Aluminium foil as AC hum shield

Discussion in 'General Electronics Chat' started by atferrari, Apr 27, 2014.

  1. atferrari

    Thread Starter AAC Fanatic!

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

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    Sep 9, 2010
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    I believe it can help, but the answer depends on where the hum is coming from. If it's brought in by the power supply, or is riding on the incoming input signal, shielding the amp enclosure will make no difference.
     
  3. crutschow

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    Mar 14, 2008
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    Foil can provide some protection against electric field interference but is of limited help for the magnetic field interference from power lines.
     
  4. ian field

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    It'll stop electrostatic pickup, but not so good for damping magnetic flux.

    Years ago I stripped some old computer gear, the shielding enclosure for the drives was sheet steel with self adhesive copper foil stuck on it - presumably this was to short out eddy currents from the digital switching on head positioners etc which would be rich in harmonics.
     
  5. wmodavis

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    Oct 23, 2010
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    If you are looking for reduction from hum - that is hum caused by local magnetic fields - you need to provide a path for the magnetic fields causing the hum to be shunted around or away from the sensitive circuit that is susceptible to magnetic pickup. To do that effectively the 'shield' needs to be of a material that has high permeability or low reluctance (they are reciprocal like resistance and conductance). The permeability of iron is .25 and the permeabilitu of aluminum is 1.25 x 10>-6. Nearly a million times less able to reduce magnetically induced hum.
     
  6. #12

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    Nov 30, 2010
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    Once upon a time, I tried to stop a cell phone from transmitting and receiving. I put it in an aluminum project box and it still worked. I put that in another aluminum project box and it stopped working. Then I took it out of the boxes and wrapped the phone in 3 layers of aluminum foil and folded the ends over. That stopped the phone from working.

    Bottom line? Aluminum works fairly well on electric fields, but it's very weak about magnetic fields.
     
  7. nsaspook

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  8. Mike33

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    If the hum is coming from an external source, however, foil IS a great shield. I line the pickup cavities of my single-coil guitars (strat, tele...) and the back of the pickguard, and make sure the foil is grounded properly. I keep it on there with spray contact adhesive, lay it in, and trim to fit. Then, when I play in a bar that has neon transformers on the stage etc., no more hum.

    INSIDE an amp, I'm in 100% agreement with the above - if it's coming from INSIDE, something is not working right, or you have wires not routed correctly (heater wires on tube amps can be notorious for this, as they usually work on AC). Can also point to a problem, like an issue with power or output transformer, or filter caps...supposed to change those caps every 10 years or so, altho few do.
     
  9. #12

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    If this is about guitars, I have failed at that!
    Trying to fix the hum problem in, "an imitation P-bass" as one guy called it.
    I did everything as right as I knew how and achieved nothing. Star washers, spray glue, the works.
    On the stage, under a fluorescent light, and it hums like it doesn't know the words.

    (That's supposed to be a joke.)

    On the other hand, I had a bad ground connection at the bridge of my 'Paul and it turned out excellent after I cleaned it up. Just sayin'. Sometimes it works and sometimes it doesn't. All you can do it try. If that fails, ask somebody that works on guitars for a living. I'm sure they know tricks that I don't know.
     
  10. atferrari

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    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.

    Amp to be fed by a 9V battery.
     
    Last edited: Apr 27, 2014
  11. Alec_t

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    Sep 17, 2013
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    Apart from your time it costs next-to-nothing to try the ali foil, Agustin. Let us know the results.
     
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  12. #12

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    OK. It's an electric field. I call it, "finger hum". Very convenient test signal!

    Just for testing, any size box will work, any sort of Faraday shield. You could put it in your mail box (if it's made of galvanized steel). Once you have it defined by defeating it, you will be confident to invest the time and money to build a shield the right size.

    and, yes, these electric fields are everywhere in a modern area. If you have electricity to plug in your equipment, you have hum in the air.
     
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  13. Brownout

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    Jan 10, 2012
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    I've used al foil to effectively shield high impdance amplifier inputs from hum. I found that grounding the shield helps trmendously.
     
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  14. atferrari

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    Thanks to you all, for time and patience!!
     
  15. t_n_k

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    And don't forget the old chestnut - the earth / ground loop.
     
  16. Sensacell

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    Jun 19, 2012
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    Copper shielding foil can be had that has adhesive on the back, this stuff is great because you can solder the seams and it's really easy to solder a ground wire to it as well.

    Aluminum foil is just 10 X harder to work with.
     
  17. atferrari

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    But hard if not impossible to get locally. :confused:
     
  18. ian field

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    In the UK; the Lidl stores occasionally have rolls of adhesive aluminium foil for £1.99, it can be soldered if you buy a reel of solder that's made for soldering aluminium.
     
  19. gootee

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    Apr 24, 2007
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    You could also try minimizing the "antennas" that are transmitting and/or receiving the hum. i.e. Sometimes, the problem is "enclosed loop area", especially if it involves the input signal and input signal ground conductors of an amplifier. If those two paths get separated at all, physically, then you have enclosed loop area, which makes a hum antenna.

    Every time-varying magnetic field in the vicinity will induce a corresponding time-varying current in the loop, with magnitude proportional to the enclosed geometric area (all else being equal). Conversely, time-varying loop current will cause a corresponding magnetic field. See Faraday's Law (or Maxwell's Equations).

    Input signal and ground usually act as a receiving antenna, whereas AC mains and transformer secondary wire pairs (and tube heater circuits) usually act as transmitting antennas. Make sure that the two conductor paths in each pair don't get physically separated, ever. Tightly twist each wire pair, all the way from end to end. e.g. Input signal ground should not connect to anything except the ground end of the input resistor, at the first active device. e.g. Run both AC mains wires to the switch and fuse (with the pair twisted all the way), even though only one wire "needs" to go there. Etc. Etc.
     
  20. djsfantasi

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    Apr 11, 2010
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    Have you tried crafts stores? Something similar is used in stained glass
     
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