EHX Small Clone LFO - 47nF cap position

[Thomeeque]

 Hi!



A or B?

Thanks, T.

[Mark Hammer]

Interesting.  The EHX drawing seems the more plausible since the cap would essentially be decoupling a bias/reference voltage taken from V+.

But I've built the Heladito successfully, as have many others, and there doesn't seem to be any problem with it.  truth be told, though, I've never used it in a loud context, so I wonder if anyone has noticed any audible ticking.  If so, then that would also suggest the EHX drawing has it right.

[nelson]

A

[Thomeeque]

But I've built the Heladito successfully, as have many others, and there doesn't seem to be any problem with it.  truth be told, though, I've never used it in a loud context, so I wonder if anyone has noticed any audible ticking.  If so, then that would also suggest the EHX drawing has it right.

I've build my own variation of SC based on Heladito and when I set RATE pot to max rate (zero resistance) I get bit weird noises (not loud but audible) in signal and when I check LFO out at IC2a pin 1 by scope I see that curve is "noisy" as well - when I turn it just little bit, noise vanishes from the audio and LFO signal are pure triangles. And now I have bumped to this EHX drawing and I see this difference.. I'll try [A] when I'm back from work today..

A

Great

Thanks, T.

[Fender3D]

B on my EHXSmall Clone mid '80...

[PRR]

Neither is absolutely needed in an ideal world.

The basic oscillator is RATE resistance against 2.2uFd. B has no effect for large Rate resistance, and isn't needed for low Rate resistance.

If the power supply is clean and the opamps don't suck, then A is just not needed.

In any real case, power rail has garbage, the 180K 68K 82K string may pick-up radiated garbage, the opamp inputs may kick when overdriven.... if there's lots of handy 0.047u then A is a good place to drop one.

> when I set RATE pot to max rate (zero resistance) I get bit weird noises

That's not a shock. We already have rolloff in the Rate resistor and 2.2u, and inside the opamp. Integrators are worst-case for NFB stability. Adding a third rolloff is really asking for trouble.

The '358 is so mellow, and the main loop pole is so low, that 47K+0.047u may not be enough to destabilize it..... or maybe with another set of parts, it is. If there's no wide reports of "weird" on the Helaudito, maybe you just got an unlucky worst-case combination of tolerances?

[Thomeeque]

B on my EHXSmall Clone mid '80...

Thanks for checking Federico!

Neither is absolutely needed in an ideal world. ..

Thanks for analysis/disclosure Paul!!

..The '358 is so mellow..

I have actually used 1458 in my build, I'll try to replace it by 358 eventually - I'll postpone my experiments with A configuration after that probably..

If there's no wide reports of "weird" on the Helaudito..

Well, it's maybe questionable how many people would be bothered by that noise - it's actually pretty silent (LFO signal gets "ironed" by 4k7/10uF LPF on it's way) and it happens only at extreme rate which is not very useful - I would not get much interested as well if I would not check LFO output by oscilloscope (which is another thing most people probably won't do).

T.