07 January 2010

Yusynth (Part 18)

Today I finished another Yusynth module. This time a Steiner VCF module. It is quite a versatile filter module that you can switch between High Pass, Low Pass, Band Pass and All Pass using a rotary switch. So actually you get three filters, because the all pass is kind of the same as your input signal :) Besides that it has two CV inputs with a potmeter to regulate the amount and also a potmeter for the input signal. I was wondering if this was the first module Yves designed by the way since it looks just that little bit different than the other modules that I build so far.

So here is the module all wired up. It is all quite straight forward. Yves provides a really neat wiring scheme on his website that makes wiring so easy. He even included the correct marking for the rotary switch so you don't even have to think :) Well after wiring it was time for testing. I applied power to the unit from my lab power supply and for the first time my current restriction kicked it. Lucky that I have it otherwise that would have meant magic smoke I guess ;) I did a quick visual check and found out quite quickly that I mounted the IC socket for the TL072 the wrong way around :( Luckily it was on a socket so I could turn it quite easily. But what a stupid mistake this time :(

Well nothing was damaged luckily. After applying power again everything looked OK and I started testing. I put a noise signal on the input from the PC Function Generator. I tested all three filter modes and that all worked like expected. There is only on 1 turn potmeter on the PCB for 1V/Oct tuning, but when I wanted to fiddle around with that I noticed something strange. When I up the resonance to get auto oscillation it didn't work at first. But only when I also fiddle with the cuttoff pot as well. I tried it several times and the same result. Maybe Yves can shed some light on this? His other modules start auto oscillating by only putting the resonance to 95% or so. And usually there is a trimpot to calibrate this. Ah well no big deal at all. Then I put a square wave at the input and actually got some very neat results. It is not a very clean filter, but that makes it exciting I think :) So I hope I didn't make any mistakes except for the wrongly mounted IC socket. It pretty much works like I expected besides the auto oscillation thing.

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