Lately I’ve been working on building a mutant version (that is: with non-vintage components) of the timbre/symmetry section of the famous Buchla 259 complex waveform generator.
I used the schematic Mark Verbos posted on his website as a starting point. I replaced all opamps with the TL072CN. All other components are common resistors (I used 1% metal film for all resistors), caps and diodes.
When I finished the circuit I didn’t get any good waveform on the output on my first test. I tried inserting a sine wave at the input of the folding section and got better results after increasing the amplitude of the input signal. After checking the input circuit I found a 1:20 voltage divider at the audio input. So my 4 Vpp input sine wave was immediately attenuated to 200mVpp. After adjusting the voltage divider everything worked like it should.
As far as I can tell from stuff I’ve seen on YouTube of real Buchla 259’s the waveforms definitely resemble the original. I’m happy with that as it was not my intention the built a one-on-one clone of the original.
Here are some screenshots of what comes out of the timbre section with a sine wave as input.
And the output with a triangle wave as input.
And now the most important thing: how does it sound!?
Here are two recordings of the timbre section. In the first recording I’m using a sine wave as input signal. The second one uses a triangle wave as input. During both recordings the controls are tweaked in real-time.
The circuit has 3 main controls: Timbre, Symmetry and what I call Timbre Sensitivity. This sensitivity control is a trimmer on the PCB on the original Buchla but I decided to bring it to the front panel as it provides in a nice range of waveform diversity. Both Timbre and Symmetry can be voltage controlled.
I have to do some final testing and tweaking to finalize the schematic. When that’s done I will post the schematic so you can have a go at it yourself.
I’m using this timbre section to built a mutant 259 complex waveform generator together with two 258J VCOs and some, still to be developed, additional circuitry for modulation and tracking of the two VCOs. More on that in separate post!