Summer Project: Powering passive monitors with computer power supply and car amp

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Joined: 09 Feb 2015
Location: Las Vegas

Post 30 May 2015

Not sure if anyone will ever need to do this, but someone donated their passive 10-12 year old Made-in-USA JBLs to me (JBL 4208s), and it was without a poweramp.
Never having had studio monitors before, I did not know how to get them to work (they had each a black and red banana plug at the back), so I took it to Sam Ash, and they showed me what I'd have to do to get it working. Didn't really understand, and the cost came out to about 200 bucks with the cheapest 2 channel amp and a pair of banana plugs.
Summer was around the corner, and an electronics DIY just sounded too good to pass.

Here's what I did. My friend had a couple car power amps lying around. He also had a couple computer power supplies lying around. Since he used neither, I borrowed them. The only expense I had on this was a $6.99 audio cable from autozone, a $5 wire stripper from home depot, some electrical tape and a y-cable (3.5mm from the laptop to RCA left and right to go into the amp) from a dollar store. Perhaps not the best idea to get a cable from a dollar store, but hey, I didn't even know if the speakers were in working condition!

Here's how the thing runs (tons of tutorials online on how to do this, so I'll try to keep this short): You use the computer power supply to power the car amp. Short the green and the black cable that goes to the motherboard so that the PSU starts, hook up the yellow cable from one of the molexes to the +B/+12V terminal, depending on your car amp -- this powers up the amp. Then connect the negative ground (it is usually negative, but double check anyway) to the ground terminal on the amp. Finally, run a small wire from the +B terminal to the REM terminal so that the LED on the amp turns on. I don't think you need to do this, but I thought it'd be nice to know that the amp is turning on. I mean that's what the LED is there for, right? Finally, take the car audio cable, strip the ends, and from the amp's output, send them straight to the speakers. Positive and negative each for both left and right. Since my monitors were 75 watts each, and since this particular amp sends 150 watts to both the left and right channels, make sure the volume/gain knob on the amp is less than halfway, so you're sending less than 75 watts to each. Mine is currently at about a quarter way, and even that too loud.

The setup works, the sound is amazing, and I couldn't be happier. I don't know what I did to deserve this (the people who donated them to me put 'em out for their garage sale, and no one bought them, so they just gave it to me saying they were going to throw it away otherwise!) but I am sincerely grateful. I am sharing this here for two reasons: 1) I am happy it works, and 2) to tell you that you don't - literally don't - need a lot of money to afford these things. Sure, you need to have a bit of luck (in my case this electronics major friend of mine who had amps and PSUs lying around), but where possible, try DIYs. You will save a lot of money, and if the effort pays off, you will be left smiling for days.

For me, it's been one step at a time, first a guitar, then a simple casio controller, then Reason and the REs and now this hardware. Here's to hoping that everyone else here who wishes to have a studio gets their dreams realized. :)

Cheers! Image 
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