Buchla 207 Mixer/Preamp

I built a Buchla 207 V2.0 Mixer/Preamp 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 and I do not know the details.


There isn't a lot of information or specifications on the 207.  This documentation comes from the November 16, 1981 Buchla Synthesizer User Guide by Daniel Scheidt.

Buchla 207 Documentation PDF


The preamp phone jack is a TRS.  In the up position it functions as a TS  unbalanced input with an impedance of roughly 1M.  In the down position it functions as a TRS balanced input depending on the specific transformer.  The original transformer had a turns ratio of 1:14.1 and an impedance ratio of 1:200 for an input impedance of ~5K.  I am using a Sennheiser TM003 transformer  which has a turns ratio of 1:30 or 1:15 which is very close to the original.

The mixer input impedance varies with the settings of the sliders and balance controls between ~40K and ~13K.  The external input impedance is higher at ~120K.

In addition, a high impedance monitor out signal is also sent to an MTS wire as part of the power cable.  This can be connected to the MTS input on the 227 mixer.




The V2.0 PCBs are black solder masked.  V2.1 BOM calls out 10 -100K for the 9mm potentiometers but they should all be 25K linear so the balance circuit works correctly.  The original sliders were log instead of linear.


PCB V2.0 adds six 5K SMT resistors from the wiper of the slider potentiometers to ground.  This gives the linear sliders a strong log contour but also drops the input impedance down to around 4K.  I believe the mixer works better without these resistors installed.  

R74 is a 2K2 although the BOM calls out a 2K4.  Either will work.

The transformer wires are identified by color.  Green and gray are the input primary and red and blue are the output secondary of the transformer.  The TRS input is labeled hot and cold with hot connecting to T and cold connecting to R.


There are also other minor errors on the BOM so I made updates.  I also made reference diagrams from the PCBs.

Corrected V2.1 BOM

207  Front Reference Diagram

207  Front Reference Diagram

Transistors Q1-Q2 and Q3- Q4 should be matched.  I did my transistor matching with a Tektronix 576 Curve Tracer.


The Sennheiser TM003 transformer mounted nicely on the PCB with various washers.  There isn't much room for the transformer wires so they have to be pressed close to the PCB.  The black wire to the right is a ground wire for the transformer shell.


Rotating the four jacks surrounding the transformer wires with the tip connector oriented away provides the maximum space for the wires..


This is one of the more difficult panels to install.  The holes for the wires are small, the transformer covers some of the holes, and the transformer wires interfere with the jacks.  I added a plastic washer between the transformer and PCB since the metal can is close to pads. I ran a ground wire over to the hole pad so the transformer shell would be grounded.  Since I didn't know the transformer wiring I verified correct phase with a test signal to make sure there was no phase reversal.

I did not install the SMT resistors to keep the input impedance as high as possible.


Although the preamp gain switch operated and provided three different gains, it didn't match the front panel.  R73 and R78 are not wired correctly.  I simply installed these two resistors as shown to match the front panel legends..




There is only one trimmer, TR1, which is adjusted by measuring the output on IC3 pin 1 and adjusting for 0.0V on the highest gain setting.

Channels A and F have different gains from channels B - E because of vactrols variation.  I hand selected R3, R9, R46, and R50 for matching gain to the other channels.  R3 adjusts the left gain and R9 adjusts the right gain of channel A.  R46 adjusts the left gain and R50 adjusts the right gain of channel F.




V1.1 appears to be the same as V2.0 with a few minor exceptions.  The six 5K SMT resistors are not present, there are two 10R series resistors in in series with the +/-15V supplies, and the bulk electrolytic capacitors are larger at 100 F.  All of the reference numbers are different but the layout is very similar with the exception of the MTS pad is at the top of the board.  BOM V2.0 is used with this PCB.  The four resistors for setting the gain of channels A and F are R18, R20, R55, and R59.