Modifying the Velleman MK171 Voice Changer Board
breaking out the controls and giving it a sample rate control
For this build I'm going to show you how I lightly modified one of Velleman's MK171 voice changer kits to break out the push-button controls, add what becomes essentially a sample rate control and give it 1/4" audio ins and outs.

The Velleman kit is available at your usual online retailers (and probably in actual shops too, should you be able to visit one) for about 10 ($14-ish). It has four features - it'll pitch the input up a couple of octaves, down a couple of octaves, add a vibrato and the 'robot' mode. In my tests the vibrato made very little difference to the voice but did make an impact when used with synths or drum sounds. The robot mode is a kind of ring modulation effect and you need to be very careful with what you're putting through it otherwise it sounds dreadful. The pitch up and down is where this becomes most useful, especially the pitch down function - you can sound like Kylo Ren in next to no time...

Disclaimer: I am absolutely a beginner at this whole electronics / soldering / making stuff malarkey, so most of what is below is be figuring stuff out as I go along, watching Youtube videos and reading things on that there inernet - so take everything I say within that context. Also, I'm no great shakes when it comes to soldering so some of the joints you'll see below are truly gruesome and for that I apologise to you all.

Getting Started
The Velleman MK171 comes as a kit so you'll already have it in pieces before you start. You'll also need the following -

  • 250k potentiometer
  • 2x 1/4" sockets (or 3.5mm if you're a Euroracker)
  • 4x momentary push button switches
  • something to house it in


  • The Build
    The build starts off really easy. Just follow the instructions that come with the kit, but leave out the following components -

  • Resistor R5 - we'll replace this with a pot to make a sample rate control
  • The four push buttons - we'll be breaking these out
  • The microphone - this'll now be a 1/4" socket

  • You'll end up with a board that looks like this -


    Fig. 1 - The board minus a couple of bits we'll be replacing. I've left the ICs out for now as we'll be doing some more soldering and I didn't want to accidentally damage them.

    We're going to be replacing the R5 resistor with a potentiometer, so you'll need to solder a couple of wires to the pot and then to the board. For mine I wanted fully clockwise to be the highest sampling rate so I connected my wire the centre and right-hand leg of the pot. If you want it the other way then use the left-hand and centre legs. One thing to be aware of is that as the resistance gets close to 0 the board goes silent. I've not measured this but with that being a linear pot I think it's about the 5-10k range, so you might want to solder a resistor in-line with the cables to avoid it dropping to zero. You can also use a smaller pot if you don't want to drop the sample rate as much as I did.

    When you've done you're soldering you should have something that looks like this -


    Fig. 2 - the wired up potentiometer (I braided my wire purely for aesthetics).

    Wire up the two 1/4" sockets that we'll be using for our audio in and out and connect these to the board. The audio out is the 'speaker' posts and the audio in is the mic connectors. Pay attention to your polarity here as getting it the wrong way round stops the board working. The 'tip' connector on your 1/4" socket is the positive.

    Wire up the power connector. This is a 9v connector that comes with the kit. Again, just keep an eye on your polarity - I didn't wire mine backwards but I'd imagine it'd break if you did.

    This is what you'll have with all the audio and power connected -


    Fig. 3 - the board with the connections. IMPORTANT - I have the audio-in wired backwards in this picture, it's how I know that the polarity is important. Make sure your tip is postive.

    Next up we're going to wire in our push-button switches. Now this is a bit where I hit a problem - and it be fair, from my searching it seems to be a problem that has affected a lot of people. The switches with the kit are four-post switches and our replacements are just two (wires). So, you'd be quite forgiven for thinking that as your switch is essentially bridging from the left-hand pair of holes to the right-hand pair of holes you can just solder in the wires to the top two holes nearest to the IC holder. Just like this -


    Fig. 4 - wiring them like this won't work.

    I'll be honest, this one had me stumped for a while. I couldn't figure out why my buttons didn't work. I stared at the schematic, I stared at the PCB, I took to calling the whole project an every increasing amount of profanity.

    And then I spotted it. The switches have to share a common ground, and the traces on the PCB provide that common ground - just not from the holes nearest the IC holders. They get their common ground from the the holes nearest the edge. Here's what I found -


    Fig. 5 - notice how the arrowed pads join the big trace, this is where we need to connect the other side of the switch.

    To get the switches to work we have to wire them to opposite corners. Looking from the top, the left-hand side goes to the hole nearest the edge; while the right-hand side goes to the hole next to the IC holder. Here's how it should look -


    Fig. 6 - this is how they switches should be wired.

    And that's it really, all you'll need to do now is box it up and have some fun - oh, and plug in the ICs... The rate control gives you a bit of 'tone' control when using the pitch functionality. Certainly, the pitch-down benefits from a slightly lower sample rate - just to take the edge off the high-frequency aliasing noise.

    Links:
  • Velleman's UK site, with manual

  • All work © Darren Shaw 2020 (except where noted)
    darren@anexium.com

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