9v/18v mic power supply

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clepsydrae

Active member
Joined
Apr 19, 2014
Messages
44
Hi -- I'm a rank beginner, and probably in over my head, but that never stopped me before... :)

Let's say I had to replace the power section from this schematic:

http://recordinghacks.com/images/mic_extras/cad/E300-schematic.png

(As I understand things, it uses a couple of 9v batteries (NiMH or NiCad) as a backup to handle higher current draws than phantom supplies can often handle. It can also run without phantom.)

Would it be all that difficult to build an external power supply that took 120VAC mains and provided stable/clean/adequate 9VDC and 18VDC for the V9 and V18 points, and let me skip recreating the whole power section, batteries, etc?

I just wanted to figure out if such a project was a matter of finding well-known schematics on the web, ordering some parts, and going for it, or if a 9v/18v power supply is an unusual configuration and would require custom designing, etc etc. Meaning, if I just need a transformer or two and some filter capacitors, I'm all over it, but I'm not savvy enough to design my own circuit, really.

(I'm assuming that grabbing a 9v and an 18v wall wart and hacking those together is a stupid idea.)

Thanks for any ideas!
 
Ok, here's what I have come up with so far, having copied/modified from that other thread...

psu.png


Any glaring issues? Will it be clean/nice power for the mic?

Thanks!
 
If you want to get rid of the batteries & have a wall PSU & lead to your mike, why not make a good P48V mike PSU instead?

It will be less messy than your proposed arrangement and useful for other mikes as well.
 
ricardo said:
If you want to get rid of the batteries & have a wall PSU & lead to your mike, why not make a good P48V mike PSU instead?

It will be less messy than your proposed arrangement and useful for other mikes as well.

Thanks for the reply -- two reasons; first, in this mic the power section as shown in the schematic is not present (it's a small board unto itself). The mic operates on 9V and 18V internally, as I understand things, so I need to supply that. I was hoping to avoid recreating the power section as shown in the mic schematic because (besides not understanding it) as it is designed it requires the batteries to be present to work (the E350 apparently only uses phantom to recharge the batteries). Since I'd have to build an external thing anyway (no room to fit a recreation inside the mic) I thought I could skip recreating it and make the 9V/18V myself.

I'm not real concerned about the messiness, though I do see that point -- this project is more for my own edification, really. I'm more concerned that the schematic I designed not have tons of buzz/hum/etc, and not melt anything, including me. :)

I.e. do I need to do a bunch more filtering or is that schematic already clean enough for such an application? The sizing of the capacitors is one of the many things I'm clueless about...
 
Well, that was a great question indeed. I had glanced back and forth and thought the part that was missing was Q1/2/3 and the associated parts, but I was quite wrong. Just checked in greater detail, and the green shaded area is what is missing (see attachment, also at http://lacinato.com/pub/mic_schematic_missing_parts.png).

I read that LP2951 is apparently also a voltage regulator... does this mean that that section is responsible for the capsule voltage? So I could leave the batteries and all that, and theoretically provide a voltage for the capsule (whatever voltage that may be? 5V, according to spec sheet?)

Something tells me that my little circuit above may not be up to that task?

I can't tell if this project just got easier or harder. :)
 

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The missing bit generates 48 - 60V to polarize the capsule.  It doesn't need to give any current but must be very low noise.

This circuit isn't the best way to do this as it generates a lot of noise.  The best voltage generator for this is Dip Ing Wuttke's Schoeps circuit.

Zapnspark's BJTVgen7.pdf in his Yahoo MicBuilder's Group Files directory is good.  You have to join.
 
Thanks ricardo! -- I have applied to join, and I look forward to checking out the suggested circuit.

Just out of curiosity, for other non-capsule microphone needs, was the circuit i posted correct? Meaning, if the other part of the circuit was what I was replacing, would that have worked?
 
clepsydrae said:
Just out of curiosity, for other non-capsule microphone needs, was the circuit i posted correct? Meaning, if the other part of the circuit was what I was replacing, would that have worked?
Yes it would work but is far too complex for what it has to do.  I would use 7818 and a resistor chain for the 9V midpoint and maybe some simple RC filtering.

But I haven't looked at the circuit in detail to see if this simple approach would do.  Depends on how much current is needed on the 9V

In MicBuilders, I've got a little OPA preamp circuit under LNprimer in my Files directory which is for use with a single PP3 9V and I derive 4V5 with 2 resistors.
 
I have searched and searched for that schematic amongst Zapnspark's files and subdirectories, but I don't find it... has it perhaps been renamed or removed? Any tips on where else to check? Thanks! -c
 
clepsydrae said:
I have searched and searched for that schematic amongst Zapnspark's files and subdirectories, but I don't find it... has it perhaps been renamed or removed? Any tips on where else to check?
Zap recently re-organised his directory.  It's now called

GenChinaPolCirc.pdf
 
Brilliant, thanks -- got it.

If you have the time, Just a few hopefully-trivial questions... if you don't have the time, I'll stumble through it ok on my own. :)

- obtaining the 6.8V from mains power? Should I use the circuit I posted above (tweaked for 6.8v obviously)?
- can I use a 10-20 foot lead from the polarization circuit to the capsule, or is that asking for trouble?
- that circuit gives ~50VDC. That's within the 48-60 you said the missing circuit gave, so all good there?
- I may be replacing the capsule with something from microphone-parts.com... do I need to mind the capacitance values of those capsules, or any value will work? The original is 65pF.

