Tuesday, June 21, 2011

Buchla 106 Mixer

I've had a gent CAD up PCB artwork for a modified version of the late 1960s model 106 mixer. Again, I await Don Buchla's permission to publicly share this new PCB artwork; I will not do so without it. Here's hoping that he will; I would enjoy publicly sharing these builds with everyone interested..


Edit: I've recently posted the new schematic and new artwork here:



I have schemos of the CBS and pre-CBS 106 mixers. Off the top of my head, the only difference is a typo in the CBS version, C8, which repeats throughout the circuit (C4 and C11), and is correct in the other schematic, and that the earlier version states 47pF caps for C2 and C6versus the later CBS version with 5pF.


Due to the roll-off in the 10uF caps throughout the design, which I'm told is at about 70Hz per cap, I've opted for 100uF throughout, which should be ten times lower.

Each section of the mixer inverts, and I wanted each section (1-3, 4-6) to have both inverting and non-inverting outputs, for various reasons (inverting AC modulation/waveforms, resonant feedback loops, etc.). I have confirmed functionality for the regular sections and that inversion works and now have to determine the optimal gain structure(s).

As stated in the schematic, I used HFE matched PN3565 (cheap at Mouser.com). I was hoping to use THAT transistor arrays but was dissuaded by the gent doing the layout owing to voltage differences. I do note that the THAT is used by Dustin Stroh (J3RK on mw.com and e-m.com) in his Verbos/258J build, for the last two 2N3565 which IIRC are involved with the square or saw shaping, and it works just fine.

I do not know if there would be any sonic differences in using other transistors such as 390X and BC55X types.

I also note that it distorts with even a single oscillator input (my 258B redo seems to have 1.7VRMS for that particular build, which is somehow above Buchla's nominal rating of 1.1). A CD player also distorts the circuit with the gain wide open. It =might= be a useful clipping in certain instances, but overall, is not immediately "musical" as some fuzz and distortion pedals at minimum settings.

I've had the 68K input resistors placed on-board to avoid lots of front panel wiring. It all fits on a single-sided 4"x6" PCB, which you'd expect from such a sparse circuit.

Why not go with a simple opamp circuit, which could also be DC-coupled for mixing control voltages? I wanted to see if the vintage, discrete circuit had any particular character (or lack thereof). It's too early to tell but yes, I'll later add a DC mixer circuit to this circus...

And I am indeed using AB resistors in all of the stated places. No photos as I don't own a camera or equipped cell phone.