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Old Shure Unidyne Impedance Question

Discussion in 'Microphones' started by lostindundee, Mar 26, 2012.

  1. lostindundee

    lostindundee Active Member

    Trying to figure out of I can use this thing with a preamp. I'm not too up on all this impedance stuff - you probably noticed. :tongue:

    I've found one of my father's old Unidyne III B545 microphones. It's from the late 1960s and still has its original red felt pouch and information leaflet. I thought it would be nice to use for recording something given my father (RIP) once gigged with it for years when I was a little kid (I'm now now 40+).

    The cable appears to have a 4 pin Amphenol connector at one end (as does the mic fortunately) and a what looks like a TS at the other end. I'm guessing that, without hacking it open and soldering, I'm gonna at least need a TS >>> XLR adapter. The cable isn't that long either (probably 3m). The specifications in the leaflet say that it's rated as 'High Impedance'. However, I'm confused by what the following means:

    "The models B545 and B545S microphones and cable assemblies are supplied wired for use with high-impedance amplifiers. The microphones have terminals available for connecting into microphone inputs rated at 25-200 Ohms. Low-impedance connections are recommended where long cable lengths are required or under conditions of severe hum disturbance. In this case, use two conductor-shielded cable and connect to microphone as shown in 'Internal Connections'."

    Can anyone shed any light on the above? Isn't 25-200 Ohms low impedance?

    Basically, is this going to be like a sm57 but just older? Or am I wasting my time?


  2. Boswell

    Boswell Moderator Distinguished Member

    Those mics could be used as either high impedance unbalanced or low impedance balanced, depending on the how you wired the connector. From what you say, your cable and connector is wired for the high impedance unbalanced configuration, and this cannot be used usefully with a low impedance XLR microphone input. I would try plugging the TS connector on your mic cable into the DI input of a pre-amp as though the mic were a guitar pickup.
  3. lostindundee

    lostindundee Active Member

    Thanks for the info Boswell

    It seems to work fine via DI. It looks like it would be major PITA to change to low-impedance. At least I should get a vocal or guitar cab recording the way it currently is.

    Thanks for the info Boswell. Much appreciated.

  4. lambchop

    lambchop Active Member

    I got a Unidyne III a while back based on a recommendation of a friend. I had an old Ampherol cable and just took the Ampherol connection and soldered it to a spare Switchcraft XLR. I've been using it as a snare top mic with an SM57 on the bottom.
  5. moonbaby

    moonbaby Mmmmmm Well-Known Member

    I used to use 545's all over the place; they work well on guitar cabs, snare/toms, and brass - the usual suspects. I went to the Shure factory in Chicago back in the 70's when I worked for a dealer and they gave us a tour. They explained that the same cartridge that was in the newer SM57/58/56 (yes, the SM56 !! ) models was the same model that they put in the 545/565 models, but the 545/565's got the cartridges that were ranked as "seconds" during their QC testing. BTW,my favorite Shure mic during that time was the 548, which had a different head and grille assembly than the others.
    I don't remember exactly what wires went where, but it wasn't really hard to convert the Hi-Z down to the Lo-Z (yes, < 200 ohms). Maybe with the age of the mic and the thin gauge of those wires, this might not be such a great idea, so if you're happy with the DI thang...
    Yours sounds like real gem!
  6. Boswell

    Boswell Moderator Distinguished Member

    If you wanted to try the alternative low-impedance wiring, you don't need to modify the mic - you just make up a separate cable. Using a low-impedance connection would allow longer cable runs without loss of high frequencies compared with the high-impedance connection.

    It's relatively easy to wire an Amphenol MC4M connector using standard 2-conductor microphone cable and terminate with an XLR(M) plug. The wiring should be: MC4M pin 4 to XLR pin 2 (+ve), MC4M pin 3 to XLR pin 3 (-ve) and MC4M pin 1 via cable screen to XLR pin 1 (ground). Leave MC4M pin 2 unconnected. After wiring the two ends of the cable, use a continuity tester to check that neither of the two signal wires shows a connection to the screen or to the other signal wire.

    This site has some information and wiring diagrams of the 545.
  7. lostindundee

    lostindundee Active Member

    Thanks everyone for giving info on these mics. I knew you guys here would know. :smile:

    Thanks for the link Boswell. I'm not a good techy person type and will probably not make my own cable. However, I may learn this stuff and do it at some point in the future. Link bookmarked!


  8. lukpac

    lukpac Member

    I've heard that before (the SM mics getting "selected" capsules), but...are you sure that's what they said? Shure flatly denies it today, and the 5xx mics and SM mics take different cartridges. The 5xx mics have copper voice coils, while the SM mics don't (I believe they use aluminum). The published response curves and polar plots aren't the same either.

    A bit of a tangent, but there were a ton of 545 variants. I won't list them all here, but in addition to the original models with Amphenol connectors and later/current models with XLR, there was also the "L" (lavalier) model with a shorter body and hardwired cable, the 544 gooseneck models and the 546 with XLR and a shock mount.

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