296M Programmable Spectral Processor

This 296 Programmable Spectral Processor is a complex design featuring 16 filters, envelope follower, programmed outputs using VCAs and special CVs generated from frequency and width controls, and transfer switches to map odd (or even) envelope follower CVs to the even (or odd) VCAs. It's best to read the manual to understand this module.

Buchla 296 Programmable Spectral Processor Manual Transcribed from Buchla Synthesizer User Guide 11-16-1981



This 296 Programmable Spectral Processor follows the original through-hole layout. The motherboard does not yet have the sliders installed. The LEDs are the two sets of pads in the upper right.


These resistors mount on the rear since they are beneath the sliders.


There is minimal clearance around the three standoff mounting holes in the motherboard so I turned the standoffs to ensure adequate clearance.


The LEDs do not have pads for mounting so need to be fastened to the front panel. I used epoxy and 30 AWG wire to run to the PCB pads in the upper corner.


Here is the finished motherboard. The 1S and 10C labels are to remind me which way the cards install. The mods in the upper right corner were for the initial design.


The SSM2020 VCA is nearly unobtainable so I used a 2164 Quad VCA daughter card. The circuitry works out nicely where only component changes and omissions are needed on the filter cards.


I use pins pulled from a PCB wire header which nicely match machined pin sockets used for IC3.


Card 1: <100 Hz lowpass and 150 Hz bandpass filters. The "white" resistors are 0R jumpers. The metal can JFET near the center of the PCB has the leads bent to mount further away from the Molex connector since the motherboard pins pass through and can short to the can.

IC5, the TL074, is used in a very high impedance circuit on all the filter cards. Very slight amounts of flux around this part can add significant offset voltage to the envelope follower outputs. Small amounts of flux can be trapped under the socket so it probably best to solder the TL074 directly in.


Card 2: 250 Hz and 350 Hz bandpass filters.


Card 3: 500 Hz and 630 Hz bandpass filters.


Card 4: Comb Filter and Attenuator Outputs and Program Control CV circuitry.


Card 5: 800 Hz and 1 KHz bandpass filters.


Card 6: 1.3 KHz and 1.6 KHz bandpass filters.


Card 7: Programmed Outputs and Envelope Follower LED circuitry.


Card 8: 2 KHz and 2.6 KHz bandpass filters.


Card 9: 3.5 KHz and 5 KHz bandpass filters.


Card 10: 8 KHz bandpass and >10 KHz highpass filters.


This build is long and tedious. The filter cards take about 2.5 hours each. Cards 4 and 7 are shorter at about 1.5 hours each. The motherboard and panel takes about 3 hours and calibration takes an hour for a total of 27 hours.


296 Mouser BOM




This 296 calibrated quite easily. There was no noise on the Programmed Outputs and the Frequency and Bandwidth controls operated smoothly. I've seen these issues on some of the clone modules. This is a great sounding and operating module.