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Old 25th June 2003, 07:41 AM   #1
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Cool LM3875 GainClone & 4 ohm speakers

Hello All,
I am very keen to build an active system using the gainclone amps.
I am planning to use the vifa XT25TG30 tweeter and a peerless 850467 which I allready have.

I have just noticed when looking at the spec sheet for the chips that the output power vs load resistance decreases dramatically as the impedance decreases.

The XT25 has a nominal 4 ohms impedance is there anyway to get around this problem as I would like the tweeter amp to have app 30 watts output but this would not seem possible with the LM3875.

Any help would be appreciated.
TIA
Blair
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Old 25th June 2003, 01:39 PM   #2
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Default Re: LM3875 GainClone & 4 ohm speakers

Quote:
Originally posted by BlairHatcher
Hello All,
I am very keen to build an active system using the gainclone amps.
I am planning to use the vifa XT25TG30 tweeter and a peerless 850467 which I allready have.

I have just noticed when looking at the spec sheet for the chips that the output power vs load resistance decreases dramatically as the impedance decreases.

The XT25 has a nominal 4 ohms impedance is there anyway to get around this problem as I would like the tweeter amp to have app 30 watts output but this would not seem possible with the LM3875.

Any help would be appreciated.
TIA
Blair
I use the XT25 tweeter in a number of applications so I know it well. Often the crossover padding means that the amp doesn't always see the full lowish impedance of this tweeter (use only series resistance to pad down), unless the speaker sensitivity is near that of the XT25. What I would be more concerned with is the overall sensitivity of any 4 Ohm speaker and the potential negative phase angle at any frequency where the impedance does drop down near 4 Ohm or lower.

I have used LM3876 based amps into five pairs of 4 Ohm speakers, none of them having difficult loads, from 85 ro 91dB
sensitivity in average size rooms. No problems!

JR
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Old 25th June 2003, 01:59 PM   #3
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Default Re: LM3875 GainClone & 4 ohm speakers

Quote:
Originally posted by BlairHatcher
The XT25 has a nominal 4 ohms impedance is there anyway to get around this problem as I would like the tweeter amp to have app 30 watts output but this would not seem possible with the LM3875.

Any help would be appreciated.
TIA
Blair
On the National Semi datasheet you will see the calculations for gain factor you need vis a vis load and operating voltage -- just simple math.
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Old 25th June 2003, 02:45 PM   #4
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If I had 4 ohm speakers I would do it with the LM3886, with a 2x22v transformer.
Just a suggestion.
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Old 25th June 2003, 10:49 PM   #5
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Hi Carlos

Quote:
Originally posted by carlosfm
If I had 4 ohm speakers I would do it with the LM3886, with a 2x22v transformer.
Just a suggestion.
That is certainly an option. I have not yet done a sonic comparison between 3875 & 3886, although I will be doing a project with parallel 3886. The current rating of 3886 is about 75% higher, that is nothing to snicker about.

JR
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Old 26th June 2003, 12:22 PM   #6
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Quote:
Originally posted by Joe Rasmussen
Hi Carlos



That is certainly an option. I have not yet done a sonic comparison between 3875 & 3886, although I will be doing a project with parallel 3886. The current rating of 3886 is about 75% higher, that is nothing to snicker about.

JR

Yes, and the LM3886 almost doubles the power with 4 ohm speakers.
I did two modules for my father with the LM3886, because he has Allison 4 ohm speakers.
He's going to finish the amp, with a nice box and the 2x22V/12A toroid (that's 528VA! ) I ordered for him.
When he finished it, I'll take some photos.
And let's see how it sounds.
They sounded very well, at least with my test speakers, and my bench PSU.
But I didn't compare to my LM3875-based GC on my main system.
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