ELectro Harmonics Talking Pedal schematic

[Gus]

Circuit fragment  looks like a circuit on page 154 in the Active Filter Cookbook by Don Lancaster first edition ISBN 0-672-2116-8

For people that want to work out the math

[John Lyons]

Hmm...
The tap is simple enough if we can figure out the tapped resistance of the 11k pot.
The resistance shouldn't be hard by tapering the total resistance with a parallel resistor (assuming the pots are linear).
That being said:
Using the LED/LED controller would be good because of no pot to wear out.
I'm not sure i'm up to the task at this point. If anyone has any other ideas for the optical controller I'm all ears.

Thanks for the observations Gus

John

[Gus]

Here is an Idea

Two Leds with a adjustable constant current drive for each one.  Constant current so the LED output does not change with the power supply voltage dropping if using a battery

Two LDRs

Now two pieces of gray translucent  plastic in grooves or a tube that have a painted shape on them.  One might be a taper triangle shaped >
the other  two triangles  with the center more clear ><.

Now getting the tapers correct and have them track might be some work the constant current drive to the LEDs should help this be more stable

Maybe two  PVC tubes that the vanes go up and down in. Felt with a slot for light blocking at the top.  Two holes drilled thur the tubes one for the led one for the LDR

[John Lyons]

Ok, I get it. Thanks Gus. Those are good ideas. Similar to how some of the morley wahs work.
One thing I'm not sure about is the tapped pot. How should I go about working that in?
I'm not really sure what it's doing in the circuit.

John

[Gus]

OK the tapped pot
I don't have EH TP schematic and I have not seen the wha. So the following is a guess
from the circuit fragment
It looks like the tapped part should go a buffered 1/2 9V ref because of the 1Ks in the filters

Not knowing if the sections of the dual pot are linear or audio taper

Now if the wiper is at the tapped part of the pot trace it is close to 0 ohms from the wiper to ref so that part of the filter is at 1K

If you move CW  from the tap you add more resistance to the 1K I would guess from how the schematic is drawn more than 1/2 of the pot resistance value

If you move CCW from the tap it looks like you only add a smaller amount of the pot resistance to the 1k

[John Lyons]

Gus
PM me with your email address if you want me to send you the EH TP schematic.
I'm not sure that the tap goes to a voltage reference. There isn't any notation about doing so and the ground point symbols are the same for the the pot and tap "ground" points.

The way you describe the resistance with respect to the tap makes sense to me the way I see it. I'm just not sure how to do the LDR resistance and the tapped resistance.

JOhn