Roland S700 clone: 704A, 704B VCA. www.haraldswerk.de

Overview

Roland S700 clone 704A VCA-1 front view.

Here are the 704A VCA-1 and 704B VCA-2 for my Roland S700 clone. They are used in the main console. Three 704C VCA are used int the VCF/VCA bank, Block 5 in the system and one in the Interface/Mixer, Block 6 in the system. The 704B VCA-2 is exactly the same as the 704A VCA-1 except for the internal connections at the 711A output module. The 704C VCA is exactly the same as the 704A and the 704B VCA's except that there are no internal connections. All three share basically the same circuitry. They only differ in the internal connections to the common bus and the front panel labeling. I have used the original schematic drawings to keep the audio path as close as possible the original. The sound and behavior should be the same as in the original (hopefully). I don't have the means to make a direct comparison though. Changes I (had to) made are described in detail below. And I added a few sockets for more flexible patching.

Specs and features

Three audio inputs with attenuators, partly prepatched and normaled.
Switchable ADSR CV input from ADSR1/2 with attenuators prepatched.
External modulation input
Internal and external output.
Lin and log response on the control voltage.
Green and red LED for signal condition
Initial manual gain control.
Runs on +/-15V
Power consumption below 15mA each rail

Implementation

Schematic

Roland S700 clone schematic: 704A/B control board 02

Roland S700 clone schematic: 704A/B control board 03

Description:

This is a simple VCA with the OTA 3080 followed by a push-pull amplifier/buffer. The control voltages are added together by U1A one halfe of a TL062. The expo current source for Iabc is build with T_4 and T_5. The lin current source for Iabc is build with U1B and T_3. The signal condition is done with the LM2903 and the constant current source around T_1. The comparator is driven by a precision full wave rectifier around U1A and U1B.

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Changes to the original:

Audio path:
- No changes on the circuitry of the audio path. Switched to today available components.

Other:
- Fixed an error on the original schematic drawing. Resistor R_19 must be connected to -15V, not to ground as in the original (R22 in the original).
- Mounted the PCB's parallel to the front panel.
- Replaced the system bus with individual wiring.

Additions:

Normaled sockets were added to the inputs for VCF-1 and VCF-2. This makes for a more flexible patching. Inserting a patch cable will break the internal patch.

Calibration

First make a gross test. You should have a signal at every output, pots and inputs reacting. LED working.

DC offset
- Set all faders to zero.
- Set gain pot to max.
- Measure the DC output voltage.
- Adjust Tr_1 for 0V at the output.
Gain linear
- Apply a sine with 10Vpp to any of the inputs. Set the fader to max.
- Connect a scope to the output.
- Set gain to max.
- Set switch to LIN.
- Adjust Tr_2 to unity gain.
Exp offset and width
- Apply a sine with 10Vpp to any of the inputs. Set the fader to max.
- Apply +10V to CV input. Set fader to max. You can use the voltage processor 712A if you want
- Set gain pot to zero.
- Set switch to EXP.
- Adjust Tr_3 for unity gain.
- Lower control voltage input to +9V.
- Adjust Tr_4 for -10dB gain.
- Repeat above steps until +10V produces unity gain (0dB) and +9V produces 3,15V output signal (-10dB).
- Don't be to picky with this adjustment. Our hearing is not to sensitiv to loudness.

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Building hints

Due to the double front panel there could be mechanical problems. I recommend using faders with 20mm lever and potentiometers with long shaft (20mm). If you can get the faders with 20mm lever you'll have to use normal pin headers and 12mm stand off for PCB A1 and A3.
You can get away with normal length pots and 15mm fader levers though. Then you have to use long pin headers for PCB A1 and A3 with 15mm stand offs and a 1mm washer to get 16mm span.