Wednesday, 29 July 2015

NLC - Buchla Style - Low Pass Gate (LPG) Build notes.

These are the build notes for the NLC (NonLinearCircuits) version of one of the classic filters of all time: the Buchla 292.
The 292 configuration consisted of 4 filters with CV control & a 3 pole toggle switch for each filter.
The  toggle switch allowed you to alternate between 3 modes: LowPass, Combination & Gate.
Gate is Buchla's name for a VCA.

Buchla still produce this filter in two forms: The 292h & the 292e (which goes by the grand title of  "Quad Dynamics Manager").

The 292e is the centre module. Notice that the toggle switches have been replaced with buttons.
 It has also gained velocity control & preset storage but is mostly the same as the earlier 292.

Here are some pics of the virgin NonLinearCircuit PCB & panel:

Andrew's build notes are here:


Useful Links:
1. CGS - Ken Stone's LPG
2. Buchla Tech - Verbos's excellent blog - 292c Secrets with schematics
3. Synth.NL.Blog - 292 5U build notes
4. Modular Synthesis - Build notes for Thomas White's LPG clone.
5. Doepfer A-101-2 LPG filter
6. Natural Rhythm Music - Thomas White 

The LPF filter design is a variation of the Sallen Key. Ever since professors R.P. Sallen and E.L. Key described it in 1955, the Sallen-Key low-pass filter has become one of the most widely used filters in electronic systems.
It's another form of what is known as a voltage-controlled voltage-source (VCVS) filter used to
implement a 2-pole (12 dB/octave) response.

Most VCVS use unity-gain operational amplifiers where the output voltage is the same as the input voltage.... At first glance this seems useless but the advantage is that they don't take current from the input source.
The input & output sides are in effect isolated.

The controlling element for frequency (in LP mode) and amplitude (in VCA mode) is the vactrol.
Basically a LED & photoresistor packaged in a light tight box. I reckon this is a really neat way to get voltage control over a variable resistor.

Andrew suggests experimenting with different types of vactrols. He has been using Silonex with good results. The NLC PCB is designed for a dual vactrol. Some of my earlier Buchla builds (eg the 208) use dual vactrols like the VTL5C3/2, and I have a few spare. So I'll probably use these first. Dual vactrols are actually two photo-resistors and a LED in the one package ....which is perfect as they sit in where R1 & R2 go. I might later experiment by using two single vactrols or even make my own vactrols if I feel like the challenge. There are lots of ways to tinker with this beautiful circuit.

Install IC headers & the Euro power connector first.
Then resistors & caps.
These 2 resistors (marked with and asterix) can be altered to suit your taste.
The caps marked 472 & 221 are 4.7nF & 220pF respectively..
I think MCCs (monolithic ceramic caps) are fine, others might insist on polys.

The 33k can be varied up to 51K .... It sets the Cv level.
I left it at 33k.
The 10K sets the gain of the output signal.

The switch must be mounted as close as possible to the PCB & watch the orientation (it's up/down, not left/right).

Sounds quite good ..esp the LP filter.
I reckon varying the CV level resistor is a good idea. The frequency pot is very responsive at the upper levels (8-10) , but volume drops away quite quickly below this.
I experimented with audio(log) vs linear pots and think a log frequency pot sounds better. (just my humble opinion).
I'll probably change that resistor to 51k.

It's done for now.
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