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April 16th, 2007

Mini power - the DC hurdleLocation Sound

Portable Computer Based Recording System

During prep for Butterfly Dreaming, I had to convert the sound cart to a computer-based recording system. Many production sound carts have the ability to run from either AC or DC (battery). But computers don’t normally run off batteries. Because I wanted to preserve the flexibility and portability that comes with a battery-based recording system, I made my decision to build out the cart to enable DC-powering the Mac Mini, monitor, and external hard drive without the use of an inverter or AC.

Powering the mini

The main issue: the Mini wants 18.5 VDC natively, and the external firewire hard drive demands not only +12VDC, but ALSO +5VDC to operate. The remainder of the components for the computer (monitor and external DVD burner) required +12VDC, which was compatible with the audio equipment on the cart (Sonosax mixer, MOTU Traveler interface, video monitor, PD-4 DAT recorder, etc.). I considered building my own multi-voltage supply from a +18V source, using resistors to get to +12V and +5V. But that seemed overly complicated and not very efficient. I also considered separate battery systems, one +18V source for the computer, and a +12V source for the rest. But that means more batteries, more chargers, etc. I wanted a solution that would minimize the number of battery systems on the cart.

The Carnetix Solution

After a good bit of research, I found myself in the world of mp3 car afficionados, staring at the Carnetix CNX-P1900. Apparently, the idea to replace your standard car stereo with a mini-sized computer has snowballed to such a degree that there has been quite a demand for folks who needed to power their Mac Mini’s with a car battery. So Carnetix decided to build this magic little device, designed to take in a +12 DC voltage, and output +19VDC, +12VDC, and thanks to a recent add-on upgrade, +5VDC. I dove in. I received the CNX-P1900, with the +5V add-on, and started wiring. After an evening of splicing into the power supplies for the Mini and the hard drive, I successfully ran the Mini and drive from a 33Ah wheelchair battery for around 6 hours. Above is the frankenstein version during its test run. The Carnetix is the small slotted metal box in the center. All systems go!