phasers (was: Mutron)

[Don Tillman]

   From: Haible Juergen <Juergen.Haible at nbgm.siemens.de>
   Date: Tue, 17 Mar 1998 16:34:53 +0100

   This was a great list of phasers you posted !

Yes indeed!

   > - Op-amp phase shift stages, with CD4069 (operating in linear
   mode) used as 6 voltage-controlled resistors (Don Tillman's clever
   design) 

   Never seen this one - has Don published it anywhere on the web ?

Nope.  (I really need to get around to writing some of this stuff
down!) 

Many years ago, around 1974 or so, I designed a phase shifter for a
small electonic music company.  It was basically a copy of the classic
4-stage MXR design but with a couple improvements -- one improvement
was an extra buffer stage (I forget the details here) and the other
was that the four individual FETs were replaced with a single CD4069
CMOS hex inverter chip.  Leave the Vdd terminal unconnected and you've
got 6 matched N-channel MOSFETs with their sources tied together to
the VSS terminal, their gates are the inverter inputs, and their
drains are the inverter outputs.  The FET bias circuitry was the same
as on the MXR; a little trimpot.  I suppose we could have gotten
really clever and wired up one of the unused FETs to automatically
control the bias voltage, but it was nice to be able to adjust the
bias to personal taste.

The idea of using the FETs in a hex inverter wasn't entirely original,
I saw it metioned in an old issue of Radio-Electronics magazine or
something, but I had never seen it applied to a phase shifter before
or since.

The advantages are that it's very inexpensive and the FETs are
matched. 

Ampeg wanted to get into the guitar stomp box business and ordered
1000 phase shifters from us, these were black metal boxes with white
printing on them labeled as the "Ampeg Phazer", but they decided later
not to continue in that market.  (If anybody sees one of these
around...) 

  -- Don