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Old 01-27-2007, 09:11 AM   #7
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Join Date: Sep 2005
Location: Minneapolis/St. Paul, Minnesota, USA
Posts: 3,308
Originally Posted by JackG
Not that simple. For example, if the load you're looking for is 4 ohms, but the speakers are going to give you a 2 ohm load, you'd need a 2 ohm resistor in series with the speakers to bring it back to 4.

However, you've just divided the power between the speakers and the resistor. In simple terms, if you had 40 watts available to drive the speakers, now you're applying 20 watts to the speaker, and 20 watts to the resistor. Fortunately, you haven't halved the sound produced by the speakers, you've reduced it by 3 - 6 dB, depending on how it's measured. You can probably adjust the fader to balance things back out.

The other problem is the resistor. I seriously doubt Radio Shack will have it, as it will have to be a Power Resistor. It will be dissipating, as heat, up to 20 watts peak in my example above. That's a pretty specialized, big, and hot resistor. A little 1/4 watt resistor from RS won't do the job.

Best thing to do is to stay within design limits and avoid bandaids like this.

Thanks for the caveats. I knew, but didn't mention, the loss of power and heat generation associated with adding a resistor to the line. There are such resistors though and even radio shack sells them. About $1.25 for a 2-Ω, 20W resistor. But, I wouldn't use them for all the reasons you state, just stating that you could.

The issue about matching impedence is really one of preventing overheating, or local hotspots, in the Amplifier. This was especially true with Tube driven ones. But, modern solid state IC amplifiers do not use matched impedances at all, contrary to myth.

The driver amplifier has a low output impedance such as < 0.1 Ω and the loudspeaker usually has an input impedance of 4, 8, or 16 Ω - many times larger.

This type of connection is impedance bridging, and provides better damping of the loudspeaker cone to minimize distortion, which is why it's used. Old style tube type audio amplifiers, required strict impedance matching for proper, reliable operation.

So, to directly answer Huckster's question, with an IC AMP, you can usually go up at least one, and often 2, level(s) of impedence with no real ill effects. I would go one for sure, (stepping up another would not be for a unit which plays loud or base-filled music because you could get into trouble).

Watch for Clipping and if so (which I doubt), turn down the gain on the Amp. You generally cannot go down an impedence level with an IC Amp (though you could with a Tube Amp). Hope this helps...

Happy Motoring!... Jim'99
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