Zlob Dual VCA


I had 2 HP of space left in my Synthrotek portable Eurorack case, after shuffling some modules.   Looking around for 2 HP modules, I found this little gem, from Zlob modular.  Here’s the description of it on Modular Grid.

I was going to order an assembled module, but Control only had the DIY kit in stock, so I got that.  And I’m glad I did, because I was able to make a few modifications.

  • Upgraded the 5% resistors to 1% from my resistor stock.
  • Upgraded the IC sockets to machined-pin type.
  • Replaced the diode reverse power protection with 0.1 uf bypass capacitors.
  • Hardwired each output into the Sum, instead of having each removed when patching the channel output jack.

The machined-pin sockets raised the profile just a tad, but only up to the level of the trim pots.  As for the reversed power protection diodes, I don’t need them, and I wanted to add more bypass capacitance, since there are only three 10 nf caps per rail.   The biggest mod was removing the jack switching, where if one of the channel outputs is patched, that channel is removed from the Sum output.  I could not see a point in that, since there are only two channels.  With the original design, if you patch both channel outputs, there will be nothing on the Sum output.  The white wires show the jumpers that hardwire, so that both channels are always present in the Sum output, no matter how the outputs are patched.



All the jacks plus the two LEDs mount on one PC board, with the rest on the main board that attaches by right angle headers.  I omitted the two header pins that brought back the switched output signals to the summing op amp, since I jumpered those on the board.

The panel is atypical for Euro, being made out of Laser Cut Acrylic.  It’s flexible.  Looks a bit bent in the pictures, too, but this wasn’t a problem.  It’s also thick.  I shaved off some of the material behind the panel from the jack holes, so that the barrels would protrude farther and the nuts could find more purchase.

No instructions came with this kit, nor was I able to find any on the Zlob website.  No matter.  I started by drawing out the schematic (which I won’t share, since they don’t give it).  All the supplied parts matched the PC board, except for the two resistors that set the brightness for the red and green LEDs (which just monitor the CV inputs).  The PC board gave 1K2 for the green and 1K1 for the red, but the resistors supplied were 1K and 3K3.  I guessed wrong and put the 3K3 on the green LED.  It was not very bright, and the red one was too bright!  So I swapped in 1K8 for each to see, and the red was still too bright.  So I set that to 3K3 and it’s still brighter than the green, but okay.


I discovered that unity gain occurs with about 2.5 volts on the CV input.  So with a typical 5 volt control signal, the gain is times two.  I don’t mind having an amplifying VCA.  And I have a Mutable Instruments Shades triple attenuator right next to this VCA, if I want to control the CV amplitude going in.


Each channel has a trimpot to set the zero point, which I set just a tad below zero, so that the channel cuts off completely with zero volts of CV.








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