EH Talking Pedal pot, workaround

[markusw]

The workaround is based on simulations in LTSpice and verified to work on breadboard. The idea was to find a way to replace the special dual pot only with standard analog parts and a normal single pot.

Here's a link to the original schem:
http://www.freeinfosociety.com/media/pdf/4101.pdf





Opamps are not LT1001 but TL072 or NE5532. For some reason a NE5532 worked better for the rectifier than a TL072. Otherwise I don't think that opamp type matters. 
V+/2 is done with an opamp buffered voltage divider.


Basically, the two halves of the Talking ledal pot are replaced by to BC550 transistors used as voltage controlled resistors (like in Dr. Q). Only difference is the base resistor which is much higher than the 22k in the Dr. Q.

Control voltage is generated from a normal expression pedal or Wah pot and centered around V+/2.
For the high freq filter (for which there is a normal up sweep when you press the pedal) cv is just buffered (by U7), inverted plus amplified (U8) and then fed into the base (cv1) via a 500-1000k resistor (1M trim pot).
Chaning the gain of U8 changes the width of the sweep, adjusting the resistance of the 1M pot allows for setting the start freq.
Adjustment works astoninglishly well.

For the low freq filter in the original pedal the resistance starts high (5k5; assuming that the additional tap of the pot's second halve is at 50% in the original) , goes to zero and then back up to 5k5.
For the workaround cv is therefore precision rectified (by U10 and U11) which gives a V-shaped cv, which is inverted to an A-shape and if needed amplified (by U12). The resulting cv2 is fed into the base of  the other BC550 via a 1M trim pot plus a 1M resistor.
Freq adjustment of the lower filter is done as for the other one.

Tweaking the filters btw works pretty cool with Visual analyzer software.

Response however is a bit different that original (at least according to the simulation).
For the workaround freq travel of the filters is more or less linear (also in the sims), for the original it seems to be more log-like.
See modified image below taken from R.G.s site (I hope he don't mind).
Maybe an expo converter would help?



Red...simulation of the original,
green.... measurement of the breadboarded workaround.
dots where not placed with high accuracy but the curves should look at least similar 




The sound the breadboarded version produces fits nicely to the green curve. Sorry, no clips at the moment, you wouldn't want to hear me play guitar 
You have to believe me that it makes a pretty cool "yaya"


I assume that the workaround can be simlified and improved. Therefore I'm hoping for your input 
Would also be cool if someone could help me with:
a) why the sweep is linear for the workaround and "logish" for the original
b) why I had to go much higher for the base resistors (also in the sims) than the 22k in the Dr. Q
c) whether the additional tap on the original's pot is really at 50% resistance
d) would you expect issues with temperature dependency for the BC550s resistance?

Peace,

Markus

[CynicalMan]

This is a bit OT but where did you get the pot model?

[John Lyons]

I don't have any answers for you but I'm interested in where this goes.
Thanks for the work so far!

john

[markusw]

This is a bit OT but where did you get the pot model?

There is a LTSpice yahoo group with lots of user models. Definitely worth a look 

[markusw]

Although I promised I wouldn't post a sample, here's a short one
Please ignore my crappy playing. 

http://www.aronnelson.com/DIYFiles/up/sample_1-3c.mp3

There is some background noise from my professional recording technique (alligator wires connected to sound card).

[phaseman]

cool, very simiral to the original (sond sample on the internet), and very similar too mine prototype.

I've been waiting to finish the pedal with further testing, to box it, draw the layout and post it on the forum like other projects, but with special dose of happiness because I love
this pedal from the moment I heard it for the firs time. Anyway, I breadboarded the original circuit. I used a stereo pot, 10 kohm. I left one side to be regular 10k, and on the other I soldered 25k pot in paralel, as a variable resistance. That way, I could "tune" the pot. Also, I used NJM4558 sil. I've mounted the pot into an old whaufuzz box, and I've been playing with it for weeks, but for the last month I've been very busy and can't find the time to finish the pedal. 10k mono pot is used to control the mix of fuzz and clean tone. Fuzz it self is great, too. I'm writing all this for all those who are freaks for filters, just like I am. Bilding this pedal is not harder than any wah pedal. 
As for mods, I tried many:
1) bypass the filters and lead the signal to a BMP tone control, and out - very cool.
2) play with cap values for difrent tuning of the filter
3) add a output buffer
4) add a gain control (500k resistor + 1M pot in series)

best regards
Marko

p.s. caaaan't wait to finish mine

[markusw]

I used a stereo pot, 10 kohm. I left one side to be regular 10k, and on the other I soldered 25k pot in paralel, as a variable resistance

I assumed there was an easier way than mine 

Mind explaining how exactly you did the pot with the additional tap?
As far as I understand for the original talking pedal this pot starts with high resistance (0% travel), then gets down to 0 (at 50% of the travel) and then up again (100% travel) .
(The other halve of the dual pot just goes high (0% travel)  to low resistance (100% travel).)

Peace,

Markus

[CynicalMan]

Although I promised I wouldn't post a sample, here's a short one
Please ignore my crappy playing. 

http://www.aronnelson.com/DIYFiles/up/sample_1-3c.mp3

There is some background noise from my professional recording technique (alligator wires connected to sound card).

It sounds like the overprocessed vocals that are so popular in pop music today.

[markusw]

Just found the schematics for the Colorsound  Dipthonizer.
Knew it could be done easier