Sunday, July 14, 2013


     This is a neat trick for professional balanced audio applications, where a stereo signal needs to be "summed" to mono or a mono one split to two destinations.
     It depends on all the sources and loads being the same impedance, which the resistors match, 600 Ohms in this (typical) example.  Isolation between the two inputs (or outputs) is very good and insertion loss is low.

     Modern practice tends more to quite low-Z sources[1] and very high-impedance loads.  With that, "splitting" one source to several destinations can be a simple (if sloppy) process of paralleling but look out if any one of them gets shorted!  This "Bridge" (as in Wheatstone) pad can be adapted to the low-Z source/High-Z load system by "building out" the pad inputs with pairs of series resistors and adding a fourth 604 Ohm[2] resistor on the load side, but insertion loss will go up as will complexity; a "branching" pad is probably easier--
     --But the isolation between ports is worse.  Still (and especially with a modern very-low-impedance source), a short on any one branch won't silence the others.  (Expanding this to more ports results in increased loss and different resistor values but simple algebra will tell you what they are.)

     This is a little off-topic but analog audio, especially balanced audio, has become more and more "black magic" as the digital age advances.  Perhaps it will be of use to some of the ham operators running AM, who sometimes use older audio mixers and feed multiple transmitters, some of which are former AM broadcast rigs.

     Most sound starts out analog and ends up that way, too.
1. Some "zero-Ohm" sources are more zero than others.  Back when Purdue University's WBAA still used Ralph W. Townsely's amazing homebrew (tube-type!) audio console, the line outputs were very high-fidelity audio power amps set for extremely low output impedance and technically-savvy visitors might be handed a paperclip by the (irreverent) student operators and invited to "short out the Line audio."  All that happened was the paperclip got hot; the audio level never changed.  Please don't try that with the "0 Ohm" analog outputs of most pro audio gear -- your results will be quite different.

2. Why 604 Ohms?  It's the nearest 1% metal-film resistor value to 600.  1% resistors cost pennies these days but you could use the closest from the (even cheaper) 5/10/20% carbon resistor bins, either 560 or 620 Ohms.  If you're feeding something critical like a passive equalizer, you may be able to tell the difference.


    You really helped me with the first picture! Clear painting by the way, nice.
    I've build two of it for stereo (IN1R & IN2R, IN1L & IN2L), and I'm super happy with it!!
    Thanks a lot!