Can I get some help from some amp gurus

[nicholas_23]

I aquired a JBL PB-12 powered sub with a nasty hum when the amp is plugged in to AC. I pulled the amp from the cabinet and plugged in another speaker to it and get the same hum. There is no source connected to the amplifier for what its worth.. so as it sits I've tried another speaker and no source going to the amp... still get the hum. Nothing looks chared on the board with the exception of one the smaller "transformers" , this is a little dark around the base of the board but nothing too alarming. Anything else I should be looking for or at? fuse is good , caps looks to be good.

Hoping maybe Aaron is still around , I know he gets into amplifers. I know Curt also works with electronics quite a bit also..... if you guys are out there I'll owe ya one.

Thanks
Nick

[Ludo]

Re: Can I get some help from some amp gurus

I aquired a JBL PB-12 powered sub with a nasty hum when the amp is plugged in to AC. I pulled the amp from the cabinet and plugged in another speaker to it and get the same hum. There is no source connected to the amplifier for what its worth.. so as it sits I've tried another speaker and no source going to the amp... still get the hum. Nothing looks chared on the board with the exception of one the smaller "transformers" , this is a little dark around the base of the board but nothing too alarming. Anything else I should be looking for or at? fuse is good , caps looks to be good.

Hoping maybe Aaron is still around , I know he gets into amplifers. I know Curt also works with electronics quite a bit also..... if you guys are out there I'll owe ya one.

Thanks
Nick

Nick, I am not familiar with this amp but does the hum go away when you turn down the volume/gain control? If not, then I would suspect there might be a problem with the power supply. If yes, then there could be something wrong with the input wiring to the connector. Louis

[johnnyrichards]

Re: Can I get some help from some amp gurus

I used to have a 60 Hz hum in one of my subwoofers, I actually had to tighten the RCA output jack screw on the receiver, it went away. I suspect you will not be so lucky though. Have you tried different outlets? It may be something as simple as a ground loop, which can often be remedied without soldering.

[nick29498141]

Aaaaarrrrrooooonnnn!

Think of it as "SHARON!" call from Ozzy... C'mon Mr. Envision, I bet you know what's up here...

[AJ]

Re: Can I get some help from some amp gurus

You're going to have to do some troubleshooting as there could be 101 causes for humming. (Someone should write a book...)

I would be looking for bad solder joints (even use a magnifying glass), leaking or bulging caps, and bad physical connections at jacks etc... If you had an oscope this would be easier as you could see if the hum is on the power supply rails.

Good luck.

[curt_c]

Re: Can I get some help from some amp gurus

Since you have nothing else plugged into it, I'd suspect the power supply filter caps are failing.

–However….

Doing a little internet search on the PB-12 shows it has had a propensity for amplifier issues. Probably the best solution is to get your money back, OR replace the amp…:D

C

[Hi Fi Guy]

Re: Can I get some help from some amp gurus

The humming as a symptom is common with expensive repairs. It is also common to other situations. You must troubleshoot. Does it hum because the hum is introduced at the input, or does it hum because of internal failures? Sub woofer amps tend to have switching power supplies and even switching amp circuits. These are cheap to manufacture, light and cool in operation but very difficult to troubleshoot and repair. Even an experienced amp 'guru' would have to do tricky inspection to find the problem. No easy answer here.

[Hackomatic]

Re: Can I get some help from some amp gurus

Because of their very design, switching supplies are not known to produce 60 cycle hum. If it is, then you'll have to dig deep into its bowels to find it. I will concur with others that you could make a career out of trying to fix this, ultimately succeed, AND STILL HAVE A PIECE O' JUNK! :eek:

I've been a JBL man for decades, but their consumer grade HT stuff is CRAP! :(

[envisionelec]

Re: Can I get some help from some amp gurus

I aquired a JBL PB-12 powered sub with a nasty hum when the amp is plugged in to AC. I pulled the amp from the cabinet and plugged in another speaker to it and get the same hum. There is no source connected to the amplifier for what its worth.. so as it sits I've tried another speaker and no source going to the amp... still get the hum. Nothing looks chared on the board with the exception of one the smaller "transformers" , this is a little dark around the base of the board but nothing too alarming. Anything else I should be looking for or at? fuse is good , caps looks to be good.

Hoping maybe Aaron is still around , I know he gets into amplifers. I know Curt also works with electronics quite a bit also..... if you guys are out there I'll owe ya one.

Thanks
Nick

Ah yes, this is a fun one. You should feel lucky that it only hums and isn't sending 63V to your subwoofer. I have schematics for it, but you'll find that the problem is embedded in a metal canned module (some have heatsinks on them) with an AMI number. Someone over at DIYAudio disassembled that thing (it's epoxy potted don'tchya know!) and drew up a schematic. In basic terms, it is surprising that the design ever worked at all!

Best bet? Replace the plate amplifier.

[Æ]

Re: Can I get some help from some amp gurus

I'd short out the input (RCA jack) to see if that is where the problem originates. If shorting out the input makes the amp quieter then you know where to begin fixing. Inputs are often very susceptible to picking up radiated 60Hz.

They actually make shorting plugs for use in unused inputs.

[nicholas_23]

Re: Can I get some help from some amp gurus

Thanks for the input guys! I think at this point it would probably be best then to look into a new amp. This ones kind of an odd ball in that it is shaped like a rectangle and not a square. Good luck finding something like that aftermarket right....

I'll see what its going to take to hack off the back side and replace it with one of PE's amps and another hunk of MDF.

Thanks again guys , I really appreciate all your time !
Happy new years all !

Nick