Modular Synth Panels

On 5/12/08, wjhall11 put forth:
>OK - now you've done it!  <LOL>  Will had been asking me about maybe a
>driver circuit and I had been ignoring him.  I'm not an engineer and I
>haven't a clue how to make one.  Can you tell us how to do this?  That
>really would be fun.  Maybe we could figure out how to do it with an
>MUUB PCB and post it on our Tau Pipe construction page.  Bill

Well, I'm not an engineer either, but when has that stopped us?? :)

In order to figure out a driver circuit, we need to know exactly what
the output of the LFO is.  JH posted this to the E-M forum:

"The CV range:

Positive only, up to about +10V.
Exact value in manual control depends on exact value of PITCH potentiometer.
Exact value in LFO depends on saturation of your specific TL072
you're using for IC6. But roughly 10V."

While that makes sense, as a negative voltage wouldn't get past D1, I
don't see how the Down LED would light unless the LFO went negative.
So could someone measure the output of the LFO??  It goes to one side
of the "Osc Level" pot.  While you are at it, could you also measure
the output of the manual circuit (which goes to CV out in the stock
circuit)??  :)

Looking at the driver circuit for the MOTM-320, it uses a diode so
that the current for each colour is set with a separate resistor.
I'm guessing the green LED is inherently a bit brighter than the red
LED??  An OP-275 is a good choice of op-amp because it can output a
relatively large amount of current, but is unlikely necessary unless
you want the LED to be unnecessarily bright.

You don't need anything as sophisticated as a MUUB-2, as you don't
need trimmers or bypass caps.  Noise and precision are not issues if
all you are doing is driving an LED. I used 741's I found in my parts
bin to add LED's to my 800's.  I tapped the power where the MAT
connector is soldered to the board, and then built little driver
circuits on little pieces of plain perfboard the size of postage
stamps.  I didn't put the LED in the feedback path or bother to add
in the forward voltage of the LED's so they do not light when the
output of the 800 is very low (when sustain is below "2"), but other
than that they work fine.

So we need to find out what the output of the LFO is, so that we know
the voltage in order to calculate the current-limiting resistor, and
so that we have a signal that goes from a negative voltage to the
opposite positive voltage, so that current flows through the LED one
way then the other.  Once we know that, we can build an inverting
summing amp and add in an offset voltage to convert the signal from
0 to 10V into -5V to +5V (if that is the case).  Then take the output
and send it through the LED through a resistor to ground, or put it
in a feedback path to compensate for the forward voltage of the
diode.  Notice that unless the JH 20-Pole Phaser LFO is already
bipolar and symmetrical around 0V, we can't just borrow the MOTM-320
circuit as is.  Don't worry about inverting the signal, you can just
flip the LED around :)

Also, I don't have any bipolar LED's except the ones already
installed in my 320's, so I won't be able to test the circuit.
Although, I guess I could sort of build one from two regular LED's,
but then again, I haven't started building my phaser either...