A few experiences with Phase 90 mods

[Ptron]

I just built a phase 90 knockoff using the Tonepad layout. http://www.tonepad.com/getFile.asp?id=42 At first I wired pots coming out if it every which way to try a bunch of different mods. I was kind of surprised by the results.

So called intensity control – I replaced the red 1M resistor with an  820k and 1M pot in series. If the bias trimpot is set right, the pot has very little effect. Maybe someone can explain to me why it should. Doesn't this resistor just feed the bias voltage to the jfets?? If the bias trimpot is mal-adjusted the effect is drastic. From no to full-out phase but the "shape" of the sweep is whacked. Seems to me there has to be a better way to add an intensity control but I'm a little confused by the topology of the LFO and how/where its signal gets to the gate of the jfets.

22k feedback resistor – I replaced it with a 250k pot. Someone posited somewhere (sorry I don't remember where) that the purpose of this resistor was to compensate for unmatched fets. Mine were all matched within a couple percent. This pot seemed to alter the tone a bit. It made it darker when the resistance was higher. However the it did not have a big effect until I got to the lower settings. If I turned it low enough the effect would squeal which is odd because it's negative feedback. (Hmm, did I have that backasswards?) Anyway I measured the resistance where I liked the sound best, and it was a little over 30k, oddly enough. Not to far from the 22k everybody says should be taken out.

Mix – I replaced the two 150k resistor that mix the clean and phased signal with a 250k pot and two 27k resistors, clean and dry to outer lugs via 27ks, wiper to output. I found this a lot more effective than the intensity control. Still would like to try a genuine intensity control if I could figure out how to do it.

[Belt]

I am interested in adding intensity control.  Have you any suggestions?

[petemoore]

  See Tonepad, IIRC [but I don't, partially because there is no schematic to my build]..
  There is a 1m resistor IIRC between bias pot wiper and the Jfets.
  I believe the byoc 'depth' pot has made that resistor a V/R pot.

[Belt]

I have looked into the BYOC phaser and think I will like that pedal.  It seems to have all the flavor that most people are looking for in the phase 90.  Thanks

[petemoore]

  I think the 741's make it sound a little different than using dual opamps, the depth mod is a nice control.

[Belt]

how would the wiring for a 741 differ from a dual opamp

[tungngruv]

dual opamp

single opamp



To use a TL072 instead of an LM741:

The connections that connect to Output pin 6 of the 741 connect to pin 1 on the TL072

The connections that connect to V(+) pin 7 of the 741 connect pin 8 of the TL072

The rest of the connections are the same.

Always use IC sockets on yourboard. That way, you don't expose the IC to heat when soldering and you can always substitute a different IC later. For example you can try an RC4558 or TL082 etc...