B&K microphone power supply - help needed

Hello,

Thanks for those links, pretty astonishing what those units cost, delivered to our door & all.

Most useful here would a simple interface box that starts from +48V Phantom Power (XLR-3), some back of envelope drawings show that this should be well possible.

Haven't found the opportunity to check if the B&K unit actually still works, the lab supply, R & C should answer that.

When that's on the table, having a few boxes with each different powering-options (battery as well as P+48V) sure wouldn't hurt.

Best regards
 
Member
Joined 2004
Paid Member
48V Phantom power should work fine however you will need to figure out an inversion for the -in. You can cheat and use it single ended which would work fine but TOO simple.

4 mA is no power. Basically why you can't get the 200V bias or the heater with that interface.
 
Administrator
Joined 2004
Paid Member
Hi Demian,
The only problems with the Phantom power is that some are noisy. I can't remember the boards, but we had to redesign the phantom power supply completely. The other issue is that often the phantom power is less than the specified 48 VDC. I'd use a meter to confirm just what voltage you have to begin with.

Just a note to make people aware that you can't assume the phantom power is what the oft-quoted spec is. It was a rude surprise for us. No, these boards were not defective, they were designed that way. Possibly could have been Soundcraft, but don't quote me on that.

-Chris
 
A battery powered version would make a lot of sense. It would not be hard. I would use a cheap DC-DC converter from China + a Lipo battery. Then probably a depletion mode mosfet plus a selected current diode for the current supply. Supply current does have an effect on the output voltage. With the B&K mikes they don't need gain so much less effect on the output. The output is high enough that you can use a follower inside and get a low output Z.

LM2587 DC-DC Boost Converter 3V-30V Step up to 4-35V Power Supply Module MAX 5A | eBay

I could sketch out the rest later if you want.

Actually if you want to detail the design I would be interested.
 
48V Phantom power should work fine however you will need to figure out an inversion for the -in. You can cheat and use it single ended which would work fine but TOO simple.

I figure single ended would be fine indeed.

Just curious how you meant too simple ?
As in: too easy to solve this question to come up with a circuit between the P+48V & the 4mA resulting at some 12V at the BNC connector of the 2671?

4 mA is no power. Basically why you can't get the 200V bias or the heater with that interface.

Yes, the 4155 capsule is prepolarized and I guess the 2671 has little more inside than a FET or a FET and a BJT. (Curious if a schematic will ever pop up, I don't see an easy way of opening the 2671, so rather not risk damagaging the unit)

Regards
 
The other issue is that often the phantom power is less than the specified 48 VDC.

Thought the standard (IEC 61938) allowed for 44 ... 52 V, so @ 44V the 4 mA should still be possible to deliver for the interface circuit (although B&K's own battery-box itself seems to lower the current to slightly over 3 mA as well).

Or are you meaning those 'phantom powered mic inputs' (seen them from Roland & Peavey, to mention a few) that happily go for 12V DC ? :(

Regards
 
Member
Joined 2004
Paid Member
I'm fiddling with several variants for this. Single ended with a fixed supply may give the best results. Its want I was thinking originally. Phantom power is more of a PITA. If you ignore the -in and shunt it with a cap to ground and add a Zener to protect the mike from 48V its also simple (1 Zener would make it fit in a cable). Originally phantom power was intended for transformer coupled circuits but that seems silly for this application today. I have something working in sim for phantom but I'm not happy with it yet. It would be easier if the - from the microphone weren't also the shield/ground. And a good solution is dependent on the implementation of the supply circuit.
 
I'm fiddling with several variants for this. Single ended with a fixed supply may give the best results. Its want I was thinking originally.

Fully agreed, separate supply the best.

Yet a nifty/decent P+48V powered insert would greatly increase the usability (read: less setup-hassle, so more chance of using it for a recording session - session of the type: a few more mics OK, but let's be quick - other bandmates waiting, you'll get the picture)


Phantom power is more of a PITA. If you ignore the -in and shunt it with a cap to ground and add a Zener to protect the mike from 48V its also simple (1 Zener would make it fit in a cable). Originally phantom power was intended for transformer coupled circuits but that seems silly for this application today. I have something working in sim for phantom but I'm not happy with it yet. It would be easier if the - from the microphone weren't also the shield/ground. And a good solution is dependent on the implementation of the supply circuit.

I need to dig up my back-of-envelope circuit again. Thought that sketched approach was to use one leg of the P+48V for the current source (JFET, or a (2) PNP current mirror with a certain ratio to save on current, and kill the AC-signal on that point with a big cap to GND. From the BNC-signal node with another cap (AC-coupling) to the other XLR-input. Might be overlooking something here, and also need to address protection, but I recall I thought it could at least function OK - assuming a decent P+48V supply.

Should make a drawing, and also interested what you've been thinking of so far.

Have a good weekend!
 
Member
Joined 2004
Paid Member
The Phantom power version is the challenge. Its an interesting problem to make a linear inverter where your power source is also the output. I'll try to post some of my sketches tomorrow (after I'm sure I didn't make too many obvious mistakes).

The external battery supply version is much easier. However I can add an output buffer which would help lower the output impedance. Something I'll do as well.
 
