Ugly Face with mix control

[soggybag]

I reworked the Ugly Face to add a mix control. To do this I replaced the 386 with a dual op-amp, used one op-amp for the input and the other for the mix control.

I set the input stage up with a gain of 200, this matches the 386 in the original.

The output stage mixes the first op-amp and the 555 through the output op-amp. I figure the signals going in here are pretty hot so I gave the output a gain of about 4.

I feel like I need an output cap between IC1-B and VOL?

[antonis]

I feel like I need an output cap between IC1-B and VOL?

Your feelings are right..

Also, a cap in series with R18 to GND should help a lot for non- saturating respective amp..

Also, IC1_B should need a +4.5V bias so move mix pot wiper from GND to R8/R9 joint..
(and put a R18 series cap to GND, as above mentioned..)

[soggybag]

Thanks Atonis that's really helpful.

If R18 needs a series cap to ground should I put a cap in series with R12?

[antonis]

Both R18 AND R12 need caps to GND 'cause they're 2 independent gain stage resistors..

Series caps serve both for outputs DC roll-off to unity (+4.5v instead of +4.5V times particular stage gain, hence amp saturation..)  and forming HPFs of corner frequency of your taste..

edit: Some good design practice suggestions..

[soggybag]

Thanks again Atonis your advice is gold! As always!

Why change the input cap to 10n? I bet it's something to do with impedance?

[soggybag]

Another question, why different values on the R12 and R18 caps? On the input you suggested 220n to 1u bu on the output you suggested smaller values 15n to 100n?

[antonis]

As for input cap:
C3 and R10 form input HPF of corner frequency 1.6Hz for 100n and 16Hz for 10n..
Taking into account 82Hz lowest frequency of interest, 10n cap should be adequate while saving cost/space..
(In theory, any cap bigger than 2nF should do the job..)

As for R12 & R18 caps values:
Both resistors form HPFs with respective caps..
Given the different resistor values, shouldn't respective caps follow..??
e.g. say you wish for 338Hz corner frequency for both IC1_A and IC1B..
R12 series cap should be 470nF where R18 series cap should be 33nF (closer value to 470/15) 'cause R18 is 15 times larger than R12..
(for any two variables given product conservation, one's increase requires the other's decrease by the same amount..)

P.S.
I presume you DO know 1st order RC filter corner frequency formula, don't you..??

[soggybag]

Thanks again Antonia, I'm going to study up. I feel like I have spent too many doing this "paint by numbers".

So you know your time and energy has not gone to waste I will do my homework!

1st order RC filter corner frequency is: 1/(2🥧RC)

Now for some practice!

R12 1k C 470n = 1 / (6.28 * 1000 * 0.00047) = 0.338

R18 15k C 33n = 1 / (6.28 * 15000 * 0.000033) = 0.321

R10 1M C3 10n = 1 / (6.28 * 1^6 * 10^-6) = 0.0159

I feel like my math is off by a thousand each time?

Looking at R1 and C2 what's the calculation there? Tried looking this up and couldn't find an example of exactly this with a formula. I know I must be missing something because I see this all the time and know it's a low pass filter roll off some highs.

Is this similar to what is happening with C10 & R11, and C11 & R6?

Are there any more RC calculations in this circuit I should examine? Give me another problem to solve!

[antonis]

I feel like my math is off by a thousand each time?

Indeed they are..!!

Check capacitor values..
470nF is 0.00000047 and not 0.00047..
Same stands for 33nF..
Also, 10nF is 10^-8..

[soggybag]

Thanks for helping out!

I woke up this morning and realized I thinking of 1µ as 1.0 but, µ is micro so 1µ id 0.001!

[antonis]

I woke up this morning and realized I thinking of 1µ as 1.0 but, µ is micro so 1µ id 0.001!

I think you have to wake up again 'cause 1μ is 0.000001 and not 0.001 (which actually is 1m = 0.001)

[soggybag]

I've never used a cap in milli-farads!

Looking at C10, R11, and R12 this is high pass filter? What's the calculation for the corner frequency? I need to practice my math!

Is this the same formula 1/(2🥧RC)? -> 1/(6.28*R12*C10)

would this use C10 and R12 or is there another way to look at this with C10 and R11?

[m4268588]

This formula is not accurate.
R1+R2 and R1//R2 are not strict. (Do you want to know the exact formula?)

