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Old 13th April 2009, 02:25 PM   #1
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Default How can LM1875 drive loads lesser than 4ohm?

Hi,

I am planning to use LM1875 chipamp to drive the mids of a line array. The LM1875 says 4 ohm is the minimum required load. I am ruling out series/parallel combination of drivers to get 4 ohm since sonically it is inferior to parallel combination. Now, parallel combination yields a very low impedance. So, is it possible to drive impedances lower than 4 ohm if the supply to LM1875 is reduced. The datasheet mentions that it can happily run from a +- 10 volts supply. Since its a line array, I dont expect to draw more than a couple of watts of power, if that helps

Thanks in advance,
Goldy
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Old 13th April 2009, 02:32 PM   #2
AndrewT is offline AndrewT  Scotland
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Can you rearrange the wiring in the line array to achieve an ~8ohm impedance?
The amp will repay for this favour by delivering lower distortion and with a better transient overhead.
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Old 13th April 2009, 02:35 PM   #3
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That would mean wiring in series (my drivers are 4 ohm each) which is exactly what i dont want to do for sonic reasons.
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Old 13th April 2009, 02:52 PM   #4
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So maybe you could use multiple amps?

The amp will work with 2ohms if you don't push it too much. It will need good heatsinking though.
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Old 13th April 2009, 04:26 PM   #5
cuibono is offline cuibono  United States
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Could you wire them in series/parallel? By playing with the combinations, you can get various impedances.
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Old 13th April 2009, 05:32 PM   #6
AndrewT is offline AndrewT  Scotland
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How many 4ohm mids are fitted to each channel?
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Old 14th April 2009, 02:17 PM   #7
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Quote:
Originally posted by Peter Daniel
So maybe you could use multiple amps?

The amp will work with 2ohms if you don't push it too much. It will need good heatsinking though.
Hi Peter, As I said its for a line array which would mean lots of amps. I was hoping to get away by using very few amps if it was feasible. Given that only a 4-5 watts (maybe lower coz freq band is 300hz to 3.5khz) of power would be required, is it really possible to lower the impedance to 1 ohm (or 0.5)? Ist'nt the chipamp only bothered about peak current thur it, which can be lowered by reducing the power supply?

Quote:
Originally posted by AndrewT
How many 4ohm mids are fitted to each channel?
Hi Andrew, The number of drivers per channel is 24.

Thanks
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Old 14th April 2009, 02:26 PM   #8
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Quote:
Originally posted by goldyrathore
Hi Peter, As I said its for a line array which would mean lots of amps. I was hoping to get away by using very few amps if it was feasible. Given that only a 4-5 watts (maybe lower coz freq band is 300hz to 3.5khz) of power would be required, is it really possible to lower the impedance to 1 ohm (or 0.5)? Ist'nt the chipamp only bothered about peak current thur it, which can be lowered by reducing the power supply?
You need to try it out. I know for a fact that LM3875 may work perfectly fine with 2 ohm loads even when pushed hard, but the heatsinks reach 50 *C or more.

With reduced power and frequency band, 1 ohm shouldn't be a problem either, but it's hard to say what could happen with 0.5 ohms.

Maybe instead of LM1875, 3875 could be more resonbale choice?
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Old 14th April 2009, 04:33 PM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
24 drivers powered by a single 3886 will get <=2W each.

It does not matter whether they are wired up as 2ohm or 3ohm or 6ohm or 8ohm, they still get their share of the 48W maximum that is available.

The amplifier will work better if you load it with a higher impedance load. 6drivers in series and 4sets of 6 in parallel give 6ohm.
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Old 14th April 2009, 05:53 PM   #10
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It is a trade-off. The drawbacks of serial connected speakers versus the drawbacks of amplifiers that work into (too) low impedances.

The German magazine Klang&Ton developed a line array two years ago. http://www.lautsprechershop.de/hifi/...ntyfive_en.htm
In that project the drivers are first arranged in parallel groups, and then the groups connected in series. That way the tolerances level out in the parallel connection and the drawbacks of unmatched impedance curves have less impact on the series operation.

The bridged-parallel speaker arrangement gives you the opportunity to try the speakers first with a single amplifier channel per speaker. If the speakers leave you really unsatisfied, don't pursue that project further. If the speakers leave you a little unsatisfied, add more amplifier channels to see, if they improve. If you are satisfied, enjoy the savings.
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