality electrolytics where designated rated for 25 to 35 volts.

Inductors

None

ICs

None

Transistors

The original used 2N5172 NPN transistors. These are still available. In any event, you can use any standard NPN transistor and they should work.

There is 1 SSM2210, Q4 on the schematic. This doesn't have to be a matched pair, but it won't hurt. I laid it out to accommodate a monolithic pair if you want to. Matched or unmatched NPNs will probably just work fine.

Diodes

Nothing special, 1N4148s are fine.

Pots

Your choice for your panel. If you use the panel I laid out, the holes and spacing will work for the Alpha 12 and 16mm pots. You can probably use nicer BTI, Bourns, etc. 9mm pots with "pot chiclets"

Trimmers

Use good trimmers, please. A good Bourns multi-turn trimmer like Bourns 3296Y series will fit the pad layout and work well.

You will probably have trouble finding a 250 ohm trimmer. You can  use a 200 ohm trimmer. You might also be able to use a 500 ohm trimmer and drop R2 and R3 to 2k.

Jacks

For the panel I laid out, a good 3.5mm or 1/8 inch jack will work. I use the Switchcraft 42A Tini-Jax true 1/8 inch jack. These are switched jacks and they work with 1/8 inch plugs and 3.5 mm plugs.

Build Notes

Soldering

I assume you know the basics of soldering. I like to insert the low lying parts first, like resistors, diodes, etc. After these, I install the IC sockets. Next capacitors, transistors, connectors. Use a good solder, either an organic flux, which you should wash regularly, or a no-wash flux.

Take a break every so often, wash off the flux if you are using a flux which required cleaning. Double and triple check orientations, pins, and solder joints.


Power Supply Regulation/Filtering

This PCB requires the Power/Regulation daughter board. I did it this way to allow this PCB to be parallel to the front panel which allows it to mount in shallow cabinets.

See this page for details on this module.

For the VCA, install the extra capacitors and resistors. The VCA uses ONLY this extra filtered supply, so you don't have to connect the +12/-10 volt standard supply, just the GND, V+, and V-, Pins 1, 4, and 5.

Because the VCA uses extra filtering on the power, you will need to install C4, C8, R2, and R6. C4 and C8 are 150 uF electrolytics, R2 and R6 are 360 ohms.

Build this PCB first and get the regulators (if you use them) trimmed to +12/-10 volts.

Some additional comments here. These modules are tested to run on +/-12 VDC. The original power supply in the Synthasystem was +12/-10 VDC due to how Nyle designed the -10 volt section of the power supply, not for any magical requirement to have -10 volts. I have, however, had some advice this module may not work the way it is designed on -12 volts. So, I would suggest installing the -10volt regulator to be safe.

With that said, the power/regulation PCB has 2 voltage regulators on it which can be set to +12/-10 (or +/-12 volts) depending on your needs. If you are coming from +/-15 volts, you need both regulators and you may as well set one to -10 volts.

If you are coming from +/-12 volts, technically you don't need the regulators, but if you want, install the negative one and set it to -10 volts. When this board is not connected to the main VCO PCB, you have to have a minimal load in order for the regulators to regulate, that's the purpose of the LEDs. you can install these for fun if you want, but are not needed if you don't use the regulators.

Important... if you don't install the regulators, you have to install a jumper between pins 2 and 3 as shown on the Power/Regulation PCB or you won't get power to the VCO.


VCA/Mixer

Component Placement


Mounting

This PCB has four holes to allow flexible mounting configurations. The FPE Euro panel is setup to allow this PCB to be mounted parallel to the panel using some 3/4 to 1 inch (typical) standoffs. The mounting holes are connected to ground. The Power Regulation PCB will mount to this PCB using standoffs.

Calibration

There is on trimmer to reduce the bleed through of the VC signal.
  1. Set R1, 250 ohm trimmer to mid point.
  2. Null the input signal with the front panel gain control.
  3. Apply an envelope for modulation.
  4. Adjust R1 for a minimum CV signal at the output.
Panel Notes

There is nothing too special. I suggest using connectors on the PCB and jacks on the flying wires. The spacing and holes are setup for Alpha 16 mm or 12 mm pots. The jack holes are 0.25 inch in diameter.

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