Made a lot of progress since the first post about the Dual Benjolin project. At that point I had one assembled and tested board. Since then I’ve built the second board, fabricated a bracket for holding the boards behind the panel, wired the first board to the panel, finished the design and build of a buffer board (details below), and completed the whole first half.

The Buffer Board

This handmade board has six each of two different buffer circuits. One of them buffers the Q6, Q7, and Q8 bits coming out of the CMOS shift register. Rather than use the onboard output structure, the bits go through these buffers, which provide protection for the CMOS chip and cut the output level in half. The outputs are bipolar, about +/- 4V.

The second set of buffers are the LED drivers for the Red/Green indicators. Each oscillator triangle output has one, and there’s an indicator for the “PWM” output, which I call T1>T2 on the panel. I decided to call the PWM output the Coupler, since it’s a lot like the coupler on the Serge SSG.

Above is the buffer board wiring. Took about two days total, including prototyping the LED driver circuit and selecting the LED. I had a flashback when cutting and placing all those short wires. (I got this great Teflon insulated wire at a surplus store about 40 years ago!) My stint as an IBM computer board repairer in the 70s came to mind. In that job we had to cut and solder even smaller wires. For $1.80/hour wage we were expected to do 20 wires per hour. Sound easy? It wasn’t.

Above, the board is installed. There are 12 inputs and 12 outputs. Fortunately I had some MTA-100 six pin connectors that simplified it greatly. The red/black twisted pair on the right side are the wires going to the LEDS. The little black circles indicate where the LED wires go through the board to be soldered. When I do the second Benjolin, I’ll need to bring over the three bits for buffering and add the three LED connections.

I haven’t drawn good schematics yet for the circuits on this board, but I’ll be adding them later.

Here’s a shot of the completed wiring for one Benjolin board. Note the power connectors at the back. These are compatible with the MOTM distribution boards I use. Right now I’m running this off +/- 15 volt regulated power. The buffer board uses it directly, while the Benjolin regulates down to +/- 9V. This panel will run off +/-12V too, with no problem. I have a plan where this may come in handy in the future.

Above is another view.

Above is the panel of one Benjolin. The second is almost identical. I ran into a little problem with pot spacing in the filter. The Frequency pot lugs didn’t clear any of the surrounding pots. I tried putting electrical tape between it and the Resonance pot, but the tape didn’t work and the middle lug of the Frequency pot shorted to the housing of the Resonance pot. I only found out when the panel was put into the rack, where the panel touched another panel that was grounded.

I had planned to use the same nice, black knob for the filter Frequency as those on the Oscillators. But there wasn’t enough finger room, so I went the Davies 1900 clone.

Talking about the filter, I should mention how great it is to have the band pass output option. Turns out that the filter will oscillate if the band pass output is patched to an input and the resonance turned up.

The obligatory video demo

The patch in this demo uses two Bugbrand oscillators to drive the Rungler. One of the Oscillators goes into the Filter and the other Oscillator is used as an LFO. The Rungler out goes to the audio Oscillator and also to the Filter CV. I just play around a bit to give a flavor of the sounds. It’s really quite a good filter.