Hi Everyone,

I thought I'd run something I've been working on past you all. It's a basic optical tremolo based on the StompLFO chip. Here's the circuit as I have it currently:




As you can see, it's not big or complicated. One dual op-amp and one 8-pin StompLFO, plus a vactrol.

The input buffer has a trimmer so you can add more or less boost. With maximum boost, you get +14dB and it boosts mostly midrange:


I deliberately played with the tone on this since I think the mid-forward sound makes it sound more like a vintage amp. Playing my friend's old Korg M1 through it, you could almost believe I'd got a genuine Rhodes, and you wouldn't say that about the M1's electric piano without some effects!

I've got a few niggles remaining. One is that volume control on the output. Does that look ok to you? I worry about the output impedance when it's turned down a bit, but perhaps I needn't.
The other is the waveforms. Although the StompLFO has eight waveforms, most of them don't sound that great for Tremolo. In fact, the best ones are the classic square, triangle, and sine. Square has that nice chop, triangle is a simple up/down sort of thing, and sine is the same except subtly more 'throbby'.

I wonder about adding a second vactrol so that it could be run as a panning circuit instead of just a tremolo (it'd still be a trem if you only used one output). That way some of the other waveforms (like the Ramp up/down, Random Levels/S&H, and Random Slopes) would make more sense - having your signal randomly drift between the two speakers might be quite cool, whereas having the volume randomly vary up and down doesn't sound that great.

Anyway, any thoughts or feedback appreciated!

Thanks,
Tom

Nice! A super-simple trem with a super sexed up LFO. I don't get the need for a gain trimmer AND a vol pot at the end.

Quote from: Ben N on June 18, 2020, 01:25:38 PM
Nice! A super-simple trem with a super sexed up LFO. I don't get the need for a gain trimmer AND a vol pot at the end.

You might need some gain because you can chop quite a lot of the sound away with the trem. So then the volume control lets you match that up with your clean signal, or lets you have a bit of a boost when you bring the trem in if you prefer. It also means you can turn the Depth down and you've got a boost, so it adds a little bit of versatility.

But sure, you could put fixed gain or no gain on the buffer and drop the volume pot to make it even simpler.

LDRs are going out of style. Selection is poor.

A cheap LDR won't load a 1k very much. Even classic broadcast limiters used 10k+. While you do not need limiting depth, there's no reason to go 1k.

LDRs distort. And of course more with higher voltages. They are not heavy hiss sources, though the series resistor has calculable hiss. The usual practice is to NOT have much gain in front, take gain after. (Of course you need buffer in front to interface e-guitar.) Depending on the LDR idle point it may have 3dB-10dB average loss which you want to make-up, plus "more" cuz you never give a guitarist ANY loss.

Quote from: PRR on June 18, 2020, 02:26:33 PM
LDRs are going out of style. Selection is poor.

What?!? Now I have to worry about using fashionable components too?!?

Xvive make a ton of options, and anyway, I always use the VTL5C3 (since that's what I have) and then I adjust other things to fit. Can't do anything about the rise/fall times of course, but so far that hasn't been a deal-breaker.

http://www.xviveaudio.com/opto-couplers-p0065.html

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A cheap LDR won't load a 1k very much. Even classic broadcast limiters used 10k+. While you do not need limiting depth, there's no reason to go 1k.

Sorry Paul, I don't understand this remark. Can you explain? I take it you think the 1K in the R7/vactrol divider should be larger? What's the advantage?

Quote
LDRs distort. And of course more with higher voltages. They are not heavy hiss sources, though the series resistor has calculable hiss. The usual practice is to NOT have much gain in front, take gain after. (Of course you need buffer in front to interface e-guitar.) Depending on the LDR idle point it may have 3dB-10dB average loss which you want to make-up, plus "more" cuz you never give a guitarist ANY loss.

It's such a simple circuit that noise isn't really an issue. A bit of distortion here is "character" not a fault. In fact, it probably makes it sound more like those old amp tremolo circuits that everyone loves so much. A Fender amp tremolo is not exactly a super-clean VCA.
So...I'm willing to live with some "faults" if it means I can do tremolo with two components instead of a whole VCA circuit.

I've played around with a similar idea before Tom, you're right that the differences between waveforms is less prominent than with other effects. In the bi-phase I've been working on I have a switch which takes the second LFO and instead of shifting phase it just goes to a transistor on the output wired like a switch to dump signal to ground. Not sure how wise that kind of set up is but it works pretty well, I just need to tweak it some more to remove the slight thumping. Might be worth exploring if you want to avoid ldrs

I don't want to avoid LDRs! I *like* LDRs! *Paul* wants me to avoid LDRs!  hohoho!

Can you use the ramp up waveform to get that old Vox Repeater sound?

Quote from: ElectricDruid on June 18, 2020, 05:54:30 PM
Xvive make a ton of options, and anyway, I always use the VTL5C3 (since that's what I have) and then I adjust other things to fit. Can't do anything about the rise/fall times of course, but so far that hasn't been a deal-breaker.

http://www.xviveaudio.com/opto-couplers-p0065.html

Are there any datasheets on Xvive optos? How do they compare to Elmer Perkin originals?

Quote from: rankot on June 20, 2020, 03:34:58 PM

Quote from: ElectricDruid on June 18, 2020, 05:54:30 PM
Xvive make a ton of options, and anyway, I always use the VTL5C3 (since that's what I have) and then I adjust other things to fit. Can't do anything about the rise/fall times of course, but so far that hasn't been a deal-breaker.

http://www.xviveaudio.com/opto-couplers-p0065.html

Are there any datasheets on Xvive optos? How do they compare to Elmer Perkin originals?

I've got the datasheet for the VTL53C that I use, but I can't find a link on their site for it any longer. It must be there somewhere...along with the others, presumably.

https://electricdruid.net/datasheets/XviveVTL5C3Datasheet.pdf

I also did some measurements in this thread

Thanks! What I am most concerned are rise/decay times. It seems they're considerably shorter than on original vactrols. But for this particular one they seem to be the same. Did you try their VTL5C9/C10?

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Did you try their VTL5C9/C10?

Nope, I've only tried the VTL5C3.

Ultimately, I wasn't looking for something that was a direct clone of the earlier vactrols. I just needed something that would be fairly versatile (there's lots of things you can do with vactrols, after all) and which would be easy to get hold of. That meant Xvive or Coolaudio and I think at the time Xvive were cheaper for the numbers I needed.