Phasers (AW: noise of arrays)

Haible_Juergen#Tel2743 HJ2743 at denbgm3xm.scnn1.msmgate.m30x.nbg.scn.de
Mon Jan 22 21:20:00 CET 1996


>I built a phaser based on the ARP ladder in 1988. I did try
>substituting the 3086s for BC109s, but for some reason it didn't
>work very well. I'm not sure, but I probably used BC109Cs, which
>have a much higher beta than the 3086. I then tried using BC184Ls
>which worked ok,

Thanks Joachim, that's good to know! I think I'll stick to arrays, too.
(Haven't got the time to experiment. Just asking on DIY for suggestions,
drawing a PCB on the Atari, etching it and hope it works or only needs
minor changes, is all I can do.)

> but the circuit was still quite noisy, so I
> stuck with the 3086s and put NE570 companders on the input and
> output.

I thought of using the original compander. Have You made comparisons
with the 570 and the ARP circuit, too?

> For the post-amp I used a differential amp built out of
> 3/4 of a TL074.

Funny, that's where I stopped and took breath for a moment, too,
when I looked at the schematics!
Took me some time to figure out how the ARP circuit works. Seems
that the DC balancing is done with the bipolar transistors (array ...), 
while
the AC path is determined by the FET's ... maybe two discrete FETs
could work here as well (?).
An Instrumentation amp built of 3 FET-opamps would be ok, too, I
think.

All in all I think I will be as close to the original as possible - call the
project a "vintage ARP" phaser (;->).

There is this other phaser, however, which I have in mind since many
months, that should be more on the "High End" side:
This one will consist of 12 Stages like the Moog, but  with SSM2024
as gm stages. And instead of *switching* between the different stages
like the Moog (one switch for output mix, one switch for feedback),
I will use two Interpolating Scanners for this. So I can choose to use
the scanners in a static way, just as switches, and modulate the all pass
line, *or* I can  keep the all pass line fixed and modulate the scan 
position
(similar to the Hammond Chorus Vibrato). Of course I can also modulate
both at the same time, from different modulation sources ...
I will make the second scanner path switchable between Feedback and
a second output tap (for true stereo effects).
This will need 3 x 12 OTAs. I ordered a bunch of SSM 2024's today ...

JH.



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