Chaos Machine with Stomplfo

[KreoPensas]

I ordered some Stomplfo chips. https://electricdruid.net/product/stomplfo/ (I am planning to build a Calypso at some point. )

I think I want to replace the pedestrian LFO in the Chaos Machine with the Stomplfo.

I've been looking over the schematic, I think I want to remove the original LFO components from the board. The Stomplfo will get a 'daughter board'.

I thought I would put this here and see if there is any wisdom to be volunteered before I rip anything apart... then the thread can serve as a guide or warning.

 

[t1redhands]

I'm curious about this process as well. I haven't done much studying of basic LFO circuits and their behaviors, but I think one of the points to consider is the voltage swing of the LFO cycle. The STOMPLFO datasheet has a couple of output filtering examples - a passive one for a 0-5v swing, and an active filter for 2-7v. I'm sure it's super obvious to determine the output range of this basic triangle LFO circuit.
For the Chaos Machine, it looks like you'll also want to retain the individual analog controls over depth/intensity and use the CV inputs on the STOMPLFO to control rate and shape.

 

[KreoPensas]

One of the first tasks for me is to measure the output of the existing lfo. Scaling the output is relatively simple. The Stomplfo datasheet does indeed have an example of how to do this, and I have some experience in hand rolling circuits that scale analog control voltage.

 

[KreoPensas]

You are correct, the depth / intensity control and the frequency controls are part of the filter 'channels'. The only original control replaced would be the rate control. I don't have any physical layout worked out, I only know there's an abundance of extra space in the enclosure. I'll probably grab a pedalPCB breakout for a 1p8t rotary switch like the Arachnid (etc ) uses., and that's probably going next to the foot switch. I have no desire to vero board one of those 1p8t swicthes, but the rest of this mod is likely to be.

 

[KreoPensas]

Probe is grounded to board ground, and the voltage trim on the sillyscope channel is wherever it is. Swinging 1 volt - ish above and below.

 

[t1redhands]

Probe is grounded to board ground, and the voltage trim on the sillyscope channel is wherever it is. Swinging 1 volt - ish above and below.

Above and below the reference voltage (4.5v)? So that would mean that the 2-7v active filter is needed, I think, and the STOMPLFO output will likely be stronger than the stock LFO.

 

[KreoPensas]

I'm not interpreting the scope that way. I also won't profess to be an expert, but when I take the probe away from the LFO output, it settles to the center of the LFO swing. The probe ground is clipped to the board ground.

I have a little McSillyscope slated for a eurorack installation. It's all automagic... stand by...

 

[KreoPensas]

Here's a different view.

 

[KreoPensas]

I had the scopes reading AC, therefore those readings are... subject to interpretation

What was that about not knowing what you don't know until you start explaining it? Soaking in it.

 

[KreoPensas]

 

[KreoPensas]

MMMkay, all this makes considerably more sense. It makes a bit of sense of the example in the Stompflo datasheet as well.

 

[KreoPensas]

For comparison, Here is the output of the LFO opamp on a Madbean Stage fright. I am seeing how the suggested Stomplfo active filter is, as ya might expect, in the ball park of general use.

 

[KreoPensas]

For reference, here is a "better" measurement of the Chaos Machine LFO in which the waveform is rendered in a more legible manner

 

[t1redhands]

Rad, this gives me hope for my project!

 

[LukeFRC]

I'm enjoying this all

 

[KreoPensas]

Since no one came back from the future to stop me, I proceeded to remove the LFO specific parts. One pot was desoldered for access.

I am using these "round" type header pins to extend the IC socket. This brings VCC, GND, connections to and from the rate pot, and the output of the original LFO all up to the daughter card. Components will face each other to save space, murphy's bylaws not withstanding. A little adhesive backed foam to remind me where not to put a few things and protect the caps below. Adhesives aren't my favorite things to rely on but I'll put a block of foam on the enclosure bottom for support and fashion a contoured foam spacer to put between the boards.

I am still awaiting the Stomplfo chips.

A design thought: why not just use a regular pot for wave selection? Who needs indents? Sure builds easier than a ladder.