Buchla 281 Quad Function
Generator Module

I built a V2 Buchla 281 Quad Function Generator module for someone else. They sent me a mostly complete kit of parts and I assembled and tested the module.  Many of the components are sourced through Mouser but specialized parts, panel, and knobs have specific sourcing requirements. See Modifications at the bottom of the page for a corrected schematic and modifications.

 

Assembly of PCB1 is straight forward.

 

The power cable wires into discrete pads on the rear and I wire tied it to one of the connector holes.

 

I used sockets for the CA3080 OTAs as they are both rare and fragile.

 

This photo shows the completed three PCB module.

 

I routed the flat cables a bit different than other photos I have seen.  I routed the cable that goes around the middle PCB on the side that had additional clearance.  This cable needs to be split to fit around the center standoff.  I made the other cable long enough that I could extend the PCB out for service.  I used 20mm spacers between the panel PCB and the center PCB and 15mm spacers for the rear PCB.

 

 

Quadrature

Quadrature mode can be a bit confusing. This explanation comes from the 1981 Buchla Synthesizer User Guide.

 

This explanation comes from the Allen Strange 1983 2nd edition of Electronic Music - Systems, Techniques and Controls.

 

I believe quadrature came from the desire to easily create four panning CVs for driving a 227 for quadraphonic sound. This patch consists of using all four envelope generators in quadrature mode. All four are all set to transient. A Pulse Out connects to B Trigger In, B Pulse Out connects to A Trigger in, and D Pulse Out connects to C Trigger In. Four synchronized CVs are generate that overlap for quadraphonic panning.

 

This view of all four CVs overlaid is a bit easer to understand.

 

Let's examine one pair in quadrature. Both are set to transient. B Pulse out connects to A Trigger in. When A reaches the maximum, B is triggered and A is held in sustain until B reaches the maximum. When B reaches the maximum, A starts the decay and B is held in sustain. When A reaches minimum, B starts the decay. When B reaches minimum, it generates a trigger pulse which starts the cycle over again.

 

Things start to get complicated if one envelope generator is in cycle mode. This is the same setup but B is now set to cycle. B free-runs since but holds during the A decay. When A reaches the maximum it is held in sustain until B reaches the maximum (which happens very shortly with these control settings). When B reaches the maximum, A starts the decay and B is held in sustain. When A reaches minimum, B starts the decay and continues to cycle until A again reaches the maximum.

 

 

Operation

This image shows A triggering B.  You can just barely see the magenta trigger pulse on this scope image.

 

 

The red banana jack output is a trigger at the end of a cycle.  Zooming in you can see it is a +15V pulse about 150 S wide at mid-voltage.

 

This scope image shows Out A - D in quadrature mode.

 

This scope image shows the same Out A and B as above and the Sum outputs.

 

I calibrated Section A to 10 seconds and then adjusted the slope of Sections B - D to be parallel.

 

 

V3 Black PCBs

The V3 PCBs have a few issues that are easily corrected.  The BOM for the 10V regulation resistors are swapped.  R169 should be 270R and R270 should be 1K8.  Pin 1 of IC7 and IC14 are driven from a saturated op-amp output through a diode.  IC7 and IC14 are powered from 10V so you need a 47K resistor in series with pin 1 to limit the current.  You can lift the pin and add the resistor  but since the parts were in sockets, I chose to cut traces and add the resistor on the rear.  All cuts are on the front of the PCB.  Cut the trace to cathode of D2, to cathode of D7, right side (towards center) of R43, right side (towards center) of R109.  On the rear of PCB2 add wires to connect the cathode of D2 to the left side (towards center) of R43, and the cathode of D7 to the left side (towards center) of R109 (the left side because you are working on the rear of the PCB).  Then add 47K resistor from the left side (towards center) of R43 to pin 1 of IC7, and the left side (towards center) of R109 to pin 1 of IC14.

 

There are four op-amp inputs that are driven by diodes so they float when the diode is cutoff and noise will affect the outputs.  Simply add four pull down resistors to ground.  Add 100K resistors to ground to IC6 pin 3, IC6 pin 5, IC12 pin 3, and IC12 pin 5.  Nearby decoupling capacitors provide ground pads.

 

I've also been sent a couple of modules that wouldn't cycle continuously.  They might cycle for seconds or minutes but eventually would stop.  What was strange is that different AC/DC adapters would either cycle or not.  The LM317L has a Vin-Vout specification of 2.5V which cannot be met using the +12V supply.  I lifted the input leg of IC15 and wired it over to a +15V decoupling capacitor and the module cycled correctly.

