AM 8019 Module - ARP 2600 VCA

Overview This is a replica of the ARP 2600 Voltage Controlled Amplifier built into a single FracRac module. I was orginally going to use the Oakley Sound 4019 PCB, but Tony decided to design his module with 5x PCB mounted pots which simply won't have looked right in my ARP2600 FracRac. So reluctanatly I decided to take the time to design my own version from the original ARP blueprints and schematics. To make the project more interesting I fitted a
3-way slide switch that enables the 2 control signals to be switched from Exponential to Linear response.

Original Circuit The original design first appeared in the ARP 2600 in 1970, but the circuit originates from the ARP 2500 VCA module. It is a VCA that makes use of matched transistors and a single buffer Op Amp at the output stage.

AM8019 Module My FracRac module replicates the original design exactly except for upgrading to SSM2220 and THAT340 matched transistor pairs, and reduced the size of the capacitor over the final Op Amp inputs as suggested by Tony Oakley. The transistors are have been switched to European versions (BC550/560) and high quality capacitors have been used throughout. To ensure the linear and exponential gain of the VCA are at unity I needed to make two change to the circuit:

• The Linear Gain Trimmer is 100K as per the ARP2600 Service Manual
• The Exponential Gain resistor has been reduced from 470K to 220K

I have enhanced the original design with a couple of new ideas. A 3-way slide selects the response of the two control voltage inputs, the positions are:

• Expo/Expo
• Expo/Linear
• Liner/Linear

And Signal A passes through a 470nF capacitor which acts as a high pass filter at 10Hz, thereby blocking any DC offsets. The ARP2600 VCO's have a DC offset which is effectively removed via this input (post VCF mixer) and thereby eliminates ADSR thumps. The signal inputs have been set up as follows:

• Signal A is AC coupled
• Signal B is DC coupled
• Signal C is inverted and DC coupled

The module has the following front panel controls:

• Initial Gain
• CV1 Level
• CV2 Level
• Signal A Level
• Signal B Levek
• Signal C Level
• CV response slide switch

There are six 3.5mm jack sockets mounted on the left hand side of the panel, these are:

• Signal A Input
• Signal B Input
• Signal C Input
• CV 1 Input
• CV 2 Input
• VCA Output

Front Panel The front panel is a 3" wide FracRac with Alpha potentiometers and push fit mixer style knobs.

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Original ARP 2600