Escobedo Q&D VCF goes chaotic

andrewF · Posted: Sat Nov 07, 2009 6:47 pm Post subject:

basically the same, just the two changes i mentioned above. It is worth having the CV input for synth use.
you can still get chaos with the published version - no worries.

some pix here
http://www.sdiy.org/pinky/chaos/chaos.html
and the circuit is still here
http://folkurban.com/Site/QDVCF-714.html

Uncle Krunkus · Posted: Sun Nov 08, 2009 12:12 am Post subject:

Those screen shots of the CRO are beautiful. Shocked

andrewF · Posted: Sun Nov 08, 2009 2:37 am Post subject:

cheers - this is an excellent cct for studying chaos, simple to build and responds very differently to the amplitude, frequency and format (tri, sq, trapezoid, etc) of the input signal.

good aggro filter too..........Tim E. is a genius Very Happy

frijitz · Posted: Sun Nov 08, 2009 12:42 pm Post subject:

andrewF · Posted: Sun Nov 08, 2009 4:56 pm Post subject:

bottlenecks! Thanks Ian - I was actually spending all my time trying to get an image like the one you posted on pg 1 of this thread, that one is quite stunning.

To get these the input is a low frequency trapezoid around 20-30Hz (Don Tillman's VCO). They are great to watch, the 3rd bottleneck is quite unstable as you can see it looks quite scratchy. It gets spat out, wobbles around for a few seconds and is gone.

I'm getting pressured to work out the maths for this circuit, not exactly my favorite job, but will post it here when (and if) it happens.

frequencycentral · Joined: May 25, 2008 Posts: 185 Location: UK

Folk Urban: http://folkurban.com/Site/GuitarEffects-681.html

thepaddiscuile · Joined: Jul 11, 2007 Posts: 7 Location: ireland

Hi guys.. Anyone know how you.d get a highpass response from this..

Be cool to have both running in series lp-hp

andrewF · Posted: Sat Jul 24, 2010 8:51 am Post subject:

In a sense it already has the hi-pass.
In a regular twin-t filter, the two sections deliver a low-pass and a hi-pass function, combined the result is a notch filter.

Of course the added diode means the Q&D is not normal.

From http://en.wikipedia.org/wiki/RC_oscillator

The second common design is called a "Twin-T" oscillator as it uses two "T" RC circuits operated in parallel. One circuit is an R-C-R "T" which acts as a low-pass filter. The second circuit is a C-R-C "T" which operates as a high-pass filter. Together, these circuits form a bridge which is tuned at the desired frequency of oscillation. The signal in the C-R-C branch of the Twin-T filter is advanced, in the R-C-R - delayed, so they may cancel one another for frequency f=\frac{1}{2\pi RC} if x = 2; if it is connected as a negative feedback to an amplifier, and x>2, the amplified becomes an oscillator.

Hope this doesn't put you off, I'm quite interested in the Q&D and would love to learn any new tricks with it. On my breadboard at the moment I have a version with the op-amp replaced by a 4009 inverter, it filters but no chaos yet.