NLC - BBx 291 VCF - build notes

 Finally starting this build of the Nonlinear Circuits 291 filter.

This is inspired by the Buchla 291 bandwidth/bandpass filter.

The BBX291 in Eurorack format.

I don't own an original 291. They are extremely rare.

However I have built a few "clones" ... whatever they call them these days.

 + 291r - build pics

Don B didn't do many filters. I think he preferred waveshaping over filtering.

This however is a great sounding module, and well worth having in your arsenal.

The original 291 (and the clones) used vactrols - four for each filter. They were VTL2C3 vactrols which have a 10M1k range.

Andrew's version uses his "black box" idea.

Rather than use individual vactrols, the LEDs & LDRs are housed in a light-tight enclosure built into the PCBs. It's a really neat idea.

This is a pic of a LPG module that uses the same idea.

Andrew used some clear lens red LEDs and GL5516 LDRs (0.5M-5k) for this filter module.

I'll see what i have.

His suggestions are that if you are using any LDR with an off resistance less then 5MΩ you can leave off the two 10MΩ resistors as they will be superfluous.

Ones that get down to 1k or less are prob best

You can read a bit about vactrols here
https://djjondent.blogspot.com/2017/12/vactrols-optocoupliers.html

Here are some pics of the non populated PCBs and front panel.

On the panel  the two CV pots are sort of attenuverters, so 0 is at mid-point.

Ok back to the build.

Some NLC words of wisdom

ICs 1st.

There are five TL 072 op amps 

Resistors next

Caps next

S1JL, Schottky, power rectifier

To be continued......

BBx291 - Nonlinear circuit build notes part 2

Links

+ NLC wiki

+ Build notes & bom

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You can find more NLC builds here.

http://djjondent.blogspot.com.au/2015/03/non-linear-circuits-ncl-index.html

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