Op Amp Question?

[modsquad]

I am troubleshooting a Ross Phaser with LM13600Ds using the tonepad schem.  I am looking at the voltages and see that for IC3 pin 3,4 have 3.98, 3.96 voltages.  Those are the input and inverting input of the opamp.  This is making the output voltage about .5v.  I am not sure this is correct.   Any ideas?   I seem to recall that it should be about 6v.

Stan

[Sir H C]

The op-amp is balanced with the output at .5 volts.  What is driving the op-amp is likely in error.

[Mark Hammer]

...and THAT's why we stick DC-blocking caps between stages!

[modsquad]

I take it then either the inverting input or the input then is wrong, so I should look at the circuit leading up to those two.   Mark, for my edification could you expound on the idea of blocking caps.  I am assuming you are refering to then blocking any wayword voltages from getting to the next stage.

Stan

[modsquad]

An added note, any idea what the output of the stage should be?

[grapefruit]

I assume the outputs of A2 are somewhere around 4.5V (running from 9V supply).
That's not a huge difference in dc input voltage.You'd need high gain to create that much offset. I'd check the power supply and Iabc pins for that chip.

That circuit would make more sense to me with DC blocking caps between stages, as it looks like the buffers that are fed with a Vb biased signal drop two diode voltage drops and then go to Vb again. Apparently this circuit works though...

Cheers,
Stew.

[R.G.]

Any ideas?

Yes. The output voltage of an OTA is not related to the DC voltage of the two inputs like a normal opamp is. The DC output voltage is whatever the output load pulls it to. What you have told is is that your output loading on the OTA is pulling it down to 0.5V. You have a loading problem.

[Mark Hammer]

I take it then either the inverting input or the input then is wrong, so I should look at the circuit leading up to those two.   Mark, for my edification could you expound on the idea of blocking caps.  I am assuming you are refering to then blocking any wayword voltages from getting to the next stage.

Stan

Yep.  Op-amps, especially the cheap kind we tend to use in pedals, are imperfect.  When we stick in copious amounts of gain, they get really imperfect.  Sometimes you WILL see direct coupling of op-amps via a resistor, but more often than not you will see a cap between stages and a re-biasing.  The cap effectively "presses the reset button" to assure that, no matter what you did and what you used to do it in the previous stage, there will be no stray DC to muck up what you have planned for the next stage.

Ironically, the very soldering iron I use today was the result of learning about stray DC over 20 years ago.  I had gotten to know the tech in the department where I was studying and he let me use their nice and spacious workbench and machine-shop for my projects.  One of the profs, whom I had known when he was a grad student in another university, was trying to build some sort of EEG instrumentation preamp from a circuit schematic he had (ever try amplifying a bioelectric signal of a couple of microvolts?).  It was not going well.  I certainly knew much less then than I know now, but got lucky and suggested he try a better op-amp like a CA3140.  I had a dozen of them and gave him a couple.  The preamp worked like a charm and the stray DC that had been plaguing his circuit all but disappeared.  In gratitude, he bought me a soldering iron.

[modsquad]

Great story Mark...You know everyone who reads this thread who is a noob should know I knew squat a year ago when I started this.  This project I'm doing now of putting a phaser and vibe on one circuit has taught me a lot about how opamps work (and don't work  ).  I researched opamps using Google asked a couple of questions and learned what Vb, Iabc, inverting inputs, gain, etc. were in relation to the circuit and opamps I am working with.

Long story short, before asking why it doesn't work, do some research educate yourself as much as possible then when you can't go any further ask questions.   You will learn so much more and be more productive in the future.  So many times we have no clue where to begin troubleshooting but this gave me a good place to start.  I may not get this to work right but I will be more productive trying to figure it out.

Stan

[modsquad]

Well got the voltages up to where they need to be but now the volume is very, very low.  The weird thing is the voltage from pin 7 of the 4558 that goes to the output measures 4.7 volts.  I am using my ruby amp to test, could it be impedance issue?

Stan

[grapefruit]

Don't forget what RG said about OTA's. IC3 is not a normal Op Amp, it's an OTA. How did you get it to work?

Is lower than the bypassed signal?
If the circuit is actually working correctly but the volume is lower than you'd like you could just increase the feedback resistor on the output op amp, but you will also increase the DC offset!
You could put a coupling cap in series with each of the 27k resistors feeding the output op amp. Or (this may be one for RG) could you just put one cap on the inverting input? I think I've seen this done before but when calculating the cutoff frequency I normally think of the resistor coming before the cap...


Stew.

[grapefruit]

Oops, I meant normally the cap is before the resistor.
The inverting op amp -input is virtual earth so I don't see how it could work, but I'm sure I've seen circuits like this but never tried it...

Stew.

[modsquad]

I forgot that the he said the LM13600 is an OTA.  I resoldered and scraped the traces to pins 3,4 of IC2 and IC3.  That got the voltages to where Pin 5 is 5.8v.  I have gotten this to same circuit working before but never had the problem of volume being low.  Okay "stupid question alert"...

If the output voltage of the Op amp IC1 is 4.5v how come the drop in volume.  Or is gain and volume not related?

[Paul Perry (Frostwave)]

If the output voltage of the Op amp IC1 is 4.5v how come the drop in volume.  Or is gain and volume not related?

Are you measuring that 4.5 wiht a meter?
Because, if so, you are just measuring DC voltage, not hte audio superimposed on it. Which may be mere milliviolts.

[Crevil]

Hi modsquad

You have to remember DC (the current from your battery, power supply etc.) and AC (waves like sound) is two VERY different things.

When measuring on your OP-AMP you can not meassure right on the pins, but after some kind of cap to get your actual signal specs.

I haven't read this topic from the start but just make sure to have this in your mind when measuring. DC and AC can be in the same "path" or "lead" but your are not able to meassure or see this if your not using a scope.

Well, you can with a DMM by setting it for AC millivolts i guess