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So I want to buy a Shure Earmic (condenser) which has a mini XLR connector with 4 pins, one of them in the centre. And I want to connect it to an AKG PT40 body pack which has a three pin mini XLR connector. I tried plugging one in directly and it doesn't work. I figure the problem is its how its wired.

Shure pinouts:

 1. Shield
 2. Connected to pin 3 via 2.2K resistor
 3. Connected to above resistor via 1uF cap
 4. Not connected

PT40 pinouts:

 1. Shield
 2. Audio (inphase) and +3.8V
 3. Audio

These connection schemas come from the online manuals. My guess is that the bias voltage on the Shure mic is supposed to come in on pin 3 via the cap. Or am I wrong?

What I am thinking of doing is getting a male and female connector and a three inch long piece of shielded cable, and simply making up an adaptor.

Can this logically work?

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  • \$\begingroup\$ Are the pinouts you give exactly the original text or your interpretation? Can you quote/picture and link the originals? \$\endgroup\$
    – Kevin Reid
    Dec 15, 2017 at 16:41
  • \$\begingroup\$ There are eBay listings for an adapter cable...ebay.com/itm/261935225922 \$\endgroup\$ Dec 15, 2017 at 16:50
  • \$\begingroup\$ Thank you Kevin for looking at this. I googled the pdf manuals. The Shure one had a diagram. But I didn't know how to post pics here. David, that's exactly what I had in mind. Thank you! Pity that item doesn't ship to UK! Couldn't find one this side. \$\endgroup\$
    – Graeme
    Dec 16, 2017 at 17:27

1 Answer 1

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You've got it backwards. A capacitor blocks DC and passes AC (audio), so the bias voltage would pass through pin 2 with the resistor, and the audio would pass through pin 3 with the capacitor.

Since we don't have a precise schematic it's hard to be sure this is exactly right, but assuming there are no surprises:

You would want to connect both pin 2 and 3 of the Shure pinout to pin 2 of the PT40. If you connect only pin 2 through, then the audio will be attenuated by the resistor; if you connect only pin 3 through then the bias will be missing.

On the PT40 side, assuming the two audio pins are differential (balanced, opposite phases), you will need to connect pin 3 (PT40) and pin 1 (PT40 and Shure) together, otherwise the input will be floating and you get no audio through. For best noise rejection, make this junction at the mic end of your adapter.

Again, please update your question with schematics or exact original wording if you have them to be sure this is correct advice.

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