[sdiy] How does resonance work?

[Jack Jackson]

Thanks,

So a potentiometer wired as a potential divider, or a variable resistor? Also what sort of value do you recommend?

Jack
________________________________
> Date: Fri, 11 Oct 2013 21:43:20 +0200 
> Subject: Re: [sdiy] How does resonance work? 
> From: cheater00 at gmail.com 
> To: jackdamery at hotmail.co.uk 
> CC: synth-diy at dropmix.xs4all.nl 
> 
> 
> Hi Jack, 
> 
> On 11 Oct 2013 21:24, "Jack Jackson" 
> <jackdamery at hotmail.co.uk<mailto: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 assume you're asking about how to build resonance control on a filter 
> via CV. 
> 
>> 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. 
> 
> Resonance (the concept, not the knob) happens when a linear system, 
> such as a filter, is put in a feedback loop. 
> 
> A delay with 3 seconds feedback? Yup, that's resonance. Nce. Nce. Nce.. 
> 
> This feedback needs to be controlled somehow. The easiest ways are to 
> make sure there is very little of it (you add a resistor) or that it is 
> limited by a nonlinearity which will clip it (e.g. back-to-back 
> diodes). The resistor can be dynamic. Yup, you guessed it, that'll be a 
> VCA. Or a Vactrol. Or something like that. Control of linear systems is 
> described by a theory in mathematics called control theory. 
> 
> If you do not control the feedback it will grow until it's eaten the 
> full gain*bandwidth product of the system (filter), figuratively using 
> up all of the filter's capability. This happens most often in cheap 
> Behringer mixers in op amps that oscillate at 1 MHz eating up the 
> channel's headroom. In filters that's called "self-oscillation", in 
> mixers its called "why is the lowest light in the LED bar always on". 
> 
> I suggest you first try your luck with a potentiometer and only then 
> try for voltage control. 
> 
> Cheers, 
> D.