Ground Busses
Eeyore wrote:
Ian Thompson-Bell wrote:
Eeyore wrote:
Ian Thompson-Bell wrote:
Henry Pasternack wrote:
Andre Jute wrote:
A star ground is just a circular bus or a very short bus.
Not really, ANDRE. A star ground is the abstract notion of a purely
equipotential (equal voltage) single point connection that eliminates
ground loops by reducing the resistance of the common ground bus to
zero. It's impossible to achieve in practice, though, and trying to
take the approach too literally is likely to cause more harm than
good.
The key phrase here is 'reducing the resistance of the common ground bus
to zero'. it is all very well connecting all grounded leads to a single
point but if you then connect that by a single wire to the zero volts
point of the power supply you have immediately introduced a long no zero
common ground bus. So a star ground really only works if the star point
IS the 0V point of the PSU i.e the ground of the input filter cap.#
In power amps this is relatively easy to achieve and running a thick bus
from the PSU 0V past the power section to the preamp section should work
just a s well since the power currents will not flow through any of the
preamp sections of the bus.
But still a compromise.
In separate pre-amps or mic pre-amps or tube mixers where the supply is
remote from the pre-amp this is clearly not possible and there is the
added problem of separating power currents from input/output signal
currents (unless floating transformer inputs and outputs are used). One
solution I have seen is to have a local pre-amp power supply - indeed
this was very common practice in many tube based recording mixers of the
50s and 60s. Each pre-amp would have its own mains transformer and PSU
along with balanced floating inputs and outputs. All earth loop problems
were therefore local to the pre-amp and you could connect them together
however you liked to build a mixer without ever having to worry about
ground loops. An expensive option to be sure, but one that worked very well.
Far simpler is to have 2 (or more) stars and use these to direct current flow.
In particular, the PSU should have its own star and this is where ALL ripple
currents must flow. This can now be tied to the input 'star' with a
*non-current-carrying conductor* (or at least carrying only the B- smoothed DC).
Presumably you mean no *signal* current carrying conductor.
Well ... if there's no current, then there'll be no signal too for sure. What I meant
basically was a link to simply ensure that the 2 stars are at the same potential.
But that link carries common mode current from one star to the PSU 0V so
how does that solve the problem?
This still leaves a common mode conductor carrying output signal current
which is of course supplied by the power supply. I am not clear how
multiple local stars solve the problem.
It is *essential* to keep power and signal paths separate for hum free
operation.
Which can only be achieved with transformers?
Not necessary. Transfomers introduce their own problems.
Graham
So how do you isolate signal currents from power supply ones? given that
output currents flow from the PSU through the load and back to the PSU
0V with or without transformers?
Cheers
Ian
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