[sdiy] FW: Re: Ladder filters and gain drop, that old chestnut

Richie Burnett rburnett at richieburnett.co.uk
Thu Aug 27 20:22:57 CEST 2015


> > What I was wondering was whether inverting the output and using positive rather than negative feedback would make it more convenient to use the input-signal-to-resonance-VCA trick to boost the input level as the resonance increases.
> 
> Yes, the magnitude of the input signal needs to get bigger, nor smaller, as the feedback gain is increased, for the volume compensation to be effective.
> 
> You can either invert the output from the filter before feeding it back to the resonance VCA.  Or if the resonance VCA is something like an OTA you put the filter output into the inverting input and the drive signal into the non-inverting input. So that each signal has the correct phase when it comes out of the resonance VCA and hits the first pole of the cascade.
> 
> > Seems like with negative feedback, sticking the input in there is going to *reduce* the input level, as well as cancel the lowpass signal coming from the output, making it highpass-ish.
> 
> Not sure about that. It's only highways-ish if you listen to the output from the resonance VCA directly with the resonance cranked up. It's still lowpass at the end of the cascade of four poles where you would normally take the output. 
> 
> -Richie,

---- Original Message ----
Subject: Re: [sdiy] Ladder filters and gain drop, that old chestnut
Sent: 27 Aug 2015 17:58
From: Richie Burnett <rburnett at richieburnett.co.uk>
To: Tom Wiltshire <tom at electricdruid.net>
Cc: 

> What I was wondering was whether inverting the output and using positive rather than negative feedback would make it more convenient to use the input-signal-to-resonance-VCA trick to boost the input level as the resonance increases.

Yes, the magnitude of the input signal needs to get bigger, nor smaller, as the feedback gain is increased, for the volume compensation to be effective.

You can either invert the output from the filter before feeding it back to the resonance VCA.  Or if the resonance VCA is something like an OTA you put the filter output into the inverting input and the drive signal into the non-inverting input. So that each signal has the correct phase when it comes out of the resonance VCA and hits the first pole of the cascade.

> Seems like with negative feedback, sticking the input in there is going to *reduce* the input level, as well as cancel the lowpass signal coming from the output, making it highpass-ish.

Not sure about that. It's only highways-ish if you listen to the output from the resonance VCA directly with the resonance cranked up. It's still lowpass at the end of the cascade of four poles where you would normally take the output. 

-Richie,



More information about the Synth-diy mailing list