Wednesday, 4 December 2019


The first filter I built and not one of the best in my opinion.

[EDIT] Before we start: I'm writing this about 7 months after I published the article below and I can tell you now that I removed this filter from my synth. I couldn't get it to self-oscillate and I was not impressed with the sound of it. Maybe that's due to the fact that I was just starting out with synth building when I made this but if you have the AS3320 chip I advise you to use it for the Digisound 80.6 lowpass filter. That is an amazing sounding filter!

Okay on with the original text:

So now we get to the heart of the matter. The thing that defines the sound of our synthesizer; the filter.  I've considered many filter designs, and there's a lot to choose from if you search for schematics on the internet, but I've picked out 3 filters that I want in my synthesizer. Two of those are LowPass filters and one has the choise between Lowpass and Highpass. That's the Korg MS20 filter We'll get to that one later.

Now we will concern outselves with the Sequential Circuits Prophet 5 (and Sequential Pro One and many others) Lowpass filter based on the AS3320 chip.
This chip (originally the CEM3320) is at the heart of many famous filters and there's a good reason for that. It sounds fenominal!
I went to the ElectricDruide website and found a page full of filters and amongst them I found this circuit:

There are also versions which are re-configurable with the flick of a switch but they were a bit too complicated for my taste and I thought this would be a great project to start with. As it turned out, I was right because it worked straight away and worked very well!

I made a layout on stripboard which you can find in the picture below. All potmeters viewed from the front.:

The first version I made of this had capacitors that were 220pF instead of the 150pF as seen in this schematic. I thought that would give me more control over the low frequencies and I was right, LOL! It brought the top cut-off frequency down to about 200Hz. Way too low. So I soldered in the 150pF and everything worked fine after that.

The three inputs in this schematic have different values but I did some testing and the 120K resistor worked the best on the input so I put those in all 3 inputs. I think you should also do this if you decide to build this filter because these schematics reflect the circuit that is used in the synthesizer itself and that's why the inputs have different values because they receive signals with different amplitudes. In our synth all the VCO's produce signals with a set amplitude of 10V peak to peak, therefore the input resistors need to be the same value.

There's not much more to say about this. The opamp in the schematics can be any low noise audio type. I used the TL072 for that because I have a lot of those in my components collection and they are the opamps most used in synthesizer projects. But you can also use the TL071, 081, 082, etc.
If you use a chip that has more then one opamp, make sure to properly connect the opamp(s) you don't use. Connect the inverting input to the output and connect the non-inverting input to half the voltage of the power supply. In our case we use a -15 to 0/GND to +15 Volts power supply and so we can connect the + input to ground, because that represents the middle value. Should you use a power supply that is just +15 and ground then you should make a resistor voltage divider using two resistors of the same value and connect the + input to the middle of that.
You can use this filter with a dual 12V powersupply without any changes except for the current limiting resistor to pin 13. Change it from 1K5 to 1K2 for -12V operation.

This filter sounds really good and smooth although it is not as versatile as the Moog or Korg filters. For one, this filter is not self oscillating (at least, the one I build isn't) so you can't make it scream but it does produce that metally high-end synthesizer sound, if you know what I mean (LOL).  It's just a different filter to the Korg or the Moog Ladder Filter which is an other filter I've put in my synth build. It's more a filter to round off certain sounds you've created on your synth, to make them fuller. I don't know, you have to listen to it to know what I'm on about. It should be capable of self oscillation if you read the text that goes with the schematics. (If you have this filter and it is self-oscillating I would like to hear from you in the comments and tell me how to alter it for oscillation.)
EDIT 20-March-2020: I just finished work on the Digisound 80.6 lowpass filter using the AS3320 chip and it sounds fenominal. If I were you I would build that filter instead of this one although this filter is by no means a bad filter. It's just not as aggressive as the other one

Here's a little video of the filter in action with some deep bass-like sounds. This video was not made as a demo video for this article, it's simply a recording of me trying out the filter for the first time:

Here are some pictures of the PCB and the panel I made:

The filter is the one on the right. The other one is the VCO. I had yet to add the text to the panels.

You can see the inputs for oscillators one, two and for noise on the left and below that the resonance control voltage input (for the LFO for instance). That input has a level control and there's a switch for internal or external resonance control. On the right is the audio output. The top potmeter is for the Cut-off frequency and the one below is, as mentioned, for resonance.

Enjoy the build and if you have any questions then just put them in the comments and I'll get back to you asap. :)


  1. Hey Eddy, how would change these modules to a 12v system? I'm fairly new to diy and i love the designs and layouts, easy to follow! Thanks again!

    1. You can in most cases simply connect them to 12V without any changes. The AS3320 chip has a current limiting resistor from pin 13 to -15V, if you want to use it with +/-12V this resistor needs to be changed from 1K5 to 1K2, that's all. I'm so glad you like my articles. This filter was a bit of a disappointment to me and I am currently building a new filter with the AS3320 chip; the Digisound 80.6 Lowpass filter. Expect an article on it within the next week.

  2. Dang, im excited can't wait!

    1. Cool! Well I've been testing it today but I can't get it to work (yet). But that's usually the case with anything I build. In 80% of the cases I get it to work eventually though. ^____^

    2. I've got it working my friend and it sounds AWESOME!! Working on an article with demo video right now. :)

    3. Oh sweet, i'll be ready. I'm sourcing parts to start building. Another thing, for the other modules what would i need to change to make it 12v? Probably just being paranoid. i'm scared of the magic smoke. Ha Ha.

    4. If there are specific things that need to be changed for 12V, it's usually mentioned in the article plus you can't get magic smoke by taking away 3 volts from the power supply so no worries ^___^