Author Topic: Trying to make a resonant low pass filter. NOOB WARNING  (Read 990 times)

Bryce McVaney

Trying to make a resonant low pass filter. NOOB WARNING
« on: February 01, 2018, 09:14:28 PM »
Hello. I am trying to build a nice little synth with an Arduino. So far, I have the Arduino generating some pulse waves and digital saw and triangle waves, with the help of some ttl chips. Now I'd like to make a nice analog filter, and I have a neat plan. I'm going to use the PWM function to control a vactrol and make a nice ADSR envelope generator - but that's not important right now because I have a problem. All I have to make the filter are about 100 3904 NPNs and some LM386s (and a lot of passives). I want to get a response like the resonant filter here if possible:

Like I said, this is all running off my Arduino, powered by my computer right now. So that's 5V and 450 mA I think. Keep in mind, some of that is used by the chips. The Arduino generates a square wave signal (with the Tone function) which goes into a counter and then into some logic chips. There are 4 ICs on the perfboard so far. If I need more power, I can get it with a different supply.

Does this even sound possible? Maybe it's really easy? I'm probably not being specific enough (sorry!) so if you have any questions, just ask. Thanks in advance.  :)


Re: Trying to make a resonant low pass filter. NOOB WARNING
« Reply #1 on: February 04, 2018, 06:10:18 PM »
If all you have is a big pile of transistors, you're probably going to be building a Moog transistor ladder filter!

Depending how careful you want to be you could learn about how to match the transistors for the top and bottom pairs of the ladder (there's no need to match the middle pairs, honestly, despite what people say). But if you don't bother matching them, the filter will still work fine and probably sound great, so don't worry about it.

Some example schematics are the Frequency Central Raging Bull (full disclosure - I designed the PCB for this project):

This uses a OTA as the differential amp to get the signal out of the ladder, but there are plenty of other versions that either use op-amps or pure transistors like this one direct from Moog:

If you add some RC filtering to your Arduino's PWM "analog" output, you can generate a variable 0-5V control voltage which will control the filter cut-off.
« Last Edit: February 04, 2018, 06:12:34 PM by ElectricDruid »