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Old 14th August 2009, 11:18 PM   #1
defect9 is offline defect9  Ireland
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Default Voltage conversion, 12v-48v

I've been trying to find information on how to easily/cost effectively make a 12v to 48v power converter, capable of about 1A max (probably 500mA is fine).

My intent is to make a mic pre for my computer that runs off the power supply of the computer, so that I can load it all into a CD drive bay. I'm not expecting to be able to rival rackmounted tube-pres or anything, but something simple, effective, and easy.

after all the googling I've done, for some reason I just can't find anything on it. could be my search terms are either too specific, or not specific enough, i dunno. If anyone could offer some help, i would appreciate it.

*sidenote* I had found one schematic online at one point that was perfect, but the person's server is down and I forgot to save it.

thanks in advance!
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Old 14th August 2009, 11:41 PM   #2
ChrisA is offline ChrisA  United States
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Default Re: Voltage conversion, 12v-48v

Quote:
Originally posted by defect9
I've been trying to find information on how to easily/cost effectively make a 12v to 48v power converter, capable of about 1A max (probably 500mA is fine).

My intent is to make a mic pre for my computer that runs off the power supply of the computer, so that I can load it all into a CD drive bay. I'm not expecting to be able to rival rackmounted tube-pres or anything, but something simple, effective, and easy.

after all the googling I've done, for some reason I just can't find anything on it. could be my search terms are either too specific, or not specific enough, i dunno. If anyone could offer some help, i would appreciate it.

*sidenote* I had found one schematic online at one point that was perfect, but the person's server is down and I forgot to save it.

thanks in advance!
The simple way is to buy a ready made DC to DC converter. I'd look up the part for you but I doubt your spec is right. Do you REALLY need 48 or 24 watts of power. Just go to Mouser or Digikey and search for "DC/DC converter" If the 48V is just for phantom power a milliamp is overkill.

OK if you must build one the trick is a simply oscillator that drive s small one transistor amp. This amp drives a large inductor and diode in series. The inductor's fly back voltage is used to charge a large capacitor. Then a voltage comparator looks a the cap and turns off the amp when the voltage is high enough. You can buy an IC that does all of this. The Google term is "switched mode power supply" You can get up to 250 VDC this way. But better to buy the thing ready made. You can buy DC/DC converts that have very, very clean power suitable for high performand mic pres. No cheap but you need it for a mic pre.
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Old 14th August 2009, 11:55 PM   #3
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I would split the power supply and use 24V for the mic pre signal path and
a separate +48 for the phantom power.

Phantom power is fed to mics via two 6k8 resistors in parallel, resulting
in a short-circuit current of about 14 milliamperes, or 0.014 amps.

48 volt output DC-DC boost converters are somewhat rare. Maybe Recom
makes one, but it's not in the catalogs. You might try series connection
of two 24V (+/-12) converters. Recom makes some 1W converters for
under $10.

For DIY, try all-in-one ICs from Micrel. You probably want a flyback boost
converter topology.

Michael

If you use a single ended signal path maybe you can use one higher
power 24V unit for the signal path and put another isolated small one
in series for the +48.

Edit - I just checked the specs on the 24V mic pres I have from Telefunken,
Neumann, UA, etc. and they are typically about 40mA at no signal and
about 100mA driving +22dBm into 600 ohms.

If you're only driving a consumer sound card input you can probably
use a 12V rail for the signal path...
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Old 15th August 2009, 02:53 AM   #4
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Default Re: Re: Voltage conversion, 12v-48v

Quote:
Originally posted by ChrisA

The simple way is to buy a ready made DC to DC converter. I'd look up the part for you but I doubt your spec is right. Do you REALLY need 48 or 24 watts of power. Just go to Mouser or Digikey and search for "DC/DC converter" If the 48V is just for phantom power a milliamp is overkill.
The problem is it needs to be quiet if it's for mic circuitry. The packaged DC-DC converters are typically not all that quiet and probably also throw off some EMI even if you can clean up the DC. Of course the inside of a PC is full of EMI already so you may have other problems there.

I assume one of the $100 - $200 external USB AD/DA boxes that already have phantom power, level controls, mic inputs, metering, line outs, etc. won't work for your needs?
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Old 15th August 2009, 05:29 AM   #5
defect9 is offline defect9  Ireland
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needs isn't really the word. I could easily just buy a rackmounted anything and be fine. But the project is more of a "I want to see if it's realistically possible". The ultimate goal is to make an option that is viable for the bedroom musicians without a shedload of cash (aka: my friends, or at least the ones who can solder). If my $20 outboard mic pre can do phantom power, why not my computer?

and then, of course, I'm also looking at powering something like a 12au7 off the 48v tap so I can do a mic tube pre. I was also looking into just using the 12v for the tube, but... if I'm going to design in a 48v source, why not attempt it? if the 48v is enough to make a tube sound good in a computer (shielded, of course), then it might also be possible to make a decent sounding tube headphone amp.

It's all for the fun of doing something slightly different, even if the idea has been around for a while.
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Old 15th August 2009, 07:24 PM   #6
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It can certainly be done. There are USB powered devices that generate phantom 48v from the 5 volt USB power. I think most probably use a charge pump rather than a true DC-DC converter. But if you need more than a tiny amount of current, you need to go the DC-DC route.

If low cost is a goal, the packaged converters are not that cheap. I think most start around $15 or $20 which might be your entire budget for the pre-amp?

I suspect getting the noise down to anything close to what you can get externally in even a $45 Behringer Eurorack or Zenyx might be tough--especially on a tight budget? But it's an interesting design challenge.

If you're interested in charge pumps, which should have less EMI than an inductor design and also tend to be cheaper, check out Maxim appnote AN725.
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Old 16th August 2009, 04:25 AM   #7
star882 is offline star882  United States
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It's very easy to make one based on a boost topology. I made a very simple one about 3 years ago to convert 12v from a PC power supply to 48v for some surplus 48v fans. It used a UCC3807N-1 controller along with a IRF510.
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Old 18th August 2009, 12:21 AM   #8
defect9 is offline defect9  Ireland
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I think I'm going to experiment with both charge pump and boost ideas, as I believe i have most of the parts for a boost style, but I like the concept of the charge pump.

Once again, thanks for all the help!
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Old 18th August 2009, 02:30 AM   #9
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Power Over Ethernet POE is usually at 48V and lots of Watts....
You might find a regulator for that purpose could be adapted...

Or you can find a ready made 802.3af brick for about $25.
Switched down from 110 or 220, rather than up from 12V...
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