AW: AW: To earth, or not to earth...

[Tim Cockram]

The balanced output stage is indeed a problem.  A common solution is the cross coupled Howland current source,  Like 
everything active it relies on good resistor matching for a good CMRR.  A British broadcast mixer manufacturer uses this 
topology with off the shelf sil resistor packs for the "matched" resistors.  The brochure claims ZERO output impeadance for 
the circuit (can't work as the op-amps are 553X which won't drive 600R too well)..

There is a halfway house though...pseudo balanced.  The inputs use a differential amp whilst the outputs are regular ground 
for the screen, signal and a reference ground.  The differential amp at the input gets rid of (some) of any interferance 
induced in to the cable.  This technique is used internally in some pro mixers (for the mix busses) and externally for some 
semi-pro stuff (there's some soundcraft stuff that uses this technique).
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Tim Cockram
E-mail: tim at redragon.demon.co.uk 

Phone:-UK-1222-384041
Fax:-    UK-1222-390326


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