🚀 go-pugleaf

In article <258@celerity.UUCP> billd@celerity.UUCP (Bill Davidson) writes:
>In article <9143@ihlpl.ATT.COM> nmju@ihlpl.UUCP (55244-Urban,M.J.) writes:
... [Original poster's hum problem is likely grounding, which is a tough 
    problem to solve]

>I have a related question.  Why don't audio components (and video
>too for that matter) use grounded plugs?  It seems like such a
>simple thing to do yet I've never seen any audio equipment that
>used them.  My reciever-amp has a ground connection for the antenna
>(which is hooked up) but I'm not sure this is the same.  Even if
>it is, it couldn't do anything for the rest of the components.
>	--Bill Davidson
>....!{ucsd|sdcsvax}!celerity!billd

Some observations, based on a bit of experience.
1) Most "Professional" or "Industrial" equipment does have a grounded plug.
   Might be a cost factor here.

2) I've seen schematics for some such equipment, and the design goes to some
   trouble to isolate the power ground from the signal ground.  Power ground
   goes to the chassis for shock protection as usual, signal ground to the
   shields on the connectors.  Often these two grounds are connected through
   a resistor, which sometimes can be shorted by  rear-panel jumper.
   I suppose trying both jumper positions (open/closed) is useful in fighting
   a hum problem, although I've never used then.

3) Much of this professional/industrial equipment uses balanced audio inputs
   and outputs, with 3-pin XLR connectors.  If everything used transformer
   balanced I/O, there would not be very many cases of hum trouble.
   Electronicly balanced I/O also usually solves the problem.
   Unfortunatly, good transformers run $20/each and up.  Electronicly
   balanced inputs and outputs still usually take an op-amp for an input and
   at least two for each output, or a bunch of discrete parts, all of which
   cost.  Even the XLR connectors run at least $2 in medium quantities.
   Here, I'm certain cost is the reason you don't see these things in
   "consumer" stuff.

3.5) Balanced signal interconnections are good for two reasons
 A The input stage cancels the 60hz carried or induced in the ground (shield)
   because the input signal is taken to be the difference between the two
   signal wires, not the difference between one signal wire and ground.
 B Such connections are usually low impedance, (like 600 ohms instead of 10K)
   which reduces the levels that are induced on the cables by things like
   inductive coupling to nearby power or signal cables.

4) I can recall one case where connecting the ground screw on an audio
   mixer to the screw on a connected stereo receiver solved a low-level
   hum problem.  The mixer had a grounded plug, the stereo did not.
   Inputs and Outputs were unbalanced.

5) Unfortunatly, video is still usually unbalanced through the BNC connector,
   which can lead to visible hum and noise in the picture.  For this there
   are available video isolators an video transformers, that couple
   the signal without allowing a direct connection on which unwanted ground
   currents can flow.  Unfortunatly, these are expensive.
-- 
Steve Tell:  senior, Duke University school of Engineering (please hire me).
Former Chief Engineer, Cable 13 / Duke Union Community Television.
sgt@dukeac.ac.duke.edu;     !mcnc!ecsgate!dukeac!sgt