AM8130 Module - Roland 100M Single VCA

Overview This module is a clone of the VCA from the legendary Roland 100M modular synthesizer. The VCA appeared in two formats:

• a dual VCA with linear and exponential modes in the 130 module
• a single VCA with linear mode in the 110 module

It is this former circuit I have cloned but as a single VCA, but with both exponential and linear modes.

AM8130 Module The AM circuit is a careful replication of the original design with both linear and exponential control modes. The VCA is based on the BA662 OTA chip, which I have replaced with the easier to find BA6110 which has the following specifications:

• 86dB Signal to Noise Ratio
• 0.5% THD

Whilst this is not a leading edge VCA, these specifications are fine for use within a modular synthesizer and are certainly better than the venerable CA3080 used in many early 1970 VCA's. I have modified the CV circuit to make use of easily available USA transistors (2N3904/06), and a TL071 Op Amp in the CV circuit. I have added a simple LED driver to display the post VCA audio level. The circuit was designed using Eagle CAD and laid out onto a 80mm x 100mm PCB, which is the standard AM format for FracRacs.

The module has the following front panel controls:

• Initial Gain
• Signal Input Level (x3)
• CV Input Level (x2)
• LED for post VCA audio level

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

• Signal Inputs (x3)
• CV Inputs (x2)
• Signal Output

Front Panel The front panel is a 3" wide FracRac with Alpha rotary potentiometers, with 15mm diameter plastic mixer knobs.

 

Roland 100M

AM8130 Front Panel