[Premium-Rx] WJ 600 Ohm Line Matching

[John USA]

Hi group,

The short reason that manufacturers, and you too, may not do a bunch of matching has to do with source impedance vs destination impedance.  In the earlier days, audio sources from tube gear were mid-Z, that is 600 ohms after being matched by a transformer...in order to terminate the transformer, and the audio cable run, a 600 ohm resistor (actually either 570 or 601) was soldered across the hi & low wires, usually inside the connector.  Failure to do so would leave the unterminated source transformer working into a random load---side effects include miscalibrated levels, thumpy base response etc.  The frequency response of a full blown broadcast circuit was targeted to match the "NAB" curve (National Assoc o' Broadcasters) and was somewhere along the lines of 100Hx to 15KHz +/- 3db.  A distribution amplifier was required at all times one source fed more than one destination.  This setup is a power system, operating on a "dbm" scale.  (decibel milliwatt).

Now, with source IC's having transformerless outputs on the order of 2 to 4 ohms, a bridging load is negligable.  This is where the previous response comes in...you can feed a bunch of bridging destinations (10K to 100K ohms impedance) with a single IC output.  There is no need to "match the source and destination any more, although sometimes one has to limit the frequency response to something less than the typical 4Hz to 100KHz capability of many IC amps.  This non-terminated system operates on a voltage scale, calibrated in dbv.  (decibel volts).

Our premium rx's usually offer 600 ohm audio, which may be presented as coax unbalanced or as two lug balanced.  I suggest using a 600 - 600 ohm transformer to balance any coaxial audio right at the rear panel of the rx.  With all the noise sources in shacks these days, balanced audio is far more immune to problems with EMI.

You can get a more comprehensive essay on this with a Google search for "broadcast audio", "analog audio", or look at manufacturers such as Sony, McCurdy, Thompson CSF, etc.

I hope this helps.

regards

John Collins
__________________________________________
  ----- Original Message ----- 
  From: stephous 
  To: premium-rx at ml.skirrow.org 
  Sent: Saturday, July 08, 2006 1:14 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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