[sdiy] Roland TB-303 filter responses graphed

Mattias Rickardsson mr at analogue.org
Mon Nov 2 10:42:51 CET 2015

Thanks Richie for clarifying this.
I'm sorry for not getting it the first time. :-)

/mr  -  a bit weedy in his response

On 26 October 2015 at 16:42, Richie Burnett
<rburnett at richieburnett.co.uk> wrote:
> The weedy bass response of the 303 VCF is due to the under-sized capacitors
> C25 and C27 used to recover the signal from the top of the ladder network.
> (These are marked as section "2" on Tim's diagram that you linked to.)  They
> give an open-loop pole at around 60Hz, so that's a high-pass filter with a
> cutoff frequency of 60Hz when the resonance control is turned right down.
> (These poles move to a *much* lower frequency in the single hertz when the
> resonance is turned up and negative feedback is applied around the ladder.
> They only contribute audible high-pass "low-cut" action when resonance is
> near zero.)
> If you make C25 and C27 larger you get back most of the bass lost in the
> VCF, but loose the characteristic nasal sound from the filter.  (The other
> poles that Tim numbered on his diagram are all at much lower frequencies and
> don't contribute audible bass loss in the filer.  They are valid from a
> stability analysis point of view, and actually cause an alarming resonant
> peak at about 8Hz when resonance applied.)
> Capacitors C21 and C22 where the VCF feeds into the VCA are an additional
> source of major bass loss due to being undersized.  They result in a
> high-pass filter with two poles (and one zero) when the resonance is turned
> up.
> -Richie,
> -----Original Message----- From: Mattias Rickardsson
> Sent: Monday, October 26, 2015 3:09 PM
> To: Richie Burnett
> Cc: synth-diy
> Subject: Re: [sdiy] Roland TB-303 filter responses graphed
> On 26 October 2015 at 13:46, Richie Burnett
> <rburnett at richieburnett.co.uk> wrote:
>> You can bring back much of the bass that has gone AWOL in the factory
>> build
>> TB-303 by making C21/C22 and C25/27 larger, but it destroys some of the
>> nasal character of the TB-303's sound.  You end up with something that
>> sounds a lot more like any other analogue synth.
> Not entirely. If you turn down the resonance, you still have the
> inherent nasal "weedy bass response" (TM) of the diode ladder
> topology:
> http://www.timstinchcombe.co.uk/index.php?pge=diode2
> /mr
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