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Old 27th August 2004, 05:37 AM   #1
TheoM is offline TheoM  United States
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Default B&K microphone power supply - help needed

I've got myself into a possible problem.
I bought a B&K 4000 series mic. It was cheap - on ebay $250 (1300 dollar mic). It was a measurement mic used in a lab, complete with the base/preamp. Flat as hell, basically omni. I figured it might sound good for recording (was I crazy?), but I need a good measurement mic anyway, so I bought it. A B&K is going to be quality gear - so I took the risk. This is a prepolarized mic, so it does not need polarization voltage, only a little current to run the "preamp" - in the base of the mic (the 2671).

It has BNC a connection. 2 wires. Turns out the correct power supply costs like 500 bucks. yikes. I forgot it might not be a standard phantom power (48V).

So, the question is, how can I build my own power supply? The issue is not the supply voltage/current since I can look that up - (I assume I can get a wal-wart that will be close enough from an electronics supplier), but the conversion to xlr (the 3 pin connector which is the interface to the mixing colsole). Which of the two wires is hot (signal or ground), and how do I keep the current out of my board? If I apply current to the ground, it will short. If I apply it to the hot, I will hurt my mic pre or the mic, maybe. But the power supply has to use one of those wires. B&K can not answer these questions in their support (they tell me to buy the power supply). I don't know exactly how phantom power works - so this will give you an idea of my state of ignorance.
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Old 27th August 2004, 07:28 AM   #2
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Which mic is it? I used to use a B&K 4133 mic for measurement work, and this was a true unpolarised capacitor mic that needed a 200V polarisation voltage. (From a little PSU that used 'D' cells).

If yours only needs a standard phantom feed then this is quite easy. Have a look on the web for the application notes on the SSM2017 (or the replacement SSM2019) from Analog Devices and this shows the schematic of a pre-amp with phantom feed to the mic.

If yours needs a higher voltage it's quite easy to arrange.
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Old 27th August 2004, 02:41 PM   #3
TheoM is offline TheoM  United States
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Default Its the 4188 + 2671 preamp

The 4188 is prepolarized. The power goes to the 2671 preamp module which is the base of the mic, as far as I can tell. xlr phantom uses a three wire xlr connector, but the 4188/2681 only uses bnc which is 2 - so that's the main issue.
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Old 27th August 2004, 03:10 PM   #4
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The 2671 pre-amp runs from a 4mA constant current supply.

I don't know the open circuit output voltage required though, but it is above 12V.

You may find that a 24V supply, running through a 3.3k resistor will work. Tap off the audio through a capacitor just like you would to power an electret mic capsule.

I guess you've already downloaded the data sheet from
http://www.bksv.com/pdf/Bp1446.pdf

Chris Morriss.
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Old 27th August 2004, 05:27 PM   #5
TheoM is offline TheoM  United States
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Default something like this....

l
l
Mic + -------------- 12V ------------ cap 10mF ---------- audio +
Mic - -------------------------------------------------------- audio -


I had previously found the document you found, but I didn't understand the language "swings around" the dc.

I assume the cap keeps the dc out of the audio input?

This document: http://www.hut.fi/Misc/Electronics/c..._powering.html

has some electet wiring diagrams which seem like the solution you offer.

Anyway - this is hugely helpful - thanks in advance for your energy on this.
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Old 28th August 2004, 04:42 AM   #6
BrianL is offline BrianL  United States
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Theo,

The 4000 series are not measurement mics. They are very
high quality mics, used by some for measurement, but are
not the measurement mics. They were designed for recording
applications.

Go to the Danish Pro Audio web site. They (ex-B&K guys) now
market the mics, though they are still manufactured by B&K.
There are two versions, one that's designed for phantom power
and one that's got its own power supply. Sounds like
yours is phantom. There are any number of sources of
information about phantom powering. It's very simple.

Also, Danish Pro Audio should be able to provide you info.
You might even be able to download info from their web site.
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Old 28th August 2004, 05:51 AM   #7
1audio is offline 1audio  United States
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Default B&K Preamp supply

That preamp is part of a new family of products from B&K designed to work in the industrial 4-20 mA systems.

To use it you need a constant current supply of about 24 V open circuit. I would set the current to 20 mA to get the most drive (and best signal). The easy way to make the supply would be a good low noise supply (a wall wart will introduce too much noise) and 4 5mA current diodes in parallel. Couple the output with a good sized quality cap (there are unlimited recommendations here) into an input strapped as unbalanced on your console (pin 1 tied to pin 3 to the shield of the cable and pin 2 to the center conductor).

Ask if you need more details.

-Demian
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Old 28th August 2004, 06:35 PM   #8
TheoM is offline TheoM  United States
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Default constant current etc.

Demian,
would something like this:
http://cgi.ebay.com/ws/eBayISAPI.dll...ayphotohosting

work? an HP lab constant current supply. If I can get it for 50 dollars it could be generally useful around the studio. I'll check some electronics catelogs too.
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Old 28th August 2004, 06:59 PM   #9
1audio is offline 1audio  United States
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That will work fine. However I doubt it will be $50 at the end of the auction. Keithley makes some as well that you can find on eBay. Be careful of the leads, thats where noise will get back in to the system. Set the compliance voltage to no more that 24V, 18V may be more than enough.

The mike you got, B&K 4188, is a measurement mike. It will probably sound a little dry. It will roll off around 15K (but so do all of the 1" recording mikes that are so popular). Its sensitivity is a little low so noise can be an issue, keep it fairly close to the source. Have fun.

-Demian
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Old 29th August 2004, 08:21 PM   #10
TheoM is offline TheoM  United States
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Default almost there.....

I found the right current regulating diodes in the Mouser catelog.

Could you posibly recomment a source and spec for the power supply and the cap type and value for the audio coupling? Then I think I'm set to go. Many thanks. Ted.
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