Karplus Strong Opamp Circuit

Posted on Feb 7, 2014

It`s basically just an all-pass filter with two components added: a capacitor and a resistor. And it`s making percussion sounds! wow! See the attachments below for a schematic of the unmodified all-pass filter circuit, a waveform captured via Audacity, and a short audio sample with a drum beat every 4 seconds approximately. The Added components are

Karplus Strong Opamp Circuit
Click here to download the full size of the above Circuit.

a cap from the noninverting input to ground and a resistor from a pulse source to the noninverting input. The resistor allows the input of a stim pulse a-la-KS normal stimulus, and the capacitor closes the KS loop with a first order low-pass filter. Component values: R is 330k, C is 0. 1uF, Rx is 1Meg in feedback position and 100k in input position. Added parts are resistor = 330k and capacitor = 2. 2uF. That`s it! well, that and an LM358 opamp does the job. This is a fascinating discovery because it is a truly minimal KS implementation if it turns out that I am correct in understanding how it works. The theory is that the all-pass filter acts as a delay line. Adding the resistor merges it with an inverting summing amp for stim pulse input, and adding the capacitor puts a low-pass filter in negative feedback. All three subcircuits are sort of morphed together into one beautiful KS opamp circuit. wow! I will definitely be adding this circuit to my "things to try out when I get round to it" list. I want to build some kind of simple as possible percussion box and this idea seems to fit the bill. Presumably I could make 4 different percussive sounds with one LM324, by varying the R and C values. Need to add some white noise generators in there somehow and a sequencer. As a side issue, I was looking at another site recently (which shall remain nameless) and I found a whole thread dedicated to the hatred of LM358/LM324. I use...

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