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Background speaker noise (1 Viewer)

Old Dog

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Just sharing a recent experience.

I have long been aware of the need to have a very “quiet” cable between the SUT and pre amp in my HiFi set up, but was rather surprised to find a background noise issue with the centre speaker when I recently switched my HT amplifiers around.

Switching a Parasound HCA1500 to bridge mode to power the centre produced an audible hiss from maybe a couple of meters away. I am not sure why. It is something I have never noticed before, but then I have never used the Parasound in bridge mode before either. I have to assume it had something to do with the fact the amplifier was bridged, which sounds a bit crazy! When operating in stereo mode, powering the fronts, there was no background hiss through the fronts at all with the amp. The mono powering the centre at that time didn’t generate any hiss either. The Parasound was in the same position on the rack when powering the centre and the fronts, so cable routing was very similar if not exactly identical. The only real difference I can think of being two interconnects and two sets of speaker cables in stereo mode and only one of each in mono. Why would that make any difference?

I tried various interconnects (Atlas Symmetrical, Atlas Asymmetrical, Audioquest, QED, XLO, Cable Talk, Mark Grant and some handmade Furukawa cables), all of them bar one produced different levels of hiss through the centre. The worst was the XLO, the best of the “noisy cables”, Atlas Symmetrical, Cable Talk and the Furukawa. The silent one was the Mark Grant HDX1. Fortunately, (or maybe because of), it was also the Mark Grant cable that gave the clearest audio.

It can’t be the signal coming from the amp, otherwise the hiss would still be there with the MG. I assume therefore, it must be the cables picking up interference, but why would being bridged make any difference? Or is there another explanation?
 

JohnRice

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Is the level of the hiss consistent, or does it go up and down with the volume setting?
 

Old Dog

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It is irrelevant what the volume control does, the noise remains constant. However, it isn’t as bad now as when I first installed the Parasound in bridge mode. Tried two cables, XLO and Atlas Hyper Asymmetric. XLO still the worst. The Mark Grant remains silent.

Pretty sure the cause has to be “mechanical”, ie. the cable is picking up something. Would you agree?

Not sure where it would be coming from! I have screened mains cables supplying the Denon, the Parasound and the UB DP9000. Only the screen and the two “temporary” monoblocks, at the moment on the floor in front of the rack, have unscreened cables. The mains block is on the floor to the left of the rack and the mains cables for the monoblocks run underneath to feed the monos. The screen mains feed doesn't really come anywhere close as far as I can see.

Could be the equipment itself, receiver or power amp, perhaps?
 

Old Dog

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Cause found?

It’s quite a pain in the butt getting to the cables at the rear of my rack. There is no access from the sides, simply no free space. When I installed the rack, the mains and speaker cables went in first (everything nice an orderly) then I started adding the equipment and interconnect cables shelf by shelf as I built it up. Over time I have changed things and it meant pulling the equipment forward, requiring “spare” cable at the rear to allow me to do so. When pushing the equipment back in place, this excess cable has to go somewhere and I don’t really have any control over how it compresses. It’s a bit of a mess at the rear regrading cables, as you can probably imagine.

This morning I took a look from the other side of the rack where there is another mains block. None of the HT is connected to this one, but I have an antenna signal booster that is. The unit (transformer) sits on the third shelf well up out of the way, but this morning I noticed the interconnect from the Parasound to the receiver is hard up against the mains feed to the booster, pushing this power cable against the wall. All of the other mentioned interconnects are quite rigid, except the Cable Talk, and were most likely doing the same or similar. The MG is certainly a screened cable, not sure about all of the others, but the XLO most definitely is not, relying simply on criss-crossed conductors. It possibly explains why this was the worst cable of the bunch (it’s a great cable otherwise!).

Anyway, I have presently convinced myself this is most likely the cause. It’s just a coincidence the occurrence happened when I changed the Parasound to bridge mode for the centre. Although I don’t really need to (using the MG cable), I’m going to re-route the booster power feed and make sure it is well out of harm’s way for the future.
 

JohnRice

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It is irrelevant what the volume control does, the noise remains constant. However, it isn’t as bad now as when I first installed the Parasound in bridge mode. Tried two cables, XLO and Atlas Hyper Asymmetric. XLO still the worst. The Mark Grant remains silent.

Pretty sure the cause has to be “mechanical”, ie. the cable is picking up something. Would you agree?

Not sure where it would be coming from! I have screened mains cables supplying the Denon, the Parasound and the UB DP9000. Only the screen and the two “temporary” monoblocks, at the moment on the floor in front of the rack, have unscreened cables. The mains block is on the floor to the left of the rack and the mains cables for the monoblocks run underneath to feed the monos. The screen mains feed doesn't really come anywhere close as far as I can see.

Could be the equipment itself, receiver or power amp, perhaps?
That's just a troubleshooting step. If the hiss changes level with changes to the volume control, it is not from the amp, but the preamp or something before it. If it stays the same, it's something after the preamp. Hiss can come from so many places. BTW, I'm guessing this is a hiss, not a hum.
 

