Adding a 90/45 switch on a MXR phase 90 Rev.F

[nooneknows]

Hi,
bought this phase 90 second hand, quite good, already did the script mod (cutting R28), I'd like to put a switch to convert it to phase 45, shorting the last two stages, I'm getting trouble finding the correct points to solder the two wires on my Rev.F board, full of SMD, has anybody done it already?
thank you in advance

[Mark Hammer]

SMD boards are an absolute bear to mod.  Partly because the component values are sometimes hard to read, but also because the boards tend to be double-sided, making it hard to decode what components are doing what, when there are several of the same value distributed around the board.

Note that one of the major differences between the 45 and 90 is the use of additional components around each FET in the 45 to prevent clipping.  The P90 does not have these, although they do not change the basic properties of the notch.  Additionally, there is the matter of removing the feedback from stage 4 to stage 2, that is found in the P90.  A 3PDT toggle shold be able to do the conversion for you.

[nooneknows]

I was thinking about doing something like this, I don't know the source but it seems reasonable:




fact is I don't know which of the TL064 op amp are the 2nd and 4th stage and where the R is located...

[Mark Hammer]

It looks like it should work.  Note that it omits the additional components of the Phase 45.  No big whoop, just not "authentic".  Note as well that it also omits feedback entirely.  Again, not a problem, just not authentic.  The vibrato/phase switch will work.

I will also recommend replacing R26 with a 500k pot (wired as variable resistance) and a 510k fixed resistor, in series.  This will allow you to adjust the offset or "range" of where the phasing takes place.  It's a simple and very useful mod.  Set the pot/resistor combo for max resistance (around 1M in total) while tweaking the trimmer.  Once you've set it for the lowest, most "gurgly" phasing, THEN you can start playing with the 500k pot.

[nooneknows]

so the R26 mod you suggest acts as a sort of depth control? that would sound really good, one of the 'issue' I have is sometimes the excessive swirl on the oscillation peaks

[Mark Hammer]

No.  R26 sets how much bias current is applied to the JFET gates.  Think of it the way one uses the initial delay setting on those flangers or chorus pedals that include one.  Varying the bias current lets you shift the region in the spectrum where the notches/peaks are moved around, from low and "gurgly" to higher and "swirly".

The extent or "width" of the sweep is set by the 3M9 resistor, coming from the LFO.  That limits how much current is applied to the gates from the LFO.  The LFO current sums with the bias current, such that the FETs change their drain-source resistance, starting from THIS value,and extending out to THAT value.  MXR designed their phasers to have a fixed sweep width that would be suitable for all speeds; not too wide and not too narrow.

[nooneknows]

could the double 150K R be used to mix less wet signal? for example connecting 2 100K in place of the 2 150K to the ends of a 100KB pot and the center lug to the 150K output resistor?

[Mark Hammer]

You can certainly vary how much of wet or dry is combined in the final mix, though it remains to be seen how much of each can be attenuated and still get a desirable sound.  Because chorus yields so many more notches, I often include a wet-level pot to put the wet signal more in the background.  But that's chorus, not a 1 or 2-notch phaser.  In the absence of feedback in the version you're building, the phase-shift/wet will be rather subtle, so reducing the wet level even a little will quickly make the phasing effect almost inaudible.  At least that's my bold prediction.

So yes, one can do what you're suggesting.  Worth the effort?  Personally, I doubt it, but it's your pedal and your aesthetic.  Certainly a simple dry lift, to achieve vibrato will be worth the effort and easy to implement.

I don't wish to dampen your enthusiasm.  But you may find yourself singing "Is that all there is?" along with Peggy Lee after you've put in the effort.

[iainpunk]

i know its not a phase 90 but i have modded my green Russian small stone phaser with a mix controll, one knob from full wet to full dry. i like being able to control the mix, but on the other side, i mainly use it with way more wet than dry, to give a bit of that vibrato, the other side (more clean that wet) is just a bit too bland and gets drowned out quite easily. the only setting i like the clean half of the pot is when the rate is maxed out, normally, with maxed rate, it get really oppressive and overwhelming, so turning the mix down works wonders, but that's a very specific situation, not commonly used. i think i'd be better off if the pot only covered the 50 to 100% wet range so its easier to dial in.

cheers, Iain

[nooneknows]

I was thinking about doing something like this, I don't know the source but it seems reasonable:

just for the records, it worked really fine, I had some trouble identifying the resistors but the switch actually convert the 90 in something really close to the 45, enough for my taste at least.

Unfortunately I broke the fiber pot base bending it back to box the board again (a resistor where one of the wire is soldered is under the pot, but this mxr pot is really really cheap, never seen a quality so low), I used a 100kB instead, I currently don't have a 470KC around, so my happiness is not so high as it could be.

As soon as the new pot I ordered will arrive I'll took some pics of the mod, with the position of the two resistor to use.