Why is my amp humming?

[bob p]

sometimes i just have too much time on my hands. it shows, eh?

[analogic]

Hello again guys,

I did some additional testing today, but the mysery continues. Since some of you suspected that the problem was due to the double amp setup (azur 340 + sub), I disconnected to sub completely, no signal input and no power. Unfortunately this did not effect the humming in any way

I also tried connecting a super short 3.5 mm (20 cm) cable to the amplifier, and although this seemed to reduce the ammount of humming a little bit, it was not at all as low as if I previously had tried without any input at all. HOWEVER I did notice something interesting while doing this. In contrast to what I said the other day that the humming goes away almost entierly when disconnecting the 3.5 mm cable, this was not the case today. I still had very noticable humming without any input to the amplifier and with the amplifier ONLY connected to my speakers. The reason for this strange behavior must be that the humming, as I've stated previously, is highly affected by the way the external cables (speaker cables, 3.5 mm cables) are placed on the ground. If I bundle the cables up, I can make the humming cancel out completely. This must be the reason that The noise disapperared the other day when I disconnected the 3.5 mm cable, the other cables must have been placed in such a way to cancel out the humming.

The screw terminal is not present on your amp. It's there in some amps to provide a separate ground to (say) a record deck.

To test if you have a ground loop problem try using your setup with a battery-powered device that is not connected to the mains. See if it still hums.

Unfortunately, I do not have access to such a device

If you have another amplifier setup, try using the Record Out to that amp.
Or the Record Monitor.

[ATTACH]47584[/ATTACH]

Since this problem, after additional testing, does not seem to be dependant on the subwoofer amp, I guess this wont help? However it was interesting to learn I can connect the sub in this way, I didnt know that. The only problem is that Rec out is not affected by volume knob right? So I would have to adjust the volume on the sub everytime I change volume on my speakers?

I'm with Mick. I think it's a ground loop. VERY common in multi amp systems. Since you have no hum with nothing plugged in and the hum level is constant when your computer is plugged in I have to conclude that there is a ground in the power amp of one or both amplifiers that is shared with the input ground of one or both amplifiers. If this is the case, ground lifting one amp may help. Try using an AC ground lift adapter first with the sub. If that doesn't help try it with the 340A, but do not ground lift both because that could leave the only safety ground connection through that long cable to your computer (bad). If ground lifting doesn't fix it the next easiest solution might be to make a custom input cable that is shield isolated on one end.

Component stereo can be an a$$pain (there go those dollar signs again) when it comes to ground noise.

Me? I'd do the custom interconnect before I'd ever lift a ground on a piece of AC powered gear. Lifting grounds has safety repercussions and is best avoided.

When you pay for those expensive "directional" HiFi cables, you're getting what Chuck mentioned -- they have the ground lifted at the send side of the cable.

Sometimes you can use a low power rated "fusible link" resistor to elevate chassis ground over earth ground to address ground loops. I'd rather do that than lift the chassis ground entirely. It's just not safe.

To be honest I have no idea what you guys mean when you talk about lifting the ground. I've got some electronics experience but it's mainly from messing around at home and studying by my self, and tbh it sounds just a bit too risky for me to try personally. And since my additional testing have ruled out the possibility of the humming being due to the subwoofer amp, I guess this wouldnt have any effect anyway?

However I'm still interesting in the custom input shield isolated cable, is this something that could help, given the current status of the problem? Could you explain a bit more in detail what it is, and how hard and expensive it it so make it?

Also, do you guys think it is possible that the "noise cancelling" properties of the amplifier are just bad by design, and my amplifier are picking up more noise in the way I have it placed now as compared to before, making the noise more obvious? It was a very cheap amplifier after all.

edit: I noticed that if I run the 5 meter 3.5 mm cable along one wall, as opposed to the other (my amp is placed in the corner of the room) the humming goes from very noticable, to not noticable at all.

[bob p]

Unfortunately, I do not have access to such a device

there are all sorts of battery powered devices that could work as a source: CD player, MP3 player, smart phone, etc.

The 'special' interconnect that we were talking about is nothing complicated. Just a conventional interconnect that has it's shielded cable disconnected from the plug at one end, the end that gets plugged into the audio source.

if you want to assure that your hum is not being coupled to the inputs, unplug all of the inputs and see if the amp still hums.

edit: if your 5 meter 3.5mm cable is an input cable, and it's running along a wall, chances are that it's picking up AC noise from the house wiring in your wall. replace a low quality cable with one that has high quality shielding, and route the cable away from AC power sources. that might help. high end audio uses balanced lines to avoid these kinds of problems.

[analogic]

there are all sorts of battery powered devices that could work as a source: CD player, MP3 player, smart phone, etc.

The 'special' interconnect that we were talking about is nothing complicated. Just a conventional interconnect that has it's shielded cable disconnected from the plug at one end, the end that gets plugged into the audio source.

Oh I see, I thought he mean I should try running the main power chord to a battery powered device at 230v. However, since the humming persists wether I have the other end (the 3.5 mm one that goes into the computer) connected not, I dont think this will make a difference.

Alright, I will look into this interconnect, however, given the current circumstances, do you think it will help at all?

Since the humming is not really disturbing while listening to music, it's not that big of a deal really.. It just that I have to turn the amplifier off everytime I'm not listening too it because it gets very disturbing after a while. It would be interesting to know whats causing it though.

[bob p]

IME computer soundcards are notorious sources of AC noise, which makes them poorly suited as a hifi signal source. If the noise goes away when you unplug the wire connecting the PC to the amp's input, that gives you an answer. replacing the AC-powered computer/soundcard with a battery powered device would be a good test to perform.

