Tweaking the LFO on CE-2, TR-2 (and possibly many others)

[ptkw]

Hello.

I'm currently working on two cheap copies of Boss pedals, namely CE-2 Chorus and TR-2 Tremolo, trying to perform on them basically all the possible modifications I can come up with.

Since I operate in the experimental/noise genre and weird sounds are of special interest to me, I'm particularly fond of a simple hack that is available for both of those pedals, which transforms them to faux ring modulators. This is achieved, of course, by increasing their modulating frequency to audio range. Here are the schematics on which I marked the critical parts for these well-established mods:



I have achieved some fairly satisfying results by adding the switches that toggle between the original "blue" caps and the ones much smaller in value. This solution, however, is not perfect for me. I would prefer to have a much broader range at disposal in each mode, i.e. I'd like to be able to dial between normal chorus/tremolo and crazy ring mod with just one turn of a potentiometer. Ideally, there would be two modes enabled by a switch: first with "normal" LFO range from extremely slow to very fast, and second starting exactly at the frequency where the first mode ends, and ending somewhere in the cosmic ring mod territory.

Basing on my experiments with different cap and resistor values, I'm pretty sure that it is perfectly possible to achieve something like I described above. However, it would require knowledge that is quite beyond my current understanding: I have to admit I don't understand how the LFO exactly works and what is the exact interrelationship between all the parts' values and resulting frequency. I have an intuition that some potentiometer substituting would be necessary for my means - but then again, I don't know what the potentiometer actually adjusts here. Could someone help me by explaining what part does what - apart from what I already figured and put into the image? Perhaps there's some handy formula to calculate the modulation frequency basing on given values?

Additionally, I'd like to ask about possibility of adding CV ins and outs to those two pedals and possibly some other LFO-based stompboxes that I may modify in the future. Say I'd like to set the modulation rate on one effect, and have the same rate on some of the others. From what point in the schematic should I "borrow" the control voltage? How would I bypass the internal CV with one from a connected device? Or perhaps it's all but a dream and something practically impossible to achieve?

Many thanks for your help!

[PRR]

> "normal" LFO range from extremely slow to very fast, and second starting exactly at the frequency where the first mode ends, and ending somewhere in the cosmic ring mod territory.

Can you put Hz numbers on that?

What I see: "normal" is maybe 1Hz-10Hz, a 1:10 range. And "wild" may be 50Hz-2,000Hz, 1:40, a wider range. OTOH, if you start at "top normal" 10Hz, and go 1:10, you only get to 100Hz.

Maybe I flog ring-mods differently than you do.

The basic (fastest) speed is a function of (in CE-2) R32 C19. R32 is somewhat constrained by load on pot VR1. So change C19. If top speed is now 10Hz for C19=0.1uFd, then C19=0.01uFd gives 100Hz top speed, and 1nFd approaches 1KHz (it will run a little lower than calculated because of stray capacitance and finite chip speed).

Both versions are currently rigged for 10:1 range by the 100k+10K pot and stopper. (Actually 11:1 range.) That's great for "normal tremolo"; better than classic rigs which struggled for 1:4 range. But ring-mod effects may run into musical pitches, many octaves, and there's just 3 octaves in a 10:1 range.

You may increase the range simply by reducing stopper R31. With hi-gain opamps, I believe this can be reduced to absurdity, 1000:1 range, and it will work. (You may have to boost C20 to clamp the bottom of R31 at slow speeds.) With a normal linear pot, it will get too-twitchy at the slowest speeds. VR1 could be changed to 100K Audio, R31 to 1K, and get a 1:100 range, all at the slower end. To make normal normal again, C19 must be 0.01uFd. Now normal covers 1Hz-100Hz, and the "tremolo range" is 0-5 on the dial.

[Mark Hammer]

Given that these are chorus and tremolopedals, you have a bit of a head start, since neither are any bloody use when the LFO rates get much slower than maybe 0.8hz.  A 100:1 modulation range without switching anything to do it would get you modulation speeds of 0.8-80hz, which is far enough up into the audio range to produce clearly audible ringing effects, even if not quite an EHX Frequency Analyser.

I contrast this with a flanger or phaser, where speeds as slow as 0.1hz are quite useful, and modification to a 100:1 range would only bring you up to a slight trill at 10hz.

You will note that the same LFO produces not only triangles, but square waves as well.  Consider tapping the other op-amp output in the LFO for the square wave.

[thehallofshields]

I saw that same suggestion to lower (jumper) R31 here:
http://bounav.free.fr/wp/?p=254
Apparently this can push the LFO into self-oscillation.

What I'd like to figure out is a way to push the self-oscillation to provide a stable rhythmic drone affect.

Listen to Sandy Bull use a Tremolo effect in the late 50's and early 60's to set a backdrop for Arabic scales.
http://www.youtube.com/watch?v=OxyYB0grU38&feature=related
He might just be getting a 60hz buzz feeding into his amp and tremolo effect, but it sounds awesome.

I'd kill for a mod that could do that.

[PRR]

> Sandy Bull use a Tremolo effect in the late 50's and early 60's

The electric sets are distinctly mid-1960s. Before that he was acoustic, played a mean oud. The first album Fantasias is worth extended listening. I had it on my Garrard when new and parts of it are on my iPod. He was a different person then, and another person by Demolition.... Greg and I just looked at each other "whaaa??"

Like many of us, Sandy combined drugs with electronics to create (or at least hear) sounds which can not be duplicated. I hear tape-effect in the piece you cite, but I can't crank it because the dogs are asleep.

> I'd kill for a mod that could do that.

Same sound on this snip:
http://www.amazon.com/gp/product/B000QMSRNE/ref=dm_dp_trk9?ie=UTF8&qid=1326261639&sr=301-4

There's some electronic talent, and strong drugs, and now-antiquated gear, not just "a mod".

The tone on Memphis is more refreshing and seems to be just a Fender Reverb/Tremolo, switch-pops and all. (Yes, the bass line is underdubbed.)