120studio.com
April 3, 2015



Intro

In a previous article, I talked about phono pre-amps and how a cheap power supply can introduce noise.  In this article we're going to look at some other noise sources, and how to fix them.


Disclaimer


Working with electricity can be dangerous.  Don't fry yourself.  Mains power can be lethal.  If you don't know what you're doing... don't do it!!  I accept no responsibility for what might happen if you choose to follow any of the ideas outlined here.


A Quick Note


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Loud Hum

More Noise Sources

Speaker Crackle

Cheap Cables

Peripheral Power Supplies


Conclusion




Loud Hum


Someone tries to plug an RCA cable or mic while the power is turned on... and it makes an extremely loud hum that sounds like it will blow the speakers.  (Actually, it could.)

Never connect speakers or audio devices while the power is turned on.  It wouldn't be a bad idea to unplug your amplifier and everything else from the wall before you connect any device to another device.

Here's why you should never hot-plug RCA cables.  The center pin of an RCA cable will connect before the ground does, because the ground is that outer shroud which doesn't protrude as far.

If you have ever heard someone hook up an auditorium mic when the power was on, you've probably heard that awful noise for a brief instant.   Through a lecture hall P.A. system, it can be deafening.


Ground Loops


Basically, the different pieces of equipment are supposed to have the same ground potential at all times.   A difference of even a few millivolts can cause hum.  That small voltage difference can be amplified through the audio circuitry, just like any other audio signal.

You will notice that many pieces of stereo equipment have a connector on the back that says "GND".  Many stereo owners don't even seem to know what to do with this connector.  This is the chassis ground, where you'd connect a ground wire to tie together all the equipment. You could even run the wire to the main ground if you wanted.  This could eliminate hum, although it's hard to predict if it will work for you (too many variables). 

If you have multiple devices all connecting to the same ground, that doesn't always solve the problem.  The path to ground is unequal in the different devices.  There can be more resistance on one of the ground connections than another. 

Here's another common source of noise.  Let's say you have the RCA cables strung loosely between a few different pieces of equipment.  AC electromagnetic fields (which are common in all households) will have different intensities at different distances from the source.  Since the cables are far apart from one another, that guarantees there will be different voltages induced in them.  So right there, you've got a ground loop without even trying.

Sometimes you can fix this kind of ground loop by bundling all the RCA cables together closely.   Turning the slack portion of the cable 90 degrees can also help sometimes. 



Noise Filtering

The 60 Hz line-power can pollute your audio signal with hum.  Or, it can carry noise of other frequencies.   Noise can get through the transformer and into your power circuit.  From there,  it gets into your audio signal.

If you were building your own audio power supply, the first thing would be to use a filtering capacitor across the hot and neutral lines going into the transformer.  This would be an X1 or X2-class capacitor, usually ceramic, and maybe 0.1 µF.   Some of them are as high as 2.2 µF.  They should be rated for at least 250 volts AC.

When my old Panasonic CD player went bad, I removed the transformer and noticed it had one of these capacitors across the input lines.  Most audio devices have one, but I'm not sure all of them do.  Probably, some of the really cheap "wall wart" power supplies do not have one. 

Filter capacitors and inductors are also used after the transformer, after the rectifier stage, and probably elsewhere.   Better devices have all these.  But sometimes, there is still noise.

There are pre-made "hum eliminators" that try to filter out noise.

Actually, there are at least three types of hum eliminator

1.)  An isolation transformer for your AC line.  This plugs into the wall and filters out some "line noise". Make sure it's rated for enough wattage to support your whole stereo system.  (But see below.)

2.)  A differential amplifier.  This also plugs into the wall, but it tries to correct out any "ground loops" in your power circuit. 

3.)  This kind, which goes between two devices and tries to filter out hum that just won't respond to anything else.  There are some cases where even tying all the grounds together won't eliminate the hum, but one of these devices will.  Another alternative is this unit.   These actually work by isolating DC grounds from one another, which is the opposite of tying the grounds together.  Sometimes, that's what it takes to fix a ground loop!

An AC line isolation transformer is not necessary if you're already using one of these, which you should have anyway if you're even semi-serious about protecting your stereo from power surges.   Probably you won't need the differential amplifier, either.  I never have, anyway.  You could still need the third type of device, though, because a ground loop could still happen between your RCA cables.

Don't underestimate the importance of a good surge protector and AC line filter.  I've lost count of how many times I've seen people with tons of high-end stereo or home theater equipment, and they think they're going to protect that with a "surge protector" based on a 25-cent metal oxide varistor.  What they should have done was get one of these immediately;  it's worth it.  It also filters out most line noise.   And if for some reason you still have audio hum after that, then you could first move the RCA cables around or bundle them together.  And if that doesn't work, grab one of one of these or one of these for your RCA cables.

    





Speaker Crackle

If the crackle starts after you hot-plugged an RCA cable or a speaker wire, then it's probably a damaged speaker element.

Otherwise, check all the connections.   Maybe when you recoiled from the stereo to get away from that awful sound, you pulled something loose.

Another possibility is that your speaker has an electrolytic capacitor that finally decided to go bad. 

Another cause can be a bad potentiometer somewhere.  If the contacts have become oxidized or grimy, there can be crackling when you adjust the volume or some other control.


Cheap Cables


Cheaply-made RCA cables can actually be a major source of noise.  Cheap cables have very poor shielding, which means they act as antennas for interference.   This will cause hum and noise.

Cheap connectors can tarnish or corrode easily.  That will create a contact-resistance.  If you hear static, hum, or other noise when you twist the cable connections, it might be that you'll benefit from better cables. 

Remember, a contact resistance can cause ground loops... which can cause hum.

Get at least some semi-decent RCA cables such as these.    Sometimes, good cables can even fix a noise problem without your having to do anything else.   And if they don't, at least you'll know it won't be that in the future.





Peripheral Power Supplies

Many of these are built around the basic bridge-rectifier circuit.  That will be noisy unless it has good filtering design and components.  Read more about it here.

Sometimes the power supply is not peripheral.  A lot of stereo equipment has its own AC power-handling circuitry inside the chassis.  There is no easy way to swap this out for something else.  If you really know what you're doing, you could always solder in new capacitors.   Just remember that AC line power can be lethal.  And some devices have DC capacitors big enough to be dangerous. If you use good line-filtering devices, you may not need to do anything.

              

Conclusion

This has been a look at eliminating various types of noise that can haunt your stereo system.  Some people might think it's easier to use an all-in-one solution, such as a boom box, but you'll find that a real stereo system is worth the extra effort. 

Not every source of hum is easy to fix, but at least now you know how to fix the most common ones.

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