Hi
Regarding 'item 5'.
EVERY amplifier amplifies the difference between it's 'input' and a
reference point. In a 'balanced' amplifier the refeence is most visible
as it's 'cold' or 'antiphase' input terminal. Peturbations of THIS
amplifier's ground are largely ignored (the amount being dependant on
design and implementation).
An Unbalanced input works in essentially the same way but the REFERENCE
is the ground of THIS circuit.
In a simple system with few circuit blocks and sensibly laid out and low
impedance grounding things work pretty well. Many, practically all
mixers have this system internally, certainly at channel level and
because it is a mostly 'enclosed' system it works.
Problems start to appear if external connected equipment (cabling)
having significant ground current flowing in it being either mains hum
or 'RF' which then start to add to what should be the clean reference
point due to the inevitable resistance of the 'ground' be it wire or
circuit traces. If there were to be say 1 amp of 'ground loop current'
it would only take a resistance of 0.01 milliohm in the 'wrong place'
for the hum to appear at -80dBV. OK 1 Amp sounds excessive but then
getting a resistance as low as 0.01 milliohm is also improbably low.
From this, what I am trying to emphasise is the importance of the
'reference' following the signal so that unwanted signals are cancelled.
Having a 'balanced' input with even only 30dB 'rejection, pays dividends
in reduced noise and unwanted 'rubbish'. This is not to say that
unbalanced connections cannot work but great care must be taken to
ensure stray unwanted currents are not applied to it. The worst scenario
is with a mixer 'tape' return where you have typically 24 signals
following the same physical route (same interference) and summed, where
they will add. 2 inputs giving 6dB worsening in noise, 4 another 6 dB, 8
another and so on. The 'high quality' gear will take these constraints
and ensure that the 'ground reference' is taken to the connectors and be
suitably low resistance to withstand some 'interference' from outside.
Having a signal flow from one amplifier connected to another in a 2
channel preamp is relative childs play. Having say 10 circuit blocks in
one channel strip then attached to another 23 channels in 3 dimensions
spread over a few feet becomes a logistical nightmare. Bear in mind that
any conductor with LENGTH is an aerial. A few inches is associated with
microwaves (your mobile phone).
The rules for successful interconnects are low impedance grounds which
physically follow the signals as closely as possible. An 'unbalanced'
input on a desk CAN reject interference if the 'hot' and associated
ground both get to the input stage at which point it is the design of
the stage which determines whether the interference is amplified or
rejected.
Matt S
Donald Put wrote:
On Thu, Feb 18, 2010 at 8:38 AM, Ike Zimbel <[email protected]
<mailto:[email protected]>> wrote:
5) And finally, I wish everyone in the whole audio industry would
stop equating "un-balanced" with other negatives in our language
like "unclean, unstable, unsafe, unreliable, unexceptional etc." I
think "not balanced" would be more accurate! (end of rant >:o )
As usual, Ike hit it out of the park. As for the "unbalanced" issue,
it's arguably a cleaner signal path (fewer number of parts in the way
of the audio signal), and unless you have interference issues I
wouldn't worry about it at all. With the exception of mic inputs, my
whole studio is wired unbalanced and I don't have any problems with
noise. Of course, I'm in a rural area so RF et al. has never been an
issue.
Cheers,
--
Don
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