More Low Pass Filter Questions (more "general" than moog specific tho)

Nyarlathotep · #1 10-09-2009, 01:29 PM

So. I've been looking up on low pass filters. Thinking of making my own one day.

My first question is what does the "poles" number define in the LPF world. I know that 1st order R-C LPFs are 6dB/octave 20dB/decade, and AFAICT they seem to be referred to as "1 pole" LPFs.

Im also aware that 2nd order appear to = 12dB/octave, and im assuming at this point that 3rd = 18dB and 4th = 24dB/oct.

Quick Edit: Another thing. If you run an X-pole circuit into a Y-pole one, both set for the same knee/cutoff freq, does it become an (X+Y)-pole?

If thats the case, it would explain a LOT as to how the moog works and is constructed. Yes, I couldn't go a whole post on LPFs without bring the MF-101 into it

2nd question, which is totally moog related (

): What does the resonance on the 101 do? In fact, what does "resonance" generally do/mean? I've noticed that res is on everything from LPFs to flangers.

Thanks

bongomania · #2 10-09-2009, 01:51 PM

Poles are the number of stages of processing, irrespective of the values chosen for each stage.

Resonance is essentially feedback. It is a condition where specific frequencies amplify themselves in a repeating "reflective" manner. In a filter, it is not the exact same as audio feedback, in that the audio output signal is not fed back into the audio input. Instead the filter circuit itself has resonant properties (acting on the electronic signal) which can be measured and controlled.

Nyarlathotep · #3 10-09-2009, 01:54 PM

Quote:

Originally Posted by bongomania

Poles are the number of stages of processing, irrespective of the values chosen for each stage.

Resonance is essentially feedback. It is a condition where specific frequencies amplify themselves in a repeating "reflective" manner. In a filter, it is not the exact same as audio feedback, in that the audio output signal is not fed back into the audio input. Instead the filter circuit itself has resonant properties (acting on the electronic signal) which can be measured and controlled.

Cool

bigchiefbc · #4 10-09-2009, 03:07 PM

Quote:

Originally Posted by Nyarlathotep

Cool

Just to add in a bit of info: if you were look at a frequency spectrum of your signal going through the LPF, you would see a small peak just before the signal hits the cutoff. The higher you turn the resonance knob, the taller that peak would be. With the Moog specifically, when the resonance knob is 8-10, that peak gets so high that it self-oscillates. You can actually use the Moog LPF as an oscillator, and get a cool sine-like tone, even with no input signal going through it.

Nyarlathotep · #5 10-09-2009, 06:23 PM

So, it becomes more like a band pass with more loss on the treble side than the bass?
Here's a site I found on res filters: http://www.allaboutcircuits.com/vol_2/chpt_8/6.html The LPF on that site is based on LC design with a resistance (could be variable I suppose) to decrease "resonance". I also suppose that a variable cap could be used to select the knee freq

Interesting.

Something else interesting from perusing Wiki: