Hum from guitar amp

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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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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.
 
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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it might hum when guitar is unplugged

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

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

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
 
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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Joined 2005
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 :D


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
 
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?
 
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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.
 
It seems your grasping. Changing random parts won't do you any good. Do you own, or do you know anybody with an osciloscope?? It can tell you where the noise originates from. Maybe it's a massive PSU ripple, maybe a faulty component. I'm still betting on a bad ground connection/bad ground design/ground loop type issue.
 
I don't own an oscilloscope but could see myself buying a used one. Would you start at the transformer and move up the chain? I'm afraid this amp is dead because I removed a second small push-in IC in case that was the source of the noise. Wasn't. I lost it. But this made in China amp deserved to be put out of its misery. I see no bad soldering joints. It's annoying because the board is so small, very few components. Could be that the lost IC surfaces if i look for it.
 
But this made in China amp deserved to be put out of its misery.
:D hahaha

It depends where I would start measuring, but a clean DC supply for the PSU is easy to check. Just probe it at e.g. the supply pins of the opamps and you're done. Next, I would check where in the signal path the noise would become apparant. etc. etc.
I wouldn't want to miss my scope. Vital piece when you'er serious about working on electronics. Second hand, there are some real gems at a very reasonable price.
 
:D Second hand, there are some real gems at a very reasonable price.
You are'nt kidding!
Just looked for fun...
The Tektronix I bought 30 some years ago when I started that cost me $900 can be had for $250.

The one I wanted, but was $2,000 (2465DVS) is on a BIN for $350 for ome that was calibrated less than a year ago from an avionics firm.

It would be great if you could borrow one to have a peep at your power, and then work your way into amd through the amp.

The fact that you say everything quiets down when you plug a guitar in and either touch the strings or turn down the guitar does sound like something poorly grounded at the input stage.

Which could be a simple design flaw.

I think a very expensive cable coule be the solution to all your problems, but only if you buy it from me!:D

We make a special one with two conductors and molded ends...

Or is that moldy ends?:scratch1:
 
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If it goes away when you touch the strings, your instrument may not be properly grounded inside.
Check the continuity (ohms) between the ground of the 1/4" jack (the outer edge) and the bridge. Zero means it's grounded. No response means it's not. If it's not, it needs to be.

Also, move closer and away from the computer monitor or TV. If it changes, you may need shielding as well.
 
a few days ago i rewired a bass guitar, and forgot to connect the ground wire coming from the bridge ... :eek: now I know why its there :p

Never leave a previously connected wire unconnected. It increases the probability that the smoke will leak out. Then you'll need to get a smoke technician to seal the leak and refill the system with smoke. Oh, you'll rarely see smoke coming from a guitar. They run at a very low level (ie. very little smoke) and it's mostly spread through the gobbazillion winds of those teentsy wires wound up in the pickups. It could take a long time for it to leak out, so you'd never notice. Ever seen someone replacing transistors and such in failed circuits? New smoke. In the old days we used vacuum tubes to keep a vacuum on the system, keeping the smoke in. If one failed, the smoke might escape, so we used several tubes in most designs. But we also used things like capacitors filled with paraffin so we could occasionally enjoy a good smoke-letting, and to keep the users scared, and in awe of our spectacular courage for sticking our hands in places where smoke recently came out. Now, we didn't really have to crouch down on the floor behind the old DuMont console to do this, but when we saw that it all started with their kid dropping pennies through the vent slots like it was a big piggy bank, we had to hide our red faces lest we laugh out loud. And THEN when we saw that apparently mom already knew what happened, and tried to get the pennies out, but in so doing saw all the dust whiskers growing on all the hi-V components, well she was CERtainly not going to let ANYone think she was a poor housekeeper, and so she took a vacuum cleaner to the insides. Which explains why several of the tubes are broken off at the base. So rather than laugh at her right out loud we had to hide behind the TV and take a hit off of our smoke refill canister to calm down. Which is WRONG, just WRONG, you should never take a hit off your smoke canister. So we'd take two, to even things out.

Nowdays everything is digital. It's all ones and zeros. If a leak happens, the ones leak out (because zeros are, after all, nothing, right?). Then you have to get a ones specialist to re-insert the ones in the right places. That takes an awful lot of specialized knowledge. You have to be able to count up to one, over and over. And sometimes it takes kids with weird eyes standing in a corn field saying " L i n u x i s e v i l . R e i n s t a l l W i n d o w s ." over and over, But I digress, which is the opposite of congress, or maybe not. And congress is part of politics, which is derived from poli (poly, meaning many) and tics (ticks, blood sucking parasites). How they got into my smoke refills, I have no idea. I hope they didn't get into my pudding. Nurse? NURSE! Where's my pudding? The ticks stole my pudding again! Wait, I think I found it. They hid it in my Depends. Oh ... that's not pudding.

-- Smoke technician since 1962. Some of these stories are true. The names were changed to protect the innocents from me, or the other way around. Wait. What's my name? Oh, here it comes, in digital: onezerozeroonezerooneonezeroone... crud, I lost count. Can we start again? Hey, did you ever notice that ones have a pointy part on one end, and a small perpendicular crossbar on the other? Kind if like a diode symbol. I think maybe ones have polarity. Someone call Art Bell.
 
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