Moog Werkstatt: improving the VCO and VCF direct outs
There are three ways to get audio from your Werkstatt: the VCO direct out, the VCF direct out (both on the pin header), and the main audio out (the 1/4″ jack on the rear panel).
The VCO Out signal is a sawtooth or pulse, depending which wave the VCO is switched to, at 0-5V. This is pure, dry VCO with no further processing, though of course it will be pitch- and/or pulse-width-modulated, depending on your modulation routings. The VCF Out is taken directly from the output of the filter, bypassing the VCA, and is nominally -2V to +2V. The main audio out is at typical line level (a couple of volts peak to peak), and comes through the filter and VCA.
If you want to use the Werkstatt as an extra oscillator for a modular, for example, you’ll probably want to use the VCO direct out. If you’re running the filtered sound into an external VCA for more varied amplitude modulation or to use with a high-pass filter maybe, you will probably want to take the Werkstatt’s audio directly from the VCF out. If you’re using the Werkstatt as a standalone expander, the main audio out will do just fine.
If signal levels were the same all the way along, none of this would be a problem. However, as with other aspects of the Werkstatt’s design, it needs some tweaking to integrate perfectly. Here’s how.
VCO direct out
Let’s say you’re using the VCO direct out. Eurorack has typical VCO signals of 10 volts peak-to-peak (see Doepfer’s Signals in the A-100 section, for example), centred around 0V (that is, -5V to +5V). To get the closest match sonically we want the Werkstatt’s output to match the other oscillators you’re using. Not all modules with mixers on board will boost as well as cut their inputs, so we can add a small circuit to give a true -5V to +5V VCO Out on the Werkstatt. The schematic below shows both the VCO and VCF mods. More on the VCF shortly.
How It Works
The VCO out mod is a basic non-inverting amplifier with an offset to make the positive-only signal bipolar. The gain is set by the two 20k resistors (1+20k/20k = 1+1 = 2) and the unity-gain reference point is at 5V. That is, 5V in gives 5V out. 0V in would be a difference of -5V from this reference point, so this is multiplied by the gain of 2 to give a difference of -10V, which taken from the +5V reference gives -5V out. In this way, the 0-5V input becomes -5V to +5V out. You can see it in action at this link. Below is a screenshot of the simulation.
Likewise, if you want to run the Werkstatt’s VCF output into an external module, boosting the signal to match requires just a small circuit, almost identical to the first. The signal level drops as resonance is increased, but to keep our circuit simple we won’t worry about that. The schematic is on the same sheet as the VCO output, posted earlier on this page.
How It Works
This is also a non-inverting amplifier, but this time with no offset as the VCF signal is already bipolar – all the amplification happens around a 0V centre point. Positive signals get more positive, negative signals get more negative. The VCF direct out is normally about 5V peak to peak at maximum, so we just double that to get the more useful 10V range. The gain is set the same way as the previous circuit, and we get an output of -5 to +5V maximum.
Installing the mod
I built both these circuits onto a small piece of stripboard mounted onto the panel with one of the minijacks. There’s just enough room, as can be seen in the photos. This allows the use of both halves of a dual op-amp so nothing goes to waste. There’s also plenty of room on the experiment pads at the top of the Werkstatt’s PCB, though you may find it a bit cramped if you’ve already got a couple of mods in there like I have…
The photos show the locations on the PCB of the various supply rails you’ll be wiring up to: -9V and +9V to power the op-amp, +5V for the VCO amplifier reference, and GND. These are all labelled on the top side of the PCB anyway so it’s easy to find them. I shared the ground that my existing mods were already using, which is connected to the nearby screw post via a solder tag. See my CV mod for details.
U1: TL072 or equivalent
R1, R5: 10k (1/4W 1% Metal Film used here, but it’s not critical)
R2, 3, 6, 7: 20k
R4, 8: 1k
jacks, wire, stripboard: as per your own choice