[sdiy] Roland TB-303 filter responses graphed

[Richie Burnett]

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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