AM8110 – VCA (100M)

Prototype AM8110

Overview This module is a clone of the VCA which appeared in two formats in the Roland 100M. A dual VCA with linear and exponential modes in the 130 module and a single VCA with linear mode in the 110 module. It is this former circuit I cloned in 2009 as part of my 100M replica project as a single VCA with both exponential and linear modes. This is a nice useful VCA with a clear sound and two CV control modes.

EuroRack Module:   14HP wide

Description The original Roland 130 VCA is based around the BA662 OTA chip, which I have replaced with the easier to find BA6110 which has 86dB Signal to Noise Ratio 0.5% THD. Whilst this is not a leading edge studio VCA, these specifications are fine for use within a modular synthesizer and are an improvement over the 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 design was updated in 2017 to use a single PCB and vertical pots and jack sockets.

Outcome & Availability The arrival of Behringer clones of the 100M modules in 2020 means I have no plans to produce a 100M VCA “130” module. The Behringer 130 module is very good but surprisingly expensive! There are trimmers for aligning the module, but unfortunately they are inside and are SMD. They are marked LIN1, LIN2, EXP1, EXP2.

B130 Trimmers

The panel needs to be removed, to access them, undo the rear screws using a small posidrive screwdriver. With care it possible to set the exponential and linear modes to give the same signal output at the maximum gain setting (equivalent to a +10V CV signal input). The expo trimmer is very sensitive to a change in rotation, just like the original Roland. Behringer made a number of resistor changes in the linear and expo drivers, which work very well and make the alignment easy. There are two additional trimmers to set the DC offset of the VCA chip (V13700).

AM8120 VCF & VCA In 2023 AMSynths launched a new set of 100M modules in Eurorack, to complement the Behringer System 100. The AM8120 module (VCF & VCA) contains the VCA from the 130 using a AS662D chip, and the AM8135 provides a quad DC coupled VCA.

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