Newsgroups: rec.audio.tech
Subject: Re: Hook up multiple power amps
References: <[email protected]> <[email protected]>  <[email protected]> <[email protected]>  <[email protected]> <[email protected]> <[email protected]>     <[email protected]>
"Phil Allison"  writes:
> 
>    **  This is the post that he now REFUSES  to discuss.
> 
> 
>        It is FULL of misconceptions.
> 
>     1.   Hum current in the ground wiring is NOT a commom mode input
> signal - it bypasses the balanced input - even a transformer one.

Hum current on the ground wire of balanced connection will have 
two effects:

1. Common mode voltage in the signal wire because of shield resistance 
   and hum current flowing on it cause voltage difference
   (voltage = humcurrent * cableshieldresistance)
   - ideal balanced input can stop this, real life implementation 
     will attenuate this typically 60-90 dB 

2. The hum current will cause this current to flow from the 
   input connector signal ground (groun usually XLR pin 1) 
   to mains connector ground throug some route on the equipment.
   - Depending on the equipment deisgn this can cause considerable 
     hum problems or not
   - If the grounding of signal connector is carefully designed 
     (very direct path to ground, not routed through circuit 
     board signal ground wiring) this current does not have much 
     noise generated inside equipment

It is the cause number 2 that passes through the balanced 
connection. And it it this what is sometimes solved by cutting 
the signal ground on balanced cables (the "ground lift" switch).

 
>     2.   There is NO relationship between balanced  DC supply rails and
> balanced signal circuits excepting only the word itself.

You are absolutely right on this.
 
>    3.    Supply ripple rejection is OT - (amps with a single supply reject
> it just as well as those with dual.)

Yes.

--
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/