[Premium-Rx] WJ 600 Ohm Line Matching

Sun Jul 9 08:36:05 EDT 2006

Hi,

The famous old Hammond Organs (I've two B3s  from 1960 here) also have been using
a balanced 600 Ohm preamp output.
Thus a twisted pair of unshielded audio leads could be run beside unshielded mains leads
in the same cable several meters to the high impedance *input* final amplifier
(like a Leslie) without any hum problem from the mains.
A high impedance *output* has also limitations for the cable length:It forms a low
pass with the cable's capacity and you loose high audio frequencies with longer
cable runs.
Therefore since decades in audio studios 600 Ohm balanced lines are standard.

73
Clemens
DL4RAJ

  ----- Original Message ----- 
  From: John USA
  To: premium-rx at ml.skirrow.org
  Sent: Sunday, July 09, 2006 9:27 AM
  Subject: Re: [Premium-Rx] WJ 600 Ohm Line Matching

  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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