Subwoofer causes noise from the speakers

[conqueeftador]

Been chasing a noisy system. I have 2x Musical Fidelity X-A1 with a subwoofer plugged into the preout, when this is plugged in, I get crackling and random noise from the speakers, not the usual hum from the sub. Happens on both amps using the same subs. Is there any way to resolve this?

 

[DMill]

Been chasing a noisy system. I have 2x Musical Fidelity X-A1 with a subwoofer plugged into the preout, when this is plugged in, I get crackling and random noise from the speakers, not the usual hum from the sub. Happens on both amps using the same subs. Is there any way to resolve this?

I'm not familiar with that sub. Are you speakers connected through your sub or through an amp?

 

[conqueeftador]

That's an integrated amp, subs are Monitor Audio VW8. The amp has preouts and the sub is connected to that with a QED RCA. Still happens with no power to the subs.

 

[DMill]

First off. Sorry for my confusing initial response. Too many kids running around. I recently had a similar issue with an SVS sub I set up 3 days ago. Turns out I torqued one of the speaker connections while installing the sub. Once I fixed the banana plug connection everything was fine. I hope your issue is something that simple.

 

[conqueeftador]

Thanks for the response, but have you got the speaker cables running through the sub or are you using RCAs?

What's odd here is it goes away when the inputs aren't in use, regardless of volume, which source is selected or what is plugged into the sources.

 

[freemansteve]

"I get crackling and random noise from the speakers"

are you referring to your main speakers or the subs here?

It sounds like a loose or poor connection to me, on a line level cable picking up noise.

 

[conqueeftador]

Main speakers

 

[DMill]

I use RCAs to the sub from an integrated just like you. Speakers are connected directly to the amp - not through a high level speaker sub connection. Your subs don’t appear to have that option anyway. While messing with all the wires on back of the amp I must have inadvertently stressed one of the speaker cables. In fairness I was feeding cords behind a heavy entertainment unit. Do you have an extra set of RCA cables for your amp to your sub? I’ve had RCA cables fail - especially overpriced ones

 

[conqueeftador]

That is interesting, I will have a look at the cables. Seems odd both amps are doing it though.

 

[freemansteve]

That is interesting, I will have a look at the cables. Seems odd both amps are doing it though.

But what test are you doing? Changing amps but with the same cabling in place?

 

[antcollinet]

I'm pretty certain you have a ground loop.

Do your USB or (presumably powered) speakers have earth connections on the mains. How are the mains connected to the subs? Also RCA?

What is your source into the amp (being used as preamp?

 

[conqueeftador]

Nope, I have 2 of the same amps and 2 of the same subwoofers.

I also thought it was a ground loop from something else, but I systematically unplugged everything from the amp leaving only the subwoofer and speakers plugged in. No sources plugged into the amp at all and the noise doesn't go up/down with volume, makes the crackling sound even when the amp is on min volume.

Speakers aren't powered, the X-A1 is powering them. This is fed from a topping DAC or a Denon 3700H. RCA from the X-A1 to the subs from the preout.

 

[freemansteve]

Trying to visualize the topology:

The normal setup has gone noisy, but which used to work ok (?) is - DAC output to amp input with RCA, (power) amp output to main speakers, preamp output to powered sub for each of L + R channels.

Questions

What feeds the DAC, and is it fed with coax or optical?

Is the crackling noise there when you unplug either of the RCA cables to the either or both of the subs at the preamp out? (NB turn the subs off or they will buzz)

Is the DAC powered by a mains power pack or does it have direct mains connection?

For a test, what happens if you move each sub in turn and connect it to the amp with a short, known to be fine RCA lead, and power the sub on?

Are there any other devices connected to any of the amp's input, such as TT etc?

Do the subs have any other input connections beside RCAs ?
---
See, I'm assuming the subs have no connection to ground other than through the amp via the RCA wires, and you can test this with a meter, by one probe on the the outer collar of an RCA input plug and then the other on each of the prongs of its mains plug in turn (unplugged, obviously). The point is to establish if a ground loop is even electrically possible through the subs. This can be repeated with other devices. I'm still leaning towards a cable issue or some other fault (TBD), rather than a loop, as if I get your setup correctly it's a completely standard one that normally works without ground loops occurring....

 

[antcollinet]

Can you sketch all the connections in your system. Difficult to visualise now how it is connected.

 

[conqueeftador]

In short: It is all new old parts in a new setup, so nothing new has brought the noise in. I was also blaming the other inputs, or DAC but removing all inputs from the amp still makes the noise. This is when it is just X-A1 (integrated amp) feeding the speakers and the preouts from the X-A1 plugged into the sub via a 1M RCA with nothing on as no sources are connected. The noise is irrelevant of volume, so does not go up with it and when it is on min/0 it still makes the noise. Noise only stops when the sub RCAs are unplugged from the amp at either end. No noise or hum seems to come from the sub.

In long, I have the exact same setup behind me, same amp, sub, speakers and cables. It also makes the same crackling noise with no inputs. These are being used as Zone 2 and 3 in the same room for an office. Mostly fed from a DAC in stereo, but the X-A1s combine for a 4.2.4 desk atmos/gaming setup using a phantom center being fed from the Denon 3700H. This also runs some power amps for the front 3 and top 4 which make a home theatre 7.4.4 setup, but it'll make the noise when all 3 sources are unplugged, hence I doubt they have much to do with the issue.

 

[antcollinet]

To me it sounds like you have a ground loop from the amp earth, through the RCA connection to the sub, and back to earth via the sub earth (unless the sub has no earth connection - in which case it must be something else). (Does the noise go away if you disconnect the sub from the mains, but leave it connected via RCA?)

If the noise is from the speakers (with standard speaker terminals), then the noise pickup must be internal to the amp, after the volume control. I've never seen this happen before though, so far from certain.

Long shot, but what happens if you short the inputs (center pin of each RCA input to screen/ground of the same RCA) of the selected input to the amp (Low impedance 0V on the input).

What happens if you plug the sub into the same electrical socket as the amp?

 

[conqueeftador]

The sub is plugged into the same socket as the amp. The amps power supply has a ground on the IEC cable, whilst the sub does not. When the subs power is not plugged in but the RCAs are, it still makes the noise.

Maybe someone can make sense of this. With another, different line of QED RCA, it also makes the noise, but switch over to a no name digital audio cable and it goes away. Found 2 different digital audio cables to use both preouts does not make a noise either.

I vaguely remember reading about an impedance difference between RCA cables. Could this be the issue?

 

[Zek]

I vaguely remember reading about an impedance difference between RCA cables. Could this be the issue?

I think no.
It seems to me that the problem is in the RCA inputs on the sub.

 

[antcollinet]

Ground loop ruled out then.

Weird. Don't think it is cable impdedance. Expecially if no signal.

 

[antcollinet]

I think no.
It seems to me that the problem is in the RCA inputs on the sub.

Agreed.

But what can they be doing if there is no power on the sub? And how does a different cable fix it.


EDIT: Does your cable have a "floating internal ferrite jacket"?

Performance Audio 40

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(No idea if this could have an influence, but...)