[sdiy] EM radiation -- current or power ?

[Magnus Danielson]

On 12/19/2010 12:29 PM, Bob Weigel wrote:
> On 12/19/2010 1:46 AM, Magnus Danielson wrote:
>>
>> You do want the guitar cable to be twisted too. :)
>>
>
>
> And speaking of which.... there's a cool trick we can do (not sure if
> anyone mentioned it in this context) to reduce noise with unbalanced
> lines. In many devices the unbalanced line has an op amp driving a 1K
> resistor or thereabouts to the output 1/4" jack for example. If you can
> get a balanced input to run it to, you can noise cancel by simply
> putting pin 1 to shield as always and run the balanced cabling back to
> the 1/4" plug! But instead of hooking pin 3 direct to ground, put a
> matching 1k or whatever resistance between it and the shield on the end
> of the device. That way you have two lines running alongside each other.
> Both will get the same amount of EM interference roughly. And now BOTH
> have the same resistance to ground and virtual ground!! So the mixer
> cancels most of the noise away.
>
> Now what I haven't tried is doing this with a guitar. I imagine it would
> work fine if you set the guitar up with a dual pot perhaps for volume.
> Of course the tone control would come into play also...
>

The point of impedance matching is found in Rane tech-notes, which 
people should read regularly.

For H-fields it is a problem when ground is a better receiver than the 
signal... so you want the same common mode termination impedance for 
both and then take the diff between them as input. Then you get most of 
the differential drive benefits relating to H-field inducting into the 
cable.

For a guitar you do want true differential relative to the shield to 
reduce cable static noise... which is an issue doe to the high 
impedances involved.

Cheers,
Magnus