See Modifications at the bottom of the page for a corrected schematic and additional modifications.

 

Transient mode is shown in this scope image.

 

This scope image shows the sustain mode where the attack is held at 10V until the input goes false.

 

On the V3 boards which use the THAT340 I rarely can get a full 10 seconds with the trimmers. I have sometimes simply increased the value of the timing capacitors but more recently I have changed the series resistor with the trimmer.  These are on PCB2 and are R4, R12, R70, and R78. The standard value is 47K and I have used 56K, 68K or 75K depending on what was required to get 10 seconds of attack or decay with the trimmer not at either extreme.

 

 

Vintage Version PCBs

There is a hand routed version of the PCBs that is built the same as V2.  It took me a while to find the reference diagrams for these PCBs which are important because there is no silk screen.  I soldered the two tantalum capacitors on the rear of the PCB for additional clearance.

 

 

Transistor Q6 is rotated 180 on PCB2/ 3 from V2 and the 220nF capacitors (large red) and 1F capacitor (yellow axial) on PCB2 and 3 also have wider lead spacing so I used different parts from the V2 BOM.

 

The 20mm standoffs are a bit tight when sockets are used as the flat cable header overlaps the IC in the socket.

 

 

 

 

R69 on PCB2 is specified as 20K in the BOM but is 49K9 on the original schematic. This generates the +10V supply and with 20K the logic runs at 12.5V. R22 and R55 are specified as 22K in the BOM but unstuffed on the original schematic. These set the  the threshold for the comparator to switch from attack to decay. I have no idea why these three resistors differ from the original schematic

I believe you can build this either way: R69=20K, R22=R55=22K or R69=49k9 and R22 and R55 unstuffed.

 

 

 

The design is very susceptible to leakage on the PCB so it has to be very clean.  It is important to use matched transistors (e.g. cut tape) for Q1A and B and Q4A and B.  Let the module come up to temperature before calibrating.

On one particular module, one channel would stop functioning if the Attack was set to maximum and reducing R9 from 1K to 910R corrected this.

The maximum frequency was just over 160 Hz.

 

This image shows quadrature operation along with the A-B mixer.

 

 

Modifications - All Versions

There are issues  with transient operation depending on the pulse level. I detail two ways of addressing the issue.

Alternative 1 - Resistor Modifications

The original circuit works by changing the threshold for the input pulse to detect either the trigger or the gate portion. The comparator is set by a 100K/4K7 divider to nominally 0.67V. The gate threshold is set by a 47K/15K attenuator nominally to 2.75V which works with either 5V gate or 7.5V gate. The transient threshold is set by a 147K/15K attenuator nominally to 7.2V. This threshold is too low to operate with a 15V/7.5V trigger/gate pulse. It is made worse by using 5% resistors which change the comparator threshold and the attenuators.

The original circuit will work with both sets of trigger/gate voltage levels if you use 1% resistors for the comparator and changes the 100K to 130K. The resulting trigger threshold is 8.3V to 8.9V which is always above the highest gate voltage and below the lowest trigger voltage.

Another benefit is that since the input is disable in cycle mode it cannot be re-triggered by the input. In essence, the circuit is still in sustain mode, but the threshold has been changed to the narrower trigger portion which needs to be shorter than the Attack/Decay time. This is how the original functioned.

There are a number of errors on the 281 schematic. I have modified it to add in missing resistors, correct the mode switch, and add the remote/local switch. I then marked it in red for the resistors that need to be changed in value and to 1%.

281 Corrected and Modified Schematics 

 

Alternative 2 - AC Coupling

You can eliminate the 100K resistors wired across the Sustain switch and add a series diode with a parallel resistor/capacitor to AC couple the pulse input. This has the advantage of working with any pulse width and amplitude above 5V.

A 2N2 capacitor will provide sufficient AC coupling but it needs a series diode to block the negative excursions. If driving the pulse input with a module like a 245 which can drive high but not low, there is no DC path to discharge the capacitor. A 4M7 resistor in parallel will provide that function.

Wire these components from the input jack to the middle switch pin as shown in these two images. These components are always in the circuit and are required for transient operation and not necessary for sustain operation. However, in cycle mode, the 281 may be retriggered during the decay cycle and this pulse input is operational. This can be considered a feature and disabled by simply removing the signal to the pulse input. This V3 image shows the removal of the 100K resistors and the addition of the three components between the pulse jack and the middle switch pin..

 

This vintage version image shows the removal of the 100K resistors and the same three components added between the pulse jack and the middle switch pin.

 

 

Modification Summary

Choose which ever transient/sustain modification has the "features" you want.

 

 

back