Saturday, July 19, 2014

Buchla 258B Clone Special Edition

Home-brew Lazertran front panel with added graphics for Waveform switching and FM depth...

* Clone of the vintage 258 version B module from the original schematic using original components (uA726, PN3565, 2N4339, 2N4248, 741 opamps, 2N4341, (modern) carbon composition resistors where specified in the schematic)
* Switchable between original module's audio-rate FM depth and extreme FM
* FM depth switch has a center OFF position so you can switch in and out extreme amounts of FM at will
* Normal'd / interrupting FM input jacks (oscillators will FM each other simply by increasing the FM Amount control, no patching required. Any patch cord inserted into the input jack interrupts that Normal and the patched signal will then modulate that oscillator. The sine/alt waveforms are what are Normal'd.)
* Waveform switching per oscillator for the alt wave, Saw or Square
* Gray "Keyboard" in jack, calibrated for 1V/Octave-ish. Should also be possible to trim for 1.2V/Octave response.
* Waveform CV in jack calibrated for 10V signals (later 200 modules, 200e systems)
* Triangle waveform audio output, single output jack on the left
* Both CV inputs per oscillator are twice as sensitive as the factory version for more extreme modulation effects
* EDAC power connector
* Switchcraft Tinijax
* Cardas Tri-Eutectic audiophile solder used throughout the PCB and flowed onto all traces for reliability; used in most of the front panel connections.

1. Detuned saw waves at unison.
2. Detuned square waves at unison, then detuned, then tuned and fine tuned to an octave apart.
3. Both sine waves, playing with original module depth exponential FM.
4. Monitoring one sine wave and FM'ing it by sine, saw, and square waves at the factory module's FM depth. Then sine modulating at Extreme FM depth. Change that wave to square. Goes out to pitched noise.
5. Pitched noise via Extreme FM cross-modulation. Monitoring only one oscillator.
6. Pitched noise via Extreme FM cross-modulation. Monitoring only one oscillator.

Here is a track using a vintage Buchla system with a pair of 258B dual oscillators, doing lots of their characteristic FM, if you need something more musical regarding their tone...

Impart / Tangent

Was considering trying a TEMPCO switched mod; maybe next time. Was concerned about going "too far" and having too many front panel switches. Have not confirmed whether this idea would bear fruit.

Clarity: There were three versions of the 258 analog dual oscillator. The A and B versions had exponential audio-rate FM (which produces clangy, robotish sounds) versus the C version, which provides linear FM but which may be modified to add a separate exponential FM path).