Will This P48 Circuit Power These Mics?

Hi Folks,
  I recently modified some of these EV mics for a friend using the circuit described here:
http://www.uneeda-audio.com/phantom/pl-76mod.htm
  It worked well and seemed to have little extra noise.
  Now I have some folks who would like their Nakamichi CM-300s adapted to use P48. I'm wondering if the same circuit would work, but instead using a 12v zener diode, since I have read that the noise floor is lower when the capsules are powered with more voltage. Apparently you can go even higher, but I don't want to press my luck.
  The Naks have an unbalanced output, so I'm not sure that they can use the circuit described for the EVs.
  Info and schemos for the CM-300s are here:
http://www.coutant.org/nakamich/index.html
    I would like to create a circuit on perfboard that fits in the battery compartment of the Naks, so that batteries could still be used, to keep the mics in their original condition.
  Thanks for any help and suggestions.

After looking at the schematic for the Naks, it seems that the P48 won't get past the transformer connected to pins 2 and 3 of the mics.
I'd hate to disembowel these things, but that might be the only way to go, other than building new bodies for the capsules...

I think you can do as you suggest - tap off the phantom power on the output side of the tranny, and run a couple of wires to a circuit in the battery compartment. I have never been inside one of those so I can't say how easy it is to take them apart.

A battery should last ages so is it worth the trouble?

Stewart

Thanks Stewart,
    That's a good suggestion. I'll give it some thought after looking at the mics closely to see how easy that might be. I'll bet the XLR connectors slide right out.
    The batteries are not too expensive (@10-15 bucks each USD), and still available, but they have all three mics, so it adds up. I've tried talking them out of it, and they seem to have more confidence in me than I have. That's why I'm trying to leave the mics more or less intact.
  Appreciate the help!

tchgtr

> The Naks have an unbalanced output

No they do not. True transformer balanced, finest-kind.

(The -cables- are unbalanced, but you won't use them.)

I did the mod below and was very happy.

Yes, you rip out almost everything. Power on/off is not needed. That bass-cut was always dubious and in recent times I've always found bass knobs/switches at the board or post-production. I think I used a little slip of black cardboard to mask the switch hole. The Rs, Cs, and Zener can be air-wired. I think I fit them at the XLR and ran bits of PC-LED wire to the head-end.

I ran 9V Zener, respecting the original plan. I don't see how 12V would make any real difference for the good-- FET bias is fairly unaffected by Vds. The max input level is more about FET threshold voltage than supply voltage. (Use the cap-pad for Rock!!) I don't think 12V is bad for the mike, but 9V leaves a wee bit more margin against 24V "Phantom".

There may be some reason to match the 4K7s to part-percent, but I just grabbed a pair of 5% that were 2% apart and never ran into a problem. 

Nakamichi CM-300 Fix

> batteries are not too expensive (@10-15 bucks each USD), and still available,

When I got into it, apparently ~~2006, the batteries were not readily available. I'm glad that's changed. I would seriously stick with batts if the supply is reliable.

Another thought was to find a coin-cell of good diameter, stack-up to good voltage, add a brass slug and a rubber sleeve to fit the compartment. Battery life should be many-many concerts. Coin cells can be astonishingly inexpensive in bulk.

And you can make a "battery" out of a dowel and two thumb-tacks, boff a hole in the side, run wires out to a common 9V batt taped to the mike. Would work great. Looks are in the eye of the beholder.

> a circuit on perfboard that fits in the battery compartment of the Naks, so that batteries could still be used, to keep the mics in their original condition.

This is tough. The FET and the load-resistor are grounded; therefore the battery must "float". You can re-string things for grounded power, but that makes a mess of going back to battery power. Now that Phantom is everywhere, burn your bridge, lose the battery option.

Thanks PRR,
  This is pure gold! I don't think the owners would be upset at all if I used your circuit. I have not proven to myself that the batteries are readily available, and you've already discounted much of the info I've gleaned from the net.
    A quick look at the mics shows that they come apart easily, and I'm thinking of trying this on at least one of them, which seems to have fallen apart on it's own.
  I had considered some battery options similar to what you suggest, but I think this bridge is burnt!
    Having modded my Radio Shack PZMs for P48, using a circuit readily found by searching that subject, I was very pleased at the improvement. It was quite easy using a spent transformer box to contain the electronics, and they sound very clean now.
  Cool!