[sdiy] EM radiation -- current or power ?

[Juergen Haible]

It depends.
Current will create hum in low impedance circuits (especially in 
less-than-optimal ground systems).
Voltage will create hum in high impedance circuits (worst case: 
1-meter-sized electrostatic pickup of a Wurlitzer e-piano)

JH.

----- Original Message ----- 
From: "Bob Weigel" <sounddoctorin at imt.net>
To: "Andre Majorel" <aym-htnys at teaser.fr>; "synthdiy" 
<synth-diy at dropmix.xs4all.nl>
Sent: Saturday, December 18, 2010 11:15 AM
Subject: Re: [sdiy] EM radiation -- current or power ?


> On 12/18/2010 12:31 AM, Andre Majorel wrote:
>> Does the amount of EM radiation depend on current or power ?
>>
>> In other words, will two wires carrying 1 W at 18 VAC induce
>> more 50 Hz hum in neighbouring audio cables and circuits than
>> two wires carrying 1 W at 230 VAC ?
>>
>> In other other words, are there any drawbacks to putting the
>> power transformer and the rest of the power supply in separate
>> boxes ?
>>
>> Thanks in advance.
>>
>>
> Hi Andre!
>
> It's current.  coulumbs/second.  The current is a result of voltage and 
> load factors.  Say you had only a 12V system with huge wires that would 
> support the current :-).   Ideally of course a household needs a certain 
> amount of *power*.  They could locally convert the voltage with very 
> efficient transformers to whatever they need it to be...but 100W light 
> bulb needs 100W of power to be the right intensity. So you have to supply 
> enough voltage so that it's impedence will flow enough current so that I x 
> E = 100W.  (I being current, and E voltage).
>
> Anyway so let's say a houshold needs 1200W average.  The 12V supply would 
> have to flow 100Amp to accomplish this.  Were we to up the voltage to 120V 
> then of course only 10A would be required.  And thus there is a lot 
> smaller magnetic field component generated in the process.  Wires usually 
> are higher voltage than this when they travel any distance so that 
> currents are scaled down even more.  This results in needing smaller wire 
> diameter as well of course.
>
> But anyway..the closer wires are running on a push/pull pair, the less 
> field they will generate at a distance.  Electric fields are easy to 
> shield but magnetic fields require high permeability materials.  It always 
> helps some to keep the currents as low for as much of the run as you can 
> of course.  So if you're talking putting a transformer that steps down far 
> away, then yes that's more wire with higher current.  As to whether it 
> will be audible somewhere is another question though.  Depends how much 
> current and how close the wires are to each other and how well shielded 
> they are.
>
> -- 
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