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.  You need to solder these on the back/component side and cut the leads off flush with the front. The Alps slider insulators need to fit nearly flush with the PCB so you don't want a solder bump on the rear.

Note I have plastic headers on my Molex connectors. This increases the vertical space and this module will not fit in a 3" deep boat. Please see the construction section at the bottom.


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.


They now have a LED PCB which mounts on the two banana jacks with a second nut to mount the LEDs.


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.


The resistor "buried" under the 4.7 F capacitor is shown as a 2K2 on the schematic. It produces a gradual peak at 66 Hz with a rolloff at 100 Hz.


For comparison, the 296R does not include this resistor and its frequency scan shows a more gradual rolloff.


I chose a 1K8  which had a flatter overall response with a rolloff at 100 Hz. Note this value may work better with the tolerance of my specific capacitors.


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 module is deep at 3.125" which will not fit in shallower 3" boats. My boats are from Jason Butcher and his are made deeper at 3.2" to fit the 296. Here are the measurements from my module


I made a second one to investigate lowering the profileto fit a 3" boat. I found that removing the plastic headers from the male pins and using 14 mm standoffs was a reasonable solution.



Low Profile Modifications

There are two areas to reduce depth of the module: reducing the height of the front panel spacers and removing the plastic headers from the motherboard pins. While the switches will fit with 12 mm standoffs the PRV6 style potentiometers may not so you will have to check. Reducing the spacers by 3 mm is insufficient to fit a 3" deep boat.

The most practical method is to eliminate the plastic headers which saves 0.125" and is the space savings needed. The Molex female connector without the plastic male header provides about 0.080" clearance between the card and motherboard which is ample for most leads and wires but interferes with some of the 33K resistors, wires, and banana jacks.


I found this the best method for a new PCB build. You can do this after axial components are installed but before the sliders, switches, and potentiometers are installed:

The Molex pins (Mouser 538-26-20-2101) are 0.045" diameter and while other individual loose pins are available, they are extremely expensive. The only reasonable cost option is to use the pins from the 538-26-20-2101 Molex headers. Use pliers on the bottom (solder) side of the plastic header and pull the pins individually through the header to remove. The pliers will leave marks on the pins so keep them organized so you know which is the "clean" end. Insert the good end into a Molex 0009481104 female connector (Mouser 538-09-48-1104) and push them through so they extend beyond the header.

Once 10 are installed, then press the female connector against a flat surface on the opposite side so the pins are aligned flush with the top of the connector. There is some slop in this connector so align each pin to be perpendicular and centered. 

Carefully insert this from the rear and solder the pins on the front. You can see the pin alignment in the hole so make sure they are all centered before soldering. The connector will push flat against the PCB making sure they will be aligned vertically perpendicular to the PCB. After soldering, cut the pins off flush with the PCB and then quickly resolder making sure the hole is completely filled. Pull the female connector off and realign any pin that is not vertical. I would not use this female connector on a card since it has been subjected to the heat of soldering 198 pins.

This photo shows the pin alignment using this method. While the pins all fit the connector there are a couple that could be straightened. Most of the pins have a lower solder profile on the rear. My pins measured 0.448" in height.

The vertical alignment is quite good.


If you have machine tools available you can use this method:

Mill the plastic header down to 0.2" width. This can be installed from the front and soldered on the rear and will fit between the sliders. If you mill too narrow the header will fall apart. Since you will be soldering on the rear connector side so be sure to not wick solder up the pins.

A variation of this method is to pull the plastic header instead of machining it narrower:

Insert the header from the front and solder on the rear. Be sure to not wick solder up the pins. Use hot air to heat up the plastic header. Use pliers across the narrow portion of the plastic header and rock it back and forth with a pulling motion to pull it up just a bit. Work your way down the header and repeat until you get the plastic header away from the PCB. You may have to reheat it. Eventually it will pull off easily. Cut the pins flush to the PCB.

This photo shows the pins alignment and the solder fillet from soldering on the rear of the PCB. My pins measured 0.387" in height.

The vertical alignment is quite good.


 Here's half of the pins inserted. It is slow but steady work.


