Mutron

[Sean Costello]

At 12:24 PM 2/26/98 +0100, JH wrote:
>Thanks to all who offered Biphase schematics.
>I got a copy yesterday.
>
>* On the schemos is the ARP logo - was Mutron part of / bought by ARP ?

Bought by ARP in the late 1970's, I believe.

>* The schemos suggest that 6 LDRs share one LED - did they manufacture a
>special "hex Vactrol" then ?

Probably.  My Mutron Phasor II has what looks to be a "quad Vactrol" in it -
a weird black disk about 2 cm across and 1.5 cm thick, with 10 legs coming
out of it.  My guess is that it is just an LED with 4 photocells glued to
it, surrounded by black plastic to keep out light.  Slightly more advanced
than the Univibe, but not by much.

>* The allpass stages look rather basic - not that different to a modern
>DOD box, for example. But if all 6 LDRs track in vakue and in response
time, this would be some important advantage.

How so?  Would it make that much difference to the sound?  (This isn't a
challenge, just an honest question - I'd really like to know what the
advantages are to matched LDRs).

BTW, the Phasor II I have sounds GREAT.  Do people on this list have any
preferences for phaser styles?  LDR's, FET's, OTA's, etc?

Sean Costello

P.S.  A quick list of all phaser topologies that I know of, for those that
are interested:

- Transistor phase shift stages, with photocells acting as
voltage-controlled resistors (Univibe)

- Op-amp phase shift stages, with discrete FETs acting as voltage-controlled
resistors (Maestro Phase Shifter, MXR Phase 90, many others)

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

- Op-amp phase shift stages, with Vactrols used as voltage-controlled
resistors (Mutron Bi-Phase, Mutron Phase II, MXR Phase 100)

- Op-amp phase shift stages, with ultrasonic PWM switchable resistor
thingies controlling phase shift amount (ADA Final Phase)

- OTA's acting as allpass filters (Moog Phaser, Electro-Harmonix Small Stone)

- LM3900 configured as phase shifter (per Serge Tcherepnin's patent - anyone
tried it yet?  Looks like the easiest method of all)

- Allpass transistor ladder (ARP Quadra)

- Frequency Shifter + static op-amp phase shifting network (Harald Bode
patent for Barberpole Phaser - anyone work with this yet?  Any tips on
whether the 90 degree phase difference network can be left out for a
Barberpole phaser?)

Any other phaser methods I left out?  I left out some of the ones I really
don't understand, like the Hammond vibrato.  How about the Magnatone amps -
did their vibrato use phase shift stages?