Speakers & Watts & Amps. Ohm My!

[JackG]

Originally Posted by MNBoxster

Hi,
One thing you can do is to wire a resistor into the circuit. Go to a high-end custom stereo shop (or even a Radio Shack) and get the proper resistor to drop the load into the required range. Hope this helps...
Happy Motoring!... Jim'99

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.

[MNBoxster]

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.

Hi,

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

[JackG]

Originally Posted by MNBoxster

Hi,

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.

Happy Motoring!... Jim'99

The problem with the new amps is not so much impedance matching (as it was in older tube amps), but the specs of the final output devices (or ICs).

It's basic ohms law. Current is voltage divided by resistance (I=E/R). Power is voltage times current (P=ExI). An audio amp will attempt to send the same voltage out to the speaker regardless of the impedance (resistance) of the speaker attached. If you cut the impedance in half, at the same voltage, you double the current flow. Since P=ExI, you've doubled the power. That's why most amps that are rated to drive 2 ohms show they deliver nearly double the power at 2 ohms than at 4 ohms.

The problem is that if the amp is not rated to drive 2 ohms, the final output stages of the amplifier will not be able to deliver that much current. It's internal power supply can also be a current limiting factor.

The results are overloaded electronic devices in the amp, which results in heat and device failure. Also, while the amp is still working, it will be clipping (as it runs into it's own capability ceiling) and that's the worst thing you can do to a speaker. You will damage a speaker much quicker by clipping the amplifier than by overpowering the speaker with clean power. Speaker voice coils *hate* DC current, and that's what a clipped signal is. Clean audio power is essentially AC current that is changing continously to match the vibrations of the sound of the music it represents.

Whew! Enough of that crap. I'm going to drive the Box.

[husker boxster]

Once again, great info - but we're starting to swim at the deep end of the pool. I think I've got the idea why you don't want spkrs with fewer ohms than the radio or amp is putting out. I will wire my spkrs in series and have ohms to spare.

I bought some Kicker KS35s today and have been working all afternoon / evening trying to get them in. They are bigger in all dimensions than the OEMs. Thought I had a soln all worked out but I ran into a clearance problem during a final check. Plan B also had a clearance problem in a different location. Went back to the original plan and modified it. Looks like everything is fitting properly now - on one side. The other side is untouched but now I have a pattern so it shouldn't take long. I was hoping to have the whole project done today as I have a nice 4 hr road trip on the agenda for tomorrow. Guess I'll have to enjoy the new spkrs on my trip in to work on Mon, but it's always better to do a good job versus doing a hack hurried job.