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[Art Welter]

I was asked whether the Talent DM1 microphone, which looks very similar to a SM-58 was high or low impedance, measured DCR on several units was 486 ohms, it's spec sheet states 500 ohms, close enough.

The 2015 Shure SM58 spec sheet lists "Rated impedance at 150 ohms, 300 ohms actual", but the four units I own all read just under 25 ohms, as do four Beta 58A mics. Seemed odd, so measured several of my older dynamic mics, PG58, original Beta 58s, EV RE10, RE-27 all coming in between 160-250 ohms DCR.

The SM 58s mics I tested do have transformers.
Anybody know when the DCR of the Shure mics dropped so low, and why, and why the spec sheet did not say "new and improved" when it did ;^)?

Art

[Mike Caldwell]

I know the old SM58's were dual impedance, 50 and 150 ohm with jumper that would plug on to 2 of the XLR connector. I have a couple of the dual impedance 58's, I'll measure one set on 50 and see what it reads. I'll also measure a newer one and see what it reads.

[Pete Erskine]

I was asked whether the Talent DM1 microphone, which looks very similar to a SM-58 was high or low impedance, measured DCR on several units was 486 ohms, it's spec sheet states 500 ohms, close enough.

The 2015 Shure SM58 spec sheet lists "Rated impedance at 150 ohms, 300 ohms actual", but the four units I own all read just under 25 ohms, as do four Beta 58A mics. Seemed odd, so measured several of my older dynamic mics, PG58, original Beta 58s, EV RE10, RE-27 all coming in between 160-250 ohms DCR.

The SM 58s mics I tested do have transformers.
Anybody know when the DCR of the Shure mics dropped so low, and why, and why the spec sheet did not say "new and improved" when it did ;^)?

Art

What equipment are you using to measure the impedance?

[Art Welter]

What equipment are you using to measure the impedance?

Standard VOM, as mentioned I measured DCR (DC resistance) not actual impedance. The odd thing to me is the DCR has gone down by a factor of 10, my first thought was there was no transformer in the newer mics (like the short-lived SM-78), but there is a transformer in the 25 ohm (DCR) mics.

[George Friedman-Jimenez]

2 SM58s were 23 and 16 ohms, Fluke 179.
4 SM57s were 17, 17, 16, 23 ohms.
All 5-8 years old.

[Lyle Williams]

DCR is only relevant if you are trying to record 0 Hz sounds!  :-)

[Mike Caldwell]

Finally got around to digging out my old dual impedance SM58.
The 150 ohm side measured 25.7 ohms DC resistance and the 50 ohm side measured 10.8 ohms.

[Ivan Beaver]

DCR is only relevant if you are trying to record 0 Hz sounds!  :-)

Agreed-for absolute measurements.

But for relative measurements, it can be a useful quick tool, to see if you are in the ballpark

[Geoff Doane]

Agreed-for absolute measurements.

But for relative measurements, it can be a useful quick tool, to see if you are in the ballpark

At one of my first jobs (broadcast transmitter maintenance) we had log books detailing the DC resistance from every pin of every tube socket to a reference point.  The idea was that if you were completely stumped as to what the problem was (no dead tubes or obviously burnt resistors), you could get out the VOM and start comparing readings, and hopefully zero in on whatever was the problem.  It was important to use the same model of VOM that the original measurements had been taken with, and also the same resistance range, in order to collect meaningful data.

When I started there in 1980, there were still a lot of transmitters in which the only solid state components were rectifiers.  Everything else was glass or metal tubes.  Often, simply looking for the tubes that weren't lit up was all you had to do to pinpoint the problem.  Other times I had to wrestle with 3 phase motors to get the thing working again.

GTD

[Art Welter]

Finally got around to digging out my old dual impedance SM58.
The 150 ohm side measured 25.7 ohms DC resistance and the 50 ohm side measured 10.8 ohms.

Mike,
Any idea the age of your dual impedance SM58?

[Mike Caldwell]

Mike,
Any idea the age of your dual impedance SM58?

Late seventies maybe 1980, I go it second hand in the early 80's.
 
After a quick bit of research I found the dual impedance SM58's were produced from 1966 to 1984 and starting in 1985 production moved to Mexico.

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