WAH Pedal question

Thread Starter

AlaSH72

Joined Apr 3, 2022
29
hi there
I'm trying to implement a DIY WAH Pedal(which in fact is a musical manipulation device that distorts the sound of guitar)
I've followed a tutorial here DIY WAH Pedal

I've wired up everything on breadboard. and instead of TL972 I've used TL072.

based on schematics shown in website, I get the signal until right before OpAmp, say at negative In which is common with 160K. but at Output, I dont get anything, seems like the sound is buried deep in noise...

why is that? and how can I solve it?
 

AnalogKid

Joined Aug 1, 2013
10,054
1. Add decoupling capacitors around R1-v and R2-V. The GND node must be a zero-ohm point with respect to both the + and - rails at all audio frequencies. The caps should be as close as possible to the opamp power pins.

Other than that, I don't see any obvious errors in the schematic. Please post photos of your assembly.

ak
 
Last edited:

Ian0

Joined Aug 7, 2020
5,103
Their pinouts are different. Have you double-checked your wiring?
How have you connected the unused op-amp in the package?
Their pinouts are the same (nearly all dual op-amps have the same pin out), but that begs the obvious question - should it really be a dual op-amp? Only one section is used.

Assuming you have used pins 2 and 3 for the inputs and pin 4 for the negative supply, was the positive supply on pin 7 or on pin 8?
And was the output on pin 1 or pin 6?
 

Thread Starter

AlaSH72

Joined Apr 3, 2022
29
Their pinouts are different. Have you double-checked your wiring?
How have you connected the unused op-amp in the package?
I've checked and both have same pinouts

is there a "unused" Opamp?
how can this be?
I mean, I can choose to use one of Opamps or only one of them...
 

Thread Starter

AlaSH72

Joined Apr 3, 2022
29
1. Add decoupling capacitors around R1-v and R2-V. The GND node must be a zero-ohm point with respect to both the + and - rails at all audio frequencies. The caps should be as close as possible to the opamp power pins.

Other than that, I don't see any obvious errors in the schematic. Please post photos of your assembly.

ak
I actually did that too. adding some ~1nF decoupling caps around R1 and R2 which was helpful but still nothing in Output...

I didnt get the
The GND node must be a zero-ohm point with respect to both the + and - rails at all audio frequencies. The caps should be as close as possible to the opamp power pins.
more explanations maybe?
 

Thread Starter

AlaSH72

Joined Apr 3, 2022
29
Post pictures of your pcb then we can assist you further, you might have missed something..
actually I've wired everything on a breadboard, which seems too clumsy with wiring and stuff...
but I can say that I've double checked the circuit and I dont think I've missed anything...
 

djsfantasi

Joined Apr 11, 2010
8,323
actually I've wired everything on a breadboard, which seems too clumsy with wiring and stuff...
but I can say that I've double checked the circuit and I dont think I've missed anything...
A breadboard audio circuit will perform poorly. All those patch wires act as antennas picking up radio/ electrical noise from the atmosphere.
 

Thread Starter

AlaSH72

Joined Apr 3, 2022
29
Their pinouts are the same (nearly all dual op-amps have the same pin out), but that begs the obvious question - should it really be a dual op-amp? Only one section is used.

Assuming you have used pins 2 and 3 for the inputs and pin 4 for the negative supply, was the positive supply on pin 7 or on pin 8?
And was the output on pin 1 or pin 6?
check out the TL072 pinout from its datasheet
according to this image, VCC+ is on pin8 and not pin7.
and yes, I've used pin3 and pin2 as + and - input respectively and pin1 as output
TL072.JPG
 

Ian0

Joined Aug 7, 2020
5,103
You should use the spare section of the op-amp to buffer the virtual ground.
But don’t attempt that until you have the circuit working!
 

Thread Starter

AlaSH72

Joined Apr 3, 2022
29
You should use the spare section of the op-amp to buffer the virtual ground.
But don’t attempt that until you have the circuit working!
Ok but not really sure how should I do that?
I'm a bit confused... is the spare Opamp too deadly that I should do something about it???
 

Thread Starter

AlaSH72

Joined Apr 3, 2022
29
Yes, and you have two excellent suggestions on what that something is. For getting the circuit to work, refer to the TI article I posted. Then, you can improve it with Ian0’s suggestion.
Really sorry but can I ask you to explain it through a schematic? I mean buffering the virtual GND

Also refering to the TI article you sent, in the article, for example fig3. I see two Opamps which are both wired somehow to make the configuration you said...
But I can not see why should I use both of Opamps to just config one of them... I'm confused...I mean, I might not be familiar with the things you said, can you explain more?
 

Ian0

Joined Aug 7, 2020
5,103
Really sorry but can I ask you to explain it through a schematic? I mean buffering the virtual GND

Also refering to the TI article you sent, in the article, for example fig3. I see two Opamps which are both wired somehow to make the configuration you said...
But I can not see why should I use both of Opamps to just config one of them... I'm confused...I mean, I might not be familiar with the things you said, can you explain more?
Leave the virtual ground buffer alone until you get the rest of it working.
You need to keep the unused op amp under control. If it starts to oscillate, or amplify noise, then it might interfere with the rest of the circuit which you are trying to debug.
So connect output to inverting input, and connect the non-inverting input to your ”virtual ground” (junction of R1 and R2). Then you know it isn’t getting up to no good, and you can concentrate on getting the circuit to work in the knowledge that the spare op-amp isn’t frustrating your efforts.
 

AnalogKid

Joined Aug 1, 2013
10,054
I actually did that too. adding some ~1nF decoupling caps around R1 and R2 which was helpful but still nothing in Output
Not nearly large enough to help. Place a 100 nF ceramic and 100 uF electrolytic in parallel with each of the voltage divider resistors.

One of the underlying assumptions of all opamp circuits is that there is a very low *signal* impedance between the power supply pins and GND. This gets down into the mud of how a transistor amplifier actually works.

ak
 
But I can not see why should I use both of Opamps to just config one of them... I'm confused...I mean, I might not be familiar with the things you said, can you explain more?
Your circuit requires one op-amp.
The IC you selected has two op-amps.
Wire the unused op amp as Ian0 described.

His description is similar to the examples in the TI article and will work well. There are several ways to wire an unused op-amp. The takeaway is that any unused op-amp MUST be wired in one of them.
 

Thread Starter

AlaSH72

Joined Apr 3, 2022
29
Not nearly large enough to help. Place a 100 nF ceramic and 100 uF electrolytic in parallel with each of the voltage divider resistors.

One of the underlying assumptions of all opamp circuits is that there is a very low *signal* impedance between the power supply pins and GND. This gets down into the mud of how a transistor amplifier actually works.

ak
I actually tried ~uF capacitors as decoupling but seems to be too large and vanished the whole signal...
 
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