use 24v wart for phantom

i want to use my 24v psu (wallwart) to power my fetboy and phantom. can i use a voltage doubler like this?
http://hyperphysics.phy-astr.gsu.edu/Hbase/electronic/voldoub.html

any ideas on the cap values?

thanks
jason

Hi Jason,

Yes that voltage doubler ciruit is often used for getting >48v DC from a 24 volt AC transformer. Be sure your wallwart spits out AC - not DC. The doubler circuit will only work correctly using 24v AC.

Some people would say that 63v caps will work OK but that's pushing it too close to the edge. You are safe using 100v caps in this kind of circuit. 470 uF will be an ample size but you might want to use a parallel pair of 470 uF 100v. caps for each shown in your circuit diagram.

You should expect to measure somewhere around 65v DC across the circuit's output before regulation. Yes it's a "doubler" circuit & yes that's more than double 24v., still ...

You will definately need regulation of some kind or you're likely to fry your expensive condenser mics. Look at the TL783 adjustable regulator for this. It works very well in this application.

Bear in mind that you eat up double the current with this kind of circuit ... luckily phantom powered mics use very little current, < 14 mA in most cases.

Fred Forrsell has a nice PS circuit on his web site using a 48v center-tapped transformer (24v-0-24v) for a mic preamp. Just ignore the -24v part & think of the transformer as having a single 24v AC output. He includes a doubler circuit on one half of his transformer to get 48v for phantom. Good Luck,

Skip

dam its a dc wart. its required for the circuit since there is no rectification on board. i could try with ac but i already have a nice tamura 24v dc wart. any suggestions for a dc doubler?
thanks

There's an article on Gyraf's site with circuits for voltage multipliers using CMOS gates: http://www.gyraf.dk/schematics/Voltage_multipliers_with_CMOS_gates.pdf
You would have to be very careful not to inject noise into the whole circuit when designing with this type of dc-dc converter.

Remove me....

i don't understand????

dc voltage doublers are complicated and one has to know how to implement it to avoid noise.
but there´s simple way to use your wallwart as is: just take out the rectifier and move it into your fet boy.

Maybe something like this would work??



It isn"t my circuit so I can"t tell you how valid it is but it looks like it should do what you want it to but you might have to rip the rectifier out of the Wallmart to use the circuit the way it is as it needs AC Input...



Cheers

that looks all right, at least the voltage doubler part. i´m not sure about the lm317 for 48+ , i never used it in this application. its not rated for 48+.

[quote author="ioaudio"]i´m not sure about the lm317 for 48+ , i never used it in this application. its not rated for 48+.[/quote]

It doesn't see 48V.

Peace,
Al.

[quote author="alk509"]It doesn't see 48V.
[/quote]

in case of a short it would, no?

[quote author="ioaudio"][quote author="alk509"]It doesn't see 48V.
[/quote]

in case of a short it would, no?[/quote]

Yes, shorting it would kill it. Plugging the output directly to AC mains would kill it also. I've never seen either happen.

Peace,
Al.

I have a circuit that uses an LM317 voltage regulator that I designed when TL783's were difficult to get hold of. It can be safely shorted - and can supply enough current for 1 or 2 channels.

Send me a PM if you would like a copy of the schematic.

Remember that most microphones will be quite happy with 24 volts DC (or even less) - so you could use your 'wall wart' as it is - if necessary!

John

Hi,

From looking at the data sheet, the LM317 appears to be able to regulate a RANGE (raw input to regulated output) of 37v maximum. It will probably work if used to regulate from about 70v to 48v as you're doing in that schematic - a 22v range.

So maybe you can use an LM317 that way but why not substitute a TL783 regulator from Texas Instruments. Just in case my interpretation (read assumption) of the 317's data sheet is incorrect, the TL783 is rated for way over 48 volts ... 125v is it's maximum.

Also I'd change some resistor & capacitor values in that schematic. Both of the 330 ohm resistors could be made 100 ohms and the 12K ohm resistor be made a 10K trimpot. That way you can adjust the supply to give you a true 48v at the XLR pins - after any additional RC filtering at each 48v phantom switch & after the 6.8K voltage divider resistors feeding the voltage to both of the XLR pins. Also a smaller value for that upper 330 ohm resistor is supposed to allow tighter regulation on the output (but this is hear-say).

At the reglulator's output, in place of the 2.2uF cap, I'd suggest using a 47uF 63v or larger electrolytic bypassed with a .1uF film cap. The original value is kinda small & 50v is too close for comfort. The 10uF cap from the #1 regulator to ground is 50v too so I'd feel more comfortable with a 63v cap there. Additionally you could double the 1st two caps' (doubler section) values but it would not make that big a difference, especially if board space is an issue & you're only powering a couple of mics.

Skip

Dozens of people here have built my power supply, which uses an LM317 for 48V...

http://www.beatbazar.com/guests/ssltech/kps-1/index.htm

Yes, you can kill it. ... ...-so don't kill it.

I've never heard of one dying yet...

If you use the SAME wallwart as is being used to power something else. be careful that it doesn't 'short past' any of the caps in the multiplier circuit... Best thing to do if in doubt: use a separate wall-wart for the 48V.

Keith

Well I personally have blown a whole bunch of LM317 Phantom Regulators but most of it was because of being clumsy with my DMM and shorting the LM317 by accident....They do Blow quite easilly though so just be carefull that you don"t accidently short them or maybe use a small Fuse that will blow before the regulator does.....


Cheers

Well well...

Then maybe I'll add a note suggesting that people buy two in case they're similarly unlucky, but I must way, I've never killed one. You live & learn... -Hey, call me lucky!

Keith

Assuming the max I/O diff of the 317 is 37V (too lazy to look it up right now)... 37V worth of zeners across the 317--cathode to input, anode to output--would provide some protection against shorts. It's crude but better than nothing. A resistor, fuse or even a small lamp at the 317 input, to limit current under a short condition, ain't a bad idea either. Of course, there are also slightly fancier schemes involving a pass transistor at the input to the regulator, to fix the maximum I/O potential to a safe value.

Does the 317 tend to fail open? (It's been a while since I've had to troubleshoot a PSU that uses one). If so, then there might be some merit to not going out of your way to protect the relatively-cheap regulator, instead letting it act as a fuse for the somewhat more expensive mics :wink:

Hi,

Here's a schematic of a 48v phantom power doubler circuit that uses any 24v AC secondary - either 1/2 of a 24-0-24 or all of a 12-0-12 transformer with the secondaries in series. I've used it several times & it's perfectly clean with very low ripple (almost non-existant).



I usually include a 100 ohm & 220 uF 63v RC filter for each XLR to isolate the power for each mic from the other(s). Of course the usual matched 6.8K resistors dividing the voltage to each pin is used as well. I've used this same circuit with an LM317T regulator & it's still going strong. I just feel "safer" using the higher power rated TL783.

Skip

i was trying to use the 24v dc wart specd in the original hamptone article. Since everyone is pushing towards 24v ac im wondering if i need to give up on 24v dc to 48v dc (seems like it should be simple though)

thanks