The Phantom power version is the challenge. Its an interesting problem to make a linear inverter where your power source is also the output.

Interesting riddle indeed!
The signal-inverter would be a nice bonus, but (apologies for stating the obvious) as long as the source impedances as seen by each of the XLR-3 signal inputs are alike, the benefit of balanced is obtained.


I'll try to post some of my sketches tomorrow (after I'm sure I didn't make too many obvious mistakes).

Nice, would be interesting to see your thoughts, and nothing wrong with some imperfect stuff still in it, it can develop from there.

Can post that back-envelope sketch as well (also after a bit more checking for mistakes, but sketches may have a few ;) )


The external battery supply version is much easier. However I can add an output buffer which would help lower the output impedance. Something I'll do as well.

That's indeed a 'mysterious' part of the 2671-story: what would be the resulting output impedance... the better the current source, the lesser lowering of output-Z (enough stating the obvious... :D )

Perhaps the Schoeps-phantom circuit topology approach can be of use here to provide that output buffering ? (attached pic: T2, T3)


Have a good weekend
 

Attachments

  • Schoeps.gif
    Schoeps.gif
    234.1 KB · Views: 126
Last edited:
Member
Joined 2004
Paid Member
The Shoeps circuit did give me some new ideas. With the current based source the shield and the negative supply are the same which limits some flexibility but I still may be able to get there.

I suspect the output Z of the microphone will be low. The B&K capsule has higher output so they don't need gain. My guess is that is on the order of 100 Ohms or less. When you get ti running it would be interesting to know what you find.

The capsule output is 50 mV at 94 dBSPL which is the same as the 1" capsules and lots more than the 1/4" Panasonics. The circuit may be a simple follower internally, which would also make sense if you need some calibration predictability.

More later today.
 
Member
Joined 2004
Paid Member
Attached is a circuit borrowing from the Shoeps circuit (Shoeps are known to sound good so its a valid source to steal from).

I modeled the B&K as a voltage source with a series resistor bypassed with a cap.

I used a Jfet as a current source to bias the microphone. I would also look at an LM339 + a depletion mode Mosfet for a higher impedance current source but that's probably overkill.

The Zener diode is the first one in LTspice's list at 24V. I might use a shunt regulator for lower impedance but again, possibly overkill.

It all works with a 48V supply. Since the internal rail is 24V I don't think it will work for anything much less than 30V. The mike bias is the real limitation.

Tweaking the base resistors on the PNP's adjusts the current. its not a great solution since it will be temperature and device sensitive. The value shown seems to work across a range of load currents (changes to the Jfet current source).

I looked at pJfets as an alternative to the PNP's but they are not well suited to this. it should be pretty insensitive to the quality if the 48V since the outputs are low Z and the only high Z is the Jfet and it has an isolated supply. The caps shown seem to be OK to 10 Hz.

I'll post the other single ended solution later.
 

Attachments

  • Phantom Balanced 1 dm.asc
    3.2 KB · Views: 40
  • Phantom power interface.PNG
    Phantom power interface.PNG
    22.2 KB · Views: 109
Hello,

That looks good, thanks for posting. Using the FET for the current source is attractive - since the number of builds is low, the adjustment/selection for current no drawback. The Schoeps is indeed a proven & tested circuit. So having both signal polarities available to send to the XLR-3 is no real hassle, comes at modest parts cost.

I've attached my back-of envelope circuit, just quickly re-drawn, not simulated etc for now.

Regards
 

Attachments

  • B&K_PH_00.pdf
    6.8 KB · Views: 59
Member
Joined 2004
Paid Member
Your circuit is close to where I started. Getting the mike to work is pretty easy. Getting an inverted output with no power source etc is not. However the Shoeps circuit lent itself to this task.

Today i usually start with the simulator. Finding paper and pencil have become harder in the new world we live in.
 
I guess in the circuit I posted the (2) PNP current mirror could use a zener to stabilize the voltage and from there the current fed to the B&K.

Or replace the whole BJT-current mirror by the JFET current source.


I modeled the B&K as a voltage source with a series resistor bypassed with a cap.

Just wondering about C1, you're using that to reflect that the AC is available LowZ ?

Best regards
 
Member
Joined 2004
Paid Member
C1 is to emulate (as a guess) the source Z of the microphone. Need real numbers as some point. The current mirror has a few drawbacks. First polarity. It doesn't invert phase. Second is getting the same amplitude and current. No solution got there without degrading the opposite phase. If the low side of the mike were available its easier but no shield and not practical.

Sent from my LG-H811 using Tapatalk
 
C1 is to emulate (as a guess) the source Z of the microphone. Need real numbers as some point.

When I have soldered a bit I'll measure for Zsrc of the 2671.


Fully agreed, the current mirror without further stuff (buffering) is not without drawbacks, but I'd say mainly because of the unequal source-impedances as delivered to the cable between interface & the XLR-3 input. That's a weak point.

Equal source impedances, that's the main principle of a balanced link I'd say. The possibility to transport an additional signal with opposite polarity is a 6 dB bonus on top of that.

In other words, I'd rather have the equal source-impedances (or a short & very well shielded cable) than the added signal-polarity inversion.

The nice thing of the 'Schoeps approach' is that we can have both.

Best regards