Monday, 30 March 2020

Synthesizer Build part-25: UNIVERSAL CV EXPANDER.

A very practical module that replicates any voltage on the input and splits it 4 ways. This is the first module I designed myself and it works like a charm. (Not that that's anything to boast about because the design is so simple a 9 year old could think of it ^__^)

I have finished the case for the second stage of my DIY synthesizer so now I can continue building modules and writing about them here. So the first thing I wanted in the new case was a CV and Gate expander. I have built a number of filters that need a 1V/Octave signal but I only had one output to provide it so I designed a universal CV Expander. I call it universal because you can connect anything you want to the inputs: Audio signals, LFO signals, 1V/Oct. CV, Gate signals, you name it. Any voltage presented at the input is replicated at each of the four outputs and if you need more you can connect one of the outputs of the first stage to input 2 and so get a total of 7 outputs for one input.

Here is the schematic drawing showing one stage. As you can see it's just 4 non-inverting buffers with all the inputs connected together. The module is just two of these on one piece of stripboard. I did not include any safeties so make sure you do not connect an output to the input of the same stage coz you might blow up the chip that way. (I'm not sure what happens if you do that and I'm not gonna try it either :-p  )

Here's the verified stripboard layout. Just two IC's and a load of wire bridges. I recently added de-coupling capacitors for the chips because it was kindly remarked upon in the comments below that they were missing and it is good practise to include them although, if you use a normal linear powersupply, there's really no need for them. But many people use switch-mode power supplies and in that case it's good to have them included. I also added 1K resistors on the outputs, eventhough they are not included in the schematic. I thought it would be better to include them to make sure the output impedance is 1K, which is the standard for modular synths. If you do experience a difference in voltage between input and output then replace them with wire bridges but there's practically no current flowing through them so you shouldn't experience any problems with this.

(Last revised 14-April-2020: Added de-coupling capacitors for the ICs on the power rails. 18-May-2020 Added 1K resistors to the outputs.)

Print only:

Bill of Materials:

This module is entitled "CV and Gate Expander" but as I mentioned before it'll work with any Signal. It's just opamps after all. Very useful to have in your setup because sooner or later you're going to need at least one of these, just like you need at least one mixer/passive attenuator.

Here are a few pictures of the finished product. I made a little L-Bracket (also visible on the layout) so I could mount the print at 90° to the panel.

Okay, that's an other one done. Number 25! A bit of an anniversary for my synth build :)
Got any questions or comments just leave them in the comments below please. See you on the next one!


  1. what about the two capacitors next to each opamp? value? purpose? thx.

    1. What capacitors? There are no caps in this design.

    2. they are not in the stripboard schematic but I can see them on the fotos! connecting plus and minus 15 to ground?

    3. Yes, you mean the decoupling caps. You can put them in. They are usually 100nF between plus and ground and between ground and minus. But if you use a normal linear power supply like I do, there's really no need for them. But it can't hurt to put them in you're right. You know what, I'll adapt the layout and put in those caps so you know where they should go. Thanks for mentioning it!