Why do amps hum and receivers dont? (1 Viewer)

[RyanDinan]

I have a quick question that's somewhat related to this:

I have a JBL PB10 subwoofer - it's about a year old. Just recently, I've noticed that I hear a very audible buzz from it. This buzz continues even if I unplug the audio cable running to the receiver, so I don't think it's a ground loop problem.

The sub's power cord is NOT grounded (only a 2-prong). As soon as I plug the thing in, I hear the buzz. I've tried different outlets around the house, and I get the same result.

Someone told me the power transformer may be loose, or going bad. I opened the sub up, and tightened the power transformer's mounting bolts, but that didn't seem to help either.

Im thinking the thing may just be going bad. If I leave the sub unplugged for a couple hours, then plug it back in, the buzz isnt as loud - But it gets louder after a hour or so...

Does anyone have any ideas? I've contacted JBL, and they pretty much told me the same thing....and to have it serviced (I think it's out of warranty now too...my luck).

Thanks!

-Ryan Dinan

 

[Matthew Speed]

Two very important things to have when running a system of separates:

1) A great Line Conditioner and/or Voltage Stabilizer. Remember, your amp is stupid, it will amplify noise right next to signal and not even feel somewhat guilty. Garbage In, AMPLIFIED garbage out. Also will eliminate ground loop interference, and clean up all your AC power. Monster's HTS-5100 is my choice.

2) Use good quality Interconnect Cables from the processor to the amp, and make sure the power cord is as far away from the cables as possible. Used balanced cables whenever possible. Whatever you use, the more shielding, the better. If the power cord must cross the interconnects, make sure it does so perpendicular.

 

[Richard_s]

Many here talk about HUM and BUZZ and it is important to know if we are talking about noise from the device or from the driver (via the ampplifier). I have the PB12 and when the system is in standby "red lights on" there is a low HUM if I get down on my hands and knees and listen. This Hum is definitely comming from the driver so even in standby there is hum. I also turnrd on my system so that the PB12 came out of standby and then "muted" my receiver and the hum from the driver is the same as when in standby.

One of the concerns I have with the PB12 (only have mine a month) is that even in standby the system draws 26 watt and the transformer is running at 60 deg C and the near by capacitors are also being kept quite warm. Also the fact that I can hear a hum from the driver in stanby tells me that the amplifier circuit is also being kept somewhat alive. I was concerned that over time by leaving the PB12 in standby that the continuous heat situation might cause the plates in the transformer to loosen and/or devices like capacitors to start to get internal leakage currents which will cause problems. As others have said it could be the transformer but if the buzz you talk about is from the driver then it could be the capacitors starting to fail.

I always turn my PB12 off when I turn my receiver off. I can't prove if this is better but I feel it is better in the long run. I built a relay box that plugs into my receiver. The receiver trips the relay ( the connection has an inline fuse to protect my receiver) and the relay contacts are wired to a separate wall plug that supplies the current to the subwoofer using the relay output contacts. I did this since the switched outlet on receivers can not supply the wattage required for subwoofers. This Relay box cost me $20 parts approx and can handle 10A @ 120volts (1200 watt).

One more thing on grounding. If the receiver or any other two prong device has a WIDE blade then that is the neutral and most likely is also the ground for that device ( in a receiver this would mean that the pre-out sheilds may be grounded via the neutral). This neutral ground is tied directly to earth ground at you circuit breaker box per code. Even two prong devices can enter into the ground loop issue. Check your equipment and see (using an ohm meter) if the RCA jack sheilds are tied to either one of the blades in the plug (even if both blades in the plug are the small size). If one is tied to the RCA jack sheilds then plug the unit in with that blade in the "wide" slot on the wall plug. This may help some grounding issues. Just a thought.

EDIT: I took a quick look at the pB10 and PB12 schematics and the grounding scheme used is not dependant on wall plug (that is why both prongs are small). The ground system is balanced by using the center tap on the transformer but there are large capacitors (create the equivalent of a ground for the amplifier circuit) that if they start to have trouble will cause problems for the amplifier.

 

[RyanDinan]

Thanks Richard -



The sound I am hearing is more like a buzz, rather than a hum. Kinda like flouresant lighting fixtures....

The sound seems to come from the back of the sub, and not the speaker itself - But it's kinda hard to tell. When I had it opened up, I plugged it in, and sounded like it could be from either the electronics, or the driver.....



Like I said, the sub can be completely disconnected (no cables connected to it, or other equipment), and as soon as it's plugged into an outlet, it buzzes... It definately sounds like a power issue and not a ground loop issue, since I have nothing connecting to it...

Is there anything I can look for on the PCB with all the electronics and power transformer? Everything looks okay....no loose connections, etc...

The transformer was still secured very tightly when I initially opened the sub, but I tightened it even more for good measure...

Or is there something actually inside[/] the transformer that gets loose and causes a buzz?

I have a digital multimeter - If it's the capacitors that are failing, how would I go about checking them to make sure they're operating correctly?



Thanks,



-Ryan Dinan

 

[Mark Austin]

Ken,

Mark,

Does that cable ground breaker work with digital cable? I have one built into my surge suppressor but it cuts out about half of the digital cable channels.

Yes, it does work with digital cable, and it doesn't block out any of my channels. Maybe try a different isolator?

 

[Richard_s]

Ryan:

If it's the capacitors that are failing, how would I go about checking them to make sure they're operating correctly?

Can't check this with a DVM. Unfortunately I do not know amplifiers well enough to know how to test the circuit using an Oscilloscope etc. to isolate the source of noise. Since you problem gets worse as it heats up after being off it seems likely to be the transformer or a capacitor. Sorry I can't help you isolate the problem. Here is a link to the JBL manuals but it may not be working.



Link Removed



I will try to e-mail you the manual for the PB12 maybe that will help.