[sdiy] Phasing circuit

[Czech Martin]

No I do not mean the dome filter, but the multi phase
network. (I know the Weaver dome filters from EN).

m.c.

-----Original Message-----
From: Magnus Danielson [mailto:cfmd at swipnet.se]
Sent: Donnerstag, 24. April 2003 20:33
To: Czech Martin
Cc: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Phasing circuit


From: "Czech Martin" <Martin.Czech at Micronas.com>
Subject: [sdiy] Phasing circuit
Date: Thu, 24 Apr 2003 19:06:20 +0200

> Has anyone built a "phasing" circuit for audio applications
> (instead for amatuer radio)?
> 
> This is a passive RC network wich will deliver four signals
> with 90 deg phase difference on the outputs.
> It is driven by balanced input.
> 
> It is described in "Art of ELectronics" and was supposed by Weaver.
> http://members.tripod.com/michaelgellis/mixerscom.html
> figure 14.

Please see the Dome filters/phase splitters of your favorite frequency shifter.
Look either at JH's site for his FS-1 or at my site for either the Moog/Bode
FS or among the Buchla schematics that I have.

These filters all build on the principle that you have a number of
1-pole/1-zero allpass filters in sequence to create two phase-slopes, which
are 90 degrees in phase different from each other.

> Isn't the Hammond vibrato circuit very similar??

No, not really. There you have a multitap lowpass filter used as a delay line
and use a capacitive interpolating scanner to scan between the taps.
Go and fetch the Hammond schematics and check for yourself. The Hammond vibrato
actually performs a form of pitch-shifting effect this way, and since it runs
on the same synchronous motor it is also locked to all the tones ;O)

> I like the circuit because of matching.
> OTOH it has an amplitude "hump" which would need some
> compensation at the imput.

This is probably why the dome-filters got used early out for good suppression.

I haven't analysed that curcuit very much but also been attracted by its
simplicity.

If someone has good reference on the Dome filters and particular on the
original conceptions, please send me them since I want to read that.

Cheers,
Magnus