[analogic]

IME computer soundcards are notorious sources of AC noise, which makes them poorly suited as a hifi signal source. If the noise goes away when you unplug the wire connecting the PC to the amp's input, that gives you an answer. replacing the AC-powered computer/soundcard with a battery powered device would be a good test to perform.

Alright, tried it now but it made no effect in humming.

As I said before, the issue does not seem to lie solely in the 3.5 mm cable, it seems more like the humming gets exacerbated by the way all of the connected cables are placed in relation to each other. Moving the connected cables around (3.5mm/speaker cable) either makes the humming worse, or less

[Helmholtz]

Alright, tried it now but it made no effect in humming.

As I said before, the issue does not seem to lie solely in the 3.5 mm cable, it seems more like the humming gets exacerbated by the way all of the connected cables are placed in relation to each other. Moving the connected cables around (3.5mm/speaker cable) either makes the humming worse, or less

From your description it is most probable that you have a ground loop problem. Most people don't understand how a ground loop works. It requires at least one closed, low impedance path (loop), where a current can circulate and a relative strong source of AC magnetic field, like a transformer, electrical machine, power supply etc. in close vicinity. The principle is magnetic induction. The AC field induces a disturbing signal voltage in the electrical (wire) loop and as it is a closed loop, an unwanted AC current will flow. The magnitude of this current is proportional to the area within the wiring loop. It also depends on the orientation of the loop as magnetic fields are directional. Minimizing the area between the concerned wires (by tieing them together) will minimize the disturbance. As will the removal of the the outer AC field source!
A ground loop problem cannot be cured by better grounding or electrical shielding. And it can occur even in battery operated amplifiers. It does not even require a mains ground connection (electrical safety ground) as it develops also in pure signal ground loops.
For instance, the 2 stereo input cables of your amplifier form a perfect ground loop: Their shieldings are low impedance and are connected on both ends (to ground), thus forming a short circuit loop. In this special case the humming will disappear, if you remove the cable of one of the channels as this disrupts the loop.
As a cure you may just disconnect the shielding on one side of one of the cables, if tieing the cables together is not enough.

Ground loops can also involve several connected components and their mains leads. This can make things more complicated. Do not remove/open safety ground connections!


My recommendations:
1)Try to identify the receiving loop by moving the cables. Minimize loop area. Keep phono leads as short as possible. If its the two phono cables, try as descibed above.
2)Try to identify the AC field source and increase the distance or turn it off.

[Chuck H]

Alright, tried it now but it made no effect in humming.

As I said before, the issue does not seem to lie solely in the 3.5 mm cable, it seems more like the humming gets exacerbated by the way all of the connected cables are placed in relation to each other. Moving the connected cables around (3.5mm/speaker cable) either makes the humming worse, or less

So... With nothing plugged in the hum is negligible. Pluggin in your poot there is hum. Plugging in a battery powered device there is hum. Correct? If yes, are you using the same interconnecting cable for both input devices?

[analogic]

From your description it is most probable that you have a ground loop problem. Most people don't understand how a ground loop works. It requires at least one closed, low impedance path (loop), where a current can circulate and a relative strong source of AC magnetic field, like a transformer, electrical machine, power supply etc. in close vicinity. The principle is magnetic induction. The AC field induces a disturbing signal voltage in the electrical (wire) loop and as it is a closed loop, an unwanted AC current will flow. The magnitude of this current is proportional to the area within the wiring loop. It also depends on the orientation of the loop as magnetic fields are directional. Minimizing the area between the concerned wires (by tieing them together) will minimize the disturbance. As will the removal of the the outer AC field source!
A ground loop problem cannot be cured by better grounding or electrical shielding. And it can occur even in battery operated amplifiers. It does not even require a mains ground connection (electrical safety ground) as it develops also in pure signal ground loops.
For instance, the 2 stereo input cables of your amplifier form a perfect ground loop: Their shieldings are low impedance and are connected on both ends (to ground), thus forming a short circuit loop. In this special case the humming will disappear, if you remove the cable of one of the channels as this disrupts the loop.
As a cure you may just disconnect the shielding on one side of one of the cables, if tieing the cables together is not enough.

Ground loops can also involve several connected components and their mains leads. This can make things more complicated. Do not remove/open safety ground connections!


My recommendations:
1)Try to identify the receiving loop by moving the cables. Minimize loop area. Keep phono leads as short as possible. If its the two phono cables, try as descibed above.
2)Try to identify the AC field source and increase the distance or turn it off.

You solved it!!

I had a power supply for a tv box similar to this



connected to a power strip (same power strip as the main cord of amp is connected to) which was in very close proximity to one of the speaker cables. I disconnected this and the humming immediately stopped. Thinking back, it's very likely the humming started just when I got this new TV box, I just never connected the dots.

Thank you so much for the detailed answer!

Thanks to everyone else as well, great forum!

[Helmholtz]

You solved it!!

I had a power supply for a tv box similar to this

[ATTACH=CONFIG]47619[/ATTACH]

connected to a power strip (same power strip as the main cord of amp is connected to) which was in very close proximity to one of the speaker cables. I disconnected this and the humming immediately stopped. Thinking back, it's very likely the humming started just when I got this new TV box, I just never connected the dots.

Thank you so much for the detailed answer!

Thanks to everyone else as well, great forum!

You're welcome.
Great that I could help with just my second post.

The result shows that this was just another EMI issue caused by a switched mode PS. Your cables acted as receiving antennas for the magnetic interference field (via ground loops) as well as for the electric interference field (no loop required), both radiated by the PS and its own cables.

BTW, what do you mean by 3.5mm cables? This term is sometimes used for miniature phone plugs of 3.5mm diameter. But the picture of your amp only shows standard RCA/Cinch connectors.