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Old 12th March 2016, 04:45 PM   #1
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Default component magnetic field interactions

Hi all,

My equipment cabinet interior is 21" wide, 16" tall and, 15" deep . It has a fixed bottom shelf and two height adjustable shelves above the bottom shelf.

The amp is on the bottom, preamp about 2" above it and CD player about
1 1/2" above the preamp. The preamp has an isolated power supply, that is outside of the cabinet, but I think it might be good to shield it from the amp and CDP.

I'm concerned that the components, being so close, could cause some interaction related to their power supplies. I've read about Mumetal but have also read some shielding material can make a magnetic field bounce back to the component from which it emanates. What is the best way, in my setup, to shield the components from each other?

I was also thinking an open ended box put over the amp's transformer might be a good way to shield the preamp from the amp. It would also shield other components in the amp from it.

Thanks,
henrylrjr
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Old 12th March 2016, 05:50 PM   #2
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Old 12th March 2016, 06:25 PM   #3
DF96 is offline DF96  England
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Mu metal is no use for shielding power supplies as it satureates at quite low field strengths. It also needs re-annealing after any working, such as bending into shape.

Distance is the best solution, combined with small circuit loops and shielded or twisted signal wires.
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Old 12th March 2016, 06:32 PM   #4
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The system sounds quite good. Jack Dejohnette's wood sticks on cymbals sounds very real and the trumpet opening in Mahler's 5th also sounds very good.

However, when I put my ear within about 2" of the drivers I hear a very slight hum. I just thought shielding might remove that and make the speakers dead quiet.

Since shieldiing material is rather inexpensive l thought it would be worth trying. I'm looking for some recommendations instead of buying and trying a bunch of different stuff.

Thanks,
henrylrjr
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Old 12th March 2016, 06:54 PM   #5
jcx is offline jcx  United States
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"ground loop" common impedance coupling is a frequent problem when wiring together systems

debugging/localizing the problem(s) can be somewhat systemized but it helps to know most of the possible players/fails

Whitlock at Jensen obviously wants to sell you a transformer but has a good guide for debugging hum in audio component systems:

Jensen Transformers Application Notes | Jensen Transformers
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Old 13th March 2016, 04:02 PM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by henrylrjr View Post
............However, when I put my ear within about 2" of the drivers I hear a very slight hum...........
A measurement is far better in helping us to help you than using your ears.
Set your DMM to 199.9mVac and measure the output noise.
Since you mention hearing hum and not white noise, I'd guess that most of your measurement will be hum.

Is it 0.1mVac, or 0.3mVac, or 1mVac?

1mVac of hum is ~-79dBW ref 1W, 2.828Vac into 8ohms.
A 90dB speaker would be outputting ~11dB @ 1m, which is pretty quiet, but audible at 100mm
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Old 13th March 2016, 04:29 PM   #7
jcx is offline jcx  United States
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a look at the waveform/spectrum is helpful too - even a free PC audio based SW Oscilloscope/Spectrum analyzer would be fine

its useful to make the distinction between "hum" and "buzz"

with "hum" reserved for mains power frequency by itself - which can be power xfmr field coupling or chassis leakage current ground loops

and "buzz" being the rectified power line harmonics - more likely internal wiring common impedance, although coupling to rectified current carrying wiring loops can be by magnetic induction too

Last edited by jcx; 13th March 2016 at 04:42 PM.
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