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Author Topic: Inexpensive "NADY" Preamp and Phantom adapters  (Read 8800 times)

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Offline Chuck

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Re: Inexpensive "NADY" Preamp and Phantom adapters
« Reply #30 on: February 03, 2007, 09:50:33 PM »
The phantom power circuit looks like basically the same one in the Rolls 224. Oh well...

Hey Chuck do they use the same chip? to get the voltage up? and I can assume from your reaction that the Rolls does not work very well??


Yeah, the 4069 is used in the Rolls too. The Rolls box I had provided ~45v when un-loaded. But, under load, that dropped to anywhere from 15v to 24v depending on the current draw of the microphones. It powers most microphones that way, but I wasn't happy with it. I tried replacing that hex inverter with a higher speed inverter, but it introduced too much noise. I also experimented with different capacitors, but it never improved that much.

If I remember right, the Denecke PS-2 uses A medium speed hex inverter too, but with a different circuit design. That box provides consistant 48v even with high current microphones.
Reality is merely an illusion, albeit a very persistent one.

Microphones: (2) Microtech Gefell M300, (2) AKG C 480 B comb-ULS/ CK 61/ CK 63, (2) CAD GXL1200 (cardioid and sub-cardioid capsule & electronics mod), (2) Audix M1290-o, (2) Micro capsule active cables w/ Naiant PFA's, (2) Naiant MSH-1O, (2) Naiant AKG Active cables, (2) Church CA-11 (cardioid), (2) CAD C9, (1) Nady SCM-1000 (mod)
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Offline rowjimmy

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Re: Inexpensive "NADY" Preamp and Phantom adapters
« Reply #31 on: February 03, 2007, 10:06:24 PM »
These threads fascinate me. And to think I don't even own a soldering iron...
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Offline poorlyconditioned

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Re: Inexpensive "NADY" Preamp and Phantom adapters
« Reply #32 on: February 03, 2007, 10:08:19 PM »
The phantom power circuit looks like basically the same one in the Rolls 224. Oh well...

Hey Chuck do they use the same chip? to get the voltage up? and I can assume from your reaction that the Rolls does not work very well??


Yeah, the 4069 is used in the Rolls too. The Rolls box I had provided ~45v when un-loaded. But, under load, that dropped to anywhere from 15v to 24v depending on the current draw of the microphones. It powers most microphones that way, but I wasn't happy with it. I tried replacing that hex inverter with a higher speed inverter, but it introduced too much noise. I also experimented with different capacitors, but it never improved that much.

If I remember right, the Denecke PS-2 uses A medium speed hex inverter too, but with a different circuit design. That box provides consistant 48v even with high current microphones.

You realize that the phantom power goes down in voltage under use, right?  The spec is a perfect 48V in series with a 6.8K resistor.  So, the maximum current is something like 7mA, and at that, the voltage would be zero (all dropped on the 6.8K resistor, internal to the phantom power supply).

An easy test is to put a sample load resistor, say 10K, on the output.  See how many volts you get on that resistor.  According to my quick calculation, you should get about 28V on that resistor.  Any more than that and you're fine.

I tend to trust the Denecke circuit, though.  Those guys build tough gear!

  Richard
Mics: Sennheiser MKE2002 (dummy head), Studio Projects C4, AT825 (unmodded), AT822 franken mic (x2), AT853(hc,c,sc,o), Senn. MKE2, Senn MKE40, Shure MX183/5, CA Cards, homebrew Panasonic and Transsound capsules.
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Offline DSatz

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Re: Inexpensive "NADY" Preamp and Phantom adapters
« Reply #33 on: February 07, 2007, 06:06:04 AM »
Richard, actually it's 7 mA per 6.8 kOhm resistor for a total of about 14 mA as the "short circuit" current. A preamp or supply should be able tolerate that for a few seconds or minutes on one channel at least; accidental short circuits do occasionally happen. However, that requires a more robust construction than is found in many pieces of portable equipment that I've tested.

For P48 microphones, the DIN EN standard gives 10 mA as the maximum. Above 7 mA, more energy is spent heating up the phantom resistors than actually reaches the microphone, and many lightweight preamps or phantom supplies fall out of regulation. So most manufacturers don't go into that zone of diminishing returns. Nonetheless there are the inevitable wacky exceptions: many Earthworks models draw the full 10 mA, and many CAD Equitek models draw 8 mA.

Most modern P48 condenser microphones draw less than 6 mA apiece. Shure KSM-series microphones are typically specified at 5.5 mA or thereabouts. Schoeps CMC 5-- and 6-- preamps normally draw about 4.5 mA apiece from a 48 Volt supply, but the CMC 6-- amplifier, if it senses too low a supply voltage, may switch over to its 12-Volt mode and try to draw 10 mA. (It isn't a "12 through 48 Volt" amplifier; it's a dual-mode amplifier which requires either standard 12 Volt phantom powering with adequate current or standard 48 Volt phantom powering with adequate current.) Neumann TLM and fet 100 mikes typically draw 2 - 3 mA.

Neumann's older fet 80 designs (KM 84, U 87, etc.) draw less than 1 mA, as did Schoeps' pre-Colette-series CMT 5-series microphones. But the age of a design isn't a reliable guide. To many people's surprise, the original AKG C 451 from the 1970s--not the electret "reissue" (cough, cough)--tries to draw 6 mA from a standard 48 Volt supply. It really was a P12 microphone with a tolerance for 48 Volts, not a 48-Volt microphone as such.

A hint: If you're revamping a cheap phantom power supply, check its 6.8 kOhm supply resistors; make sure that the pair on each input is matched as closely as possible, since any imbalance there reduces the preamp's common mode rejection. Also, the closer the match, the better your protection against any hum or hash from the DC supply getting into the audio.

