[sdiy] A hacked phasor, with vactrols

[René Schmitz]

Hi Aaron, Christine and Xavier,

> They put in a switch to give the user the option of using a 
> knob or using a control voltage to control the mix. The CV input on the 
> mix doesn't seem to give the same dramatic change as the pot does, but 
> you do hear a change as the CV mix input is swept.

See below.

> To keep things simple, I suggested they keep the original ground-9V 
> power scheme, instead of trying to switch everything to -15/+15 V. 
> What's the most _elegant_ way for them to get 9V from the +15 V in the 
> MOTM? Of course, there's three dozen ways to do it...

Resistor divider and opamp, or emitter follower (the latter can supply 
more juice!)

> They're about to start committing to solder... any last minute advice?

I think the CV input for the MIX is a bit odd. The top LED has a 300k 
resistor, where the lower has 500 ohms. I'd expect this to be more or 
less the same value. (I'd add a transistor as a follower between the 
zehner and the LEDs, so that the voltage source is sufficiently stiff. 
With a mere ~1mA running through the zehner you can't power the diodes. 
  (That transistors base to the 5.1V zehner, C to 9V, Emitter to the 
string of LEDs via 500 ohms. An emitter follower.)
I guess thats the reason for the CV-mix not being so dramatic.
Also I'd buffer the CV, not all CV sources like to supply several mA.

This will have the same problem as in most other circuits we've seen 
here that employed vactrols. Just supplying the LED current with a 
resistor doesn't work too well, because the LED has a knee voltage of 
1.6V, below that there will be no action. So the useful range of CV here 
is expected to be pretty narrow.

(The more canonical solution is a voltage controlled current source for 
the LEDs. An opamp with the LED from output to - terminal, and the R to 
the virtual GND for example. See the mail with regard to the vactrolised 
Wah. But to get the opposing characteristics other CV circuitry would be 
required.)

> (Since this isn't going into a stompbox pedal, I suggested they go ahead 
> and use the 5532...)

Go for the 5532, its a relatively low noise opamp.

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159