Ring Mod StompFLO PT2399 as my first crazy try

[Matthew Sanford]

Hello to all of you brilliant people!
Long time listener first time caller here...

So, I am posting to go over a weirdness I made after being influenced by a post from Intensive Care Audio with all the Electric Druid time that led to the Fidel Eater, and by my kid's BF being a guitarist for 5 years with no distortion pedal! Long story short, it's always been in my mind but not budget to build, then I was defeated by the Ruby Amp for years pulling my confidence out from under the desire for more effort (turns out they are likely -1 {not marked} from China and the FET was too much for it, cause it works on my board without that). Recently I committed to testing, actually worked on having soldering skills, and stopped giving up. Also, I always do things somewhat wrong to find out why right is considered right (it isn't always).

Thank you for the therapy time, now on to it.

I've long been idealizing the PT2399, also theramins, ring mods (grew up in the 80s here and that meant Saturdays were Daleks on BBC with my dad glued to it), dynamic control of signals (vactrol, is that it?), and many other weird things too. Recently, thanks to the Fidel Eater I started digging in to the madness that is the Electric Druid (ALL HAIL!), and ordered a few StompFlo chips....so amazing! My thought was a ring mod with the StompFLO providing the modulator wave. I set it up per the data sheet without the active/passive filter portions, the CV to 0v, all else to pots. Kinda made a wobble sort of... oh, I did a simplified Ring Mod I found online (resistors are all 100ohm)



So, I decided to look at how to double the frequency (some diode LCR thing) also using an op amp to increase the gain and change it too -5v/+5v output (constant learner here) thinking the negative was not there so not making it do the full ring mod, or frequency too low, not really knowing... I also did a simple NPN buffer  and fed both the Stomp and Guitar through one each or not in different ways. Actually before all that my first StompFLO thing was using it as the bias voltage for a Distortion+ circuit made by barbarach.com based on ElectroSmash - that was a weird way to increase distortion, but not a lot else...


After that, I set up a PT2399 basic echo found online , put the Guitar through a buffer, just ran straight circuit so no mix to see what it all was, put the output of the PT2399 to the Ring Mod modulating freq and the guitar to the carrier (after boost and to PT before it) and that is when weird stuff started happening - oh, the Stomp controls the PT as well using a LED/LDR glue shrink wrap thing (thanks to a Mini-Mu Envelope filter circuit I found, not sure who but was rev1.2 from 05/09/2014). So these things were all kind of modular on small breadboards and have been connected different ways. The coolest thing so far was almost a harmonically correct movement up the scale when playing say a high G on 15th fret, it is closer to the peak of the ramp up wave that it occurs, and doesn't really do on the lower notes. It does give a bit of an octave overtone though - for that thing, I was thinking maybe it'd be better off with Druid's OneShot to just throw off weird ring mod things when wanted, instead of planning a song to match the speed of the wave (though I love doing that too, get that weirdness at just the right part, let the madness dictate the composition that may never be played the same again!!!)

Now I don't have much in good videos, took a bunch with the front face microphone on my new to me iPhone X before realizing it doesn't work so good. Here I will post a couple though, first one where at about 3:45 in it has the scale from the ring mod...uh, or not, guess gotta put it on YouTube and post here (see, I'm learning!), so if it uploads before I finish I'll put it in... oh here it is https://youtu.be/NkbLgxJBBTk . Now things I was thinking to change might be a HPF filter to reduce the frequency range going to the ring mod, currently have the double and half of the ring mod feeding either side of a 10k pot with center pin having guitar from booster tied in to the out, so maybe mix in after PT sounds prior to ring mod, and definitely set up a blend of the original tone to reduce the madness. other ideas?

At least, here are the images of the schematic I drew poorly and left values out from till I make sure what random things I threw in, as well as pics of the breadboards

[Matthew Sanford]

https://youtu.be/fnjJ1MBUc54

A slightly earlier video. Once I get it cleaned up more, with a blend and what not, I will post better videos to show all the weird from StompFLO - most of what I have done was with the ramp up. Eventually I will devise or steal (sorry Intensive Care) a knob to select wave forms properly, it'll make my life easier...but for now, turn the knob.

Oh! a bunch of noise/oscillation is caused by the LM386, it doesn't handle much well, so I plan to do a push/pull for my bench testing amp later too. I am sure the NPN boosters are helping to put it into it's misery as well...

