Doepfer

Hi M. Dimm

My A-121 behaves pretty much as you describe it: all the little odd
things you mention are almost certainly due to the CEM3320 VCF chip
that does all the work. The datasheet for the 3320 mentions (for
self-oscillation) that 'the sinewave is not clipped by the first
stage', so this would seem to equate to the  bandpass output in the
A-121 configuration - if one wants to use the filter as an oscillator,
then this is clearly the best place to take the output from (unless of
course the distortion is desired!).

The problem of the highpass output (with a low cut-off frequency)
acting more like a bandpass filter, whilst annoying, is also likely to
be a limitation within the chip. The chip contains 4 'gain cells'
which act in a similar manner to VCAs, and are connected together with
various resistors and capacitors to give the filtering action. The
gain setting is used to control the filter cut-off frequency, and the
gain they can provide probably decreases as the input frequency
increases. Thus for a low cut-off frequency, their gain is already set
quite low: passing high frequencies through them will decrease the
gain further, and thus higher frequencies get attenuated. Standard op
amps (the staple component in most modules) also behave like this, and
in fact the 'emphasis' filter in the A-199 reverb module appears to
make good use of this effect - it is really a high pass filter, turned
into a bandpass one by the falling gain available from the op amp as
the frequency increases. I don't have an A-123 high pass module, but I
note that it uses the same CEM 3320 chip - it would be interesting to
hear from other list members whether it behaves in a similar fashion.

This probably isn't much consolation to you, merely confirming that
you are stuck with this behaviour! (Can't help with the white patch
cord length!)

Tim

[The views expressed above are entirely those of the writer and do not
represent the views, policy or understanding of any other person or
official body.]


> I was under the impression that a high pass filter should let a 
> signal pass almost unchanged with the cut-off frequency all the 
> way down (the opposite of a low pass). However my A-121 
> seems to be cutting higher frequencies at low cut-off settings 
> (resonance all the way down), more like a band pass filter.
> The result is especially clear when runnig a rich external signal 
> through it, songs on the radio in my case.
> 
> When viewing an A-121's sine wave output on an oscilloscope, 
> the "Band" output seems to be the only output to generate a true 
> sine wave. The waveforms from "High" and "Low" seem to be 
> clipped at the highest and lowest points, respectively, and the 
> "Notch" output is significantly lower than the others. 
> Is this an anomaly of my module or do all A-121's behave like 
> this?
> 
> 
> And which of the 4 lengths of cables available for the A-100 are 
> the white ones?
> 
> Thanks
> M. Dimm