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Old 9th January 2009, 06:40 PM   #1
jeppix is offline jeppix  Italy
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Default +/-18V & +48V from a battery

Hello to everyone,

I have to make the power supply for a mic pre amp.
This preamp must be portable so I'd like to supply it with a battery.
In some commercial preamp with one 9V battery they obtain the voltage for the ICs (+/-18V) and also +48V for the phantom power.

I make some googling but I don't find nothing

Could anyone help me with some schematics?
I don't want to use trafo and if it's possible the ICs used have to be DIP (no surface mount integrate).

Thanks.

Andrea
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Old 9th January 2009, 06:46 PM   #2
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You can use an SG3524 or SG3525 pulse width modulator chip to boost the 9V with a push-pull DC-DC converter. Since it is for a microphone circuit you will need to clean up the output with a linear regulator (for which you will need to boost the output about 1.5V over what you need to end up with.) Elliot Sound Products has a number of designs with the SG3524 -- folks use them to power mobile electronics all the time.

Unfortunately, you need a fair amount of inductance for a relatively low current demand on the dc-dc converter.

You can use LM317/LM337 for cleaning up the +/-18VDC, and a TL783 or LM317HV for the +48VDC. Make sure that the power supply box is well shielded, probably want to put some ferrite beads on the power lines.
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Old 9th January 2009, 07:27 PM   #3
jeppix is offline jeppix  Italy
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Thanks!

but I prefer to use an IC because the preamp and the power supplymust be inserted in the same enclousure and it's difficult, for me, to make the cores.

I'va found this schematic

Click the image to open in full size.

changing the value of R2 and R3 I could obtain any voltage.

Is this schematic suitable for my project?

Thanks
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Old 9th January 2009, 10:12 PM   #4
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Quote:
Originally posted by jeppix
Thanks!

but I prefer to use an IC because the preamp and the power supplymust be inserted in the same enclousure and it's difficult, for me, to make the cores.

I'va found this schematic

changing the value of R2 and R3 I could obtain any voltage.

Is this schematic suitable for my project?

Thanks
You can try it, step-up converters of this type are a bit "noisy" -- virtually all of the big IC houses make chips which willl work -- you might want to take a look at the various design tools which Texas Instruments, National Semiconductor, Linear Tech (whose chip you have shown above). etc.

How much "volume" do you have to work with?
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Old 10th January 2009, 02:45 PM   #5
mr.duck is offline mr.duck  United Kingdom
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I think the best way to get portable phantom power would be to use 5 9V batteries in series. That would give 45v not 48v but 45v is high enough to work. It should perform very well compared to switching ICs which might be too noisy. I imagine the battery life will be quite bad with a single 9v battery and step up IC as well.
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Old 10th January 2009, 02:53 PM   #6
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Hi,

maybe this get's you somewhere:

http://tangentsoft.net/elec/vgrounds.html
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Old 13th January 2009, 05:46 AM   #7
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I have built this for 48v... it works. But I have not tested it for any noise, or actually put into use, and checked it in "real-world" situations (i.e....how long the battery will last).

Click the image to open in full size.
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Old 13th January 2009, 07:54 AM   #8
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Why don't u try with white LED boost regulators..they r more flexible..try linear technology web site for drivers upto 10 Led's by modifying the circuit around feedback u get phantom power with current limit..but am not sure abt noise.
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Old 13th January 2009, 12:20 PM   #9
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Quote:
Originally posted by sivan_and
Why don't u try with white LED boost regulators..they r more flexible..try linear technology web site for drivers upto 10 Led's by modifying the circuit around feedback u get phantom power with current limit..but am not sure abt noise.

LTC3783

Better yet -- if you can pull down the August 7, 2008 article from EDN:
"High voltage, low noise dc/dc converters" by Jim Williams -- all will be clear.
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Old 13th January 2009, 04:14 PM   #10
Eva is offline Eva  Spain
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Unfortunately the maximum efficiency of charge pump solutions (the ones based in diodes and capacitors only) is 50%. They waste at least as much battery energy as they deliver to the load.

The most efficient way to do it is a flyback SMPS with a small transformer.

Unfortunately, again, 48V phantom power and +/-18V rails would result in short battery life because a lot of energy is wasted in the op-amps and sometimes in the microphones when they contain active circuits drawing current from 48V to ground.

The 48V alone may be easily obtained from 9V through a boost converter using a standard axial inductor and no transformers. There are specific control ICs that waste very little power.

For signal amplification you may consider low-power op-amps fed directly from 9V in order to achieve reasonable battery life. The output op-amp may be "rail-to-rail" type if you really need +/-9V swing (unlikely).
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