[sdiy] Roland TB-303 filter responses graphed

[Richie Burnett]

This was measured from the line out of the TB-303.  So technically it's VCF 
+ VCA + Mixer response.

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.

Reducing R47 makes the cutoff frequency go lower.  Colin Fraser has 
previously recommended a suitable value for fitting here.

One of my 303s has the 6 mini-jacks fitted along the back for CV control, 
and you can conveniently lower the cutoff frequency of the filter just by 
plugging a shorted mini-jack plug into the filter CV input socket which is 
sometimes handy.

-Richie,

-----Original Message----- 
From: cheater00 .
Sent: Monday, October 26, 2015 11:33 AM
To: Richie Burnett
Cc: synth-diy
Subject: Re: [sdiy] Roland TB-303 filter responses graphed

Were you measuring this at the output of the VCF or later?

The bass response is fixed perfectly well by replacing the output
capacitors at the top of the PCB with much larger ones.

You can extend the range of the cutoff knob to include much lower 
frequencies.

On Thu, Oct 22, 2015 at 4:33 PM, Richie Burnett
<rburnett at richieburnett.co.uk> wrote:
>> kudos for doing this! :)
>
>
> No worries.  They're old results that I decided to share :-)
>
>> I don't know the 303 in detail, how does the output level
>> of the VCO respond to the frequency change? Does it stay constant?
>
>
> The sawtooth output from the VCO is a textbook perfect sawtooth over the
> full musical range, but the "square" waveform of the TB-303 is anything 
> but.
> It's pulse width and shape vary greatly from the low 10's of Hz up to 
> nearly
> a kHz.  It's amplitude is also much lower than the saw.  It is this lower
> amplitude of the square wave that drives the filter less hard, and results
> in more resonance from the filter when the square wave is selected.
> Conversely the sawtooth drives the filter harder and reduces the 
> resonance.
>
>> chirp as chirp can.
>> which also would explain the pronunciation of the higher harmonics
>> of the VCOs waveforms.
>
>
> Yes, there is plenty of high-pass action in the VCF and the VCA that
> follows.  This results in the "weedy bass response" that Adam mentioned. 
> To
> me the TB-303 filter sounds distinctly "spitty" and "nasal" as a result of
> it's resonance increasing with frequency and the actual low-frequency bass
> energy being rolled off.  (You can clearly see the LF rolloff caused by 
> the
> high-pass action on the left side of the graph I posted.)
>
>> What about lower filter frequencies? … or is that as low as the 303 goes
>> for cutoff?
>
>
> It goes a little bit lower.  Just for you... ;-)
>
> http://www.richieburnett.co.uk/temp/303vcf2.gif
>
>
> -Richie,
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