Also, R1C1 > R2C2 are fine.

[PRR]

I've never used a cap in milli-farads!...

Nevertheless, large transistor power amps use thousands of uFd, and the modern fad is to write 2,200uFd as 2.2mF.

[antonis]

This formula is not accurate.
R1+R2 and R1//R2 are not strict. (Do you want to know the exact formula?)
Also, R1C1 > R2C2 are fine.

Plz, do not further confuse OP..

As long as HPF and LPF RC time constants are far apart (at least by an order of magnitude), they can be analyzed as indepentend 1st order filters..

I presume Mitchell could be more easily face each filter seperately, in the way of individual RC combination "terminals"..
(R1/C1 between amp out and virtual ground and R2/C2 between vitrual ground and circuit ground..)

So, for R2/C2 faced as HPF, C2 is in series with R2 (which R2 lies in gain formula denominator) hence the higher the frequency the lower its impedance, adden in series with R2, resulting into higher gain..
As for C1/R1 faced as LPF, C1 shunts R1 (numerator in gain formula) hence the higher the frequency the lower the parallel combination of R1 & C1, resulting into lower gain..
(it also can be seen as more current leaking R1 through C1 due to lower impedance, making R1 aparent value lower than its actual value ..)

P.S.
Strictly speaking, both filters are HPFs but R1/C1 acts as LPF 'cause is placed inside the NFB loop..
(but better let it be for the time being..)

[soggybag]

Thanks again for all the help! I got this working.

I wanted to redesign Tim Escobedo'a Ugly Face and put it in a wah. The pedal controls the frequency sweep. Here's a couple pictures.









Everything is working it all fits in the wah enclosure.

The op-amp input has a gain of 200 which applies a bit of clipping, which in your boutique overdrives would sound gauche, and just a yawn in your fuzz pedals. Here it mixes well with the raucous square wave from the 555.

There are still a couple issues to handle.

These wah enclosures aren't perfect. The package comes with a round gear and a rack gear. The round gear has a round hole in it. It's not clear what type of pot to use and how to attach it.

I used a Dunlop Hot Potz pot which comes with a D shaped shaft and it's own gear. But the teeth on this gear don't mesh perfectly with rack gear.

The most important problem is the sweep is not great. The interesting portion of the sweep is bunched up at the very end of the travel. I get best results with reverse log pot. The Hot Potz has weird normal taper.

I think it might be possible to adjust the timing cap along with the pot to move the best part of the sweep to a better place in the pedal travel.

[digi2t]

These wah enclosures aren't perfect. The package comes with a round gear and a rack gear. The round gear has a round hole in it. It's not clear what type of pot to use and how to attach it.

I used a Dunlop Hot Potz pot which comes with a D shaped shaft and it's own gear. But the teeth on this gear don't mesh perfectly with rack gear.

Glad to see someone finally doing what's been on my back burner for like, forever. Though, I was thinking more along the lines of just adding expression jacks. Cool beans!

You may have to just bite the bullet and get a gear set from Dunlop. That's what I do for wah projects. The pinion gear comes in two formats; splined and D shaft. The splined version will adapt to both fine and coarse splines easily enough. I personally prefer the D version. I tend to opt for solid shaft pots, and file the flat by hand. The circlip is a bonus as well. I used my Dremel with a thin diamond wheel to cut the groove.

As for the pot, it's always an adventure when you need to go with a taper other than A or B. When the usual wah taper won't do, I end up resorting to eBay. I'm especially fond of PEC, Allen Bradley, or Clarostat pots for linear pots. Expensive, but built like tanks. Jaycar has some cool pots for wahs, but I've had to get duck_arse to buy and then ship them to me because they don't ship outside of Oz. Hey, sometimes you justs gotta do whatcha gotta do. I think a C taper, for this application, might be a beyotch to find.

[soggybag]

Thanks for the tips! These wah pots are not easy to find and the options are limited.

[anotherjim]

Not sure if you shouldn't replicate the 386 input impedance since that loads the guitar input affecting the response. Original has 100n into the 386's 50k input impedance. You could change the 1M input bias to 51k to get a similar response, but with the clean mix, the 50k loaded signal for the 555 is now also your "clean" tone in the mix.

[soggybag]

Thanks for the insights.

This sounds very close to the original but not exactly the same. The input impedance could have something to do with that.