Any thoughts very appreciated. Thanks,
-c
 
clepsydrae said:
- obtaining the 6.8V from mains power? Should I use the circuit I posted above (tweaked for 6.8v obviously)?
- can I use a 10-20 foot lead from the polarization circuit to the capsule, or is that asking for trouble?
- that circuit gives ~50VDC. That's within the 48-60 you said the missing circuit gave, so all good there?
Just run it of the 9V rail on your E300.

Zap has a twiddle pot to adjust voltage but for a one off mike, you want as high a voltage as possible that doesn't collapse the diaphragm.

- I may be replacing the capsule with something from microphone-parts.com... do I need to mind the capacitance values of those capsules, or any value will work? The original is 65pF.
Any dual diaphragm LDC should work well.
 
Oh, that's great. Going from the internal mic power is much more convenient.

- OK to use ceramic capacitors for the 6.8pF and 27pF caps in Zap's circuit? Can't find axial metal film in that capacitance...

- re: the inductors L1 (150uH) and L2 (62uH) -- any particular max current rating, max resistance rating, or tolerance needed?

Thanks, I'm getting excited to start. :)
 
clepsydrae said:
- OK to use ceramic capacitors for the 6.8pF and 27pF caps in Zap's circuit? Can't find axial metal film in that capacitance...
should be NPO/COG ceramics
- re: the inductors L1 (150uH) and L2 (62uH) -- any particular max current rating, max resistance rating, or tolerance needed?
Not critical.  Use whatever small inductors you can get.
 
Ok, built the polarization circuit!

Feeding it with a 9V battery (which tests at ~9V unloaded), I'm only getting ~20-35VDC at the output, though. (This is with the neg lead of the multimeter to the neg side of the battery, which I assume is proper.) Should I be concerned?

Going off the GenChinaPolCirc.pdf, I get 0VAC at point b (the PDF says it should be 13.6Vp-p?), and about 6VAC at point a with respect to ground (PDF says 46.6Vp-p).

I used a couple different batteries... it's possible they're both dead, but as i say they tested unloaded at ~9V.

My 62uH inductor is weird -- the one I ordered turned out huge, almost an inch long: http://www.mouser.com/Search/ProductDetail.aspx?R=4630-RCvirtualkey54200000virtualkey542-4630-RC

Could that be the issue?

Any ideas greatly appreciated! Thanks,
-c
 
clepsydrae said:
Ok, built the polarization circuit!

Feeding it with a 9V battery (which tests at ~9V unloaded), I'm only getting ~20-35VDC at the output, though. (This is with the neg lead of the multimeter to the neg side of the battery, which I assume is proper.) Should I be concerned?

Going off the GenChinaPolCirc.pdf, I get 0VAC at point b (the PDF says it should be 13.6Vp-p?), and about 6VAC at point a with respect to ground (PDF says 46.6Vp-p).
To measure the polarizing voltage accurately, you need an electrometer with input resistance of MUCh more than 10G.  :)  Otherwise your meter loads down the circuit.

If you measure 20-35Vdc with your multimeter, your circuit is probably working OK.  How are you measuring the Vp-p voltages?  You need a good oscilloscope with the proper probes.

Just connect it up and see if the capsule diaphragm is OK.  If so, you're in business.
 
ricardo said:
If you measure 20-35Vdc with your multimeter, your circuit is probably working OK.

That's what I was hoping you would say. :)

How are you measuring the Vp-p voltages?  You need a good oscilloscope with the proper probes.

Not with an oscilloscope and not with proper probes. :) (I had assumed p-p meant peak-to-peak as opposed to RMS, and that my multimeter would measure p-p.)

Just connect it up and see if the capsule diaphragm is OK.  If so, you're in business.

Awesome! When the capsule arrives in the mail, I shall do just that.

You say "see if the capsule diaphragm is OK"... meaning, that it doesn't pull in too much? Is there any simple way I should confirm the voltage of this circuit before risking damaging the capsule, if that's possible? Or just don't sweat it and see how it sounds...

-c
 
clepsydrae said:
How are you measuring the Vp-p voltages?  You need a good oscilloscope with the proper probes.

Not with an oscilloscope and not with proper probes. :) (I had assumed p-p meant peak-to-peak as opposed to RMS, and that my multimeter would measure p-p.)
No.  Most multimeters will measure 'average' (more expensive ones measure 'rms').

But there is a more serious problem.  Most inexpensive multimeters give good results at 50 - 60Hz but will have seriously drooping response by 1kHz.  The oscillator frequency is over 1MHz so you probably measure zilch.

Is there any simple way I should confirm the voltage of this circuit before risking damaging the capsule, if that's possible? Or just don't sweat it and see how it sounds...
Seriously, a real mike designer would just check the diaphragm doesn't collapse and then just see if the sensitivity is correct.  It's a PITA to measure polarizing voltage properly.

The capsule won't be damaged if it collapses.  But don't you have the old capsule?
 
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