StompLFO-controlled BBD clock

[ElectricDruid]

Hi All,

I've been doing some experiments with the StompLFO chip and the Small Clone clock circuit. The idea is to produce a modulated BBD clock, essentially just replacing the Small Clone's standard LFO with the StompLFO instead.

The reason I chose the Small Clone circuit for this transplant operation was that it works best with an LFO signal up to about 4 or 5Vs (..ish) and the StompLFO produces a 0-5V LFO signal so it seems ideal.

I started just plugging the StompLFO datasheet circuit into the Small Clone clock circuit, like this:

https://electricdruid.net/wp-content/uploads/2019/05/LFOAndSmallCloneClock.jpg


This worked straight away which is always nice, but the frequency was far too low. Swapping the 150pF clock cap for 22pF helped a bit, but it was obvious that the 10K+100K+39K resistance was causing some issue. Remember that  the StompLFO is producing a string of 0-5V digital pulses at 2MHz, so I *need* filtering of some sort.

I've now tried it with a 10uF cap to ground (like the Small Clone's original circuit has) and then used a 4K7 resistor as a simple single-pole RC filter for the StompLFO output. It still works, and we get a much higher clock frequency (I'm into 100-200KHz now) but the huge cap totally flattens all the StompLFOs waveforms since the RC cutoff is way down at 3.4Hz.

I shall keep experimenting to find some compromise option between these two extremes, but any advice or guidance would be appreciated.

As I understand it the Small Clone clock works by changing the voltage at the LFO end of that 39K resistor, and that then changes the rate that the cap discharges through it. Is that right? What's the big 10uF cap for in the original circuit? (They're not filtering the LFO are they?)

Thanks.

[idiot savant]

I messed around with the 4047 for awhile, but ultimately decided that I liked the 4046 more.

Here's a basic idea:


Since your LFO is much nicer than mine, I assume you could easily simplify it quite a bit. If I remember right, you have a DC-offset example in your datasheet...

[ElectricDruid]

Yeah, the StompLFO provides a DC offset control.

If you use the 4046, what do you do about the biphase clock? You only get one clock output, right? Seems a shame to have to add another chip just for that - that's the advantage of the 4047. It has the biphase output built in.

Anyway, I hate that VCO. It's the most variable chip I've ever worked with. If you get ten and put them in the same circuit, none of them work the same. And then take ten from another manufacturer, and they don't even work at all until you alter half the components in the circuit.

[Scruffie]

You can get bi-phase out of just a 4046 (see the zombie chorus) but I hate that chip too...

Yes the 10uF cap is for filtering the LFO in the original, but there's no reason it has to be that high if its messing with your wave.

The Small Clone clock is actually current controlled, not voltage, that diode is working as a variable resistor.

[idiot savant]

One of the other avenues I tried was to just program a second processor for the delay clock. It works, but needs more time to get it dialed in. It also made me feel kinda dumb having two processors on the board just for those separate tasks...

[diydave]

Have the same thing sitting on my bench since a month or so: the stomplfo with passive RC's directly hooked up on a small clone.
Schematic and pcb are from runoffgroove.

It worked when not in an enclosure. Even the different waveforms. Had no real issue's with frequency-sweep (but I like a rather sloooowww but very deep chorus from time to time).
But as soon as I put everything in my enclosure, I only get a kind of "ping - pong"-ish swirl (square-wave). The rate-knob works, so I'm guessing it's not the stompflo-chip itself. Haven't find the time yet to solve the issue.

Wish I had an oscilloscope now. Then I could see if the lfo is spitting out the different waves, or not.

[anotherjim]

I can't find a scheme with a 10uF cap in it - 1uF biggest filter on the LFO I saw.
Anyway, it may be a large cap so it automatically decreases LFO depth range as speed increases because a fast and deep warble can just be annoying.

[ElectricDruid]

The Tonepad schematic has the 10uF:

http://www.tonepad.com/getFile.asp?id=97

Yes, good thought. I'd wondered if it was to reduce the mod depth as the frequency goes up. That's a handy feature of a basic RC filter, 6dB amplitude reduction per octave. Unfortunately the trick only works for sine waves, so it's not going to help me any.

[ElectricDruid]

One of the other avenues I tried was to just program a second processor for the delay clock. It works, but needs more time to get it dialed in. It also made me feel kinda dumb having two processors on the board just for those separate tasks...

That's very close to my FLANGE chips, or half a FLANGE chip, strictly. Those roll the LFO up in the same chip as the NCO clock, so it's a nice neat one-chip solution. I could do a variant for chorus easily enough, but I'd thought to go a bit more analog with this one. Plus the StompLFO is already done, so it's a shame not to use it.

Back to the breadboard I guess!

[pfzpedales]

Hi! Bringing this back, as I´m trying to add de TapLFO3 to the Small Clone. LFO works great but I have a loud oscillation that follows the lfo rate. Took 39k and 10uf out and tried different values there as well. Also tried changing the values of the tapLfo filter but still the same noise...... Nothing seems to be working. Any suggestion?

[ElectricDruid]

TAPLFO3 is a more difficult chip to use in this situation. The reason being that the TAPLFO uses a PWM output which is at 31.25KHz, whereas the StompLFO uses a PDM output that goes to several MHz for most of the waveform. Hence the filtering requirement is much reduced.

[pfzpedales]

Sorry Tom, my bad...I´m actually using the STOMPLFO. I followed your recomendations with no luck! Thanks for your answer.

[ElectricDruid]

Umm, well, I don't know. I had trouble with the waveshape getting a bit mangled, but certainly no oscillations.