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Meatwad650
2007-09-22, 06:40
My setup is so simple that I'm not sure what I can do to get rid of this hum.

I've got my SB3 going out the RCA jacks into the input of my Cambridge Soundworks "Soundworks" system - it's just an amped sub with a pair of small sats. The only wires in this are:

1. Power supply for SB3
2. Power supply for sub/amp
3. RCA to 3.5mm cable
4. Speaker wire for sats

So no cable to be grounded or anything. The only thing is that both power supplies are plugged in to the same power strip. Any suggestions?

Phil Leigh
2007-09-22, 07:34
My setup is so simple that I'm not sure what I can do to get rid of this hum.

I've got my SB3 going out the RCA jacks into the input of my Cambridge Soundworks "Soundworks" system - it's just an amped sub with a pair of small sats. The only wires in this are:

1. Power supply for SB3
2. Power supply for sub/amp
3. RCA to 3.5mm cable
4. Speaker wire for sats

So no cable to be grounded or anything. The only thing is that both power supplies are plugged in to the same power strip. Any suggestions?


does the hum go away if you disconnect the power to the SB?

Eric Seaberg
2007-09-22, 10:58
A TRUE Ground Loop isn't possible because unless your powered sub AND SB3 power supply have a ground lug on the AC plug, there isn't more than one path for the ground to travel.

I'd say what you're hearing is the switching power supply of the SB3, which Phil eluded to in his post. What happens if your turn the input level to the sub/satellites DOWN and turn the output of the SB3 UP? Try running the SB3 at 50% or higher in volume. This will most likely drop the 'hum' from the noisy supply.

Phil Leigh
2007-09-22, 11:29
Erm - Eric yes that is what I was driving at. I can't see how a conventional ground loop could occur in this setup.

Meatwad650
2007-09-22, 12:37
I'd say what you're hearing is the switching power supply of the SB3, which Phil eluded to in his post. What happens if your turn the input level to the sub/satellites DOWN and turn the output of the SB3 UP? Try running the SB3 at 50% or higher in volume. This will most likely drop the 'hum' from the noisy supply.

I could buy that. I only hear the hum when nothing is playing, however, and turning the SB down didn't help. It's pretty quiet - I can only really hear it late at night when all is quiet.

One thing of interest, though - I have another pair of the same speakers hooked to my computer. With that pair, unlike with the SB3, I am using the volume control they provide:

http://www.cambridgesoundworks.com/store/category.cgi?category=acc_volume&item=c1swvolzz

and in that application if I have the volume set halfway on that thing then I get a very loud hum. That's why I decided to not use a similar connector for the SB3 and just go directly from the RCA jacks (although I use the preamp control configured on the server).

NewBuyer
2007-09-22, 21:21
I could buy that. I only hear the hum when nothing is playing, however, and turning the SB down didn't help. It's pretty quiet - I can only really hear it late at night when all is quiet.

One thing of interest, though - I have another pair of the same speakers hooked to my computer. With that pair, unlike with the SB3, I am using the volume control they provide:

http://www.cambridgesoundworks.com/store/category.cgi?category=acc_volume&item=c1swvolzz

and in that application if I have the volume set halfway on that thing then I get a very loud hum. That's why I decided to not use a similar connector for the SB3 and just go directly from the RCA jacks (although I use the preamp control configured on the server).

Sounds to me like the problem lies in the amp within your sub/sat. This happens often with them, and your experience using them without the SB3 and getting *more* hum seems to provide evidence for it.

Unbalanced (RCA) connections can allow some potential interchassis current to flow through the cable shield, but I don't think this is your issue here - it seems much more likely to me, that the sub/sat amp is the problem.

Meatwad650
2007-09-22, 23:19
So I went to make sure that the network connection wasn't causing the hum. Then I unplugged the power and noticed something.

Unplugging the power adapter from the squeezebox didn't make the hum go away. In fact, if I held the plug end (that goes into the SB3) it would make the hum louder. The only thing that made the hum go away was unplugging the power supply at the power strip. I'd think that points to the power supply pretty well, wouldn't it? It's so obvious I can't understand why more people don't complain unless mine is faulty somehow?

