[BC] audio cable termination

[RichardBJohnson at comcast.net]

Cable termination is determined by what you want to accomplish. The 
current trend is to feed audio cables
with a low impedance source and use a bridging (high impedance) 
destination. In the case where the
round-trip delay of the cable is significantly less than a wavelength 
at the highest frequency carried by the
cable, this should be okay if the device that feeds the cable has no 
problem feeding the resulting cable
capacity. This kind of setup should maximize the S/N because there 
are no losses in the source resistor
and the terminating resistor as would be the case with the old-school 
600-ohm source and destination
terminations.

However, often all is not well with this setup in areas where there 
are transmitters. Any RF getting into an
unterminated line will undergo "transmission-line effects" in that 
there were be voltage maxima and minima due to a high VSWR on the 
line at the RF frequencies. If this RF voltage exceeds the tolerance 
of the source or destination circuitry, rectification will occur and 
noise, hum, or feedback will be the result.

I suggest, therefore, that unterminated lines be used at studio 
facilities that are not in high RF fields, and
conventional 600-ohm termination of shielded audio wires be used at 
transmitter sites. It has been my
experience that FM transmitter sites induce great amounts of RF into 
random wires, possibly due to surface-
wave effects, so one should not assume that FM sites are immune. The 
de facto 600-ohm termination, even
though twisted pairs may have roughly 180 ohms impedance, is 
certainly close enough to reduce any VSWR on the lines to reasonable values.

--
Cheers,
Richard B. Johnson


  -------------- Original message ----------------------
From: Mario Hieb <mario at xmission.com>
 > Terminating audio lines and other cables is a subject that interests
 > me and tends to come up now and again.
 >
 > So I throw the question out to all of you; why do we terminate
 > cables? When should we terminate cables?