[sdiy] Roland TB-303 filter responses graphed

Adam Inglis 21pointy at tpg.com.au
Thu Oct 22 07:13:32 CEST 2015


See the bottom of this page
http://www.firstpr.com.au/rwi/dfish/303-mods/ <http://www.firstpr.com.au/rwi/dfish/303-mods/>

Apparently the unmodified 303 has a "weedy bass response”

Adam


> On 22 Oct 2015, at 2:23 pm, rsdio at audiobanshee.com wrote:
> 
> Thanks! That is certainly interesting.
> 
> What about lower filter frequencies? … or is that as low as the 303 goes for cutoff?
> 
> Brian
> 
> 
> On Oct 20, 2015, at 5:03 PM, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
>> For anyone interested, this graph shows the response of a real TB-303 filter set at full resonance for a range of cutoff frequencies:
>> 
>> http://www.richieburnett.co.uk/temp/303vcf.gif
>> 
>> There are lots of theoretical responses, mathematical transfer functions, and simulation results on the net, but i've never seen any real measured responses before.
>> 
>> These responses were generated at the same drive level that the sawtooth VCO drives the filter in the TB-303.  (The shape changes if the drive level is changed.)
>> 
>> Notice how the resonance is much stronger at higher frequencies, and runs out of steam at low cutoff frequencies.  The little peaks and dips to the right of the normal resonant peaks, are due to harmonic distortion in the filter and the VCA that follows it in the TB-303.  Also notice how the actual resonant peak of the TB-303 has a steeper slope on the right side, than the left side when the Q is high.  This "lop-sided" resonance is another artefact of the filter's non-linearity and occurs naturally in mechanical resonators as "hardening".
>> 
>> http://www.zainea.com/lowresonances.htm
>> 
>> Hopefully this is interesting to some TB-303 scrutinisers.
>> 
>> -Richie, 
> 
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