Anyone who has ever played with a delay pedal knows of the quirk where the pitch of the delayed signal changes if the Repeat Rate pot is adjusted while playing. So if you have a way to subtly and reliably change the resistance of this pot, a sound similar to chorusing can be achieved.

If you have the stones to modify a vintage analog delay pedal, here is a circuit you can add to get some modulation, à la the MXR Carbon Copy. I haven't installed it into my pedal yet because it would require the addition of two pots and a switch; even if I could fit them into the box, I refuse to drill a bunch of holes in it. I'm going to rehouse the circuit, one of these days. You could use trimmers for the Speed and Depth pots which, in hindsight, would probably be better.

By playing with all the knobs you can get sounds from almost nothing, to nice "chorusing", to seasick "tape-flange" undulations for those niche sounds.

The LFO is built on a daughter board and, aside from the power wires, a single wire gets soldered to an empty pot lug in the pedal. No need to get all uptight about "destroying its value". This mod is quickly and easily reversed. Here is the schematic and layout.
(image removed--see next post)

Notes on the LDR:
I used a Clairex CL705HL; I don't know if they are still being manufactured (the few I do have are older than I am), but from what I've seen they are still available on eBay for a decent price. It is spec'd at 28k light resistance and 18.7M dark resistance. They're made of cadmium sulfide so it is most sensitive to red light. If you use a cadmium selenide LDR instead, it will be most sensitive to green light. I used a plain 5mm diffused red LED. Using another size, color, high-brightness, water-clear, etc. would require you to mess with everything after the output of the second op-amp.
Last edited by Invader Jim at Aug 24, 2016,
Got a bit of an update here. As it turns out, things get weird when this add-on is installed into the existing circuit.

Here is the new corrected schematic.

And a scope trace (huge pic). http://oi60.tinypic.com/rtnl90.jpg

The Ramp switch was basically useless at higher speeds so I got rid of it. The 330k was added in series with the Depth pot because otherwise the delays stop and the LFO stops if there is too little resistance to ground (when the Depth is set too high). This resistor may need to be adjusted to taste. As a bonus, that extra series resistance makes the Depth pot way more usable and eliminates the need for the Intensity switch.

To make it work with the pedal's power supply I had to short the internal diode and resistor that drops a 12v adapter down to 9v (it's a common mod to make old Boss pedals that used an ACA adapter work with a modern 9v PSA adapter). For some reason, when the LFO is wired to the pedal's power supply, the LFO doesn't work and the circuit won't delay unless you short those components. Weird...

I'm actually happier with this version of the circuit. It's a bit of a pain to get to work right though; lead dress for low-frequency oscillators is tricky.
Last edited by Invader Jim at Jul 28, 2016,
Interesting. What pedal did you install this in?
I figure the circuit would require tweaking to get to work well with each type of pedal.

I wonder if the dry output on a pedal like the ad-9 could be disconnected (not sure if it is board mount) and then this plugged in there in a separate enclosure. That might make it less intrusive on the guts of the pedal.
Luckily my DD7 already has a modulation function. Looks good though, Jim.

"Show me war; show me pestilence; show me the blood-red hands of retribution..."
I was thinking that this could be built into a separate enclosure, and then converting the dry output on something like that AD9 to function as a hook up point. I could disconnect the dry output line from the jack, and run a wire to the appropriate hookup point on the circuit from the jack. It would only work if the jacks arent board mounted, but it might be easier than having it inside the existing enclosure.
Well you could cut the traces around the jack in that case. But there may be problems assosiated with having excess lead length on the Repeat Rate pot's CCW lug (this pot controls the clock freq). I haven't really tested it out though. Shielded wire there may be ok.

While it would let you use a seperate power source, the LFO may act differently. I was using my powered breadboard while tweaking the circuit but when I used the pedal's power source everything changed. Not sure why. Maybe a low-ESR BFC (big ****ing capacitor) across the power rails would fix the wierdness...
Last edited by Invader Jim at Feb 2, 2014,
Thanks for providing this! I have it on my breadboard now but didn't get to try it yet.
Until I do, I'm wondering: What kind of values and swing should I be getting from the LDR for this thing to work? I'm only seeing about a 10k range, actual values are around 10k - 20k. I tried an LED/LDR as well as an NSL32 optocoupler. I found that if I raise the current limiter going to the LDR (from 10k to 30k), I can increase the swing to a 20k range.

I know that I might have to adjust the 330k based on the value of the delay pot in my pedal (I have a couple pedals to try, I think that the delay pots are 180k and 370k), but I just don't know what range/swing I should be aiming for.

By the way, I'm not sure, but I think that the layout differs from the schematic: In the schematic, the bottom 220k resistor should connect to the node that connects the speed pot to the 100R resistor, correct? However, as far as I can tell, in the layout, that 220k resistor (I believe it to be R6) connects to the node to the right (100R/10u/+IN1). From my experimentation on the breadboard, it worked when following the way it is in the schematic.
Last edited by yonib at Apr 27, 2014,
Oh, you're right. Edited the correct diagram into the original post...

What pedal are you using? My DM-2 has a 1M pot for the repeat rate, iirc.

I'll have to get back to you about your other question. I gotta dig out and set up the circuit and test gear. Before that I gotta clean off my bench so I have a place to work... I'll try to get back to you sometime tonight or tomorrow. Very sorry for the wait...
Last edited by Invader Jim at Sep 25, 2014,
Don't go to all that trouble to check! I was just wondering if you remembered the values. I'll hook it up and see what kind of results I get. Too bad about your computer.

I think that I'm going to give this a try on my DOD FX90 (180k delay pot?) or my DOD FX96 (470k delay pot?).
Last edited by yonib at Apr 28, 2014,
Well I already got everything cleaned up and set up. Been meaning to do it for a while now, anyway.

Connecting my meters would change the characteristics of the modulation so I had to disconnect the depth pot. I measured the LDR resistance with my Simpson 260 (20,000 ohms/volt) because my 10M digital doesn't refresh fast enough. Under these conditions I was able to measure a resistance change fluctuating between 400k and 500k at the lowest speed. Note that connecting the depth pot to the circuit may change these readings.

It's not much, but it's a place to start. It's better to use your ears anyway, rather than getting hung up on numbers.
Last edited by Invader Jim at Nov 4, 2014,
Thanks for checking! I'm going to have to go by my ears on this, since I only have a cheap multimeter that doesn't refresh quickly (and it can't display over 1M). I hooked it up to my DOD FX96 and definitely got some chorus/flanging, but it made the delay time MUCH shorter. My theory is that it's because the delay pot is 500k, and since this gets hooked up in parallel to the delay pot, it's pulling the resistance down too much. So I think that I'm going to need to increase the overall resistance, but then I'll probably need a much bigger swing from the LDR for it to be able to modulate over a bigger resistance. The LDR that I tried (VT935G) has a low off resistance (1M), and the optocoupler that I tried (NSL32) has an even lower off resistance (500k). BUT I just found in my parts an LDR with a 20M off resistance (NSL-19M51), so I will try that next.
On the DM-2 the circuit is tied to the unconnected lug of the repeat rate pot so that it is sort of "in series" with the pot to ground. The pot itself is not grounded though, unless the modulation circuit is swinging. Idk how to explain it so here's the diagram.


My circuit gives a seasick tape-flange sound at the max depth and low speeds and some great chorusy flange-y sounds at various settings. The speed, depth, and repeat rate pots are all pretty interactive.
Last edited by Invader Jim at Aug 24, 2016,