MATRIXSYNTH: DIY ARP 4072 Lowpass Filter (ARP 2600)

Monday, July 20, 2020

DIY ARP 4072 Lowpass Filter (ARP 2600)


Preview of latest circuit: The ARP 4072 Lowpass Filter (ARP 2600) MrCaliforniaD

"Version 1.0 is working like a charm.

Once in a proper enclosure, I'll try to record something more substantial.

Thanks to Eddy Bergman for his unconditional support.

You can visit his website to get the schematics. Also check out his other awesome projects while you're at it:
https://www.eddybergman.com/2020/03/s...

I don't sell theses modules. Sorry!"


Some details via Eddy Bergman's website (note the schematic is from YuSynth):

Wednesday, 4 March 2020

Synthesizer Build part-21: ARP2600 LOWPASS FILTER (4072).

The best sounding filter of any I built so far! With verified stripboard layout.

A word of warning right at the start; this is an advanced project, not for beginners. You need to know your electronics and you also need to have a good multi channel oscilloscope.

The ARP2600 has become one of my favourite synths lately and after tackling the ARP's Envelope Follower I thought it was time to try out the famous 4072 filter. ARP has three well known filter types. The 4012 of the early 2600 models, which was the ladder filter with which they got in trouble with Moog over, because Moog held the patents for that. Then they changed to this filter, the 4072. And then there's the 4075 which was the filter used in the ARP Odyssey.

If you want to build this filter there's really only one schematic you can turn to and that's the Yusynth schematic. So I set to work making a layout. I first tried just starting at the lower left of the schematic and building the layout up from there. Within minutes it turned so complicated I couldn't make heads nor tails of it. So after an other unsuccessful try I came to version 3 of the layout and this time I decided to place all the semiconductor components neatly on the board first. All transistors in a row on top and the two chips in their own space underneath and wire it all up that way. This worked fantastically and after a days work I had a layout that looked really good and, more importantly, turned out to be faultless right from the get go.

However, like I mentioned at the beginning, this is not a project for beginners. It's reasonably complicated and you need to work very methodically and do things in steps. First map out all the cuts in the copper strips with a Sharpy and cut the traces accordingly. Then solder in all the wire bridges and then solder in the components. Keep counting the holes and make sure everything is placed exactly like on the layout, otherwise you will run into trouble with space on the board and things end up not being connected right. I worked from left to right soldering it all in and checking every connection with a powerful loupe. And in the end, of course, it didn't work straight away. It turns out I had forgotten to cut four copper strips near the 1V/Oct trimmer. After I cut those the filter suddenly sprung to life and started making sounds that instantly reminded me of the ARP2600.

Here's the Yusynth schematic. It looks a bit weird but the LM3900 really operates on negative voltage, in this circuit. I tested the filter on dual 12V and it works just aswell on 12V as it does on 15V without changes. I did notice I had to open up the Resonance potmeter more to get the same effect but it all works as it should..."

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