Subwoofer Shenanigans

[Video Beagle]

I've been having some issues and thoughts regarding my computer Subwoofer, and hopefully some folks here might have thoughts and opinions that might help me out.

 

Plan:

Hook up both computer sound system (PC Speakers) and entertainment sound system (AV Receiver) to same subwoofer, either thru y-adapter or switch.

 

Equipment:

Subwoofer: Advent AV009sub

AV Receiver: Sony STR -DH520

PC Speakers: Yahmaha YST-M8 (connected to iMac and PC)

BESIGN Ground Loop Noise Isolator

a plethora of cables and adapters

iPhone 5s
 

 

Issues:

I've run into several things during this adventures.

 

1) (Ground loop?) hum

With everything hooked up, I would get (usually) the following hum from the subwoofer.

Subwoofer Hum.mp3

 

a friend I've been consulting, and googling have lead me to believe a ground loop is the cause of the hum.

I say usually because while it was indeed usually there, I had some trouble getting the fail state to be absolutely consistent, but from research, the hum and the 2 inputs plugged into separate outlets and then hooked to the subwoofer on a third one seems to point to that.

 

The hum would be there if I used a y adapter, a switch, or put one device on each channel of a stereo connection

 

I got a "ground loop isolater" off Amazon to try, plugging it in at the subwoofer, but it did no good.

 

I tried plugging all 3 into the same outlet (ok for testing, not practical for day to day due to room size) and seemed to get no hum.  I used the receiver and my iPhone plugged into the speakers for the auto input. (computer too far away for to be plugged in.).

 

Putting the speakers and subwoofer back into place, I learned that I got the hum when the computer was plugged into the speakers, but not when it was not or the iphone was plugged in.

On a whim, I plugged the ground loop isolater into the pc Speakers...first between the iMac and the Speakers, and then at the output from the speakers to the subwoofer, and the hum appears to have gone away.

This works with both a Y Adapter and the switch.

 

SO, for this issue,  is it case closed/problem solved? Or is there something else I should be looking at?

 

 

2) Signal Strength/Feedback

Before learning about ground loop hums, I had thought this was an issue of feedback along the y adapter I first tried. I was reminded of this while doing the "all at the same plug" tests.

When both inputs are plugged in, the volume coming from the subwoofer is much lower than when only one is plugged in.

I assume this is due to power leakage along the Y branches.

For a while, I considered trying to build a 1 way Y adapter, using diodes to prevent the leakage, but, well, I'm not smart enough to do that.

After letting the project sit..for years..I went with the switch idea...which led to the hum, and see above :).

 

SO, for this issue,  is there a way to have both sources connected at once (thru a Y), rather than using a switch? Would I need a full on sound mixer to do this?

 

3) Lower volume from Receiver

This is more an observation than an issue.

In single input testing, the volume I get from the subwoofer with either the PC speakers or the iPhone plugged in directly is vastly louder than from the AV Reciever. The Receiver I know is an un-amplified signal.

I assume that the PC Speakers are sending an amplified one, and I have no idea what the iPhone does.

According everything I can find (and folks I consult), the subwoofer does have a built in amp.

So is the difference due to the relative volumes of the inputs? (the AV Receiver, being more powerful, is set mid level or lover, vs the PC speakers/iMac's volume being pretty high.)

Would getting a cheapish subwoofer/signal amp (like this) be worthwhile?

 

4) Power brick voltage difference

As shown in the graphic, the Subwoofer is listed at 15 volts...do to reasons not worth going into, it's powerbrick is 14.5 volts.

Is there benefit to getting one with the full 15 volts?

 

 

Thanks for taking the time to read thru this and I appreciate any thoughts and opinions on this.

[TheFlyingTraut]

I would not spend any more money into this project. You are dealing with impedance issues between receiver and source like a phone. You are going to have hum just trying to loop off of a 3.5mm y connection. There are no special in line ground thingy's w/o actually physically grounding them. You will see zero benefit with a different power brick. 3.5 is always noisy. Especially when any decent amount of gain is introduced (volume)

[Video Beagle]

4 hours ago, TheFlyingTraut said:

I would not spend any more money into this project.

---You are dealing with impedance issues between receiver and source like a phone.

--- There are no special in line ground thingy's w/o actually physically grounding them.

Thanks for the reply!

So far, I've only spent the $7 or so on the amazon isolater, which I'm assuming is basiclly a ferrite core wrapped around a wire. Despite reading, and consulting with my on-call physics professor, I've never been sure how a ferrite bead/core wapper is supposed to actually work, but assuming that's what it is, it does seem to be doing something now that it's in the right location.

 

Can you expound more on your impedance comment? I know what it is in the intellectual sense, but not quite sure how it's applying here?

[TheFlyingTraut]

Your phone out has an internal amp, giving it fake gain essentially. The receiver has to be turned up to a higher volume to match that gain, the more you turn up the non amp'd gain (receiver) the more noise introduced. Impedance was the wrong term for me to use. A hum is just noise and bad grounds.

[Video Beagle]

5 hours ago, TheFlyingTraut said:

Your phone out has an internal amp, giving it fake gain essentially.

Good to know, Thanks !