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Make it (the buzz) stop... please

Started by idle_chatterbox, August 13, 2006, 01:10:09 PM

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idle_chatterbox

Hi Teemuk:
I had a few questions before I tear into the amp again.

QUOTE:
"1. Connect the green wire from the power amp to the ground leg of a jack not connected to preamp in any way (i.e. hanging in the air). You either need a new guitar jack or have to remove one completely from the amp."


I'm guessing that the power amp is the PCB in the back. Is that correct?



"4. If you had DC you need to decouple the signal with a capacitor. At this point it is also worthwhile to check if the amplifier does the decoupling in either pre or power amp. If there's a decoupling cap in the power amp but you still have DC you have located a leaky capacitor."

Could you maybe elaborate on what it means to decouple the signal?

Thanks for the help. Just want to double-check that I know what I'm doing here....

:)

teemuk

Sorry for the delay... Yes, the power amplifier circuit is located in the PCB mounted to rear panel.

"A Decoupling capacitor decouples one part of a circuit from another. That means it makes one part of a circuit unaffected by things going on in another part of the circuit." -Wikipedia

In this case you want that DC voltage potentials in different parts of the circuit connected together will not affect each other. Since capacitors block DC and pass only AC signals they will "decouple", for example, two gain stages from each other. i.e. The DC potential in the output of the first gain stage will not pass to the input of another preceding gain stage - only the audio signal (which is AC) will pass.

idle_chatterbox

Thanks Teemuk, I think I follow. I'll give it a shot.  :tu:

idle_chatterbox

whew.... ok, finally found the time to tear into the Noisemaker 2000 amp again.

Here's what I found, using the diagnostic of swapping in a guitar jack to the two wires coming off the power amp.

1. hooked up the jack, using alligator clips. As soon as they were hooked up, checked for DCV. Didn't have any. There was a low hum. Nothing like the hum I had been dealing with (which is anything but low).

2. of course, turning the volume knobs on the front panel has no effect, but I admit I tried it anyway  ;D

3. just for the heck of it, plugged in trusty Pandora. low hum went away. As in completely silent like the amp was turned off (seriously). Now, if I switch pandora to a high-gain setting (e.g., "distortion") the hum gets reasonably loud. But I thought it was interesting that on the low-gain, clean settings, the amp is dead quiet.

4. grabbed a guitar and plugged it into the test jack. No pandora, just guitar. Reasonably loud hum. Not like the original bad hum, and has this significant difference: it varies with the signal from the guitar, which the original bad hum never did. In other words, if I play a string, the hum gets louder. If I mute the string, the hum settles down (but still hums).

5. plugged in guitar through pandora. Sort of like before; on high gain simulations, hum got louder, on clean settings, the amp was almost dead quiet, and didn't do the thing of humming louder as the strings were played.

6. grabbed another guitar. A little quieter. Admittedly, the first one does have some fairly hot pickups, and is noiser no matter which amp, than the other guitar. But as long as the guitar signal goes through Pandora, it is playable, and there's none of the increasing/decreasing hum.

any thoughts?


this amp.....         :trouble

joecool85

Sounds like you may have it as good as you can get it. I've seen plenty of amps from the factory that have hum like you speak of.  On that same note, if you work at it hard enough, its not impossible to totally get rid of hum from the amp.  However, hum from guitar you may not be able to get rid of without working on the guitar.  Single coils are known for hum, so that could be part of your issue with the guitar if its a SC setup.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

idle_chatterbox

naw, not a p'up issue. Wish it was, but it aint. It's pretty much the same noise whether I plug in a single- or humbucker guitar. Oh well. I'm tempted to gut it for parts.  xP

Got plenty of working amps, thank god, so this one was just a project.

teemuk

Joecool85 is right: The amp can be repaired to be better but it will most likely mean a significant amount of work. Judging by your description the hum is introduced by a bad grounding path in the preamplifier - probably located in one of the last stages in the preamplifier. However, sorting that stuff out in the particular amp will not be a very easy job; the grounding between the two panels is done very poorly. Actually, IMO the whole grounding is done poorly. If the hum is caused by a part gone bad you at least have some hope of easier repair job but it means pretty much "shotgunning" the design (swapping all the parts that seem likely to have failed).

"First aid" hum curing could be:
1) Replacing all jacks with insulated ones, insulating the front panel from the PCB or reducing the amount of connections to it to 1. (Could even make things worse - experiment).
2) Shorting some ground paths with jumper wires to main ground point. Again, experiment.
3) Connecting the front and rear panels together with a metal plate. Recommended.
4) Replacing all electrolytic capacitors or at least filter ones. Very likely will not solve the problem but might hide it since this reduces the amount of ripple.

Some extra work would include complete rerouting of the ground return paths. I started a thread about grounding methods and I guess you saw it as well. Following the instructions presented there you could make the amplifier almost hum free, problem is: the stuff is very difficult to implement into finished designs. Very likely you would have to do stuff like cutting PCB traces etc. so I would not bother unless the amplifier's tone is otherwise worth to die for. Personally I'd just replace the jacks with insulated ones, make a solid chassis with external metal plate and experiment with shorting the ground return paths to main ground point from few logical places. This is quite a lot of work if you get into it. (You have to trace the amp schematic to see what parts of the circuit are worth looking into etc.) ...and it may still prove fruitless.

BTW, the hum getting louder during louder signals is normal: The amplifier is drawing more current to produce its power output, while it does this the supply voltage will sag, the filter capacitors can not charge fully since the current is required elsewhere and as a result of all of that the amplitude of ripple voltage grows higher. Because you do not have a clean DC supply the output signal will be modulated by the ripple voltage. Since your amp is most likely a single supply design the effects of ripple in the supply will be even more notable. The guitar plugged straight to power amp is also most likely a serious impedance mismatch as well, which might exaggerate the effect of hum. Since this was intented to be merely an experiment to see if the problem was in the line feeding the power amplifier I guess you should not care too much about the effect. If it is a great concern two things might help:

1) Replacing the filter capacitors. Aged filter capacitors can grow a high internal resistance which will weaken their ability to charge and discharge properly.
2) Adding more filter capacitance. I do not recommend this: Large supply capacitances stress the amplifier during power on (while the capacitors charge your amp is basically shorted) and can stress the rectifier diodes even during normal operating conditions (larger capacitance requires larger supply current thus the rectifier has to dissipate more waste power).

idle_chatterbox

Thanks Teemuk. I think you're probably right. I will probably just reincarnate the amp into something else. The case is very large and sturdy, so I was thinking of turning it into a nice cabinet/speaker enclosure. I'm sure I can use the pots for my other DIY stuff too.

Thanks for all the advice in trying to save the patient!  0:)