Introduction: Phantom Battery Power
My daughter swapped some audio equipment and ended up with a condenser mic, which looks pretty nice. The problem is it needs phantom power, and there was none available on any of her equipment. There are lots of phantom power supplies out there, some of them pretty cheap. But how good are they? What are you really getting for your money?
Not much, as it turns out. Just some resistors and caps, and a 48V DC supply. I thought it would be interesting to try and build one, using stuff I had around the shop where possible. That way, I can control the quality of the components, and hopefully come up with something cheap AND good. What the heck...
It turns out I did not have everything that I needed. Some of the stuff I had to get from DigiKey. You can get these from any number of places, but if you use DigiKey, here are the part numbers for handy reference:
UKL1J100KED 493-14491-ND CAP ALUM 10UF 10% 63V RADIAL Quantity 4
IO-XLR3-M-JL 1937-1137-ND CONN RCPT MALE XLR 3P SOLDER CUP Quantity 1
IO-XLR3-F-JL 1937-1138-ND CONN RCPT FMALE XLR 3POS SOLDER Quantity 1
B32523R0226J000 495-B32523R0226J000-ND CAP FILM 22UF 5% 63VDC RADIAL Quantity 2
1N4740ATR 1N4740AFSCT-ND DIODE ZENER 10V 1W DO41 Quantity 4
The other things you are going to need:
Prototyping board. Mine is about 2" X 3".
6.2 kΩ 1% resistors, quantity 2
9V battery clip, like the Keystone 2240
Spacers to hold the board in the box
Some kind of box to put everything in
9V batteries, quantity 5
Double-sided tape or Velcro to stick the battery to the case
Step 1: 48V Supply: a Big Battery
I looked up several phantom power supplies. Most of them ran off of a custom 48V supply that was stepped down and rectified line voltage (120 VAC here in North America). Most were pretty old-school, with individual rectifier diodes and caps, and some kind of transformer. I didn't have any of that stuff, and I did not really want to play with the line voltage. Besides, all of these are going to have some AC ripple at 60 Hz, which is bad for audio. I wanted something cleaner, cheaper, and quick to throw together.
Batteries are pure DC. There is no cleaner power supply. So, I though I would try making a 48V battery, and run the phantom power from that. If nothing else, this could serve as a reference for future designs, as you won't get much cleaner.
There aren't a lot of 48V battery packs on the shelf at the local hardware store, but it turns out they are easy to make. Connect five 9V batteries in series an voila: 48 VDC. I wrapped them all up in electrical tape to keep them all together.
Step 2: Hooking Up the Battery
I made battery connections by prying apart a standard 9V battery clip, and cutting the plastic base in half. One lead goes to the "+" terminal, and the other to the "-" terminal
Step 3: Wiring Up the Circuit
I lifted the circuit from this web site. It's pretty simple. For my implementation, I placed two buses, one for ground and one for 48V, one either side of the protoboard. The big caps take up a lot of room, so I placed them first.
From what I've read, it's important to match the resistors that connect the phantom power to the mic inputs, to keep the input balanced. Matching the values is more important than the actual value itself. I measure the resistance of a bunch of 6.8 kΩ parts, and wrote down the value beside each one. I can now go back and pick out matching pairs. I chose two that measured 6.80 kΩ.
I soldered on "point to point" style, using the leads of the components to connect each part of the circuit where possible. Leads come off the buses to connect the battery and the power switch..
Step 4: Prep the Enclosure
I found this old HeNe laser supply box that was about the right size to hold the circuit an the battery. It's also pretty solid and looks kind of cool. It has a few extra holes in it, but hey...ventilation?
The XLR connectors need a big holes to sit in. The optimal size would be .875", but I don't have a punch that big. I opted for the 1" Greenlee punch. If you don't have one of these, borrow one. They make cutting holes so easy. You first have to drill a pilot the size of the bolt that hols the punch together. Then put the halves of the punch on either side of the hole and thread the bolt through the middle. A few cranks on the bold with an adjustable wrench and you have a nice, clean hole. I put one hole on the front and another in the back to bake an "in-line" configuration for the box.
I then drilled four holes in the bottom, using the pattern of the holes in the protoboard, so I could mount the board in the bottom of the box.
Step 5: Hooking It All Up
It's a lot easier to work with the XLR connectors when they are not mounted in the box, so I put some leads on the connectors before I mounted them.
The circuit has bare wire running beneath it, so I mounted it up off the bottom of the box using plastic spacers. Once the board was mounted, I put the XLR connectors in place, using the female XLR as the input and male XLR as the output. The is the convention I observed when I googled a bunch of different phantom power supplies.
I found a switch that popped right into the square hole in the front of the case. The switch interrupts the 48V, so I connected one pole to the battery lead and another to the circuit board.
I soldered the leads from the XLR connectors directly to the board. This will make it difficult to disassemble, but I don't plan on doing a lot of that.
I used double-sided tape to stick the battery to the top of the case. Then, I connected the battery, bolted the case together and voila!
Step 6: Quick Test, Results and Future Plans
It works! We were able to confirm that the mic was working, at least for a while. Either the battery died more quickly than i thought it would (phantom should really have to supply a lot of current), or the mic is intermittent. We have to do some more tests.
The next thing I want to do is add a 48V power supply. My plan is to find one that boosts 5V to 48V. Modern DC to DC converters are ridiculously efficient, and typically operate at frequencies well beyond the human hearing range, so they are less likely to couple audible noise into the circuit. 5V chargers with USB output are everywhere, so this seems a logical choice of input. Eventually, I'd like to compare the battery power to the power supply to see if there is any audible difference.
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