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Vintage threads from the first ten years

MXR Phase 90 tips?

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6/20/1999 6:22 PM
Mike Burgundy	MXR Phase 90 tips? Anyone built this? I found the schems at RG´s (90/180, 180plus); what IC´s are used in the original? The biaspot regulates depth of sweep, or am I mistaken? Any specifics I should watch out for - it all looks rather straightforward, but you never know...Thanks!

6/20/1999 9:00 PM

... well, I have...

The "original" was actually three originals as I understand it. The first version used 741's, the second used TL022 duals, which are a low power dual, and the third used 1458's, I think. In any case, the last two versions were dual opamps of the 741 ilk.

The bias pot sets the particular matched set of JFETs to the proper bias voltage to get their drain-source resistances in the range that makes for good phasing. If this is set too high, the JFETs are all almost off, and the resistances are too high; if it's too low, the resistances are too low. In either case, the phasing does not sound good as the cancellation notches are above or below the guitar range.

This also points out the need for matched JFETs. If the JFETs are not matched for resistance at a specific Vgs, then the phase cancellation notches are not present, as the phase shift contribution for the JFETs begin at radically different LFO sweep voltages.

There is a JFET matching circuit on GEO as well. You need low Vgsoff JFETs. The original 2N5292 are unavailable as far as I know, but the 2N5385 works. I bought 100 of these to make matched sets against the day I could offer board and parts kits for the effect.

The PCB for the p180 turns out to be pretty tightly packed, too tight for a beginner to do with toner transfer, so the supply of p180's from GEO will have to wait until I can get boards made.

6/20/1999 9:14 PM
Joe Fuzz	Mike: the URL for the JFET matcher is: http://www.eden.com/~keen/p180fets.gif R.G.: can you give us some nominal values to look for at the test points? Perhaps it's up at the site and I just missed it.

6/20/1999 11:51 PM

Thanks!

I suspected the FET´s needed matching (darn!)

I was thinking along lines of 3x 4558/TL072, 1458+4558, whatever. Any *minor* differences in sound detected? I doubt it, but...

On the tonertransfer; thanks for the tip, but I think I´ll enjoy designing a PCB for this one. Just for the record; how tightly packed did you get it? And what about that very tight Univibe version? I´m still curious how tiny you got that...

6/21/1999 3:19 PM

The exact opamp seems to make almost no difference except in residual noise. This is what we should expect - opamps that are not driven in a nonlinear region or too fast for them to respond are all dominated by their gain-setting elements and sound very much alike.

The Phase 180 board came out 2.95" by 1.95" and the neo-vibe plus came out 3.25" by 1.925". The packing density on both is about the same as the original P90, which fit in a 2.1 by 2.5" board; however, the p90 was a double sided copper board, and these are both single sided. It should be pretty simple to get a phase 90 onto a 3" by 2" board, which will fit into a Hammond 1590BB (following the first rule of PCB layout - figure out how it fits in the box FIRST).

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