opamp problem, nothing on output.

[fbalic96]

Hi guys,

My first post here.
So I was designing a guitar pedal by myself and the pedal was too quiet so I added a booster at the end in series. I use tl072 opamp from texas instruments.


As you can see from schematics I put 1Mohm resistor in feedback loop and 50kohm resistor. That should give me gain about A=20. I breadboarded it and it worked fine. Now when I put it in PCB I get nothing from the output... Is opamp faulty or I made mistake in picking the resistance values.

I ger reading as follows:
Pin 1: 9.18V
Pin 2: 3.68V
Pin 3: 4.4 V
PSU voltage: 9.77V

I know I could just replace new opamp but I already soldered it to board without a socket.
Did I overdo the difference between those two resistors so op-amp does not work or I should just cut it off and solder new one?

[antonis]

Hi & Welcome..

Although C13 should be placed backwards, your circuit should work..

As op-amp is already soldered, you can try using pins 5, 6 & 7 instead of 3, 2 & 1 to see if it works..

P.S.
Never let unused amp pins "open"..
https://e2e.ti.com/blogs_/archives/b/thesignal/posts/the-unused-op-amp-what-to-do

[iainpunk]

welcome to the forum

very odd indeed
the output is clearly clipping, and the (-) input is lower that the (+) input, so the output voltage should be clipping the negative supply rail, not the positive one... very odd
my guess is a shot chip in there. what is the specific type you used?

cheers

[antonis]

the output is clearly clipping, and the (-) input is lower that the (+) input, so the output voltage should be clipping the negative supply rail, not the positive one...

I________________I
Late night there..
(here also..)

[JerS]

It looks to me like the input bias of the opamp is not correct. You need to do a split power supply (like on a tubescreamer) and bais the + input of the opamp at 4.5V (Vb). The way it is now, the signal cannot swing positive and negative, so it clips hard or makes no sound.

Edit: On second look at your schematic, I see you have done a split - the only thing missing is a largish cap (47-100uf) to ground from the + input. Try adding that and see what happens.

[fbalic96]

R32 and R32 are acting as voltage devider so this is how I get 4.4V on pin 3.
also I use pins 6,7 and 8 as buffer in front of the circuit but I only drew part of the circuit that does not work.
I will replace opamp and rotate C13 as suggested and inform you about the results.

http://www.jer00n.nl/2010/07/28/clean-boost-for-guitar-or-bass/
This is  schematics i used... only changed values of 2 components.

Thanks guys

[anotherjim]

Probably is C13 polarity reversed causing leakage current. The leakage makes it as though there is a resistor across the capacitor.
This makes the -input pin2 "see" down toward 0v via R33 and so the amplifier swings the output positive as high as it needs so the voltage divider R34 and R33 gives the same voltage at the -input pin2 the opamp sees on its +input pin3. The larger R34 is compared to R33, the higher the output voltage must go. This is how the amplification works.
The opamp is doing as it's told and is ok. Note that if R34/R33 were more equal, you would probably never notice this kind of fault as it could sound ok. So, always be careful with cap polarity!

When the opamp is working, you can't spot the cause of this kind of fault on the opamp input pins, only the effect on the output voltage which ought to be very close to the +input voltage. C13 leakage current will show in the voltage between C13 and R33. Without leakage, it will be the same as +input pin3 but when leaking it will be a little lower, depending on how much leakage current. However, this error in voltage difference will often be too small to measure accurately.

[antonis]

You can pass to a more effective booster by utilizing the 2nd amp for Vbias stabilization..
(don't be scared of circuit "complexity" - it simply is different topology..)

[fbalic96]

Thanks guys! It works now!

Corrected c13 rotation and it works perfectly without touching op-amp.
If anyone is interested it's a compressor pedal that I was working on for 2 or 3 months. Now its alive finally.

[ElectricDruid]

Hey, that's beautiful! Nice work!