Aluminium foil as AC hum shield

[atferrari]

Aluminium foil lining the enclosure of an audio amplifier circuit, is it good as AC hum shield?

 

[alec_t]

Yes, providing you can find a way to make a ground connection to the foil.

 

[Nigel Goodwin]

I certainly wouldn't call it 'good'

I've seen it used a few times, but I've never seen a case where adding it reduced hum to any degree - it's more for RF shielding than hum, and not very effective then either.

 

[ronsimpson]

Where is the AC hum coming from? a transformer?

 

[atferrari]

Gracias to all replying.

The amp is battery fed in a domestic enviroment.

 

[alec_t]

Oops. I misinterpreted 'hum' as 'electrical interference'. The others are right; if the hum is magnetically induced then the foil won't work.

 

[atferrari]

So, what is the aluminium foil wrapping Belden wires amongst so many, used for?

 

[cowboybob]

Exactly as you assumed:

A grounded shield for network, audio, telephone, etc. cables, that protects them from, primarily, the 60Hz (and other low frequency) hum(s) associated with power cables, fluorescent lights, etc, that are often run right alongside.

 

[JimB]

So, what is the aluminium foil wrapping Belden wires amongst so many, used for?

Preventing capacitive coupling from the interference source to the screened wired.
To reduce the magnetic coupling, the wires are twisted together so that (hopefully) the magnetic field induces equal and opposite currents in both wires, and the currents cancel out.

JimB

 

[BobW]

A non-magnetic conductive shield will also shield against magnetic interference. It works because external alternating magnetic fields will induce currents in the shield which cancel out the external fields. (See Lenz's Law.) The shield's effectiveness depends on how conductive it is, and how good the conduction is across the seams.

 

[atferrari]

What could I be getting at home, a common domestic environment? No trafos nearby other than a small one inside my PSU on the bench and a CFL lamp. The hum is rather low but increases if I touch the mike output that goes into the preamp. Quick way to know the amp is working.

Is it electric or magnetic? I believed it is electric.

I could not find a steel made enclosure small enough from my amp thus my idea to resort to something lined in aluminium foil.

More than confused now, I realized that I ignore basics here.

 

[cowboybob]

What could I be getting at home, a common domestic environment? ...

The wiring in your house. Primarily electric fields.

 

[4pyros]

The hum can be picked up by you amp from any AC wiring in your house from any wires attached to it. Twist your power leads together and use shielded wires for the inputs and outputs.

 

[jjw]

A non-magnetic conductive shield will also shield against magnetic interference. It works because external alternating magnetic fields will induce currents in the shield which cancel out the external fields. (See Lenz's Law.) The shield's effectiveness depends on how conductive it is, and how good the conduction is across the seams.

The aluminium foil is useless for magnetic shielding at low frequencies.
The thickness of material should be much bigger than skin depth, which is in aluminium about 10mm at 50Hz.

 

[Externet]

Hey Agustín, the key source of noise is that CFL lamp. Use an incandescent. The noise path can be irradiated, conducted trough mains, or both. Those bulbs are a diarrhea of noise, unuseable around amateur radio transceivers, and around can be 100 metres!

 

[dr pepper]

You may have an ground imbalance, if the ground on your source is not eqwual potential to the amp, the difference is 'seen' as an audio signal and amplified.
A ground lift resistor sometime sorts this.