Tag Archives: DIY

Pain in the resistors

Well that was a pain in the… resistors.

Finally got the new 3320 VCF module working after tearing my hair out a little. With the lockdown, I’ve had to source the components in dribs and drabs and can’t always get the exact components stated on the circuit board.

About a week ago, I finally took delivery of the last capacitors I needed then excitedly plugged it in and heard… nothing.

A bit of headscratching later, I chanced upon the idea of sending a voltage through the CV input which initially did nothing, until I swept the filter knob, which revealed a comb filter type noise, but only when the knob was pointing straight up. Resonance did nothing to the sound, and I had silence at either end of the filter range.

Where do I start?

My first thought was that perhaps the 100k B potentiometers were faulty, or that I had bought the wrong ones and got my Linear vs Exponentials in a twist. So, I checked stock at http://www.thonk.com and fortunately they had some.

Then, this morning, I had the joy of desoldering the old pots and installing the (arguably superior) new ones. Unfortunately, it didn’t fix the issue.

I checked my soldering again – all looked sound to me – so as a last resort double checked that I’d used the correct resistors. And low and behold, the 91k resistors I had been sent (and which were in a bag labelled 91k) were actually 910k! In fairness, the difference between the two is just a red stripe rather than an orange one, and even my wife wasn’t sure what colour they were when I asked her to check. So I desoldered all four of them, and replaced with 100k resistors (the closest I could find) and I’m pleased to say the module sprung into life.

I still didn’t have any resonance though, but tracked this down to some damage I’d inadvertently caused on the board around one of the resonance knob pins. I could just about work out where the pin was meant to be going though, so added a jumper wire to link to the appropriate resistor, and I have resonance too!

Whether the 100k vs 91k resistors makes a difference to the sound, I’m not really sure, but it sounds good to me and I’m currently enjoying feeding some sample and hold through the CV input.

This was the first time I’d built a module just from a PCB and Panel, rather than a full kit, and it definitely makes me appreciate the convenience a full kit gives you, even if I did save a bit of money along the way.

Gizmophone

Today was day one of Ableton‘s #LoopAtHome and they set a challenge to build and record your own ‘Gizmophone’ instrument. Unfortunately, I had urgent gardening to attend to (?) so only managed to hack something together at the last minute, using whatever scrap items I could find. This included an offcut of windowsill, a metal Ikea leg and some old wire I found at the end of the garden. And this is the result…

I did manage to include a Piezo condenser mic too, but it had terrible background hum, so only really worked for short percussive noises which I combined into an Ableton drum kit. I also dialled in a smattering of the Corpus effect to make the sounds ring a little more, but only at 5.5% wet.

Some sounds were actually Ok!

The revelation though was the metal Ikea leg. When tapped with a spanner, it made a high pitched bell tone that was (almost) in tune. I created a patch using Ableton’s sampler instrument, sorted out the tuning then duplicated it with slightly different filter settings before panning each version slightly left and right.

Another technique that worked really well was swirling and scraping the spanner inside the wire springs, which created a lovely rhythmic texture which I layered behind the main percussion sounds. Lots of editing of warp markers in that one, as well as some Beat Repeat to add variation, but really pleased with the end result.

I created a quick demo of my ‘instrument’ so you can hear what it sounds like. Some additional kick, snare and synth bass was added to fill the sound out.

And if you fancy having a look ‘under the bonnet’, I’ve even zipped up the Ableton project folder too!

Gizmophone Project

It only uses stock Ableton plugins, so should work fine (have tested on my Microsoft surface and iMac) – Ozone Elements is on the master channel, but you can just ignore that if you don’t have it installed yourself.

3320 Filter module

Well, this house building business takes a lot more time than I had expected, and whilst I’ve become something of a dab hand at building airing cupboards and fitting out utility rooms, I’m finally coming out of the other side and getting a bit of time to work on my music.

I’ve recently taken delivery of a third Mantis case (someone should have told me this Eurorack hobby would turn into an obsession!) and am working out what modules to put into the gaps.

DIY time

As you can never have enough filters, I decided to go DIY and search for something interesting for the last 8HP of the ‘Analogue Synth’ Mantis case.

This led me to a PCB and Panel kit by Guru Gara Synth, based around the 4-pole 24dB CEM 3320 filter. The filter was used in lots of classic 80’s music technology including most notably the Prophet 5 synth (Rev. 3), the Fairlight II, PPG Wave 2 and the Oberheim OB-SX – none of which I would ever expect to be able to own! (Unless Behringer feels like cloning them of course).

The kit doesn’t come with any build instructions, but the PCB is very clearly laid out, so that’s no biggie. As the CEM3320 has been out of production for some time now, I went for the AS3320 from soundtronics.co.uk which is a modern equivalent and very reasonably priced. From the YouTube videos, it does sound rather nice!

More info on the 3 Mantis cases themselves and how I’ve configured them in a future post.