Noisy Audio Amp

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SETEC_Astronomy
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Noisy Audio Amp

Post by SETEC_Astronomy » Fri Jun 30, 2006 9:20 am

I have an audio amp that someone gave me and when I turn the volume up or down there is a terrible loud hiss/crack/static noise that goes along with it. The volume is controlled by a potentiometer. I would assume all this "noise" is coming from the pot but before I go and change it out I wanted to make sure. Also I wanted to find out if that is only part of the problem and if maybe there is more to it. Thanks.

Mike
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Post by Mike » Fri Jun 30, 2006 9:52 am

Is this with or without an input connected?

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Chris Smith
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Post by Chris Smith » Fri Jun 30, 2006 10:40 am

Several things can make a noise in a amp.

The pot naturally, the final transistors, and the dummy load type of resistors out of the mains can cause "noise". [Big low value resistors]

Many things can make noise, however.

If your noise goes up and down with the knob turning, it is most likely the pot. Especially if you can find a bad spot in the pot and constantly repeat this bad noise or hiss?

However if the noise is "on its own" and follows the sound level and input of your amp with out adjustment, its most likely an active component causing the noise.

If the noise changes with temperature, its also most likely an active component.

Also bad grounds can cause a terrible noise.

SETEC_Astronomy
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Post by SETEC_Astronomy » Fri Jun 30, 2006 11:33 am

Sorry I should have been more clear. The noise only occurs while adjusting the volume. So I am fairly certain that it is the pot and pot alone but I figured I'd ask the more knowledgeable. It does sometimes hiss and crack while just sitting there but it seems to coincide with loud spots and bass so I think the vibrations from the speaker are enough to make the pot shift slightly and crackle.

jimandy
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Post by jimandy » Fri Jun 30, 2006 2:53 pm

Before you go to the trouble of replacing the pot you might try squirting some contact cleaner in there and rotate the pot vigorously. Turn back on and see if the problem is improved.
"if it's not another it's one thing."

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Bob Scott
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Post by Bob Scott » Fri Jun 30, 2006 3:26 pm

I think you might have a combination of problems, one being a worn control pot. Beware of using contact cleaner as your only cure, even if it works. That solution usually lasts maybe a month. I think you may also have DC voltage on the control due to a leaky electrolytic coupling capacitor in series with the control. Check for DC voltage on the control and replace any leaky cap first. Spraying a SMALL amount of contact cleaner into the control should then also be a longer lasting improvement but beware. Don't use the spray unless you have to. Some controls are assembled with lubricant grease that makes the control feel smooth when it rotates. Too much contact cleaner can dissolve this lubricant leaving the control feeling "cheap" and sloppy to rotate. Fixing this problem properly is a good thing 'cause the original pot may be hard to come by.
I think this is your problem.

Other sources of noise would be noisy signal transistors. These are unusual. I haven't seen one in decades, probably due to improvements in manufacture. These were more common back in the late 1960's when germanium transistors were common. Both germanium and silicon noisy transistors are usually temperature sensitive. They can be troubleshooted (troubleshot?) by heating them up with a heat gun or soldering iron to bring on the noise. Cooling with freeze-spray takes noise away immediately, and ergo, immediately identifies the offending transistor.
I don't think this is your problem.

If it's a final output transistor, it's usually an overrated PNP device used in a cheap department store amp. These go intermittent open collector, and when the collector goes open, there is a LOUD POP followed by massively distorted sound.
I don't think this is your problem.

Good luck.

Bob :cool:

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MrAl
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Post by MrAl » Fri Jun 30, 2006 5:07 pm

Hi there,

I've had that problem too with an amp i once used a lot.
It had 'slider' type pots and they would make noise,
i would spray with contact cleaner and it would go away,
then come back like someone else said about a month later.
Spray again, wait one month, spray, month, spray, month,
you get the picture :grin:
I didnt feel like replacing the pots.
LEDs vs Bulbs, LEDs are winning.

Robert Reed
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Post by Robert Reed » Fri Jun 30, 2006 5:39 pm

There are contact cleaners and then are contact cleaners. If you can find it, try "QUIETROL". Best contact cleaner I ever used. Make sure you get the liquid form with eyedrop bottle. It is red and smells like a french cathouse. This stuff also has a longlast film lubricant in it.

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jollyrgr
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Post by jollyrgr » Fri Jun 30, 2006 7:01 pm

AHHHHHHHHH!!!!!!! Don't make this so complex!

This is clearly a dirty pot! I've fixed untold numbers of these in everything from mini radios, big stereos, and even color adjustments in older TVs. I used TV Tuner contact cleaner. (Talk about the old days!) Radio Shack STILL sells this as tuner cleaner even though mechanical tuners have been gone for years. Another source contact cleaner.

Image


This is available from:

http://www.allelectronics.com/cgi-bin/i ... _OZ_.html#

There are other brands and this is only an example. Follow the advice of jimandy and Rober Reed. MrAl is right about some of them. I have a receiver with a switch that acts that way. (Nearly every one of this type of receiver I've ever seen has this problem. The switch that selects the source goes bad, mostly the right channel. No amount of cleaning ever fixes it.)[/b]
No trees were harmed in the creation of this message. But billions of electrons, photons, and electromagnetic waves were terribly inconvenienced!

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