Phase 90 resistor question(s)

[Govmnt_Lacky]

So I built the Tonepad Phase 90 and I must say that I was able to dial in a SWEET phase effect with some precisely matches 5952s. I could not be happier with this setting.
One the other hand, the ONLY mod that I performed was to install a SPDT to switch a 22K feedback resistor in or out of the circuit (to get the script/block logo)

So this leads me to my problem/question:

With the resistor IN the circuit - I get a decent effect however, there is a sudden THUMP at the end of every phase cycle 

I would like to keep the switch available but I want to get rid of the THUMP.

Do I need to use a 24K resistor instead of 22K? Would it make a difference?

Do I need to dial-in the trimpot when I switch back and forth?

Has anyone else ran into this problem?

[Mark Hammer]

Along with the notches, a phaser will introduce peaks around the notch if there is any feedback.  The 22k resistor in question allows some of the signal to be fed back to an earlier stage.

Now, while the 10k input and feedback resistors in each phase-shift stage make them nominally unity gain stages, my guess is that, like me and so many others, you are using 5% resistors to do that job, such that there might be some gain in there without you realizing it.

That is, in fact, why so many phasers have historically included a trimpot in conjunction with the feedback/regeneration/resonance/intensity control.  You could never predict whether the highest regen setting was going to be too much or not, so each unit was calibrated by setting the regen to max, and then trimming back until the howling (or thump) stopped.

It is also worth noting that the bias trimpot on the board partly determines where the notches/peaks occur in the spectrum, such that you can shift the range up or down a bit to where it suits your tastes.

So, the solution to your problem can have as many as 3 elements:

• use a trimpot to set the amount of resonance just right
• tweak the bias trimpot to move the sweep maybe a little higher
• insert a cap in series with the feedback path so that there is less feedback in the bass range than for the rest of the spectrum

[Govmnt_Lacky]

Thanks Mark!  You are Phaser wise beyond your years!

I am hesitant to tweak the trimpot as I already have a SWEET phase WITHOUT the 22K in the circuit.

I guess that leaves me with experimentation on the correct value of the feedback resistor.

[zombiwoof]

Thanks Mark!  You are Phaser wise beyond your years!

I am hesitant to tweak the trimpot as I already have a SWEET phase WITHOUT the 22K in the circuit.

I guess that leaves me with experimentation on the correct value of the feedback resistor.

The 22k resistor was not the only component difference between the script and block version of the Phase 90, there were a few other components in the later version that had the feedback circuit.  Maybe that's why just adding that resistor in doesn't get you there.

Al

[Govmnt_Lacky]

Ok. After doing a bit of research, I have come to the conclusion that although there were some cap/resistor changes between script and block versions..... it is the Feedback resistor that is the primary difference. So, the question that I need answered from those who are familiar with the circuit and/or build is:

Which would be the best way to get a smooth sweep with BOTH options?

Should I leave the 250K trim as is and install a 50K multi-turn pot in place of the 22K feedback resistor and just "dial in" the sweet spot in the feedback or

Should I try adjusting the 250K trimpot to find a happy medium with the 22K that is installed and perhaps sacrifice some of the sweet range that I have dialed in right now?

Any recommendations from experience?