Weird opamp issue..help plz!

[dschwartz]

Hi there folks!
I have a problem biasing an opamp..symptoms:
- sounds gated and fizzy
- if i strum hard, the sound fades out a little and fades in as the input decreases amplitude..
- with gain at 0, it works ok as a buffer..
Topology:
Standard feedback clipper with 500k gain pot on fdbk, 1k ohm and 100nF cap to ground from - input..
Input from buffer--> 22n cap-->10k ohm--> +input with bias
Bias resistor is 1Meg, and the bias network is 2x 220k divider and 47uF bypass cap...

Other data :
The same bias supply is used for other 3 opamps stages, a buffer (biased with a 2.2 meg) and 2 mild gain stages, biased by 470k ohm..

Opamp voltages (vcc 8.8v):
Out=4.4v
In-=4.4v
In+= 3.9v (wtf??)

First thought was bad opamp, so i changed the tl062 for a tl072...same readings...
The only signficant change i made to the working version is sharing the bias voltage for the two tl072..before i had separate vref per chip..

Is that causing my biasing problem? Feeding too many opamps with 1 vref and different bias resistors? Afaik, that shouldnt be an issue..but maybe you have a different opinion

Cheers and thanks for your help!!!

[aron]

I know you know a lot more than I do, but the only thing I could see is the use of 220K bias resistors. Is there a reason for the higher value as opposed to the more typical 10k/10K bias divider?

[dschwartz]

I tried with 33k/33k also..same problem...

Bad capacitors may be the issue , too, but i dont know if the symptoms are congruent...

[anotherjim]

Does it sound funny when you work the vol control on the guitar - that's a clue to a bad input cap.
The + input voltage drop is your meters 10M or so input resistance in parallel with the 1M bias - nothing to worry about.
Other than that suspect your caps, but this rarely happens with non-polarized types (unless you happened to use a 0.1uF electro/tant - they do exist.

What about the output (you don't mention the components there)? It could be what you're plugging this into that's getting soaked.

[Rob Strand]

Try putting a small cap from the opamp output to the opamp - input.
Maybe check the resistance from each end of the pot to the wiper is OK when in the middle position.

[dschwartz]

Does it sound funny when you work the vol control on the guitar - that's a clue to a bad input cap.
The + input voltage drop is your meters 10M or so input bresistance in parallel with the 1M bias - nothing to worry about.
Other than that suspect your caps, but this rarely happens with non-polarized types (unless you happened to use a 0.1uF electro/tant - they do exist.

What about the output (you don't mention the components there)? It could be what you're plugging this into that's getting soaked.

Im going thru a buffer so the gutar volume works normally..also, im using a probe at the opamp output, so the latter stages are not involved..maybe i need to pull out all the involved parts and put in new ones..

[dschwartz]

Try putting a small cap from the opamp output to the opamp - input.
Maybe check the resistance from each end of the pot to the wiper is OK when in the middle position.

Im using a 470p cap there.. And the pot works fine.. Im scratching my head now and breathing slowly to not throw the pcb out of the window..this is so frustrating!!!

[bool]

TLDR

>>> 2x 220k divider and 47uF bypass cap

This doesn't feel right to me. The cap - unless it's a tant - would/could have a too-big a leakage current to get the bias right. It would/could load down the divider too much.

[Rob Strand]

I think I know what is going on with the DC voltage.  It's very simple! The multimeter is 10Mohm when you connect it to the 1MEG resistor on the opamp + input it forms a divider of 10/(10+1) = 0.91, and 4.4 * 0.91 = 4.0V.

As to why it is not working, I'm still not sure.

Try adding a 1k resistor between this stage and the next stage.

You shouldn't need to worry about your bias resistors as you have a good size cap (47uF) on the divider.

You could also just disconnect that stage and debug it in *total* isolation.

[dschwartz]

Guys...
I resoldered the board..new parts, same issue..

Until i noticed i had both leds the same orientation...

Nothing more to say

(Sobbing sound on the background)

[ashcat_lt]

I think I know what is going on with the DC voltage.  It's very simple! The multimeter is 10Mohm when you connect it to the 1MEG resistor on the opamp + input it forms a divider of 10/(10+1) = 0.91, and 4.4 * 0.91 = 4.0V.

For DC, it's more like 1.11M, which gets you even closer to the actual measured voltage.

[Rob Strand]

1.11M is technically correct.

(It doesn't actually change the answer much.  When you do this stuff a lot you learn to strip away the parts that don't matter, otherwise you get bogged down in the details.  Unfortunately it can look like a mistake.)

[PRR]

> The multimeter is 10Mohm

More-or-less.

"Proper" bench meters have 1Meg in the probe (so you don't load signal) and are 11 Meg DCV range. (Think VTVMs.)

While 10Meg is common in DMMs, my Fluke is 2Meg (they make several models with different loading), another good DMM here is 3.7Meg to 4.9Meg (I don't use that one for fine electronics), many seem to be 2Meg, and sometimes we see reports suggesting even lower resistances.

Not to mention needle-meters. (They didn't go away.)

All of which should usually be +/-2%, but sometimes worse.

Yeah, when first-guessing a voltage list I assume 10Meg and throw-way any .11 left over. If the 10Meg guess isn't real-close, try 2Meg. If that still makes no sense, the error may not be (just) meter loading, but A Real Problem.

[dschwartz]

The opamp bias was right..the issue was the clipping leds were both going the same orientation...that was causing the fuzzy buzzy flabby tone...

I'm  embarrased...newbie mistake..