NewBuyer
2007-09-23, 01:09
So I went to make sure that the network connection wasn't causing the hum. Then I unplugged the power and noticed something.

Unplugging the power adapter from the squeezebox didn't make the hum go away. In fact, if I held the plug end (that goes into the SB3) it would make the hum louder. The only thing that made the hum go away was unplugging the power supply at the power strip. I'd think that points to the power supply pretty well, wouldn't it? It's so obvious I can't understand why more people don't complain unless mine is faulty somehow?

So capacitive coupling (perhaps), and all the way into the sub/sat amp. I can't help but wonder, what happens if you plug the SB3 into another outlet on the same circuit leg (but not the powerstrip)? Also, do you have your computer, sub/sat system, etc all plugged into the same strip? Might check the power rating of the strip or try another strip...

Phil Leigh
2007-09-23, 01:26
does the hum get louder if you move the cable between the SB and amp closer to the power supply? if so it could be poor/damaged/incorrectly wired shielding on the cable.

Meatwad650
2007-09-23, 13:15
So capacitive coupling (perhaps), and all the way into the sub/sat amp. I can't help but wonder, what happens if you plug the SB3 into another outlet on the same circuit leg (but not the powerstrip)? Also, do you have your computer, sub/sat system, etc all plugged into the same strip? Might check the power rating of the strip or try another strip...

There's one other plug on the outlet but I can't easily get at it. I'd have to move the dresser.

The only thing on the power strip is the speaker system, a bedroom fan, and the SB3. I can try a better power strip. I'm just wondering if this issue is one that would be solved by spending $20 on an elpac.

haunyack
2007-09-23, 15:34
a bedroom fan

.
.

Meatwad650
2007-09-24, 10:14
.
.

If you're suggesting that the bedroom fan is causing the hum - it's been unplugged. But the hum is still there...

bobharp
2007-09-24, 10:43
If you're suggesting that the bedroom fan is causing the hum - it's been unplugged. But the hum is still there...

Is this a wired or wireless Ethernet connection?

Meatwad650
2007-09-24, 14:19
Is this a wired or wireless Ethernet connection?

Wired. I unplugged the network connection (see earlier in the thread) and the hum did not go away.

Furthermore, as I've said, earlier, just having the SB3 power supply plugged in to the power strip causes the noise - even if it's not connected to the SB3.

cliveb
2007-09-25, 02:15
The only wires in this are:

1. Power supply for SB3
2. Power supply for sub/amp
3. RCA to 3.5mm cable
4. Speaker wire for sats

Does the power supply for the amp/sub have a ground? If not, then since the SB3 power supply doesn't have a ground either, the system is completely floating (ie. no ground anywhere), and this is a common cause of slight hum.

If this is the case, you need to hook up a ground connection between the system's ground plane and the mains ground. The most convenient location on the SB3 would be to use the outer ring of the COAX digital output socket.

Meatwad650
2007-09-25, 10:48
Does the power supply for the amp/sub have a ground?

Yeah - the amp's power supply has a 3-pronged plug.

I called SlimDevices yesterday and the guys in MTV said that it was a known issue with the power supply. I'm gonna go pick one up tomorrow afternoon and I'll let you guys know if it addresses the problem.

cliveb
2007-09-26, 03:10
Yeah - the amp's power supply has a 3-pronged plug.
But is the mains ground connected to the amp's ground plane? To check this, connect a multimeter between the ground pin on the mains plug and the screen the an amp input socket. If the resistance is not 0 (or very close to 0), then the mains ground is just there as a safety measure, but is not shunting the signal ground to earth.


I called SlimDevices yesterday and the guys in MTV said that it was a known issue with the power supply. I'm gonna go pick one up tomorrow afternoon and I'll let you guys know if it addresses the problem.
A relevant issue I know of with the stock power supply is that it has a fairly high level of mains frequency ripple on its nominally 0V line. And if the system doesn't shunt that ripple to earth, then it becomes part of the signal - ie. you get hum.

If this is indeed the source of your hum, replacing the power supply with another one that has no ripple will fix the problem. Ironically, the reason another power supply has no ripple could well be because it's grounded. And it would be much cheaper to just keep the stock power supply and hook up a ground to the system.