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Old 21st April 2010, 11:33 AM   #1
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Default Phantom power supply

I found a circuit from national semiconductors with schematics for 48 volts, this is an switching power circuit, LM 5000. Is it possible to use it to build a phantom power supply in a mic preamp where i only have 2X18 volts? (switching up +18 to +48). Switching freuency is 300 or 700 kHz, so good capacitors on the output should work i think.
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Old 22nd April 2010, 07:29 AM   #2
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Originally Posted by unsolved View Post
I found a circuit from national semiconductors with schematics for 48 volts, this is an switching power circuit, LM 5000. Is it possible to use it to build a phantom power supply in a mic preamp where i only have 2X18 volts? (switching up +18 to +48). Switching freuency is 300 or 700 kHz, so good capacitors on the output should work i think.
Your have 2X18V AC or DC? If Your have 36V (connect 18V+18V) AC, it will be about 48 V DC, without "phantom power supply".
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Old 22nd April 2010, 10:29 AM   #3
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The problem is that i can't connect -18 to ground, the 18 volts is also regulated DC.

The phantom power shall connect to ground and + / - on the primary side of the input transformer.
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Old 22nd April 2010, 12:18 PM   #4
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The problem is that i can't connect -18 to ground, the 18 volts is also regulated DC.

The phantom power shall connect to ground and + / - on the primary side of the input transformer.
Then it is better to get other transformer 36-37V AC and to make a new power unit because for the high-sensitivity microphonic amplifier such food will be better, in my opinion.
It is possible to find other scheme where your can use available power unit.
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Last edited by Prophetmaster; 22nd April 2010 at 12:23 PM.
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Old 25th April 2010, 05:18 AM   #5
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by unsolved View Post
I found a circuit from national semiconductors with schematics for 48 volts, this is an switching power circuit, LM 5000. Is it possible to use it to build a phantom power supply in a mic preamp where i only have 2X18 volts? (switching up +18 to +48). Switching freuency is 300 or 700 kHz, so good capacitors on the output should work i think.

What you need is a "voltage doubler" or tripler. Use the 18V ac from transformer. This is a good way to get high DC voltage from low voltage AC. Then aply a very strong filter to the DC, At least CRC and a regulator to clean it up. There are easy to make and cheap and perfect for this use.

Voltage multiplier - Wikipedia, the free encyclopedia
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Old 25th April 2010, 06:23 AM   #6
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I would never use a switching supply for phantom power. Too noisy. And you don't need 48 Volts most of the time; half that will probably work fine.
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Old 27th April 2010, 11:26 AM   #7
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I will take a look at the tripler. Why i was thinking about a switched circuit was because i looked at some audio cards (external) and they have 48 V with either firewire power or +9 V power to all amps and phantom power (+4dB out, so 9 V isn't enought, must be higher voltage inside....)
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Old 27th April 2010, 11:27 AM   #8
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And, i wanna have 48 to make sure it works with all microphones....
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Old 27th April 2010, 08:09 PM   #9
ChrisA is offline ChrisA  United States
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Originally Posted by bob91343 View Post
I would never use a switching supply for phantom power. Too noisy. And you don't need 48 Volts most of the time; half that will probably work fine.
The reason why switching power supplies are common for 48V phantom is because many times only DC power is available to the USB or Firewire box. So what is the engineer to do?

I agree that I'd not want to use a switch mode supply only because making them noise free requires more skill and experience than I have. But it certainly can be done as thousands of Phantom power supplies in commercial units demonstrate.

One advantage the switcher has is that it runs at a higher freequency than a liner supply. The liner supply runs the transformer at 60 or 50Hz and as we all know requires some very large caps to filter of the 60Hz ripple. But the switch runs at 30KHz, well above the audio range where any noise can't be heard and the small caps and inductors can work as filters. For audio applications I wonder if a switch mode supply running at (say) 100Khz would be be best? But like I said, supplies like that are not easy for amateurs to design and build.

I prefer to use a liner suppy with a tippler rectifier and then CRC filter before the regulator becausesuch with within my competence to understand, design and build
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Old 27th April 2010, 08:33 PM   #10
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Here is the data sheet with some applications, ie 300 or 700 khz switch for 48 v

https://www1.elfa.se/data1/wwwroot/a...s/07326766.pdf
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