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Old 10th August 2013, 11:59 PM   #1
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Default Hum from guitar amp

This is a Challenge CH-BA20 guitar amp. It hums when a 1/4" phono plug is plugged in, otherwise not. What could be the cause of this?

The power is drawn from two of the wall outlet poles, plus and ground, as far as it looks to me, but I'm probably wrong. The 1/4" cable connector looks clean, functioning. Nothing burnt. I can support a helper by providing voltage readings.

Any help is much appreciated. The amp hums too when the guitar is plugged in.

P.S. It's the type of static hum that goes away when I put my finger on the plug casing or guitar string.

P.S.S. The hum goes quiet when the two areas of the plug are shorted, but with a wait before contact there is a static pop, then silence. Could a capacitor on the board be bad? Which one if so?
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Last edited by strawberry; 11th August 2013 at 01:16 AM.
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Old 11th August 2013, 06:53 AM   #2
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When your guitar is plugged in but with the volume backed off all the way, is it quiet?

A cable plugged into an input on an amp like that is an antenna for any stray RF, as typically are a guitars magnetic pickup.

Depending on the guitar's shielding design, or lack thereof, even humbuckers (twin coil pickups) can seem noisy because the entire circuit is picking up stray RF.

What you are most likely picking up, is the noise field generated by the amp its self.

When you plug in the guitar and the move away from the amp or change the angles at which the guitar faces away from the amp, does the hum get quieter and louder depending on which way it is facing?

If so, and if things quiet down when you touch the strings, it sounds like it is a properly working modestly priced/shielded setup.
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Old 11th August 2013, 08:10 AM   #3
tinitus is offline tinitus  Europe
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Originally Posted by strawberry View Post
P.S. It's the type of static hum that goes away when I put my finger on .......(edit).... guitar string.
that's the way it works when playing

anyway, never leave your instrument without turning down att knob on preamp

might be you should look at your cables first, or instrument
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Old 11th August 2013, 12:40 PM   #4
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Yes, the static hum changes when the cable is turned in the air like an antenna. One cable is noisier than the other. I should bring the combo to the music store and pick the least noisy cable they sell? It doesn't make much of a difference if the guitar is plugged in or not. I'm going to try to ground the plug casing with the amp ground next. Is the a way to shield the amp? Perhaps wrap the transformer in aluminum foil?

The tone and attack of this cheap combo is otherwise good. A $50 CL find.
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Old 11th August 2013, 12:57 PM   #5
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it might hum when guitar is unplugged

you should always turn the att/vol down when doing that
and I suppose problem gone

when guitar is connnected you will always have it strapped and touching the strings
and problem gone

might be different, but also normal
but could ofcourse be worse if you have a defective plug or cable

not sure, but maybe a worn out pot can do it
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Old 11th August 2013, 03:47 PM   #6
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Buying a $100 cable won't get rid of the hum. Any decent cable will have more or less the same amount of shielding. The problem is probably in the grounding scheme of the combo's circuitry (for example using the chassis for both safety and audio ground). Noise is coupled into the audio path because of this. What frequency is the hum??

It might be as simple as connecting the input jack's ground directly to chassis, but it could matters worse. Depending on the design it might create a groundloop or you might create the low impedance path it needs to shunt noise away from the first gain stage. But it's worth a try.

Further amp shielding doesn't seem necessary since the problem only exist when a cable is plugged in, so no interference is picked up by the amp itself.
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Old 11th August 2013, 04:48 PM   #7
tinitus is offline tinitus  Europe
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well, my amp hums too if I unplug the guitar, with volume turned up
so I never do that

its also a bit noisy when left unattended, with volume turned up
so I never do that

when holding the guitar by the strings its almost 100% noise free
so thats what I always do

but I never swing the cable in the air when playing

btw, your amp may b e mounted with input plug that shorts when the jack is pulled out, making it noise free
if you just pull out the jack from the instrument, this shorting function does not work, ofcourse
and the result could be noise
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Old 11th August 2013, 10:33 PM   #8
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It's a wide range static, angry noise. Turning up the treble or volume makes it worse. Using a shielded woofer didn't help. Semi shielding the transformer with a tin cup with ground contact didn't help. Grounding the plug casing didn't help, with no voltage reading above 0 there. Shielding, grounding the pickup well didn't help.

The noise goes quiet when the amp volume or guitar volume is dialed all the way down.

At very low practicing volume it's doable. Frustrating. I could ground my self to the bridge of the guitar with wire for less noise.

Something must be bad about the amp. How could they put this crap on the market in the first place if not?

Last edited by strawberry; 11th August 2013 at 10:36 PM.
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Old 14th August 2013, 06:37 PM   #9
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Does the amp hum if you only go in the CD input?
Is your guitar a single coil pickup by any chance?
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Old 16th August 2013, 01:34 PM   #10
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With the CD input there is limited volume and less noise, but the same type of noise. It's a two-single pickup bass guitar, P-BASS.

I also tried to replace a couple of capacitors, no improvement. Undid the contact between the cooling fin and ground, no improvement. Bypassed a hissy pot, no improvement.

I read that a bad transformer or bad resistors can cause arching popping noise. It's more noise than hum. But this problem is one others have had with this amp. It may be a design flaw. As it is I have stopped worrying about it. The IC is a LM1875T which I should be able to find an audio project for. There already is the power supply and pots, so I can pick parts and reorganize to a working amp. Or I can buy a different IC if there is a very easy circuit out there I can copy. The transformer gives 2x17 V.
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