Speakers Humming

[S&L]

umm..could someone give me a yes or no. I just want to make sure I understand the theory.

 

In general, with a live band, there are twoe source of hum and mains noise...

 

1) Guitars. They have a pickup which is, unless magnetically screened, very susceptible to picking up any electromagnetic signal that happens to be floating around. This could be the local taxi service, noise from cables nearby or the venue's induction loop. That last one is a favourite if you get feedback in the guitars that you can't seem to get rid of. Turn off the loop amp to check.

 

2) Unbalanced audio connections. ie ones that use the earth/screen to complete the audio circuit. Here the problem is usually that there is a difference in voltage between the source and destination kit's earth/screen. Connect your audio lead between the two and a current flows along your audio lead's screen which, because that screen is also in the signal path, adds to the wanted audio causing hum/buzz/etc.

 

See this picture produced in PaperCAD...

 

 

The dimmer has filtering in it which works by filtering noise into the earth connection. This appears as a current flowing down to the main earth. But the connections have resistance. So this noise current causes voltages to appear at various points. The noise flowing through R1 causes a noise voltage to appear at point VA. But this is also connected to the destination audio kit via it's earth wiring R2. So the noise appears at VB. But the source kit has a clean earth connection via R4. So there is now a difference in the earth voltages between the source and destination kit. They are connected by the audio cable and any voltage difference will cause a current to flow in the screen. This current/voltage superimposes itself on the audio signal. Result is hum/noise.

 

This is HUGELY simplified and the moment you have more than a couple of bits of kit involved the number of noises paths goes up dramatically.

 

The solution is balanced audio.

 

With balanced audio you can still have noise currents flowing through cable screens, which still ought to be tracked down, but because your wanted signal doesn't rely on using the screen the noise doesn't appear.

 

 

One day I will sit down and write 'Real World Entertainment Systems' because there is a scary amount of rubbish on the 'net.

 

all makes sense - so are we saying that if an active speaker exhibits hum when a dimmer is operating then it's likely to be an imperfection the balanced line feeding that speaker?

[Brian]

so are we saying that if an active speaker exhibits hum when a dimmer is operating then it's likely to be an imperfection the balanced line feeding that speaker?

Assuming that the output of whatever is driving the speaker is clean then 'Yes'.