[anotherjim]

OK then...
The basic PT2399 looks ok. I will say the pot across your LDR looks like it can go to zero ohm which could easily lock the chip. I'd put at least 510R in series with the pot to put a safer minimum to it. 1k is safer and around 2k is what the datasheet asks for IIRC.

The diode ring mod isn't something I know too much about. I think it only does frequency doubling with 2 similar inputs that are 90deg phase shifted apart, but you can still get clangour if the inputs are different. The PT2399 pin15 will be inverted which is 180deg phase shift but if you take the delay output from pin14 or 12 the phase will be whatever it happens to be from the delay time and that might sound more interesting. It probably needs a consistently strong signal to overcome the diode forward voltage needs but there probably is enough. You could improve it a bit with Schottky diodes for their lower voltage needs.

I don't know what the 8pin is doing driving the LED. If it's a 386 , is it some kind of oscillator working through its internal feedback which I haven't seen before?

[Matthew Sanford]

Hi Jim!

For the LDR I had put the 50k pot in series, so start up I keep it heavy but there are some lockups here and there that I am not sure are completely tied to that - well, on the other end the "basic echo" circuit I used has the repeats control using pin 6 as well, so there is interaction there too...was thinking to molest pin 2 a bit too as I've seen that used for delay time.

That 8 pin chip in the second picture with the 3 pots is the StompFLO, just a piece of amazing versatility there! If you haven't tried one yet, I can't recommend them enough.

Interesting about the phase on the ring mod. I figure my NPN boosters are switching phase, albeit 180 not 90, so that might be factoring in. I know what I'd read was the carrier should be 6-7 times the modulating signal, and I haven't even paid attention to that yet (lots to fine tune). I am figuring that the pitch modulation created by the delay time modulation combining with the original signal is what is causing seemingly harmonic (not mathematical) returns. It seems to be towards the higher end of the wave that it starts doing those kinds of combinations, maybe having more signal or because it is timing in a way that will mesh in the ring? That is the most interesting part of the circuit, so what I am trying to enhance. I had mentioned the One Shot chip as a possible replacement for the Stompflo so the effect could be timed as needed, and it seems the pin outs are the same so just drop in replacement.

Other thing I'm thinking is about the CV Offset on the stompflo, it affects the depth of the signal, so I may set it to midway voltage so instead of 0v-5v it should be -2.5v to 2.5v...I think? Was originally trying to get negative voltage from it into the ring mod so it can go both ways, and now considering it too, but I am figuring if I keep it off 0v I might not get that weird chirp or wooong sound when the LFO goes back to bottom...

More work to do, but thank you for the phase direction, I'll pursue that too. Though the stompflo and PT2399 are amazing chips themselves, I am working to make the ring mod more the defining feature of it

[ElectricDruid]

Glad to see you're enjoying the StompLFO! There's definite FidelEater vibes about that circuit.

I think you've done the right thing connecting the offset to 0V. When you turn the LFO's depth right down to zero, you want the LED to be completely off, so Offset=0 is what you need. If you go for offset=2.5V, you'll have the LED dim when the depth is low, and that means the LDR will modify your Delay time setting even when there's no modulation.
Remember that the StompLFO can't produce any actual negative voltages (it's running on a 0-5V supply, after all). With Offset=2.5V, the output is only -2.5V to +2.5V in the sense that that's how it moves from the 2.5V offset - down to 0V, and up to 5V.

As you said, the OneShot chip shares a pin-out with the StompLFO, so it would be a drop-in replacement for a different set of experiments. The OneShot doesn't have the offset control, instead it's hardwired to 0V, but since that's what you're using, that's a good match.

[Matthew Sanford]

Thanks Druid!

I'm trying to work towards bringing the ring mod out more, as I realized it's the StompFlo modulating the pitches up the scale. I found a circuit for 6 octaves alternating left and right and modulated by an LFO. I might use the Stomp to mod that and put the two channels in as the waves...I had used the StompFlo as the modulation freq and it did pulsing sort of distortion, so maybe before the rigamerole or just off the repeats, and maybe try the guitar dynamics for the PT or keep the StompFlo running it all...but after the 25th, too much to get ready this close in!

I'll have to ask for the OneShot for xmas!