[sdiy] Formant 24dB filter

[René Schmitz]

You're probably plagued by the phase reversal of the opamps. 
I have had this latchup problem in OTA based filters where I've used TL084s
as opamps. 
The solution could be to replace those opamps with ones which are free of
phase reversal. However I didn't replace the opamps, merely decreased the
input amplitudes, so that it is less likely that the opamp is overdriven,
and a resultant change in sound. Haven't had it for several years now.

Cheers,
 René
 



At 22:44 05.06.01 +0200, Magnus Danielson wrote:
>From: Brown David <David.Brown at tms-ltd.com>
>Subject: [sdiy] Formant 24dB filter
>Date: Tue, 5 Jun 2001 10:53:23 +0100 
>
>Hi David,
>
>> Has anyone who has built the Formant 24dB filter had a problem when the
>> filter tries to go into oscillation?  My filter seems to work perfectly
well
>> most of the time, but if the 'Q' control is set to maximum and the
Frequency
>> knob turned up, the filter just latched up rather than oscillates.
>> Admittedly, the construction manual states that a resistor in the feedback
>> circuit should be chosen so as to avoid oscillation.  However it doesn't
say
>> bad things will happen if oscillation starts?
>
>My Formant's 24dB filter has the same problem. I have only thougth
>about it some, but have not spent quality time on it. I too want to
>wash out this problem out of my Formant.
>
>Let's work on this together. I'm eager to learn what others have for
>an experience on how to attack this and naturally how well people
>disciplined thier filters.
>
>I beleive high frequency sweep speed caused the latchup, but it is
>some time since I tried it. However, I love when that filter slowly
>sweeps over the overtones when the Q is HIGH.... hehehe...
>
>Cheers,
>Magnus





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