[Amps] RF in the Audio

[Jim Brown]

On 9/24/2011 10:13 PM, Tom Thompson wrote:
> This does not imply that the currents in the wires are equal and
> opposite and if they are not, then a common mode current is present.

Exactly.  There is an excellent AES paper from 1994, published in the 
June 1995 Journal of the AES (Audio Engineering Society) by Bill 
Whitlock analyzing a balanced interface as a Wheatstone Bridge. The 
function of a balanced interface is the rejection of noise and 
crosstalk, and Bill shows that the performance of a balanced circuit is 
defined by the balance of the IMPEDANCES in the circuit, including those 
on each end of the line. Whitlock's paper can be downloaded (for a small 
fee) from the AES website, and a less formal version of the paper is 
available free from the Jensen Transformers website (Jensen is 
Whitlock's company).

It is important to realize that while the usual application is audio, 
the analysis is entirely valid at all frequencies. The only thing that 
changes is the complexity of the impedances (including strays) both in 
the cable and at both ends. For example, it's fairly easy to balance 
impedances at mid-audio frequencies, but a much bigger kettle of fish to 
do so from low audio frequencies to high RF frequencies, but that's what 
is required if you want the balanced interface to reject interference 
over that range. One thing Whitlock showed us, with supporting measured 
data on real cables, is that the capacitance between the two conductors 
and the shield is typically unbalanced on the order of 5-10% for GOOD 
cables. A colleague at Belden on the AES Standards Committee noted that 
some reasons are the differing dielectric constants of different color 
dyes, the tolerances on the dies that place the insulation on the 
conductors, and the centering of the conductors within the shield as the 
cable is manufactured.

Following the publication of this work, IEC Standards relevant to 
defining and measuring the balance of a circuit were revised to reflect 
the realities that Whitlock's analysis exposed, using a test method that 
he devised.

73, Jim Brown K9YC