Neat little EA Tremolo trick

[Mark Hammer]

Although the new EHX Super Pulsar can do circles around the EA Tremolo, many of us have built this delightful little tremolo circuit, and been very happy with its performance.  Listening to a sample of a very different, and more sophisticated, tremolo the other day, it occurred to me that I might like a choppier action from the EA trem.

It dawned on me that the basic triangle LFO could be turned into something a little choppier by simply sticking a back to back pair of LEDs in series with the LFO, to shave off the first and last portion of the up and down half cycles.  There are a variety of versions of the EA Trem, but I'll refer to the one below for this post.  It will work the same way on all of them.

I inserted a back-to-back pair of green LEDs between what is shown here as a 120k resistor and the input lug of the Depth pot.  They chop about 1.5V off the triangle wave of the LFO such that it doesn't realy "start" to have any effect on the JFET until the LFO reaches 1.5V.  Similarly, once the LFO goes below 1.5V, it cuts out.  The waveform is still sort of triangular (actually, more the profile of a house with rectangular walls and a sloping roof), so it isn't a hard percussive square wave, but it does have a choppier feel than the stock triangle.

If a person wanted to experiment, they could try other LED combinations,  For instance a pair of green LEDs in one direction and a Schottky diode in the other might yield a sawtooth (or ramp) wave by setting a higher threshold for one half cycle than the other.

A very simple mod that you can easily add onto an existing pedal, simply by desoldering the wire to the Depth pot input, routing it to a toggle, running a wire from the toggle to the pot again, and soldering whatever LED combo you use directly to the toggle.

[Digital Larry]

One of the issues about the EA trem is that the action of the gain modulating JFET serves to increase rather than decrease the gain.  Of course you could look at it the other way around, so all I mean is that if you remove the JFET completely, the gain stage is at a low but still audible gain.  So while changing the shape of the LFO will probably have some effect, I don't think you're going to get into real chop territory with the LFO alone.  You might want to adjust the 1.2k resistor higher which would make the "low" gain lower.  It will affect the bias as well so it may be necessary to think of something cleverer.

[Mark Hammer]

Thanks for the idea, Larry.  In this instance, I wasn't aiming for a clearcut square-wave chop, just a more abrupt onset/offset.  What Jack Orman once described as a lagniappe.  Folks wishing to have a hard chop should probably look elsewhere.  This little mod simply gives two different feels to the modulation.  In some respects, maintaining the triangle top makes it more generally usable, or at least more the sort of thing you can leave on for longer periods.

I haven't used it at any appreciable volume, but there does not appear to be an audible change in volume level, and use of a toggle to simply bridge the LED pair and "stop the chop" does not appear to produce any switching-related noise or pop.

Incidentally, I was wrong about the 1.5V drop.  Red LEDs have a 1.5V Vf.  Greens are higher.

[midwayfair]

Mark, this is a neat idea.

I think you could use a 100K pot in series with the diodes to control it in parallel with the 120K.

Okay, I need to write to a friend to ask him if he wants me to improve a pedal I built him a long time ago ...

EDIT: fixed second sentence.

[bluesdevil]

Thanks, Mark. I have my old EA trem out fixing the footswitch and would be a great time to try your mod.

[bluesdevil]

Just tried it with a pair of back to back green LED's. Didn't seem to make it choppier, just lowered volume.
Correction: lowered depth.

[Mark Hammer]

Hmmm, worked like a charm for me.  There are multiple versions of the EA out there, like the ROG version that uses a BS170 gain stage, or the GGG version that uses a 2-transistor audio path.  I know I must have easily 4 or 5 different versions on my hard drive, and the component values are not always the same.  Entirely possible that the voltage drop of the green LEDs, or the resulting current, is incompatible with a particular version.  I have no idea which version I'm using, since I perfed ti and everything is so cramped together I can't see what transistors I'm using unless I disassemble.  BUt it IS a 3-transistor version (so not GGG), and the Speed pot is 25k, if that helps.

[bluesdevil]

Hi Mark - I'm using the ROG perf layout. I still have the wire disconnected from lug 3 of the depth control, so I'll keep trying different clippers before I give up. Thanks!

[Mark Hammer]

Just in case there was any misunderstanding, the LEDs are in series with the LFO signal fed to the depth pot, and do NOT go to ground.  This is not necessarily directed at you BD, but at anyone reading this who may be more accustomed to seeing back-to-back LED pairs used as clipping diodes to ground.

[bluesdevil]

Yes, in series between the 120k resistor and lug 3 of depth pot.