wjm@whuxl.UUCP (MITCHELL) (08/06/84)
<gulp, gulp, gulp>
Yes, most broadcast equipment uses 600 ohm balanced lines (this means that
neither side of the line is connected directly to ground, but rather that
the line is driven through a transformer and the center tap of the transformer
is grounded:
_________________ line 1
hot out ___ )
( )_____
( ) |
common ___( )
)_________________ line 2 Center Tap is grounded to
signal ground.
Advantages: One can use relatively inexpensive wire (although shielded cable
with the shield grounded is still preferred in hi-fi applications
or wherever there is a problem with RFI.
Disadvantages: Unless your equipment is "semi-professional" (and thus designed
with balanced inputs and outputs), you'll have to buy N+1
transformers, 1 for the driving preamp and N for each of the
remote amps, and hi-fi transformers are EXPENSIVE (why do you
think McIntosh amps cost so much, the output transformers are
one reason).
Ron Natalie has the right attitude about RFI - the only way to stop it is to
keep it out in the first place. Connecting cables should be shielded, and
the shields should be solidly grounded (to earth via the 3rd wire of the AC
line cord, or a convenient cold water pipe). It may be desirable to only
ground the shields at one end, to prevent ground loops that can pick up 60Hz
hum.
While we're on the subject of connections, I agree that gold plated RCA jacks
are preferable to the tin variety. Whatever the material, the cables should
be unplugged periodically and the plugs twisted (hold the plug, NOT the cable!)
to remove any corrosion that may have built up on the contacts.
Does anyone have any experience with 'Tweek', a fluid which is supposed to
improve connections, inhibit corrosion, and cure whatever ails you :-) ??
It is a small tube of colorless fluid that is applied to contacts.
Note to Dick Dunn: I don't necessarily endorse transmission lines for speaker
wire, however, I would like to minimize the DC resistance (which means at least
16 gauge, 14 gauge if you have a high power (> 100w/ch) amp) of the wire, and
I don't think a low inductance configuration is going to hurt. Personally,
I'm using Monster Cable on my JSE-1's and I'm quite pleased with the transient
response - there is an improvement over the standard 14ga SJ cord I was using.
Regards,
Bill Mitchell (whuxl!wjm)gino@voder.UUCP (Gino Bloch) (08/20/84)
Re an ohmmeter magnetizing a transformer's core. Ohmmeters use small currents AND transformer people work very hard finding magnetically soft materials to make cores from. So that the transformers will work!
rzdz@fluke.UUCP (Rick Chinn) (08/23/84)
Ok...Ok...Now you've done it...I gotta speak.
First...It's very common practice nowdays for "balanced" inputs on sound
equipment to be floating. It's very uncommon for center-taps on input
transformers to be grounded, let alone exist. I think that I can say this
with 99% certainty for the equipment being sold today that still uses input
transformers.
Second...Don't you dare use an ohmeter on your input transformers. The DC
used for the ohms current source will almost certainly magnetize your
transformer's core, ruining it for anything musical. Once this happens, you
might as well add it to your collection of audio transformers being saved
for your boat anchor.
Rick Chinn
John Fluke Mfg. Co MS 232E
PO Box C9090 Everett WA 98206
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(206) 356-5232muller@inmet.UUCP (08/25/84)
#R:whuxl:-14700:inmet:2600096:000:417 inmet!muller Aug 10 21:03:00 1984 ***** Um, I guess I should look at the transformers (and wires) we use for parts of our band's PA system, but I don't think there is a grounded center tap on the primary (input) side of garden-variety, vanilla PA impedance-matching (600 Ohm) transformers. (I s'pose I could go slap an Ohmmeter on one, but... gee, they are all packed up, and well it's anawfulota' trouble, but I don't THINK so,...) Jim Muller