Old Dog

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I have had cases of hum with my HiFi, in particular in connection with the cartridge step up transformer. That really was a hum, but this sounded different, certainly a higher note, hence describing it as hiss.
There is just one other thing in the back of my mind. At times when changing interconnects I have had the feeling that I have caught a sharp piece of metal, an edge or something. Never with my fingers, but on the wrist (palm side) where it's really sensitive (equipment turned off, but not disconnected from the mains). I didn't think too much about it when it first happened, but it has happened once or twice now and I haven't found any sharp edges that I may be catching and it doesn't happen every time either. Is it possible it could be a mild electric shock?
I notice not all equipment these days actually has a mains earth connection. They must be earthed though, somehow? My two Quads, for example, have no mains earth, the Denon receiver doesn't either, not sure about the TV (fixed mains cable and sealed plug), the Parasound does have mains earth. Could this be of any relevance?
 

JohnRice

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There are obvious differences between US and GBR (correct?) electricity. What you're referring to is referred to in the US as a dedicated ground (aka: earth) wire. In general, I find that dedicated grounds are disappearing from audio gear, because they are unnecessary (two conductor electricity here is still grounded, just without a dedicated ground wire) and lead to ground loops, which is a low frequency hum, and can be quite loud at times. A hiss can come from so many places, but is generally created by the gear itself.

What you refer to could be a shock. That can feel like a sharp edge. I hope your turn off the amp any time you do anything. Still, there is power stored and "purist" amps don't generally have protection circuitry.

FWIW, with the disappearance of dedicated grounds (in fact, I used to use a "cheater" plug on amps to eliminate them) and since I got rid of cable TV (a HUGE source of ground loops) I haven't had any noise problems in my systems. They can be infuriating to hunt down.
 

Old Dog

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Definitely differences, not least the voltage. I have been to the States, but dont really know what they do with their plugs / wiring. Here we have "3 pin plugs", three rectangular pins that will only fit one way in a socket. Standardised wiring, brown = live, blue = neutral and yellow/green = earth. Because live an neutral are (should) always be connected to the same pins in any plug or socket, live is always connected to live.
When I lived in Germany for a while, they also have 3 pins, but their plugs are round, have two round pins with an earth strip on the perimeter. When they are plugged in, you can be connecting live to neutral. Clearly it is not a problem, so I don't understand why the UK feels it needs to make it more difficult here.

Equipment always off. Pleased you are not telling me there is possibly an issue :)

Fortunately, I have never had any issues with ground loops. Have been lucky I guess.

Thanks for your comments!
 

JohnRice

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As you probably know, our plugs can have two or three pins. More and more it's two unless it's a major appliance.
 

Old Dog

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Just to give an update. Unfortunately a noise reappeared through the centre. Not convinced it sounds exactly the same as before, but certainly noticable from a distance, doesn't change with volume, and unfortunately intermittant.
The amp was removed from the HT system, switched into stereo mode and placed in a totally separate stereo system. Here the noise was audible through the left channel only.
I wondered whether the stero /mono switch could be introducing the problem, a bad internal contact perhaps? With the amp off, I returned the switch to mono and then back to stereo, but the switch failed on this move and became locked in the stereo position. My hopes certainly rose the switch was the culprit.
Unfortunately, having replaced the switch, the issue is still there and it has now been suggested I check the bias. Parasound kindly sent me details on how to do this and I am presently waiting on different probes for my DMM to make the job safer / easier.
With the case on, it is quite noticable more heat is given off from one heatsink than the other, suggesting the two halves are not operating identically and / or possibly within the defined specs. My hope is the bias will be out and resetting will change this situation and hopefully eradicate the noise.
Is it likely that incorrect bias could potentially cause a noise issue, or is it more likely the noise is being generated from an alternative source?
What exactly is this bias adjusment controlling?
Many thanks
 

Old Dog

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Good news, the probes arrived and the bias was found to be quite a way out on both channels: 7.9 mV (L) and 4.9 mV (R). Bias was adjusted and set at the recommended 15mV after 30 minutes. I also checked for DC voltage at the speaker terminals and both channels read around 0.5 mV.
Re-checked a couple of days later, bias voltages remained as set.

Since making the adjustment I have yet to hear the noise. If incorrect bias is the cause, I don't understand why the noise would manifest itself on the left channel and not the right, the right voltage was actually worse than the left when initailly checked.

I am used to setting the bias on my valve amps and although a similar function I believe there is a different puropse behind bias setting on valve and solid state amps. From what I am picking up on the net, biasing ss amps appears to adjust the point at which the amp switches from Class A to Class B, rather than output from power tubes in the case of valve amps.

It remains unclear to me why the bias point in ss would generate spurious noises if it is not correct, or at least too far out, and why only the left chanel in my case. It would great to understand more if anyone can offer an expalantion.

It may be that incorrect bias is not the cause and I do indeed hear the noise again, but for now it is looking promising. Fingers crossed!
 

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