The specification says that the two resistors in any one pair (i.e. on any one input) should not differ from one another by more than 0.4%. Even expensive "audiophile grade" 1% resistors may thus be out of spec by as much as a factor of 5--a real case of misplaced priorities, which is unfortunately fairly common. The absolute 6.8 kOhm value has a 20% tolerance on it; having equal value resistors within each pair is far more important.

--best regards
« Last Edit: February 07, 2007, 06:10:06 AM by DSatz »
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Offline rowjimmy

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Re: Inexpensive "NADY" Preamp and Phantom adapters
« Reply #34 on: February 07, 2007, 08:37:51 AM »
Nice post. Even I got something from it. +t
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Offline Church-Audio

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Re: Inexpensive "NADY" Preamp and Phantom adapters
« Reply #35 on: February 07, 2007, 11:19:10 AM »
Richard, actually it's 7 mA per 6.8 kOhm resistor for a total of about 14 mA as the "short circuit" current. A preamp or supply should be able tolerate that for a few seconds or minutes on one channel at least; accidental short circuits do occasionally happen. However, that requires a more robust construction than is found in many pieces of portable equipment that I've tested.

For P48 microphones, the DIN EN standard gives 10 mA as the maximum. Above 7 mA, more energy is spent heating up the phantom resistors than actually reaches the microphone, and many lightweight preamps or phantom supplies fall out of regulation. So most manufacturers don't go into that zone of diminishing returns. Nonetheless there are the inevitable wacky exceptions: many Earthworks models draw the full 10 mA, and many CAD Equitek models draw 8 mA.

Most modern P48 condenser microphones draw less than 6 mA apiece. Shure KSM-series microphones are typically specified at 5.5 mA or thereabouts. Schoeps CMC 5-- and 6-- preamps normally draw about 4.5 mA apiece from a 48 Volt supply, but the CMC 6-- amplifier, if it senses too low a supply voltage, may switch over to its 12-Volt mode and try to draw 10 mA. (It isn't a "12 through 48 Volt" amplifier; it's a dual-mode amplifier which requires either standard 12 Volt phantom powering with adequate current or standard 48 Volt phantom powering with adequate current.) Neumann TLM and fet 100 mikes typically draw 2 - 3 mA.

Neumann's older fet 80 designs (KM 84, U 87, etc.) draw less than 1 mA, as did Schoeps' pre-Colette-series CMT 5-series microphones. But the age of a design isn't a reliable guide. To many people's surprise, the original AKG C 451 from the 1970s--not the electret "reissue" (cough, cough)--tries to draw 6 mA from a standard 48 Volt supply. It really was a P12 microphone with a tolerance for 48 Volts, not a 48-Volt microphone as such.

A hint: If you're revamping a cheap phantom power supply, check its 6.8 kOhm supply resistors; make sure that the pair on each input is matched as closely as possible, since any imbalance there reduces the preamp's common mode rejection. Also, the closer the match, the better your protection against any hum or hash from the DC supply getting into the audio.

The specification says that the two resistors in any one pair (i.e. on any one input) should not differ from one another by more than 0.4%. Even expensive "audiophile grade" 1% resistors may thus be out of spec by as much as a factor of 5--a real case of misplaced priorities, which is unfortunately fairly common. The absolute 6.8 kOhm value has a 20% tolerance on it; having equal value resistors within each pair is far more important.

--best regards

Great info I learned a few things too! T+
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Offline nem

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Re: Inexpensive "NADY" Preamp and Phantom adapters
« Reply #36 on: February 28, 2007, 12:44:07 AM »
would anyone be so kind as to point me in the direction of an inexpensive pre that has both xlr ins and outs? or is the Nady the only one?
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Offline Church-Audio

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Re: Inexpensive "NADY" Preamp and Phantom adapters
« Reply #37 on: March 03, 2007, 02:04:37 AM »
would anyone be so kind as to point me in the direction of an inexpensive pre that has both xlr ins and outs? or is the Nady the only one?

This is right now as far as I know the only one at this price point. But you have to remember its not going to be pretty.... :) I think the term you get what you pay for will apply to this product. I think if your looking for 40+db of gain I would stay away if you only need maybe 20 or so db of gain I am pretty sure this will have a half decent noise floor at that range. But the other issue is sound quality and that's one I can't answer because I have never used it.. This thing is cheap you will have to baby it if you drop it on a knob say bye bye. I still think its pretty cool though I must admit.. I think for the $60 its worth a try.. I would love to get one to mod it. But as it turns out its too much of a pain in the ass to get in Canada..
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Re: Inexpensive "NADY" Preamp and Phantom adapters
« Reply #38 on: May 23, 2009, 05:52:26 AM »
You can find the pictures here.
http://www.mp-acm.de/hitec_audio/hitec_audio_pre_mobile_2.html
That's the best I could manage with my digital camera. I hope they're of any help.
Ulrich

I had to remove the pictures [2007-03-25] due to urgent need of the webspace they blocked. If anybody does still need them, please don't hesitate to pm me.
For the archive, I just noted that afri-cola put the pictures up again at the same address:
http://www.mp-acm.de/hitec_audio/hitec_audio_pre_mobile_2.html

Meanwhile, the Nady DMP-2 has dropped to $31.98!
http://yhst-38616620066226.stores.yahoo.net/dbdmp-2.html
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Offline heyitsmejess

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Re: Inexpensive "NADY" Preamp and Phantom adapters
« Reply #39 on: May 23, 2009, 05:59:54 AM »
ive never used it with the gain cranked all the way up, but halfway, it dosent sound TOO bad....for the price, im happy with what it does.
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