[sdiy] How does resonance work?

[Tom Wiltshire]

It's sort-of simple, and sort-of not.

The simple bit is that if you have positive feedback, you get resonance. That positive feedback could just be a pot sending some of the signal back to the input.

The not-so-simple bit is that the feedback needs to be positive - e.g. it needs to be in phase with the input. With a filter, this isn't often as obvious as it sounds. There are phase shifts through the filter stages that might well require that the the feedback be *inverted* in order to produce resonance.

Once you've got positive feedback and resonance, you can control it with a pot, or a VCA, or a Vactrol or whatever other volume control element takes your fancy.

There's nothing special about VCAs. They appear in the classic synth schematics because they allow programmability, not for any other reason. If you look at classic monosynths before programability, you'll find lots with a simple pot for the resonance.

HTH,
Tom


On 11 Oct 2013, at 20:22, Jack Jackson <jackdamery at hotmail.co.uk> wrote:

> Recently I've started building the Jupiter 8 filter using the IR3109 and also with discrete OTAs. I couldn't get the resonance to work, so I looked at the other implementations of the 3109 in filter circuits. I started wondering:
> 
> Is resonance as simple as a VCA in the feedback path of the gain cells?
> 
> I know that resonance is an increase in emphasis of the cutoff knee of the filter; the highest frequency you can hear from the LPF. So, although the fundamental frequency of the signal passing through it might be quite low you hear the whooshy self-oscillation as a higher freq.
> 
> Please correct/educate me.
> 
> Thanks,
> Jack 		 	   		  
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