PRR's 24vdc supply.... 48v phantom possible?

[rascalseven]

Hi,

On the old forum PRR posted a picture of 24vdc supply that was nothing more than a 24vdc wallwart filtered by two 27 ohm resistors and two 4700uF/35v electrolytics. I have the drawing if anyone is interested (I hope Chris doesn't try to sue me for copying it! )

Anyway, this supply works brilliantly in a pair of Pultec clones with Neve outputs and a few passive mixbuses with Neve outputs that I've built so far. What I'm wondering (because I NEED someone to help me with this as I really don't yet know squat about power supplies) is can I use the same wallwart and add, in addition to the filtering, a DC/DC converter to provide 48v phantom as well? I'd really like to stick with the inexpensive wallwart but also have phantom power for some Neve-clone mic preamps.

FWIW the 24vdc wallwart is 400ma output.

Any ideas?

Thanks so much,

Joel

 

[The Kid]

rascal, have you seen the mic pre info on Eddie Ciletti's site? (tangible-technology)

 

[rascalseven]

His phantom supply uses a 25v AC transformer, so I didn't think it was helpful. Can I borrow from this design for my 24v DC wallwart?

Like I said, I know absolutely nothing about power supplies. I would be very grateful for any help.

Peace,

Joel

 

[owel]

You can use a voltage doubler circuit, then regulate it to 48V DC using an LMxxx chip (forgot the model). But you need AC voltages to work with.

Voltage doublers have poor current capacity, but we're just powering condenser mics so that will be more than sufficient. It will do.

 

[PRR]

> add, in addition to the filtering, a DC/DC converter to provide 48v

Where you going to get that? A 24VDC in 48V DC out converter?

They exist, but are rare. Unless you get lucky in surplus, you are going to pay over $100 for one.

The $5 answer is to get a second 24V wall-wart and series it with the other one to give your 48V.

FWIW: many good phantom mikes are exactly as happy at 24V Phantom as 48V. The AKG 414 runs un-changed from 12V to 52V, and doesn't really sag until 10V-9V. I sure heard no change between 48V and a 30V supply I used for years. (However some cheap mikes use Phantom as Polarization, so sensitivity will go down.)

 

[BradAvenson]

check out the schematic here for another option:
http://www.rolls.com/data/pb224man.pdf

you would be making your own DC-DC converter. Might be easier with another wall wart.

another 24v on top or get a 48V wall wart just for phatom.

 

[dr. foster]

No, no no. You guys are making this too hard.

You don't need a DC to DC converter at all. You can use a simple circuit known as a charge pump. It consists of a 4049 and a series of diodes and charging capacitors. I don't have a postable schematic here at work, but Paia uses something similar in their mic preamp circuitry to derive phantom power from a 12VDC voltage rail. This will work fine in your situation.

Sorry, I just couldn't resist adding my 2 cents on this subject.

 

[CJ]

PRR!
Welcome back!
OK, all we need is Kev and were done!

:grin:

 

[rascalseven]

The charge pump sounds interesting. Any idea where I could get more information?

Thanks,

Joel

 

[CJ]

There are some Neve 48 volt supplies over at the "Black Market"
Man, thats a cool name!

 

[soundguy]

The power supply that joe is currently selling with his JLM pre kit uses a charge pump design, might be worth a shot referencing his design on his site. If you wind up ordering a board from him, I can vouch for them being completely awesome in regards to quality and logic. You'll need an AC transformer to use that supply though...

dave

 

[hodad]

I wouldn't mind seeing PRR's little schematic. I've got a little project at hand that it might be good for.

Thanks
Tom

 

[daArry]

chek the the psu's at kevs' site: http://recording.org/users/kev/PS.htm - TK's one in particular may help :wink:

 

[dr. foster]

The charge pump sounds interesting. Any idea where I could get more information?

Sure--your first and best shot would be to look at Paia's mic pre design--the charge pump is comprised of IC1 and associated components. That's probably the simplest and most convenient implementation of a charge pump I've seen. My only recommedation over the stock design would be to increase the size of C12 to 100uf if you use this circuit.

There are other methods, too. Maxim Semiconductors make a few different IC's that are specifically designed for charge pump apps, DC polarity inversion, and rail splitting.

Here's a simple charge pump tutorial from Kansas State University's EE department.

Here's a PDF that discusses four-stage Dickson-type charge pumps like the one that Paia uses. It's a bit heady, so be prepared.

Overall, charge pumps are not usually seen in pro audio--they're much more commonly used in cellular phone applications. Some might argue about excessive IR noise generated by the use of diodes in a multiplying application, but it's my opinion that it's really very negligible in a phantom
power situation.

Hope this helps you.

 

[Mark Burnley]

Here's the schem out of the pdf Brad linked to from rolls:

Phantom Switcher PSU

It looks like there is feedback taken from the DC output of the switcher taken via the schottky diodes to a transistor to vary the drive to the MOSFET.

Nice and compact circuit.

Thanks for the other links dr foster.

Mark

 

[TobWen]

Can anybody tell me, which DCDC is used in that thing?
http://mysite.wanadoo-members.co.uk/deltaharmonic/schems/rolls_switcher_phantom.gif

I don't know any 4069 with dip14.

 

[JLM Audio]

The 4069 is a standard 4000 series CMOS IC with six inverter stages in it costing about 50cents

Joe

 

[TobWen]

LoL ... 50 Cent(s) ... I know - Eminem is EVERYWHERE
But I did not know, that a CMOS-IC can act like a DC-DC!

I think, I will build this thing and make some test.

 

[peterc]

I built a similar cct to the Rolls, & it worked. The 48v was VERY noisy though, & I was only happy once I had upped the output caps to 220u & 100u.

Peter

 

[Mbira]

Look at Kev's page on his build of the Green Pre. The power supply has the "voltage trippler" thing. It turns 15 vDC into 48vDC.

Joel