The idea of a stereo module for the CGM family has been with Frap Tools almost since the early development stage of the channel, group, and master modules. It wasn’t until October 2019, however, that actual proof of concept appeared.

Packing up

We designed the CGM series to create various configurations, but we understand that often it is also helpful to have smaller Eurorack setups for more focused works.

For this reason, we decided to pack more than one channel in a single module to provide a CGM unit that could offer many functionalities even when used by itself. On a side note, we also wanted to optimize the space-to-feature ratio.

A three-channel stereo configuration with the then-available CGM modules, for example, would have required six channels and a group for a total of 42 HP – definitely too much for a small case. We thus came up with the idea of a triple, 12-HP stereo channel, and we began to think about its interface.

The interface problem

We soon realized that we couldn’t just transpose the Channel concept into a triple module because the interface would have been too crowded.

The first thing that we removed were the individual outputs for two reasons:

  1. it would have required two more jack sockets for each channel, crowding the panel and the PCB;
  2. the module was also meant to be used as a standalone compact mixer, so a local sum seemed more appropriate, design-wise.

Then, to improve the usability, we decided to use ⌀12 mm knobs instead of the ⌀20 mm of the channel.

Frap Tools triple stereo channel prototype.
The prototype of a possible triple stereo channel

As you can see from this early prototype, we found another issue: how to fit the classic three buttons (Mute, Solo-in-place, PFL)? At this point, our idea was to get rid of the PFL and keep just the Mute and SIP. However, it didn’t sound quite right, and it still carried some other problems under the surface, on the PCB.


Even if the panel design was indeed fairly ergonomic, there was no way to fit all the components for three stereo channels in the module’s back without making it too bulky, even for our PLUS cases.

We felt compelled to choose which features to keep and which to let go of, so we started to check for redundancies.

We’ve already got rid of the individual outputs, but this wasn’t enough, so we started to discuss the control voltage inputs. We asked the opinion of many musicians regarding their send/return and panpot habits, and we found that CV over each send and pan could have been redundant.

Our idea, then, was to “redistribute” the CV capabilities across the three differentrent channels: some of them could have had the yellow send CV, some others the green, some others the pan.

Even after scaling down the control voltage inputs, there was no way of fitting the three channels into a 12 HP PCB. Sure thing, we could have made a 13 or 14 HP module, but our golden rule is always to follow multiples of 6 when designing a module to avoid those tiny little 1-2 HP gaps in our systems.

Expanding, again

So perhaps now the way out of this impasse was to add features rather than remove them!

Here’s why in spring 2020, we decided to raise the number of channels to four and make the module 18 HP wide.

The Quad Stereo Channel concept was born: those extra 6 HP made the PCB design way easier (well, at least, feasible), and the additional channel made the concept of a potential standalone mixer for compact systems even more solid.

We became so excited by microsystems that we began to think about potential features to add to the QSC without compromising the overall CGM architecture, so we came up with three things:

  1. a switch to change the panpot behavior, from classic stereo panning to dual mono crossfader; this feature allows the musician to patch two different mono sources to the left and right channels and blend them through the pan knob so that the QSC can eventually become a sort of eight-channel mixer;
  2. local mono effect send outputs that mirror the signal usually routed to the Group’s sends;
  3. an auxiliary mono input.
QSC Interface
The final prototype of the QSC Quad Stereo Channel

It soon became clear that the QSC reached its final form and became what we’ve been looking for over the past few years: a compact stereo mixer with enough features to use it by itself, but not too many to make it redundant on a broader CGM setup.

We also kept all the classic CGM features that require groups and masters to operate: PFL and Safe Solo. Remember when we thought about removing the PFL? We came up with a different approach: use a switch for the Solo-in-place function.

Solo-in-place button (C)
Solo-in-place switch (QSC)

In this way, one can use the QSC by itself, or pair it with groups and/or masters to unleash its full potential.

Read more about the QSC in its product page!