Help: Bias Problem of Simple Dual Opamp Boost

[samhay]

^Just a general question: With Dual Opamps does the Bias of one side of the Opamp affect the other? Is DC offset from Pin1-3 affect Pin5-7?

Not in my experience.

[duck_arse]

Just a general question: With Dual Opamps does the Bias of one side of the Opamp affect the other? Is DC offset from Pin1-3 affect Pin5-7?

from your first posted schem, you have both sections connected by 470k's to the bias voltage, so they should both be the same voltage or very near to. each of the opamp outputs is dc blocked by their caps, so there can't be any interaction between.

[thehallofshields]

Thanks. I'm getting signal through one side now, but the Bias is still way off.

At what point do you guys give up on perfboard projects and smash them with a hammer?

[slacker]

Have you still got R7 and R8 in the wrong places?

[thehallofshields]

YES I DO!

I jumped my Pin6 (Inverting B) and Pin 7 (Output B) and I've got Unity GAIN! Ahh so sweet to my ears.

I think my wire connections to the Pot are bad. I'm just so happy to have made progress after staring at my failure for 3 months.

[antonis]

At what point do you guys give up on perfboard projects and smash them with a hammer?

Just after they've worked...!!!

[thehallofshields]

I bet everyone on this forum is a little bit neurotic. I think this hobby demands it.

[thehallofshields]

A HA! A bad Pot!

I replaced the 250k Gain Pot B, and I've got the output side working!

It sweeps from Unity to Amp Busting Gain, and the Switch works!

[thehallofshields]

Pin 5 - 3.4
Pin 6 - 4.7
Pin 7 - 4.7
Pin 8 - 9.4

So on the working side, the Voltages are still seem off, hopefully that's Voltage-Drop caused by the DMM.

But I'm extremely happy to have gone from totally borked to partially working.

[PRR]

> A HA! A bad Pot!

Jump Var1 (pots are not 100% reliable) with something small, say 5K.

I think you may be up to Var2? But same thing.



> Pin 5 - 3.4
> Pin 6 - 4.7
> Pin 7 - 4.7
> Pin 8 - 9.4
> hopefully that's Voltage-Drop caused by the DMM.

I mentioned that also. It should be same 4.7V when not poking. You find 3.4V poked, a 1.3V drop in the 470K resistor, and 3.4V across meter. 3.4/1.3 is a 2.6:1 ratio. Your meter acts-like a resistor 2.6 times larger than 470K. 2.6*470K is 1.230Meg. This is not unlikely for modern DMMs.

Any time you are reading a point that is only held-up by a resistor anywhere "near" 1Meg, you will have loading, and should discount the exact value. Look that it isn't Zero or 9V, or rediculously teeny 0.1V, but anywhere "near" half-supply (even 3V) is probably OK when you are not poking it.

[thehallofshields]

Thanks PRR. I should have been less lazy and applied Ohms Law.

Side B of the circuit is fully functional.

I've got the same 3.4 on the Non-Inverting Input A, but 1.5 and 8.5 on the Inverting and Output.

That tells me that I screwed up the Feedback Network on Side A too.

It should just be a matter of time now...

[thehallofshields]

No Voltage at Var1.

I'm closing in.

[thehallofshields]

Pot connection was way off. Really screwed that up.

Pedal is 100% Working as far as I can tell.

What a freaking relief I don't have this failure hanging over my head anymore.

Thank you guys so much for your input.