EH Pulsar....mod to stereo?

[axg20202]

Subject line says it all. I have one of the now-discontinued EH Pulsar Reissue pedals. Does anyone know if this circuit is in any way similar to the more recent stereo pulsar pedal and, if so, is there a relatively straightforward mod I can do to make my pedal into a stereo panner? I'm guessing it would require tapping off some of the existing output and making this signal 180 degrees out of phase, but I have no idea how to do this. There's plenty of room in the pedal for a daughter board!

[puretube]

Yes I know all and everything about it,
and have a valid NDA with E-H about these items.

I am not willing to share details of my developments that touch mentioned trade secret.


I have asked the site administrator in the past, to support my interests
of not letting publish my technology on his forum.

Thanks for understanding,
Anthonie Barmentloo, aka: "puretube".

Happy Easter, nevertheless  .

[axg20202]

No problem. You can't contribute and that's fine.

If someone else could contribute this mod it would be much appreciated - I can't see why sharing information about modding this pedal would be any different from the hundreds of other mods posted on this forum which relate to commercially available pedals as long as it doesn't infringe any copyright.

Thanks and Happy Easter,

Andy.

[darron]

i doubt there's an easy way to do it unfortunately.

there's a schematic here:

http://www.tonepad.com/project.asp?id=14

a lot of it would need to be doubled i guess. maybe use some double gang pots. of course you'd need them both to run off the same lfo though, they'd go out of sync pretty fast i think!

not a very good answer, but it gives you an idea and hopefully someone will give you a proper answer.

[R.G.]

I don't know if the new pulsar is similar to the old pulsar, but the old one is easy. The modulator of the old one is an NPN transistor based volume control stage. The LFO is an astable oscillator. Astables have two out-of-phase outputs, only one of which is used in the pulsar. If you build a second everything-but-the-LFO section and drive that from the second output of the LFO, you get a stereo output.

Note that this is going to be rough and not pretty sounding, as the duty cycle of the astable departs from 50-50 at low speeds.

For a better approach to stereo panning, see "Panning for Fun" at GEO, http://www.geofex.com.

[axg20202]

Thanks folks. I have checked out other stereo panning trems but I really want to mod this one if its doable. The reissue Pulsar is quite different from the original circuit I think. The reissue is based around a TL074, an LM324 and a CA3080 (an OTA). I was kind of hoping that I could somehow tap the output of the existing board, send it to a basic circuit that reverses the polarity, and send this to a second output. Or am I talking through my hat here??

[slacker]

You could tap the output and reverse the polarity but that wouldn't give you a stereo trem in the way that you want. That would just give you 2 signals out of phase with each other, they would still get louder and quieter in sync with each other, so you wouldn't get panning. To make it pan you need to do what RG said, find the output of the LFO, invert that and use it to drive a copy of the audio part of the circuit. You could do this to an existing pedal but you'd be adding so much extra circuitry to it that you might as well build your own pedal. 

[R.G.]

Your hopes are too easy.

If that is the chip complement of the new pulsar, then the 072 does input/output buffering and signal path work, the 3080 is the gain control mechanism, and the LM324 makes the LFO signal. You can't easily just hook a couple of things up and have an inverse-working stereo output. You would need a second 3080 and possibly a second half of an 072 and some work on the 324 LFO to get the second channel.

If it is OTA based, the result will be very like my article on "current controlled amplifiers" from February 04.

[axg20202]

Thanks slacker and RG.   RG - I hadn't seen your article. Looks like its just what I need. So, to utterly simplify what might be required.....I would need to tap off the LFO output from the existing LM324, use one half of a TL072 to invert the control signal and drive a second 3080. I can see this might be a lot of effort but I'm mad enough to try  .  Could be a hiding to nowhere - I can already see that the depth pot controls the amount of LFO signal into the 3080 - not sure how to double this up without swapping the depth pot out but I'll have to see.

I'm going to try to disect the circuit regions and find the sections I need to duplicate and if it's possible without doubling up the whole kit and caboodle! Could take me a while and I'll probably end up binning the idea, but what the hell.

Thanks!

[Processaurus]

Slacker, going with your idea, what if you mix the tremmed output signal with the dry, 180 degrees out of phase (on the actual tremmed signal, not the LFO)?   Total cancellation of the new channel when the first channel was peaking.

The weirdness would be that it would be symetrical only at max depth...    Otherwise the new output would be quieter over the entire cycle.  A solution would be to always have it at max depth, and pan in the dry to both outputs to get less depth.