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Instruments and Amps Everything that makes music, Especially including instrument amps.

Hum from guitar amp
Hum from guitar amp
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Old 26th August 2013, 11:23 PM   #21
Jsixis is offline Jsixis  United States
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could be a bad jack, seen that many times on an instrument not on an amp though.
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Old 27th August 2013, 03:00 AM   #22
bparnell57 is offline bparnell57  United States
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If the hum occurs when your fingers are not touching any of the guitar's metal, it could be a grounding issue in the amp, or simply a bad grounding scheme, but honestly, unless it's excessive, that is simply the way it works, your body is used in the circuit...
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Old 28th August 2013, 12:00 AM   #23
Jsixis is offline Jsixis  United States
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if the hum goes away when the guitar is pointing north (or gets louder I can't remember) that is open coil pickups like strats have, you live with it or buy new pickups
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Old 28th August 2013, 12:50 AM   #24
geraldfryjr is offline geraldfryjr  United States
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I haven't read every post yet,But I saw that you get hum when you touch the strings.

This can happen if the ground wire that is supposed to be connected to the bridge gets disconnected.

It is also normal for the hum to change as you move the guitar around and it makes no difference if it is pointing north or south.
many times I have found it to be caused from a TV or Monitor.

Once lived in an area that this type of noise would start up at a certain time of the day every day, and I couldn't play the thing at all during those times.
But other times of the day it was dead quiet with no noise what so ever.

Sometimes CFL's can cause this type of notorious noise as well as bad cables.
I just got rid of the ones I was using because of this.

I have had cables go bad that produce humming noises and others that are dead quiet.

I bad pot in the instrument can also cause this if it is open on the ground side.



Last edited by geraldfryjr; 28th August 2013 at 12:55 AM.
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Old 28th August 2013, 05:32 AM   #25
gootee is offline gootee  United States
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The suggestions below might solve the problem completely, or make a significant improvement. Or they might make no difference. Or they might make it much worse. I think it will be the first one, obviously. But you never know.

Is there a small capacitor connecting pins 1 and 2 of the LM1875? (Pins are numbered from left to right, when looking at the front of the chip.)

If not, or if you can't tell, try a 50 pF to 500 pF (try 220 pF or 330 pF, first) cap between pins 1 and 2 (which are the + and - inputs, respectively; the LM1875 is basically a big "power op amp". The datasheet for it is at Don't leave any excess leads on the cap, and connect the cap body as close to the pins as possible. You can solder the cap's leads directly to the pin's pads, on the bottom of the circuit board, or, even better, you could solder it to the pins themselves, as long as you used something like long-nose or needle-nose pliers to heat-sink the pins, BETWEEN the chip and where you are soldering, and solder quickly (use a rubber band on the pliers handles to hold them closed for you, to clamp them on the pin you're soldering).

But as a temporary test, you could just wrap the leads around the pins, once or twice, and then cut off the excess (but make doubly sure that they aren't touching anything else!). Hopefully, you have long-nose or needle-nose pliers and diagonal cutters, maybe in two sizes each. If the temporary connection works, you could then just solder them to make them permanent. Actually, I would solder them, anyway.

If that little IC was an op amp, and it received the input signal and input signal ground before the LM1875, then ALSO install a similar cap between ITS + and - input pins. But you will have to download that IC's datasheet, to verify which pins they are. (I can't quite read the markings, in your photo. Sorry.)

A photo or two of the bottom of the PCB, and another one of the top, but from the other end, might help. Try to brace the camera on something, so the markings are clear, etc.


It could ALSO (or additionally) be the way the input signal conductor and input signal ground conductor are routed, both before and after the LM1875 (or preamp, if the 8-pin DIP [i.e. Dual In-line Pins] IC was an op amp). If those two conductors (input signal and its ground) get physically separated from each other, at all, such as by one of them taking a detour through the chassis, that makes a loop antenna.

So, to make sure that is not the case, you would want the input jack to NOT connect its ground to the chassis, and would probably also not want the volume pot to connect to the chassis (neither its body nor one of its pins). Instead, the input signal's ground conductor would ONLY connect to the ground end of the resistor that is connected from the first IC amplifier's input pin to ground (could be either + or - input pin). And if one pin of the volume pot connects to ground, it should connect only to the input signal ground, not the chassis or any other ground. So it could connect at the input jack or the ground end of the resistor at the first IC amp's input (either preamp's op amp, or, LM1875, whichever comes first after the volume pot).

If there are wires from the input jack to the PCB, they should be at least tightly twisted together, ALL the way from the jack to the PCB, with at least four twists per inch or more.

Likewise, ALL AC WIRING PAIRS, before and after the transformer, must be tightly twisted together, all the way to each end. And if one AC wire has to make a detour, to a switch or fuse, take the other one of its pair with it, even if it doesn't "need" to go that way. Separating ANY pair of wires that eventually form a loop (a circuit!) makes an antenna. But in the case of the AC wiring, they make TRANSMITTING antennas, which send out hum that will get into all of the other loops that have any "enclosed area" left open.

Also, IF that ground, from the resistor that connects between the first IC amps input pin to ground, SHARES a length of conductor with any other ground, on its way back to the common single ground point (which is possibly just downstream from the main filter caps' ground), then you will get all kinds of noise induced at the ground end of the input resistor at the IC (nasty voltages induced by all of the other ground return currents, across the inductance and resistance of the PCB trace itself), which will arithmetically SUM with the input signal (since the chip doesn't know what ground is; it just sees the difference in the voltages at the ends of the input resistor, as its input signal).

So, if the IC input-to-ground resistor DOES just connect to a trace that also carries the current from other grounds, such as the output, or decoupling capacitors (ones from power rails to ground), etc, then you could try disconnecting the resistor's "ground" end and running a separate new wire from that end of the resistor back to the main single-point ground, or just downstream of the last main filter cap's ground.

It's likely that the same IC input has a capacitor, in series, just upstream of that input resistor, with another resistor, on the other side of the cap from the input resistor, with the second resistor also going to ground. (See the LM1875 datasheet's schematic, for a picture of that type of setup.) If that is the case, then that second resistor's ground should also be disconnected, and it can go on the same new separate wire, to the main ground point. But first try it for just the first resistor, mentioned above (the one that's closest to the input pin).

If you have a preamp AND the LM1875, you would want to check the input signal grounding on BOTH of them, and make sure that it doesn't share any length of conductor with any other ground return, on its way back to the main ground point.

Needless to say, if the main common ground point is connected to the main filter cap's ground, but is BETWEEN any of the filter caps and the transformer, then the rectifier charging spikes will induce voltages across wires that are partly shared by all of the system's ground-return conductors, and the grounds will be noisy, everywhere.
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Old 28th August 2013, 10:09 AM   #26
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Guitar amps often hum because they don't know the words.
Support for Fender, Marshall and all Valve Equipment; Audio Innovations, Audiorama FU29, Quad and Leak.
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Old 24th February 2014, 11:04 PM   #27
strawberry is offline strawberry  United States
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This issue has been resolved. One pot sounded angrily noisy when agitated without its plastic knob, finger on metal shaft. Replaced the pot with another pot and the hum/noise went down to an acceptable low level.
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