twin-T

[honcho at paia.com]

>Does anyone have any twin-T filter schem's?
>>>

Ooh, ooh, I've got a good one. Take a look at 
http://www.paia.com/paia/syndrum.htm .

This twin-T circuit can be used as a cheapo filter or as a 
ringing oscillator for rhythm ace drum sounds, but it has a neat 
twist in that the center frequency is controllable with a CV. 
Adding the trigger-in circuitry shown allows the circuit to be 
triggered by a short pulse (a few mS.) and automatically do the 
down scale glide that characterizes the "synth-drum" cliche. It's 
velocity sensitive in that the amplitude of the triggering pulse 
controls the dynamics of the sweep as well as the overall output 
level.

Most twin-T circuits don't do drum sounds with long decay times 
well, but with this circuit the decay can be quite long (several 
seconds) because raising the pitch of the filter by forward 
biasing the diodes can cause the circuit to self-oscillate for a 
large part of the time. It finally sort of "quenches" as the 
losses through the T-network become greater with lower control 
voltages.  It's hard to say with just words, there are more 
details on the web page.

-John

---------------------------------------------------------
John Simonton                  PAiA Electronics, Inc
3200 Teakwood Ln.              honcho at paia.com
Edmond OK 73013                http://www.paia.com/paia
405-340-6300                   fax 405-340-6378