An individual contacted me on their method for eliminating the plastic header. They pressed the plastic header down flush with the bottom of the pins. They inverted the board and installed the pins partially through the PCB. It turns out that a 5.5 mm spacer provides the right spacing so the pins do not extend through the card connectors. They simply tack soldered the end pins, removed the spacers, soldered and cut the pins. This photo shows the header spaced with 5.5mm standoffs on both sides (4 standoffs) prior to tack soldering. This is an easier solution. This image shows four 5.5mm standoffs used as spacers.


If your motherboard is already assembled, then this is likely the only method.

Use hot air to heat up the plastic header. Use pliers across the narrow portion of the plastic header and rock it back and forth with a pulling motion to pull it up just a bit. Work your way down the plastic header and repeat until you get it away from the PCB. You may have to reheat it. Eventually it will pull off easily. You will need to cut the pins off at the appropriate height so they don't protrude through the female connector. Although the pin height above the header is 0.45", only 0.37" is needed to fully contact the female connector. The header is 0.125" thick. Cut the pins off to 0.25" above the header (cutting off 0.2"). After cutting the pins you will need to sand the sharp edges so they do not scrape the mating connector.


Without the headers the female connectors on the cards slightly interfere with many of the 33K resistors which are close to the rear of the cards. You need to trim off the two plastic tabs between pins 10 and 11 on all cards.

10 of the 33K resistors will slightly interfere with the bottom of the card edge. This doesn't allow the card to be fully seated, but it is sufficient to fit into a 3" deep case using the shorter standoffs (below). To allow full clearance, five of the 33K resistors can be rotated and one lead soldered to a trace and 5 of the resistors need to be lifted and mounted on top of the 22K resistors. Six of the resistors are fine as-is.

Another possibility is to use smaller diameter resistors, 1/8W or Mouser 660-CFS1/4CT52R333J,  which should provide ample clearance.  (untested).

Larger photo of resistors


Removing the plastic header will reduce the depth to 2.96". The card edges will likely touch the bottom of the boat so make sure there are no screw heads or any other protrusions which could contact the cards or traces. To gain some additional clearance the standoffs height can be shortened.


For PRV6 style potentiometers more than 12 mm:

The datasheet doesn't show the dimensions for the PRV6 potentiometers from the rear to the mounting flange. I did not use the 72-PRV6-50K called out in the BOM so I couldn't measure them.  I do have some of this style potentiometer and it has folded over tabs at the base which can short to PCB traces. These measure 12.42 mm so will not fit with 12 mm standoffs  A 14 mm standoff (Mouser 761-M1261-3005-S-12) is available and expensive, but only three are required. A round nylon 14 mm post is half the cost (Mouser 761-M0513-3-N). Using 14 mm will provide ~0.040" clearance between the cards and boat. I like this alternative best as it provides some clearance and the controls fit closer to the panel.

With 14 mm standoffs and no header on the pins, the cards now touch the banana solder tabs. You can bend them out of the way (I bent towards the bottom) and make sure the wire comes off the side of the tab and not the top. This photo shows the moved 33K resistors and the bent banana solder tabs.


Not Recommended
For PRV6 style potentiometers less than 12 mm deep:

You can use 12 mm standoffs which provide ~0.120" clearance between the cards and boat. I have some PRV6 style potentiometers that do not have folded over tabs and are 11.7 mm deep. This is very close so check your spacing to ensure no traces are shorted. However, this increases the issue of the banana solder tabs interfering with the cards which I did not investigate further.

I do not like the appearance of 12 mm spacers as the controls, sliders, and potentiometers extend through the panel more and the knobs sit higher.



 Substituting 9mm Alpha potentiometers:

You can use 12 mm standoffs with Alpha 9 mm potentiometers. Cut off the four tabs on the bottom and the locating pin as close as possible. Fold the narrow portion of the leads and solder short solid wire to the leads. Bend these to fit into the PCB. This obviously is a more difficult assembly of the panel with 5 potentiometers. You need to use care to make sure you don't spin the potentiometer when tightening the panel. This also has a significant cost savings. The original used 25K linear potentiometers so you can buy the correct value. However, this increases the issue of the banana solder tabs interfering with the cards which I did not investigate further.


