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December 20, 2013, 03:31:34 PM

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DIYstompboxes.com
|  DIY
Stompboxes  |  Building
your own stompbox  |  Adding
more stages to a phaser

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Topic: Adding more stages to a phaser  (Read 919
times)

Pedro
Freitas

Posts:
73

Adding
more stages to a phaser

« on: February 12,
2004, 07:12:19 PM »

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

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Michael
Allen

Posts:
459

Adding
more stages to a phaser

« Reply #1 on:
February 12, 2004, 07:26:55 PM »

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....

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Michael

http://www.freewebs.com/loudnoise/
http://photobucket.com/albums/v368/soma_hero/

ExpAnonColin

Posts: 2208

Adding
more stages to a phaser

« Reply #2 on:
February 12, 2004, 08:13:04 PM »

Once again the day is saved by
RG keen.

-Colin

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Mark
Hammer

Posts:
21032

Adding
more stages to a phaser

« Reply #3 on:
February 13, 2004, 10:28:44 AM »

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.

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Pedro
Freitas

Posts:
73

Adding
more stages to a phaser

« Reply #4 on:
February 13, 2004, 02:31:57 PM »

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

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