Hello!

I'm thinking of adding more stages to a P90 phaser and I would like to know if I should be aware of any sort of bias problem regarding the FETs apart from the usual setting off the bias trimpot.
Oh, I'm planning on having a chain of 16 phase stages. I gess I will have to buy the whole store's lot of FETs for matching  :shock:

yours,
Pedro

I believe RG has a schematic of how to replicate phase stages in the Phase 90 at Geo....He calls it the Phase180 if i'm correct....

Once again the day is saved by RG keen.

-Colin

Note as well that regeneration of the type used in most FET-based phasers relies on feeding the signal back to the second phase-shift stage.  You can add as many stages as you want but don't lose track of where you're feeding from and to.

Note as well that noise can add up over the number of stages you are pondering. Since FETs require use of a more modest input level to keep them from distorting, the S/N ratio is not as "protectable" by use of hot input signals.  A typical strategy for addressing cumulative hiss in cascaded stages is to stick in a small value cap in the feedback loop of, say, every 4th stage or so, just to keep things from getting out of hand.

So, if you used 10k feedback resistors, a 1nf to 1n5 cap might not be such a bad idea.  1nf with 10k starts rolling off around 16khz, and 1n5 starts rolling off just over 10khz.  Roll off the high end 3 or 4 times like that and you end up with some pretty steep filtering of hiss.

I was planning on using Fracisco Pena's layout for the P90, building a separate PCB with all the phase stages I would require and then use hookup wire to connet the Vr, V-, FET bias and LFO, In, Out and Ground traces. I already draw a PCB for 8 stages using the TL074 chips which makes a simple and small PCB.
Anyway, I already did this kind of mod on a Small Stone, increasing the  total stages to 8 and it worked perfectly. The only down side is that TL074s run cheaper  :lol:
But since I'm going to build a one off the modded Small Stone isn't such a bad idea.
Oh, and I replaced the whole transistor input thing on the SS with a simple opamp and I got no volume drop, no distortion, just beatifull phasing    The only thing I had to alter was the feedback resistor value because the phaser would oscilate with a higher input signal from the opamp.

Thanks for the replies,
Pedro