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Dave

Resurrecting & Powering Old Bruel And Kjaer 4125 Measurement Mic?

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Hi All,

I found an old 1980 Bruel and Kjaer 4125 measurement mic that was given to me, while going through my old bits and pieces. I've never had this mic hooked up at all but it still has me intrigued. I would like to hear it at least once before it goes back into the box. I have the accompanying "type 2810 power supply"  that appears to have a "battery" function. I cracked open the box but there is no battery slot that I can see.  The mic itself is quite DPA-like, very miniature with a mini lemo connector.   I don't see a battery compartment in the mic body either.

Perhaps I could have the connector adapted for a more common power supply? The printed info label on the mic capsule storage vial says the           following:   Type 4125 Condenser Microphone Cartridge, Open circuit sensitivity at 1013 mbar, 23 degrees Celcius and 50% R.H. 140.8 dB re. 1 V per Pa. Capacitance : 14.5 pF (typically), Polarization voltage : 28 V, Frequency : 250 Hz, Date : 3-11-80

If anyone has any ideas about how to power this mic or has used one before, I'd love to hear your thoughts.

Thanks kindly and have a great evening.

Cheers,

Dave

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On 10/01/2017 at 3:16 PM, Tom Visser said:

Some of the B&K mics of this era required 130V phantom power, but don't know if true of the mic in question.

Not this case Tom. The 2810 power supply supplies 28V to the 4125 mic (and 36V to the 2642 preamplifier - at least in the successor 2810/4130 mic set). Other B&K equipment however does work on 200V, 130V and numerous other combinations, and the pre DPA (and current?) non-measurement (music) omnis which you're most probably thinking about had 130V versions, the 4003 and 4004.

On 10/01/2017 at 4:10 PM, James Bull said:

I think it takes 4 9v PP3 batteries, and you may have to take the top case off with a screwdriver to fit them. 

Yes that's right. A basic flat head screwdriver (the screw itself is fixed with a washer so you shouldn't lose it) will loosen the top which is then lifted off. Half the space inside fits the four batteries. The other half houses all the guts of the 2810, including two little holes to adjust microphone gain from 0-40dB.

Dave, you have the full set of equipment there for one of the two possible channels of the 2810. First thing to do is open the 2810, check there's no dead batteries or battery damage inside. If not, fit four new 9v batteries (with switch in off position) then turn the switch to the Batt test middle position. If it lights, the 2810 is probably OK. Turn it back off, plug in the mic, turn back on (this is perfectly ok with cover still off) and hope that the mic and its preamplifier also still work. The output is unbalanced on the BNC - I use BNC-phono converters to connect to unbalanced XLR, phono, jack and minijack line-ins etc.

You have the older model. It was replaced by the 4130 mic (also found on noise dose meters) and the later 2810 mic lemo connection to the 2642 preamplifier became a different 5 pin arrangement.

Hope that helps. Let us know if it's clean inside and works.

Jez Adamson

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