[Premium-Rx] WJ 600 Ohm Line Matching

[Karl-Arne Markström]

You have two major cases when dealing with the 600 ohms line outputs.

One is when connecting the line output to a telephone line or multiplex equipment (carrier/PCM)
for transmission to a distant place. In this case impedance matching according to the telephone
network practices can be of some importance.

The other case is when distributing audio within i.a. a monitoring site or an equipment rack.
In this case inputs usually are high-impedance or "bridging", which enables monitoring of one audio source
by multiple users, without altering the audio level when the number of users is changing.

In communications stations (Coastal or Air/Ground) this arrangement is sometimes called a "watchkeeping matrix", where many receivers can be connected at will to loudspeakers at one or many operating positions.

When "traffic" is routed through receiver/transmitter combinations they usually are disconnected from the "watchkeeping matrix", and connected at a "one-to-one" basis to switching and level-adjusting equipment, using impedance matched interfaces.

73/

Karl-Arne
SM0AOM

----- Original Message ----- 
From: "stephous" <stephous at att.net>
To: <premium-rx at ml.skirrow.org>
Sent: Saturday, July 08, 2006 10:14 PM
Subject: [Premium-Rx] WJ 600 Ohm Line Matching


I received a number of emails regarding the purity of balanced 600 Ohm audio lines. We should look beyond what we have been told and examine each case. The manufacturers themselves do not hold to a 600 Ohm 1:1 rule. Why?

Example: Most Watkins-Johnson receivers specify a 600 Ohm audio line out, however most of the amplified or buffered WJ audio devices have a 10K Ohm input (read):
http://watkins-johnson.terryo.org/Documents/Catalogs/1997%20Electronic/GENACS/9903E54.PDF

a) Devices that are not amplified such as headphones or matched speakers derive their power from the final amplifier stage in the radio. These devices are matched to the output stages for obvious reasons. An odd impedance on the phone or line jack can cause poor performance or other ill effects. Mismatched impedances can also cause over or under drive conditions. The amplifier type will determine how much mismatch can be accepted. The WJ S-9902A 600 Ohm speaker uses a balanced 600 Ohm non amplified configuration.

b) Amplified devices usually have 10K Ohm or higher AC couple input stages. They are designed to interface with a wide range of impedances. Input signal levels are typically low. To the output stage of a receiver they might look like an open circuit. At audio frequencies they look like a piece of un terminated line. Shielding is important when coupling to these higher impedances. The WJ S-9903E and 9954 are examples of 10K Ohm input amplified speakers. They attach directly to the receivers 600 Ohm line output.

Is 600 Ohm audio line matching important? Depends entirely on each application.

Regards,
Steve Pappin

PCS Associates
949 Glenneyre Street
Laguna Beach, CA. 92651-2706
Phone: (949) 376-7844
FAX: (949) 376-7866
http://www.pcsassociates.net/


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