Tuesday, 24 November 2015

NLC - 2 x LFOs - Build notes

These are my build notes for the NonLinearCircuits Dual LFO & difference rectifier (Eurorack version).
It's a easy build as all parts are through the hole.

Andrews build notes are here:
and his blog notes are here:
Some pics of the virgin PCBs
And the rear.
There are two op-amps used : a TL074 & a TL072

 Headers first.

Resistors next.

This LFO uses a combination of integrator & schmitt trigger. It's quite a commonly used combo.

This is a simplified version of Andrew's circuit.
The op-amps "A" & "B" are part of the TL 074. "A" is the non inverting schmitt trigger. "B" is the integrator.
The output of the Schmitt trigger is a square wave. This is fed into the integrator. The output of the integrator is a triangular wave, which is fed back to the Schmitt trigger. Thus first stage drives the second, and the second drives the first..... which came first ???? the chicken or the egg ???

The last part of the module is a diference rectifier which is described by Andrew thus:
"This circuit is a hybrid of two basic op amp ‘building blocks’ .....a difference circuit and a rectifier.
 It takes the triangle waves (from the 2 LFOs) and supplies a signal that is the difference between them"

The NLC module uses two standard 1n4148 signal diodes for this rectifier circuit.

"Basically the circuit compares the voltage on the ‘-‘ inputs with the voltages on the ‘+’ inputs. (on the 072 op-amp). The difference between these voltages is fed to the outputs. If the difference is positive  it appears on the ‘+’ output, if the difference is negative it appears on the ‘-‘ output. Again, a great way to mix boring CVs to get something interesting, audio frequency wave - shaping. "

The two electro caps I'm using are rated 25V.
I've used monolithic ceramic capacitors for the two 1uF 105s.
The three 100nF 104s are straight ceramics

I've used 1k 0805 SMDs for the LED resistors. Not really sure of the exact value but they are easy to change if needed.
As it turned out, the 1K resistors seem to work well.

The pots now. I was debating whether to chose two audio 100k pots (for frequency adjustment) and two linear pots for the fine adjustments, but Andrew advised me to use all linear100K pots

I've used scrap resistor wire for the ground connectors

Testing the square & triangle waves.

Lovely !!!

Useful links.
+ Video - VCO using a Schmitt trigger & integrator.
+ Pdx.edu VCO
+ Falstad.com
+ Thomas Henry - The birth of a synth
+ The two amp oscillator
+ Circuits Today
+ What is a schmitt trigger and how it works
+ NLC build notes for the difference rectifier & neuron

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