Doepfer

oops!!!!!wrong....the grey cables are 50cm long.
sorry,
bakis.


--- stinchcombe_t <tstinchcombe@qinetiq.com> wrote:
> 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
> 
> 


=====
synthfreak(parallel worlds)
athens-greece
[Doepfer_a100] group owner-moderator

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