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Old 12th April 2008, 03:10 PM   #1
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Default 4 ohm? 8 ohm?

Hello, folks. I'm looking at the 55 watt bi-amp module over at MCM, part no 50-6277.

http://www.mcmelectronics.com/product/50-6277

It claims to run off of LM3886TF devices. It also claims to be for 8 ohm speakers, minimum. The drivers I would prefer to use with it, however, are 4 ohm. I read someone saying that they used 4 ohm speakers with it, highpassed at 100 hz and it was fine, but I'm not imagining using the 100hz high pass.

The datasheet I saw for the chip seemed to think 4 ohms was ok. How seriously do you think I should take the 8 ohm minimum rating on the MCM module?

Thanks!
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Old 12th April 2008, 03:47 PM   #2
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just use less voltage and a good heatsink and 4 ohms will be fine.
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Old 12th April 2008, 04:06 PM   #3
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Yes the chip will run a 4ohm load fine if the power supply and
heatsink are large enough. But to keep the price low I bet they
used the smallest heatsink,transformer and filter caps that they
could for 8 ohm operation.
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Old 12th April 2008, 08:01 PM   #4
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Is the danger when I push the amp too hard (loud)? The concept here is a computer speaker, so I don't see myself working them real hard.

If I try it and it doesn't work am I likely to destroy anything? I would be satisfied to gamble on the project and lose, if could disassemble it and use the parts elsewhere.

Thanks again for your input.
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Old 13th April 2008, 09:31 AM   #5
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Well, it won't blow up immediately. What could happen is that it gets very hot.

If it does that, add a speaker resistor in-series with the speaker.
Usually, only 1 or 2 ohms (added resistance) will cool it off. That's a popular thing in the op-amp textbooks, and it still works with a really big op-amp like lm3886.

Other way is to put one of the cheaper 20ga inductors in your woofer crossover (probably installs additional 1 ohm DCR). Those don't work well for BSC or notch filters, but just one of them for a 2nd order crossover's main inductor is quite doable if the woofer is 4 ohms.
For the tweeter, a 2nd order is quite the problem, with the inductor representing nearly a dead short. No problem--start off the tweeter crossover with a 3.3 ohm resistor and then a capacitor. . . before the rest of the crossover (or you can use that resistor as the negative feed for the entire tweeter circuit--opposite side of the crossover).
Now the speaker probably can't drop impedance below reference for the amplifier.

You can also look up reference materials for the "4 to 8" style of speaker. That would be good because my description above probably didn't quite cover it.

Another option is to purchase a transformer with slightly less voltage. Or you could use the factory provided transformer along with "diode drop" of inefficient diodes, like 6a05 or similar (couple of volts less than average).
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