Inverting amp, not inverting..at a loss

[chicago_mike]

Okay..im stumped.

I made an inverting amp. for a bass amp so I can bridge the 2 power amps. No biggie.

However..the inverting amp is not inverting the signal.

What are some reasons..knowing the circuit is correct, that an inverting opamp would not invert the signal?

Totally stumped here.

[Gurner]

What are some reasons..knowing the circuit is correct, that an inverting opamp would not invert the signal?

without fail ...the circuit, the construction &/or the operator.... for one thing is certain, when configured properly....an inverting amp inverts!

we'd need the actual circuit you've configured  and how you know it's not inverting! (scope?)

[chicago_mike]

I want to use the unused opamp to make an inverted signal...keeping the other non-inverted signal intact. For two power amps in the bass head.

[Gurner]

that opamp (IC202-A) is not configured there as an inverting opamp circuit....that's a non-inverting config....so if you were expecting that cct to invert....there's your problem.

[chicago_mike]

IC 202B

Leave A as it is.  

2 power amp modules in the bass amp. I want to drive one of them with an inverted signal off of IC-202B.  

[Gurner]

So like I say, when you say your config is not inverting, we need to know what you've done  because IC202-B is an unused opamp (and not configured as anything)

If you want to invert the ouput of IC-202A, then place a 1k resistor from the output of IC202-A pin 1 to IC202-B pin 6 & remove R220 (have pin 5 straight to ground).

[chicago_mike]

I get that..but its B I want to invert



this is a crappy ass drawing, but its what I want to basically do. 

Whats in red feeds power amp #2.

[Gurner]

If that input is High Z, then I wouldn't go that way....take the input into your inverting opamp from the ouput of IC202-A. (else your input impedance on the inverting opamp is gonna be about 1K & will slaughter your input)

Remove the resistor R220.....you've then got a valid/classic inverting opamp circuit.

If after that you say it's not inverting, how are you measuring this?

[chicago_mike]

It's low impedance...I think.  Theres opamps infront of this.

Heres where the signal is fed from.

The low and hi pass filters.

[Gurner]

yep, as you say the input is low Z so that's fine.....so you've still not answered....how do you know your inverting opamp is not inverting?

[chicago_mike]

I was just about to type.

I am using a scope. tektronics tds 320. 

I set the probe / scope starting from 500mv swing to 5v swing...depending on the volume of the master volume. Everythings cool. But just no invert.

I'm missing something obvious, I just wish I knew what it was.

[Gurner]

I was just about to type.

I am using a scope. tektronics tds 320.  

I set the probe / scope starting from 500mv swing to 5v swing...depending on the volume of the master volume. Everythings cool. But just no invert.

I'm missing something obvious, I just wish I knew what it was.

You are scoping the input to the opamp on Ch1 of your scope & the output of the opamp into ch2 of your scope - right?  (and you've not got ch2 set to invert by any chance? ...therefore inverting an inverted signal....)

[chicago_mike]

Well...I have one probe. I seriously need another.

not figured out how to save the wave form...but I can switch between the outputs quickly, no inverting..

Unless today is just stupid me day.

I'll check my manual on saving a signal..but yeah I also check the inputs and outputs...And I did check to make sure that invert is OFF. That would be VERY embarrassing! 

[Gurner]

Well...I have one probe. I seriously need another.

not figured out how to save the wave form...but I can switch between the outputs quickly, no inverting..

Unless today is just stupid me day.

Phase relationship is relative thing....you can't scope one place...remove the scope then move onto another place and scope another. ...and hope to know whether it's not inverting or non-inverting - that'll never work - not a chance.

You need to scope one place (the input) & another (the output) into seprate scope channels at the same time & compare them at that same instant - this is almost certainly your problem.

Until you get an extra scope lead....you can join the outputs of both opamps together via a seies 10k resistor off each opamp - at the junction of the 10k resistors  there should be no AC signal if your inverting opamp is inverting.

[chicago_mike]

Hmm....

can a make a diy probe or should I not even go there...

[Gurner]

Hmm....

can a make a diy probe or should I not even go there...

Yes, you can lash up a DIY scope lead for a quick & dirty test.....just push some suitable single core wire into your scope's CH2 BNC socket.....you don't need to worry about the ground if you've already got the ground probe of your present scope attched to cct ground.

[chicago_mike]

Okay, gonna try this out!

thank you for the help and patience! 

[chicago_mike]

I AM A DUMBASS

Now I just need to match the gain of the two outputs. but yeah, now I should be able to bridge the outputs.

Thank you for your help Gurner! 

[R.G.]

this is a crappy ass drawing, but its what I want to basically do. 

Whats in red feeds power amp #2.

Gurner is right - take that 1K to the (-) input of the second opamp to the output of the first opamp.

The input impedance of the second opamp is very close to the 1K input resistance, and that will mess up some things about the pot. The pot is 10K, and there's a 4.7K "tapering resistor" which adds a bit of log feel, even though it's not enough for a real log curve. The 1K would appear in parallel with the 4.7K and make the taper more extreme. And in addition, the 10K pot has an output impedance which varies with position. This doesn't matter much when the load is a resistor and an inverting input, but it has the potential to change things if you tap the signal with a 1K input at the wiper.

The output of the first opamp can drive 1K just fine; even then, I'd probably change both the input resistor and feedback resistor on the second opamp to 10K or more, just to reduce loading on both opamp outputs.

[chicago_mike]

I saw a schematic like that...

The invert was tapped off pin 1, and it fed to a 10k, with a 10kfeedback resistor.

The gain of both opamps / outputs  would be more or less matched.