JonDent - Exploring Electronic Music: It's 555 - Nonlinear Circuits 555 resonator

Tuesday 21 January 2020

It's 555 - Nonlinear Circuits 555 resonator - Build notes

These are my build notes for the 555 resonator eurorack module.
This module is inspired by Bernie Hutchins' CMTG or Cascaded-Monostable Type Generalized Resonator. He described this in Electronotes #93, September 1978.

I've got a couple of versions of this. .. in banana format and the early metal plate euro format.

This early one was mostly through hole.
The new module is SMD. I understand this is version 5.

Module description:
"it's 555..." resonator/drone/noise source with CV control of pulse width for all five stages (plus a CV all input)

This module takes an input signal and splits it into clusters of 5 independent pulses. The pulses are created whenever the signal transitions from negative to positive. Each pulse can have its width modulated by pot or CV and can be set to any amplitude or even inverted. The pulses are summed together at the output to give a rich strong signal, choc-full of harmonics. It is a very powerful sound that cuts through any mix.

Andrew adds in his build notes:
"this design is based on the Electronotes EN#93 Cascaded Monostable Generalized Resonator.
The main changes from the EN version are that it has been expanded to 5 stages and uses \
larger valued capacitors to allow the pulses to get wide enough to merge with each other and promote mayhem".

So whats a resonator?
According to wikipedia:
"A resonator is a device or system that exhibits resonance or resonant behavior. That is, it naturally oscillates with greater amplitude at some frequencies, called resonant frequencies, than at other frequencies."

Below is a standard monostable configuration of a 555

And here is a small section of the resonator circuit.
This is repeated 4 more times and the output from pin 3 is cascaded into pin 2 of the next 555 chip.

From what I can decipher from the schematic the input pin of the 555 is pin two. This is pulled down.
The 10n cap connecting pins 7 and pin 6 is the timing capacitor. This is also connected to external CVs which I guess is how you get voltage control of the circuit.
The 10nF capacitor connected to pin 5 is a decoupling capacitor to shunt electrical noise, to avoid instability and false triggering.

You will need five 555 ICs

These are pretty easy to get on. Watch the orientation.

Next are the BCM857
These are bipolar PNP matched transistors

Tricky to get on the pcb , and watch the orientation.
There is a dot on the top left

The BC847s are NPN matched transistors

The circuit uses four TL072 op-amps.
From what I can decipher from the schemo, one is part of the opening comparator circuit.

The input enters here. I'm guessing that the identity of the input signal (apart from its frequency) is lost when it leaves the comparator. The output is just a trigger for the following monostable 555s,

A second opamp is used as part of the voltage control.
And the last two find their way in the summing circuit ???