EH Polychorus: What makes it poly? + a couple inappropriate mod ideas

[Processaurus]

An incredibly generous friend gave me one of the stereo reissues last week, out of the blue.  I was wondering if anyone knew about what (if anything) makes this design different than other, simpler choruses that use a single BBD.  Also is it much different than the old Echoflanger/Polyflanger/Polychorus?  What does the Sweep Filter switch do?  What does the flip flop do? I couldn't find much information on it by searching, beyond that it has a lot of parts and would be hard to clone.

looking at the original designs 1978 schematic on Marcos Munkey's site http://www.geocities.com/diygescorp/  I can see one strange option on it where the input gets delayed by 2 1024 stage BBDs, then gets mixed with the input, at which point that mix goes into another 1024 stage BBD, and then that result gets mixed with the input.  Weird.

Theres a good amount of room in the chassis for some mods, plus the power switch could do something else, and theres a good spot for another footswitch if theres something that warrants it.  I liked the idea Vanessa had suggested in another polychorus thread about integrating a longer delay circuit to it.   Also there was this thread a while back about hybrid digital/analog delay that got me thinking.
http://www.elixant.com/~stompbox/smfforum/index.php?topic=31886.0
One could insert a PT2399 based digital delay in there so that each repeat has to go through that AND existing BBDs, filtering, and companding.

A Dry/Wet mix is is easy, and would be very useful for when you don't want the full poly experience.  That would be least I would inflict on this box.   Also, would a dry lift switch for vibrato sound good, or if it would need a more sinusoidal LFO (which wouldn't be hard with an XR2206 chip, especially since there is already a regulated 15v to supply it)?

Some kind envelope control of the flange would be cool too, to help with the boredom of a cyclical LFO.  Or a random LFO, like the Randomatic.  At that point the XR2206 might come in handy because both the frequency and amplitude are voltage controlled.

I already swapped out the footswitch to a 3pdt, but was happy to leave the tone sucking bypass arrangement, because the input impedance is so high (2M).  The power LED got changed to a bi-color type, to show if its bypass state.  It looks cool with the green LED showing that its on, because its kind of a green sound that comes out of the thing.

Thanks for any suggestions anyone with experience with this box can make, or crazy ideas for other mods.

edit: wrong chip #s

[nelson]

I have cloned the original echoflanger, I also own an original Polychorus.

I have the PCB files etc in a PDF if you want to compare to the reissue. I have always wondered whats different between the two.

The polychorus is my favourite Chorus effect by far, a mix knob wouldnt go amiss sometimes though.

Increased delay time would be ok, but I think that the double track/slapback delay is fine short, if you want a delay buy a delay pedal not a modulation pedal.

The LFO mods would be worthwhile. Random, Envelope, and sinusoidal.

A panner circuit on the stereo outs would be a worthwhile mod.

Since there are two BBD's there its concievable with a daughter board and some CD4053 switching you could get through zero flanging.

A rev eng schematic for this pedal would be nice for educationial purposes. Just to see what the differences are.

[Processaurus]

Thanks for the reply, Nelson.  Congratulations on cloning a circuit that complex. 

if you want a delay buy a delay pedal not a modulation pedal.

Ahh, that makes waay to much sense.  I guess the charm of pedals is that they do one thing, as opposed to every thing.  Gotta watch out for those creeping features.

Since there are two BBD's there its concievable with a daughter board and some CD4053 switching you could get through zero flanging.

That could work great, but I would be scared to put in the time to do that and have problems with two different clock signals heterodyning, maybe instead a very short digital delay on the dry signal would avoid that possibility, by having the two clock frequencies be far different rates.

[nelson]

Thanks for the reply, Nelson.  Congratulations on cloning a circuit that complex. 

if you want a delay buy a delay pedal not a modulation pedal.

Ahh, that makes waay to much sense.  I guess the charm of pedals is that they do one thing, as opposed to every thing.  Gotta watch out for those creeping features.

Since there are two BBD's there its concievable with a daughter board and some CD4053 switching you could get through zero flanging.

That could work great, but I would be scared to put in the time to do that and have problems with two different clock signals heterodyning, maybe instead a very short digital delay on the dry signal would avoid that possibility, by having the two clock frequencies be far different rates.

I understand the apprehension. however it would be relatively easy to do, if you already plan on putting your function generator in there. Given the double track setting already available to you. It also wouldnt be that difficult with even the existing circuitry, with some intelligent trace cutting and switching. I am only going on the vintage unit, the reissue could be substantially different. Given that Ron Howard has worked on "improving" it.

When I finally release my condensed layout for the echoflanger I plan on including a through zero mod for it. Aswell as stereo, mix and a simple sinusoidal LFO mod.

I say, if it was given to you, you should make it your frankenstien. The polychorus I have and my clone is an amazing noise maker. It deserves to be modded to its full potential.

I wish I had the time to realise all my ideas immediately. Damn my need for instant gratification.

The easiest mod would be to mess around with the feedback trim and get modulated delay oscillation sounds.

Like a UFO landing with an L plate learning to park.

[delusionboy]

HIya

I have a 90s reissue big chasis poly chorus that ive modded a bit with a trim pot for using it live with keyboards (its pretty much glued to my DX7)

I read here you managed to get a duel colour LED to show bypass state, Id be very interested to figure out how to do that. What is the jist of doing that mod?

thanks!

[pinkjimiphoton]

if using a standard stomp switch and true bypassing it, wire up the led cathodes to two of the unused outer poles.. whichever color you prefer. put ground to the middle. wire the anode to a current limiting resistor to b+.
now in bypass, the led will light one color, in operation the other.
you'll have to steal some voltage off the power supply to power the led, but its way simpler than it is to write about.