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.
 
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.
 
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.
 
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.
 
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/circuits/microphone_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.
 
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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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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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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What I would do is probably buy a decent small surplus linear supply. 20 mA at 18-24V isn't much (.5 Watt) so its tempting to use a modular supply. A wall wort type may couple too much line noise it. I would try one line this : Cincon TR15 (418-tr1524) from Mouser and isolate it with a cap (1000uF) across the connection where it enters the housing, series 100 Ohm resistors on both the plus and minus leads and another bypass cap (.1 uF at least) before you connect it to the circuit.

I am reluctant to recommend coupling caps here. Everyone has very strong opinions and they are all different. A 4.7uF to 10 uF film cap should be fine into a 10K load. You can have endless fun changing it to change the sound. Do not disconnect any of this with the audio up. It will give the pop you will remember.

Let me know if you need more advice.

-Demian
 
clarify and confirm

Mic

+ -
PS + 12v 20mA---l-- 100 ohm ----- l l
l l l
.1 mf filter l l
PS - ----------------l---100 ohm -----n---- l
l l
l l
10mF l
l l
Audio l l
Mic Pre + - ,g


Why the resistors? Anyway - I bought a cheap constant current supply on ebay (50 bucks), so I am assuing the above drawing is is from its leads out. Does the drawing describe what you specified? I did notice the mic has a max output voltage of 7 (seems high). Is that in the same range as typical mics for which typical mic pre's are built?

Much appreciated. Ted.
 
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I'm sorry but I got you confused between making a constant current supply and connecting the one you found. ($50 is a good price for it).

The resistors were a way to reduce the noise from the wall wort. You don't have that problem with the HP.

I have attached a drawing showing the connections. Let me know if its clear enough. (I will forever be confused on the XLR connections since the film industry uses the altrnative phase.)
-Demian
 

Attachments

  • mike connections.jpg
    mike connections.jpg
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aftermath

Wired as we talked. All the voltages and current are right, no dc going through the cap. Hums like crazy, and has almost no gain. With 60db of gain from a mic pre, some signal, but very little, mostly hum - 60 cycle.

I've tried several things to get rid of it. Mic cable for the power supply (several feet), with the shield to chasis ground. no change. Shield to DC ground, no change. A cap across the power supply, little change. Going to an unbalanced input, no change. Line voltage with no ground loop, no change.

I'm at a loss at this point. Could be the mic's broken, but B&K makes pretty durable stuff. Maybe its time to ask the seller to take it back. :). Maybe seperate dc+ and - cables from the power supply. ted.
 
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Got your note. Let me think about it. Can you detail the voltages you have measured? Replace the Mike preamp with a 600 Ohm resistor (20 mA @ 12V) and see if you still get hum. Check that the supply is working right (It could be humming all by itself) and that its operating in constant current mode- the voltage across the resistor should change as you change the current.

If the supply is in constant voltage mode then it will eat any output from the preamp.

Remove the capsule from the preamp and see if you still get hum. Turn the supply off with everything attached an see if you get hum. Make sure that the low side of the supply isn't attached to the ground pin of the power cord. (Possibly a jumper on the back of the supply).

Let me know what you find. its unlikely that the mike is dead in this way. More likely is just dead if its actually broken.

-Demian
 
The B&K power supply

I put the correct resistor in place of the mic and the hum stayed the same. When I connect the mic, the voltage drops at the power supply drops off significantly (to like 10V). Open circuit I was using 20V (should have been 28 I think, but I don't think that would make a difference.) There IS a strap on the back of the power supply connecting RV1 and RV2 (I don't know what those are, but I assume they are standard designations).

I was thinking I could replace the power supply with three 9 volt batteries and a few current regulating diodes to total 20Ma. .

btw the open circuit voltages are right at each point, and the cap does indeed take the dc out. It does seem possible that the ac (signal) from the mic goes back to the supply if the impedance there is lower than the input impedance of the mic preamp I'm using. It also seems possible that in addition, the power supply is creating that noise - although putting a cap to ground didn't change anything. The Power Supply is made by Power designs inc.

Anyway, thanks for the continuing help.
 
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Please confirm the DC voltage on the resistor and on the mike. They should be similar if we have the right settings. And the open circuit voltage should be higher that the voltage with the loads. If you are getting a lot of hum check the following-
1) make sure that the ground pin is isolated from the supply output. (Open circuit to any pin)
2) make sure that the minus terminal on the output side is connected to the mike in neg terminal.
3) Does the hum go away when the supply is turned off?

This is a vexing problem and I'm sure it must be very frustrating to you. If you were nearby I would have you bring it over.

I saw a couple of the correct supplies for the mike on eBay. They were in the $300 range I think.

I also looked and discovered that this http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=25407&item=3843628096&rd=1

Endevco 2775A will operate the mike preamp. I have one and more important its manual and it definately will operate the mike preamp and give you some control over its output. (i'm always learning something here!)

Or this: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=50968&item=3839389282&rd=1&ssPageName=WD1V will operate them. It has the wrong connectors but they can be changed to BNC's easily. Save the old ones, they are valuble to those working with accerometers.

Demian
 
dont want to hijack, but...

you guys know what your talking about so can any of you help me out with power pin diagram for a behringer shark dsp110? its a great feedback killer but i have not got a power supply and im realy confused by the pin layout / voltages. cheers for any help, steve.. ..