With the shorter standoffs the cards will interfere with the banana jack solder tabs. You can bend these towards the bottom and make sure the wires come off from the side of the tab and not the top.



I had a LED board for the second motherboard. This is the panel all wired and ready for assembly with the motherboard.


The initial batch of LED boards has the orientation of the LED backwards. This closeup shows the correct orientation.


Here is the completed low profile motherboard which will fit in a 3" deep boat.



Here is the low-profile 296 on the left as part of a full vocorder patch.




Calibration is straightforward. Use a DMM on the individual channel output and adjust each to <2mV with no signal inputs. If you can't get them to this level then sometimes excess flux on the PCB can be the cause. Do all 16 channels first.

The filter topology is designed around three cascaded single op-amp multiple feedback bandpass filters in a stagger tuned configuration. Each of the filters is a small frequency offset from the next to get a wide and flat passband with steep slopes. You adjust each of the three filters for maximum output at their specific frequency.

There are three trimmers per channel except for the <100 Hz channel. To adjust the frequency trimmers, monitor the individual filter output with an oscilloscope and apply the correct frequency input. Keep the pre-emphasis knobs at minimum (full CCW). Adjust the trimmer for maximum amplitude. The amplitudes will be different since the other trimmers for that channel may not yet be set. Also, there is some variation in overall amplitude between channels. (Note - if you have a two channel oscilloscope then monitor the output of your signal generator and trigger on channel 1 and monitor the individual filter output on channel 2. This will make the display a bit more stable).

There is a regular pattern to follow for calibration. Set the input frequency to what is shown for Trim #1 and adjust, then set the frequency for Trim #2 and adjust, and then set the frequency for Trim #3 and adjust. Then repeat for the second channel on that card. You soon learn the pattern of adjusting the lowest trimmer, then moving up two, then moving up two, then going to the next to the lowest trimmer, then moving up two, then moving up two. I kept a marker on the card I was adjusting so I didn't venture to either side which is easy to do. Calibration is pretty quick.

Trimmer Location Chart

For those that do not have a calibrated waveform generator for the calibration, SAModular has recorded wave files that you can download to use.




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 and corrected them by adding more decoupling. The original design had more decoupling and wider power traces. This is a great sounding and operating module.



SSI 2164 Parts

An individual contacted me whose 296M exhibited noise on the program outputs when the Width potentiometer is full CW. The noise was oscillation around 700 KHz and was more noticeable with SSI than Alpha 2164s. It might have been impacted by the switching power supply. It turns out the control TL072 op-amps were oscillating. He stated increasing C8  and C17 from 100 pF to 220 pF corrected the issue. 


I used SSI 2164 parts in my Low Profile built above. I found considerably worse results. I verified card 1 as soon as complete, so the module consisted of cards 1, 4, and 7. I had significant 'chopping' on the programmed output when using either the Program controls or an external CV. Increasing these capacitors did not correct this issue.


This chopping is full amplitude and occurs at about 45 KHz, so a considerably different frequency than he noted.


I modified the adapter card to the standard 220R and 1200 pF compensation network. This eliminated the chopping but now there is a  negative DC shift in the programmed output at CCW setting of either Frequency or Bandwidth control. I verified this offset does not occur with the Coolaudio 2164 parts.


I then calculated the compensation network for the input impedance per the datasheet. The audio channel requires 750R and 330 pF and the reference channel requires 180 pF and 1K4. Using these values with verification on a single card seemed to function correctly.


I changed all 64 0603 parts and I have no oscillations when just using the program controls. I do, however, still have significant (e.g. several volts) of DC level shift as I rotate the Bandwidth control. I tried to isolate the channel by turning on each VCA individually. In many of the channels, they break into oscillation at the maximum CV level and many have some amount of DC level shift. This is just becoming too much work to address when the Coolaudio parts work just fine. I recommend to NOT USE the SSI2164 parts based on this experience.



The Programmed CVs always seemed a bit mysterious to me so I mapped them out better. These are calculated values for edge pins 16 (CV1) and 17 (CV2) but come pretty close to what mine read (although I didn't do a comprehensive check). The nominal range is 0 to 8V and I suspect the voltages depend what IC was used for the AMC911 on card 4.