Odd Op-amp behaviour

[kloan]

I was listening to my cmoy, with an OPA2227 and I noticed when I touched the metal chassis, if the music was paused, I could hear a quiet squeelching sound, almost like a worn fan belt noise from a car.

Out of curiosity, I decided to have a listen to a cheap op-amp I picked up at the surplus store last year, but never got around to using it. It's a TL082CP, and the first thing I noticed was that it had a little more bass, and was not quite as bright.. which is kinda funny, because the OPA2227 isn't considered bright at all. But the second thing I noticed was that if I touch the chassis there is no more squeelching.

Any idea why this would be?

Btw, I must say I actually like this op-amp.. am I totally crazy??

 

[ECM]

i suspect some oscillation is going on. i have used the 2227 in the past and it seemed to be real picky. the tl082, as you mentioned, is a cheap opamp, however, it is more tolerant to poor layouts. the 2227 is a "faster" opamp with a wider bandwith than the th082, so try to bypass the power pins with some .1 uf caps.

ECM

 

[tangent]

Quote:

when I touched the metal chassis...I could hear a quiet squeelching sound

Sounds like AM RF noise coupled into the amplifier. Check your grounds. Also, measure the amp's current draw with nothing plugged into it. What value do you get? Does it go up when you touch the amp? Does it change when you plug headphones in? Is it there with the source unplugged?

Quote:

am I totally crazy??

Psychological diagnosis will have to wait until you get the 227 behaving properly.

 

[kloan]

2.0 at the right channel, 5.4 at the left for the TL082

1.3 at the right channel, 1.1 at the left for the 2227

Neither went up or changed when I touched it or plugged the headphones in.

For the 2227, without the source plugged in.. the squeelching isn't there when I touch the chassic, but it does make a click sound.

 

[kloan]

huh... the TL802 just died! it's now really quiet and if I turn up the volume it distorts like crazy... dunno how that happened..

 

[tangent]

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2.0 at the right channel, 5.4 at the left for the TL082

Units, units! 2.0 what?

In any case, I asked about the current draw of the amplifier. The fact that you've taken left/right measurements tells me you didn't measure that.

Break the power line, put your meter on milliamps, plug the leads into the right holes for a milliamps measurement, and use the meter to "heal" the broken power line. You should read something on the order of 20mA.

Quote:

For the 2227, without the source plugged in.. the squeelching isn't there when I touch the chassic, but it does make a click sound.

You most certainly should not be hearing clicks when you touch the chassis. You may have tied a rail to the case, or something like that.

Quote:

the TL802 just died!

IC op-amps aren't trivial to kill. You have to do something clearly bad to them.

 

[kloan]

Oops, sorry I misunderstood.. I measured the output.

But before I measure the draw... I noticed something. Instead of plugging the cable directly into the iPod, I plugged it into the remote first, then the remote into the iPod. All the noises are gone. I think the cable is crap. The jacks are metal, and come really close to the metal body of the iPod.. if I bent it just slightly to make contact with the iPod the left channel cuts out completely... so I'm thinking just having it really close to the metal on the iPod was causing the sounds.. could this be the case?

Also, I don't know what I did to wreck the TL082.. all I did was unplug it like the 2227 a few times, that's it. But there's definately something wrong with it now. The 2227 still sounds fine.. so I dunno...

 

[tangent]

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I think the cable is crap.

Could be, but it's too easy to find out to tolerate speculation. Try a different cable, and you'll have your answer.

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I don't know what I did to wreck the TL082.

What supply voltage are you feeding the amp?

 

[MD1032]

Yikes! The first thing I would do is cover the inside of your enclosure with electrical tape, eliminating ANY chance that something is grounded to the case.

 

[kloan]

Quote:

Try a different cable, and you'll have your answer.

Unfortunately, I only have the one. I'll have to pick one up tomorrow.

Quote:

What supply voltage are you feeding the amp?

I'm using one 9V battery.

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Yikes! The first thing I would do is cover the inside of your enclosure with electrical tape, eliminating ANY chance that something is grounded to the case.

I've wrapped it in a soft static free cloth, so I'm pretty sure it isn't grounding.