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Old 11th May 2005, 03:33 PM   #1
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Default subwoofer amp - lm4780

http://www.chipamp.com/lm4780.shtml

Could someone please confirm that this kit is using 2x stereo chips in mono mode. (It says x2 in the kit list). And hence is capable of 2x 120W (8ohm).

This looks perfect to drive my dual voice coil driver ???



Jim
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Old 11th May 2005, 03:38 PM   #2
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It uses two 4780's, each operating in parallel (mono) mode, resulting in stereo output. There is one chip per board, two boards.
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Old 11th May 2005, 05:46 PM   #3
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that's a yes then.

Any reason why this wouldn't be good for driving my Tempest sub(dual 4ohm voice coils) ?
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Old 11th May 2005, 05:56 PM   #4
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Nope. Just make sure you have a PS capable of delivering the current, and select a rail voltage to match the load. I think National recommends +-30V max for 4ohm loads.
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Old 14th May 2005, 03:08 PM   #5
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I will put the transformers(s) and amps in 1 box and wire a single rca socket to both amp inputs.

??What VA rating of transformer would you recommend ?
ie for 2x120W should I use 240VA or something bigger?

My AV amp handles gain and phase. So I probably don't need gain control (although I might add a passive attenuator just for the hell of it - as I've not built one before)

But I would like an adjustable rumble filter to protect the excursion on ported subs.

??How do I build in an adjustable rumble filter?
I'm guessing this is such a simple thing that no-one has asked it before!

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Old 14th May 2005, 09:15 PM   #6
G4ME is offline G4ME  United States
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is it a newer tempest? cause the originals were dual 8's but i think the ones they are shipping out now are either dual 8 or dual 4

also make sure you are feeding the same signal to both amps.

a rumble filter i just a variable high pass, look up active cross overs or digital filters and you will find tons of info.

what alignment are you using with the tempest?
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Old 14th May 2005, 10:38 PM   #7
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You're right, the old Tempest is 2x8ohm.

The current alignment is EBS(?). It's in a 220l ported sonotube tuned to 17Hz.

Currently it is being driven by a 2x80W Pioneer stereo amp which has proven adequate so far. Although this alignment would be happy up to 300W.

My main concern is going beyond xmax if i crank it up and it gets fed some sub 15Hz signals.

Can I build a passive high pass filter ?
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Old 18th May 2005, 05:55 AM   #8
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Its not practical to make one you need alot of mH so you would have to buy several like 10mH coils and series. Active is the way to go. sub sonic filter has a good one I would like to build sometime but havent got around to it.
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Old 18th May 2005, 04:39 PM   #9
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Thanks.
How much roughly would all the components cost?
And how much to get to +-15V from say a +-30V amp supply.

I only need mono.
And do I need 36dB roll-off ?

ie is there a simpler circuit I can build on stripboard ?
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Old 18th May 2005, 05:24 PM   #10
tiroth is offline tiroth  United States
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You cannot get 120W into 8 ohms with a single chip in parallel mode. The most you can get in the 60W range.

For your application, I think the best thing would be bridge parallel into 4 ohms. With 25V rails, this delivers about 230W. The only simple options are 2x60W (two chips, 35V) into 8 ohms or 1x120W into 4 ohms (one chip, 25V).

As you can see, the decision you make is important because it will determine the transformer you buy.
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