Powered Speaker Hum When Plugged In

[craftech]

M-Audio Studiopro 3 Relatively inexpensive powered speaker set.

Produces 60 HZ hum. Thought it was only when turned on regardless of volume, but it can be heard through headphone jack and when you put the speaker next to your ear as soon as you plug it in. There is one large cap which I replaced. Checked solder joints on bottom of PCB. Re-soldered a few (which actually looked OK).

Hum still there.

https://usermanual.wiki/M-Audio/MAud...433470494/html

Any ideas?

Thanks,
John

[stj]

Re: Powered Speaker Hum When Plugged In

need a foto of the insides.
it sounds like a cap though.

[craftech]

Re: Powered Speaker Hum When Plugged In

need a foto of the insides.
it sounds like a cap though.

Here is the PCB (Note: the red wire is a mod the guy who wrote the article did to bypass the amp to make then passive}

https://blog.jonandtina.net/uploads/IMG_0301.png

External view:
https://media.guitarcenter.com/is/im...00-500x500.jpg

My photos: (NOTE: The only large cap is the one I replaced that wasn't blown) None of the caps look like they are bulged or leaking. The brown smears are phenolic glue.

[stj]

Re: Powered Speaker Hum When Plugged In

that's terrible, it could even be radiating between the nest of wires!!

[craftech]

Re: Powered Speaker Hum When Plugged In

that's terrible, it could even be radiating between the nest of wires!!

Actually, when I was experimenting to find the cause I disconnected the wires and separated the chassis from the speakers, then ran them to their jacks and the hum was still there. I kind of thought it was the transformer or the transistor (IC) mounted on the heatsink.

John

[petehall347]

Re: Powered Speaker Hum When Plugged In

check the rectifier

[craftech]

Re: Powered Speaker Hum When Plugged In

check the rectifier

DMM set to Diode:

Blk lead of DMM to + of Bridge Rectifier
Red lead to each AC leg of Bridge Rectifier

Both read .486 V

Red lead of DMM to - of Bridge Rectifier
Blk lead to each AC leg of Bridge Rectifier

Both read .488 V

My understanding is that between .4v and .6v are OK

John

[petehall347]

Re: Powered Speaker Hum When Plugged In

you got a scope there ?

[petehall347]

Re: Powered Speaker Hum When Plugged In

do you still get audio playing ?

[craftech]

Re: Powered Speaker Hum When Plugged In

do you still get audio playing ?

I should have indicated that in the OP. (I'll add it in) Sorry can't edit the OP any longer.

The powered speaker set plays perfectly. It's just the hum you can hear when there is no music or quiet music and even when it is off but plugged in.

But they sound great.

Thanks,

John

[craftech]

Re: Powered Speaker Hum When Plugged In

you got a scope there ?

No

[petehall347]

Re: Powered Speaker Hum When Plugged In

short the inputs to ground and see if hum goes away

[craftech]

Re: Powered Speaker Hum When Plugged In

short the inputs to ground and see if hum goes away

The positive terminal? I tried grounding all the negative terminals yesterday.

[petehall347]

Re: Powered Speaker Hum When Plugged In

yep just short to ground .

[craftech]

Re: Powered Speaker Hum When Plugged In

yep just short to ground .

No change. Both RCA inputs and the 1/8" input as well.


In diagram 4 that I posted the terminal on the right is the 115V / 230V selector slide switch. I don't see any soldering issues. ??

[craftech]

Re: Powered Speaker Hum When Plugged In

I am pretty sure the hum is coming from the transformer.

Is there a reasonable supplier for a replacement?

I don't know the specs. The sticker says BDT-48A002(SP3)
Shenzhen Xingyaoda Electronics Not sure if that helps though?

Output 15V 1A
Two red wires out

John

[stj]

Re: Powered Speaker Hum When Plugged In

so it's a physical hum?
if it's the windings or frame that's loose then epoxy or laquer will fix it.

[craftech]

Re: Powered Speaker Hum When Plugged In

so it's a physical hum?
if it's the windings or frame that's loose then epoxy or laquer will fix it.

I don't think it is rattling. It is mounted on a piece of gasket material and the screws are covered with a resin. It's not old so I don't think the laminations are loose.

I can hear it with the unit plugged in (but turned off) if I listen close and louder when I turn it on.

John

[stj]

Re: Powered Speaker Hum When Plugged In

it does sound like the plastic coil-frame may be vibrating against the iron.

[budwich]

Re: Powered Speaker Hum When Plugged In

The first post indicated that the sound can be hear at the headphones. This makes it less likely that the transformer is cause a vibration that is producing the electric hum. You need to see if you can follow the input area towards some capacitors and test those. In my limit experience with a "humming" guitar amp, there was a bad capacitor. It did not visually look bad but tested bad.