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 PAiA 9730 VCF 
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Joined: Sun Nov 09, 2008 2:49 pm
Posts: 620
Post PAiA 9730 VCF
Hello Paolo.

It is good hearing that your system is nearly in working order!

One thing about the VCF that could make it seem like it is not right is
that it can break into oscillation if the Q is too far advanced. But
there are measures that can be taken to ensure this doesn't occur.

On high Q settings there is quite a bit of boost in the pass band, so if
the signal is very strong and it gets boosted, it can break into
oscillation. Amps, Filters, and Oscillators are all the same circuits
with different tweaks--a filter is an amp in a certain range, an
oscillator is a filter with enough boost to sustain an audio signal. So
with just minor variations, an amp can be like a filter or oscillator,
or, a filter can be like an amp or oscillator.

The 9730 is most effective if the signals sent into it are attenuated so
it has plenty of range to boost without breaking into an oscillating
condition. But it can also be 'fine tuned' to operate below this
threshold for normal input signal levels, such as the 5v waves from the
VCO. For these, it is best to avoid setting the Q above about a
mid-setting and starting with the modulation amount at minimum. Then to
be sure the Q is right, run the VCO frequencies through the range of the
sound that will be used for the patch, ie if controlled by a keyboard,
run up and down the keyboard to present the VCF with the spectrum upon
which it will be operating. If the Q is not too high so as to allow
oscillation for this range, then introduce the desired amount of
modulation and run the VCO through it's range again. If oscillation is
encountered, backing off on the Q will stop it. The VCO pitch that
causes the oscillation is the frequency of the filter passband and this
is where the boost will be the most and setting the Q below the
threshold of oscillation will be the setting for the most amount of Q
for the input frequency range and the filter Frequency control
settings. It will be more critical when the two filters are cascaded as
there will be two bands, or one particularly 'hot' band. But, at these
just under the threshold settings the filtering with some modulation is
very distinctive.

Because lower signal levels input to the VCF give it more room to
boost/peak, the Q setting is not so critical so inputting lower level
signals is another way to keep oscillation at bay. The two VCO waves
mixed through the L+R VCA are attenuated a bit and work as a good source
to the VCF input. Either the L or R VCA can work like an attenuator by
using one or the other and the Pan control to set the level resulting at
the L or R output.

Thank you.

Sincerely, Scott Lee

Vallebona Paolo wrote:
> Hello Scott,
> Now my 9700 PAiA system is completely assembled, and working;
> I have not yet implemented any of the modifications I told you, but
> thanks to your help I was finally able to trim and calibrate everything
> (almost), and can now happily play in tune over more than 4 octaves
> Only the filters are still giving some trouble, squealing everytime the
> cutoff freq CV goes high
> I heard that it is a known defect on some 9730, so maybe you already
> have a solution for it that can save me again a lot of time
> Is it true ? can you help please ?
> Thank you
> Regards
> Paolo Vallebona

Scott Lee

Mon Mar 16, 2009 3:03 pm
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