MXR Phase 100 mods

[jreading]

Hi all,

I'm new here and after searching the net countless times for mod info, I joined. I've done tons of mods over the years, built kits and dabbled a little in circuit design, but recently I've gone hogwild with mods on all my old pedals.
I was wondering if anyone has any info on modding a rev. e dunlop phase 100. Any need? Anything comparable to the r28 phase 90 mod. I was hoping to add a variable resistor...

Thanks!

[Mark Hammer]

The schematic for the original (I have no idea if Dunlop used the identical circuit beyond the status LED) can be found here: http://www.generalguitargadgets.com/pdf/ggg_p100_sc.pdf

The 4-position switch does two things at the same time, that are similar to what the Color switch on a Small Stone does. Both switches adjust how much resonance or feedback there is at the same time as adjusting the sweep width. So, of the 4 positions you can have wider sweep with and without resonance, and a narrower sweep with and without resonance.

The Phase100 has 10 phase shift stages, 4 that are fixed (IC5b, IC6a, IC3b, and IC2a), and 6 that are swept by the modulation/LFO circuit (IC6b through IC1a). Look closely and you'll see that the output of IC2a goes to the switch and from the switch through a 15k resistor to the input of the 3rd phase-shift stage (IC5b). The resistor is mixing the output of the second last stage back into a point earlier in the chain, and that's what produces the resonance.

The amount of resonance is set by the value of the resistor. We're going to changethat from a fixed resistance to a variable one, to produce variable resonance/Q/feedback. Ideally, the feedback should never be mixed in at a greater amplitude than the input signal for that stage. Since the input and feedback resistor in each stage is 10k, the feedback signal cannot pass through a resistance less than 10k or else you risk oscillation/howling. However, there are many degrees of resonance in between 15k and open circuit (the other switch position), and it is a reasonable mod to try and get them. Ideally, you'd want something that is around 11k as the absolute minimum value in place of the 15k unit, to prevent any howling. Or you can use a 10k value that measures a little over 10k, and let the added control do the heavy lifting. A pot value of 25k or 50k is a good choice, though you may find that you want 100k if you wish to have negligible resonance (and 100k will be far enough away from 15k to be almost like open circuit).

The other thing you can do is remove the 20k Centre Freq trimpot from the board, and substitute a 20k or 25k panel-mount pot to dial in the "zone" covered by the nothces.

[DeepBlueC]

Mark, is it possible to also set up a pot to vary the sweep width?

I have a '78 block Phase 100 that I still love, but I've always wished I could separately adjust resonance/regeneration and sweep width/depth rather than being stuck with four positions. I've been thinking about grabbing a GGG P100 kit and trying to create something unique... The one other thing I would be interested in that you didn't mention is a Level trim; when I've experimented with tweaking the center frequency trim on my original, it has affected the overall level slightly, so that would be good to have on tap as well.

[Mark Hammer]

In principle, it ought to be possible. But I have to say that the circuit is not intuitive for me. I'm looking at the switch and trying to figure out how to institute variable width out of it, and I just can't see it. My guess is that the two width positions that form part of the 4-way switch function do something mre than merely adjust width. Do 2 of the positions sound like the range is shifted over a bit as well?

[DeepBlueC]

I just A/Bed them and no, the center of the sweep sounds the same, only with wider range both up and down. Very much like the circular parts of the Intensity symbols... although I obviously can't speak to any of the plastic or reissue versions that came later. Could it be as simple as R18?

[jdsleep]

Hi guys, I thought I would chime in here since "DeepBlueC" clued me in to this thread. The kit we sell has frustrated many builders and I hope to put an end to this agony very soon. I will try to make it up to those who have purched the P100 kit in the past. We should be posting a much improved project for this in the very near future and it should be much easier to build and should work well and hopefully have some mods (as many as I can figure out) .

So that being brings me to the point of this post. The new proto PCB will be here this friday. If all goes well with the basic build, I plan on trying every mod that I can think of on it. Maybe next week I can report some findings.

Mark, maybe you can comment on this, Here's the mods that I can think of as I see it (some could be wrong):
1- As you mentioned on the Regen - replace R1 with a pot.
2 - Depth/Intensity control - Replace R19 with a pot.
3 - Vibrato/Phase switch - on/off toggle switch after R18

I don't understand what the other side of the switching is doing, so I haven't figured out what can be moded there.

JD

[Mark Hammer]

What has not been established is how closely the Dunlop reissue conforms to both the circuit and component numbering of the original. I don't expect it to change much, but if, for instance, the wet/dry mixing is done via a pair of resistors that aren't 6k2 and labelled R18/R19, I would be hesitant to send someone off in that direction.

That's not to say it isn't the same. I would suggest holding up at first base to see if the coast is clear befopre trying to steal 2nd base.

The list of potential mods is considerable, given the complexity of the basic circuit. It comes with 6 swept plus 4 fixed stages of phase shift, but there is no reason why one could not select between different numbers of phase-shift section to get different sounds. You'd need to know a bit about the rerouting of the feedback signal, though, to get it right.

One suggestion not mentioned yet is the panel-mounting of a 20k-25k pot to make the centre frequency adjustable. This can do a lot to making the sound produced more gurgly or glassy. Just keep in mind that the entire range of that pot may not be usable. Even so, if 40% of the rotation is of use to you, that's better than nothing.

The modulation circuit in the bottom half consists of the same basic op-amp based LFO found in the Phase 90. The rest of it is some form of voltage to current conversion to drive the LEDs in the optoisolators. I gather what that means is that any mod you can make to the P90 LFO can be made here too...within limits. So, ring-modulation-like sounds can be produced by making the .01uf cap in the feedback part of the LFO (C16) smaller (e.g., .0033uf). Be reasonable in your expectations because the optoisolators may not be able to sweep that fast.

The variable depth (i.e., how much wet is mixed in with dry; JD's suggestion #2) is also useful for those who like their phasing on the subtler side.

[Mark Hammer]

A posting on the DIYStompbox forum this morning confirms that the pair of 6k2 mixing resistors are indeed used on the Dunlop issue MXR P100. What the poster also notes is that if one replaces those with a 10k linear pot (outside lugs to where each resistor "comes from" and wiper to where they meet), you can adjust the wet/dry blend to get variations of subtler phasing or vibrato.