Phase 90 "depth" mod

[I am the Walrus]

 Hello all.  Has anyone done this "mix" mod on the Phase 90?  http://www.geofex.com/FX_images/p180plus.gif  Thanks for any input on it.

[j_flanders]

"depth" mod

"mix" mod

Could it be that you mean the "feedback" mod? It's also called the "r28 mod" or "script mod".
If so, you might find a lot more info on people's experiences when searching the forum or googling with those terms.
https://www.diystompboxes.com/smfforum/index.php?topic=78354.0 for example

[I am the Walrus]

 Thanks for the response, but no, I'm talking about the "mix" mod on the schematic. The two 150k resistors are swapped for 47ks, and a 200k pot is connected from the wiper to the transistor at the base.  I'm just wondering if the results of this change were worth while.  Thanks.  http://www.geofex.com/FX_images/p180plus.gif

[antonis]

R.G.'s mod gives you the ability of altering equal amounts of Wet/Dry signals mixing..



It's strictly personal decision if it worths the mod or not..

[Mark Hammer]

Not a fan of variable wet/dry mixing on phasers.  I do think there is value in using a trimmer to get an optimum true 50/50 mix for maximum notch depth, but I don't personally find much of value between 50/50 balance and wet-only (vibrato), so I just install a dry-lift toggle.  When it comes to chorus or flanging, the pitch wobble these produce can be distracting, so I find a wet-level pot IS of practical use for dialing back the intensity.

The "width" control shown is VERY practical, although I generally change it from a 1M pot to a 510k fixed resistor and 500k pot.  It doesn't adjust sweep width, but rather changes the offset of the phasing - the part of the spectrum where the notches are swept - from low and gurgly to higher and swirly.

The 3.3M resistor beside the "width" control in the drawing is actually the component that sets the sweep width.  Different issues of the pedal use a 3M9 or 3M3 value.  The lower value provides a wider sweep, that is better suited to slow speeds, and the higher value is what you want for faster bubbly speeds.  In my own builds, I'll use a toggle to add two parallel resistances for narrow, medium, and wide sweeps. 

It is unwise to aim for values lower than maybe 2M7 or higher than 4M3.  Keep in mind that the resistance on the "Width" arm, coming from the trimmer, and the series resistor coming from the LFO, are providing currents that sum and feed the gates of the FETs.  A higher resistance from the bias trimmer to the gates reduces that current source, and permits more current to be fed from the LFO. If one makes the offset current variable (e.g., with the 510k +500k pot I suggested), then you have to be prudent about how low you can make the current-limiting resistance from the LFO.

How to calibrate this mess:
The more combined current is fed from the bias source and LFO to the FET gates, the more their drain-source resistance drops, shifting the region where peaks and notches are created lower.  It can't go too low or else you won't hear much, if any, sweeping.  So, make the LFO and bias-leg resistances as high as they are going to be, to drop current down to a minimum, and THEN tweak the bias trimmer for the least amount of current needed to produce and audible sweep.  Probably wise to set the LFO for a reasonable fast sweep rate that  makes the modulation audible.  Once you have that, then you can play with the offset pot and lower values  of the LFO current-limiting resistance.

It's all about squeezing the most potential usable movement out of the range of drain-source resistances those FETs are capable of, without running into dead spots at the top or bottom of their range.

[I am the Walrus]

Thanks for the input, Mark and Antonis.  I think I'll try that 510k and 500k pot width mod.  Mark, does that 3.3M resistor control the speed as well? I have a CSP026 reissue and it has a 4.7M resistor in that spot. So, if I lower the value of it will I likely get some more speed out of it? Thanks.

[Mark Hammer]

Thanks for the input, Mark and Antonis.  I think I'll try that 510k and 500k pot width mod.  Mark, does that 3.3M resistor control the speed as well? I have a CSP026 reissue and it has a 4.7M resistor in that spot. So. if I lower the value of it will I likely get some more speed out of it? Thanks.

No.  It controls how far from the lowest point the sweep goes.  Smaller values will make the sweep go much higher from the lowest point.  On Many commercial phasers, that function will be a pot, and labelled as Depth or Width.

As for upping the fastest speed a bit, in some, but not all, posted schematics for the P90 (like this one: http://www.generalguitargadgets.com/pdf/ggg_p90_sc.pdf ), there will be a small value fixed resistor in series with the 500k speed pot, that sets fastest speed.  Chances are good it is 4k7.  If you drop it down to 3k3 or 2k7, you shold hear a modest increase in fastest speed available.

[Rob Strand]

Hello all.  Has anyone done this "mix" mod on the Phase 90?  http://www.geofex.com/FX_images/p180plus.gif  Thanks for any input on it.

That type of width control is good because it lets you set-up the correct range of sweep.   For example when you use FETs with VP's less than the original devices you can back-off the sweep so it sounds like the original Phase-90.     Normally that would be a trimpot, or you set it up with a trimpot and put in a fixed resistor later.

I wrote a VST phaser plug-in which has a width control like that and also an offset control.  What it those controls it lets you set the sweep so it sounds closer to one phaser or another.   (The offset works somewhat like the bias trimpot but obviously there's no JFETs inside of the software.)

[ljudsystem]

Couldn't you use small "split and blend" daughter board to mix the bypassed signal with the Phase 90?