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Old 16th November 2005, 08:56 PM   #1
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Join Date: Nov 2005
Location: New Hampshire, USA
Smile Hello from New Hampshire

Hi all --

Rank amateur that I am, I can't truly qualify as a geek bearing gifts, but I wonder if any others out there feel frustrated by Radio Shack's shrinking selection of components.

For instance, time was when you could find 7905 and 7912 negative voltage regulators at Radio Shack, but no more. And there aren't any other retailers of electronics parts near where I live. So when I found an old Radio Shack subwoofer for $10 at a yard sale, and decided to bypass its internal crossover and DIY an electronic crossover to make a biamped sub/sat system, Radio Shack's eschewal of the negative presented a hurdle.

Turns out (and with apologies to anyone to whom this may be old news) you can make a positive regulator (sort of, at least) do the work of a negative one.

My first idea had been to use an LM317 adjustable positive regulator on the negative side of a bipolar supply to run an active crossover built around several op amps. Well, it was a long strange trip, of which I'll spare you the frustrating details, but I finally got a functioning bipolar supply using a 7805 fixed 5-volt regulator on the negative side and the LM317 adjustable regulator on the positive side, and here's how the whole thing works:

If you hook up the 7805 connecting the terminal intended for the positive input to ground, and the ground terminal to the negative line from the filter capacitors, in effect you've told the 7805 that the negative input (in my case minus 19 volts) is ground and ground is plus 19 volts. Having no way of knowing you're scamming it, the 7805 dutifully offers an output 5 volts more positive than the minus 19 volts it thinks is its ground, which (in the absence of a significant load) comes out to minus 14 volts.

BUT -- that's not a REGULATED voltage. First, because it's taking its reference from a voltage that might vary instead of from the actual ground, the output voltage of the 7805 could vary with it.

Second, because its internal circuitry is oriented the wrong way, the 7805 can't maintain a steady output voltage over a range of loads the way it could if its positive terminal were hooked to the positive line and its ground terminal were grounded. So I found that the no-load negative 14 volts output climbed up toward the positive when I started putting op amps into the circuit. With the five TL082 dual op amps in my finished crossover, the voltage put out by the 7805 climbed to minus 9 volts, or about 1 volt per dual op amp.

So here's what I suggest to anyone who wants to try making the 7805 do the work of a negative voltage regulator:

First, install the LM317 adjustable positive regulator and adjust its output to about half the unregulated positive voltage from the filter capacitor.

Next, hook up the 7805 with its positive terminal connected to ground and its ground terminal connected to the unregulated negative voltage. Then connect your active components to the output of the regulators -- active components in my case being the five TL082 dual op amps that form the basis of the crossover's active filter circuits.

Then, switch on the circuit and measure the negative voltage at the 7805's output, and finally, adjust the LM317's positive output to balance the 7805's negative output. In my case, that gave me a +/ 9 volt result. Since the TL082 will work fine with anything from +/ 5 volts to +/ 18 volts, this result works very nicely in my finished project.

The finished crossover (not to mention that old Radio Shack subwoofer!) isn't exactly the stuff of The Absolute Sound, but it works well enough for music on my budget. Cheers! :-)
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Old 16th November 2005, 10:44 PM   #2
JoeBob is offline JoeBob  Canada
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Welcome to the forum. Even in a big city, there aren't any good electronics shops close to me, or close to a metro (subway for some people) station. So I usually end up ordering my components anyways. You might want to check out digikey or mouser, the usually have good prices and pretty cheap shipping as well. Much better than RS, that's for sure.
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Old 17th November 2005, 02:23 PM   #3
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Location: New Hampshire, USA
Thanks for the welcome, JoeBob. I've bookmarked digikey and mouser; they look pretty good.
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