Offset Voltage Indicator?

[Stocker]

This is probably not going to generate much interest but here goes...

If you are designing an amplifier that has potential for DC offsets on the output;

And the offset can be trimmed out;

And you would like to keep it to (nearly) zero offset...

...and somehow you are motivated enough to design this circuit but not design away the offset voltage in your amp...

How about a front panel LED that is an offset indicator? That is to say, when your output has a DC offset, there is an LED winking at you. Then a little knob beside the indicator trims the offset away until the LED goes out. I am thinking more along the lines of a headphone amplifier, but the concept works as well for a main system amp, no?

This would probably be a pretty simple cct to design using any run-of-the-mill comparator setup.

Comments?

[millwood]

would a DC servo driving an LED work?

[sreten]

Designing in a problem so you can design a solution is not a popular appoach sreten.

[sreten]

Originally posted by millwood
would a DC servo driving an LED work?

Better to use the DC servo to make offset a non-issue.

sreten.

[boholm]

Yes, it is possible.

IMHO it is quite simple: any OPAMP with a healthy gain should do the trick. Example: If +/- 50mV is max. before indication (LED switches on) then the gain should be around 3V/50mV=60. I say 3V for the LED+resistor to switch on, but all of it should be experimented with, so that the indication comes in an acceptable maner. And if you use a bicolored LED then you will know which way to turn your trimmer.

I hope I make sense . . . and no, I havent tried it myself. Just a suggestion.

[Stocker]

Originally posted by millwood
would a DC servo driving an LED work?

Originally posted by Stocker
...and somehow you are motivated enough to design this circuit but not design away the offset voltage in your amp...

If you went to the effort to design in the DC servo, the whole thing would be unneccessary, n'est-ce pas? Although I suppose, If you are a gadget freak or tweaking is in your blood (surely no one here ! ) It could be another gee-whiz feature to show off...

Originally posted by sreten
Better to use the DC servo to make offset a non-issue.

Yes, if you like a DC servo

Boholm: seems like if you are going to do it, do it all the way: go for <10mV if you are making the effort. Heck, go for 0 if you want. but yes, this was the idea. A high-gain opamp would definately be a must. A bicolor LED would be one way, and a great idea for amps with dual supplies, BUT it would always be on. If I did it I would go for a single LED so I could have the satisfaction of seeing it fade to black as I zeroed out the offset. maybe an integrator...

It would be if you put it behind a translucent face plate and you didn't see anything at all besides the adjustment knob when the LED winks out.

[boholm]

Originally posted by Stocker

Boholm: seems like if you are going to do it, do it all the way: go for <10mV if you are making the effort. Heck, go for 0 if you want. but yes, this was the idea. A high-gain opamp would definately be a must. A bicolor LED would be one way, and a great idea for amps with dual supplies, BUT it would always be on. If I did it I would go for a single LED so I could have the satisfaction of seeing it fade to black as I zeroed out the offset. maybe an integrator...

It would be if you put it behind a translucent face plate and you didn't see anything at all besides the adjustment knob when the LED winks out.

Well, I just left a little headroom, so that it wouldn't be triggered too often if the amplifier had some fluktuations (spelling? meaning ) But yes, you could go so low - I would test it out before final assembly.

Why would it always be on?

And if you go for a single LED then you would only catch the DC going in one direction depending on how you soldered in your LED . . . . oh, maybe you actually mean two LEDs (how to say?) opposite coupled? (sorry my technical english - needs brushing off )

And the fading would look good, but it shouldn't fade too slow. Otherwise you could turn the trimmer too far and past 0.

[janneman]

What you would want is an integrator that senses the DC, basically a servo but without the servo output used to null the offset (because you want to do that yourself, right). Then take this output and stick it into what is called a window comparator. Basically two opamps in open loop that switch to HI or LO depending whether the offset is below or above a threshold. Combine the two to drive a LED and you have a bi-directional offset indicator.
Look up window comparators in Walt Jung's opamp cookbook, or ask google to look it up on the net.

Jan Didden

[thoriated]

This is close to a scheme I described a few weeks back here using an LED to indicate offset & bias. For offset, the output would be fed into a full wave 'ideal' rectifier using op amps which then drives an op amp configured as a current source driving the LED whose brightness is approximately proportional to the current passing through it. You can, of course, add integration or a LP filter to the above to reject audio band energy.

[Fred Dieckmann]

Perfect. Concise with a good recommendation for changes and a good reference. Also going to a linear IC website and doing a search for comparator data sheets will most likely get a good circuit in the application notes.