reducing gain on multi-function wah-wah pedal....

Thread Starter

bigbearth

Joined Nov 27, 2023
14
thx SGHIOTO.
That's the simplest thing to try... and not to worry about damaging components when unsoldering/swapping things..
 

AnalogKid

Joined Aug 1, 2013
10,944
But need reduced gain from "drive" P2 pot....
To reduce the effect of P2 on the gain of the U2C stage, reduce R15. This will have some effect on the frequency response of this stage, so you might have to increase C13 to compensate.

As the circuit is now, the driving point impedance for R15 is the Thevenin equivalent of P2 and R16, which varies from 0 ohms to 5.5 K to 909 ohms as the pot is rotated. This, in series with R15, forms the resistor part of an R-C lowpass filter. This means there already was a frequency response that varies with rotation. Decreasing R15 will shift the corner frequency upwards. How much depends on how much R15 is changed.

You don't indicate how large a gain reduction you need. To start, try tacking a 100K resistor in parallel with R15 and see how that works out. This will reduce the max stage gain by 32% -ish. If that is too much, try other resistor values until you have one that works best. With that value we can calculate the change needed for C13 to restore the original frequency profile.

Another approach would be to increase the value of R16, but I think the R15 path is easier because you can try different values with clip leads; no soldering until you have the right one.

ak
 
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AnalogKid

Joined Aug 1, 2013
10,944
R14 helps set the threshold amplitude at which clipping starts, and softens the "knee", the onset of diode clipping. This is in parallel with the gain setting components, so the actual gain equation for the stage is very messy.

If the right side of R14 is at GND, then (ignoring the cap impedances for the moment) as a gain-setting component it is in parallel with R13; both are paths from the inverting input to GND.

AND - I temporarily ignored the presence of R14, so I take back my "infinity" remark.

ak
 
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sghioto

Joined Dec 31, 2017
5,369
Just breadboarded U2C circuit.
Max gain at 1000hz before clipping was appx 62. P2 set at minimum, gain of 13
The best way to reduce the gain is to lower the value of R14.
Changing R14 to 100K, max gain of 25, min gain of 10 through P2
 
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Thread Starter

bigbearth

Joined Nov 27, 2023
14
Very thorough answer! I will consider these mods...THX SO MUCH
Just breadboarded U2C circuit.
There is a 6.6 times increase in gain with P2 before max clipping.
The best way to reduce the gain is to lower the value of R14.
Changing R14 to 100K reduce P2 to appx 2.6 gain
Max gain at 1000hz before clipping was appx 67.
you guys are very helpful....Im reading comments and deciding which approach...
THX so much
 

AnalogKid

Joined Aug 1, 2013
10,944
Change R13 from 1.5K by adding a 4.7K resister in series with it.
R13 and C12 form a single-pole highpass filter with a corner freq of 2.26 kHz. That is a serious filter, and will attenuate *all* of the fundamentals of a guitar to some degree. As above, the low E string (83 Hz) will be attenuated by over 28 dB relative to the circuit's flat response band. That's a lot.


1701356391052.png
https://fretsuccess.com/what-are-the-guitar-string-frequencies/

Don't know why it is in there, but someone thought it was a good idea. My guess is that they want the diodes to clip only the higher frequency signals for a "brighter" fuzz effect.

Anyway - increasing the series resistance from 1.5K to 6.2K will lower the corner freq to 546 Hz, a decrease of over two octaves. That will be a significant (drastic - ?) change in the sound of the circuit.

Increasing the input resistance certainly will decrease the circuit gain, but it also will increase the bottom end frequency response when in "Distortion" mode, and darken the effected sound at high signal levels and the un-fuzzed sound at lower levels. If you want to prevent this change in tonal quality, decrease C12 as much as the ratio increase in R13. That is, if the new series resistance (R13 + Rx) equals 4 x R13, then decrease C12 by 4x.

Total series resistance = R13 + Rx = Rt

Highpass corner frequency = 1 / (2 x pi x Rt x C12)

ak
 
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Thread Starter

bigbearth

Joined Nov 27, 2023
14
R13 and C12 form a single-pole highpass filter with a corner freq of 2.26 kHz. That is a serious filter, and will attenuate *all* of the fundamentals of a guitar to some degree. As above, the low E string (83 Hz) will be attenuated by over 28 dB relative to the circuit's flat response band. That's a lot.

View attachment 308741
https://fretsuccess.com/what-are-the-guitar-string-frequencies/

Don't know why it is in there, but someone thought it was a good idea. My guess is that they want the diodes to clip only the higher frequency signals for a "brighter" fuzz effect.

Anyway - increasing the series resistance from 1.5K to 6.2K will lower the corner freq to 546 Hz, a decrease of over two octaves. That will be a significant (drastic - ?) change in the sound of the circuit.

Increasing the input resistance certainly will decrease the circuit gain, but it also will increase the bottom end frequency response when in "Distortion" mode, and darken the effected sound at high signal levels and the un-fuzzed sound at lower levels. If you want to prevent this change in tonal quality, decrease C12 as much as the ratio increase in R13. That is, if the new series resistance (R13 + Rx) equals 4 x R13, then decrease C12 by 4x.

Total series resistance = R13 + Rx = Rt

Highpass corner frequency = 1 / (2 x pi x Rt x C12)

ak
Thank you so much guys! Its a lot to chew on...but will try some things. never expected such a in-depth response. wow
 

MisterBill2

Joined Jan 23, 2018
17,791
Thank you so much guys! Its a lot to chew on...but will try some things. never expected such a in-depth response. wow
One possible alternative will be to adjust the pickups on the Stratocaster to deliver a bit less signal at those higher frequencies. There is a lot of adjustment range available.
 

Thread Starter

bigbearth

Joined Nov 27, 2023
14
One possible alternative will be to adjust the pickups on the Stratocaster to deliver a bit less signal at those higher frequencies. There is a lot of adjustment range available.
Its actually a humbucker gtr...and already has another TubeScreamer pedal before it to achieve most of the crunch tone. This Morley is for added boost for solos. Just adds some midrange sustain. It almost works, but its just a bit too hot even at min drive settings because of the previous pedal that is already distorting the signal....
 

MisterBill2

Joined Jan 23, 2018
17,791
OK, to simply reduce the drive by increasing the feedback, put another 1K resistor in series with R16 at the bottom of the drive control, or change R16 to a 2.2K resistor (red,red,red) That will allow reducing the gain a bit more.
 

Thread Starter

bigbearth

Joined Nov 27, 2023
14
Hey gents...I tried lowering R13 by attaching 4.7k across. (only because I had this value available as opposed to other suggestions on R14, R16, etc,,,)
It didn't really lower the gain/volume as hoped for. I measure it at 1.1k now....
Should I try to lower it below 1k?
Sorry, I am not sure and just trying things you gurus suggest....

Thx
 
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