[sdiy] help finding roland filter schematics?

Oakley Sound oakleylist at btinternet.com
Fri May 1 10:15:29 CEST 2009


 > It strikes me that the Oakley method of increasing the filter-block 
gains to 1.47 is dangerous when the cutoff frequency is turned way up.

As far as I know no one yet had died or suffered serious injury from 
using the COTA filter... :-)

The op-amp input summing stage of the COTA module has a gain of 0.3 thus 
preventing any major overload to occur within the cascade itself.

 > Doesn't this imply that the outgoing minus 24dB signal at high cutoff 
would be +/-8V for an incoming signal of +/-5V?

Yes, at low resonances there is a overall gain in the cascade itself. 
However, in conjunction with the input stage gain loss, and some make up 
gain at the output stage, the overall gain of the module itself is less 
than unity. ***

 > It would also imply that the cutoff control would effective act as a
volume control at high cutoff.

I don't follow you there. For a sinewave at a given frequency any low 
pass filter can be seen to act as a volume control. At high cut-off the 
cascade can be seen as a linear system, so long as we don't overdrive 
each element too much. As such any gain could be considered lumped into 
a single element at any point along that chain, including the output.

What is interesting though about this is that clearly the signal 
headroom before soft clipping occurs does get smaller as you work 
through the COTA's cascade. Does this affect the sound? Quite probably - 
but the actual 'depth' of this affect is very much dependant on the 
signal input to the module.

My ears told me they preferred the sound of this set up compared to a 
straight unity gain cascade. But ears have been known to be deceived.

***

As a matter of an aside - what should the gain of any filter module be? 
I usually make it less than one - usually because at resonance the 
signal level goes up to clipping with a 5V signal input. However, the 
Moog ladder shows a drop in gain at high resonance and has high 
non-linearities due to soft clipping, so should that be treated differently?

Its quite difficult [impossible??] to get this right for all input 
levels, all filter settings and then make it completely compatible with 
every other filter out there.

Tony

www.oakleysound.com



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