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Old 5th February 2007, 04:29 PM   #1
StalfoS is offline StalfoS  Canada
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Default Chip amp for subwoofer

Hi all,

Yesterday I managed to score a Mission 70ASA 100w active subwoofer for a lowly $30, only the amp seems to be completely busted, and blows fuses one after the other. I pulled off the panel, and it seems to be a "s53ami" module digital amp, which have a reputation for crapping out.

So I'm thinking of replacing the amp with a lm3886 following an active low pass filter. The transformer that came with it has 33V secondaries but seems to be sort of small-ish for an amp that is supposed to be 100w. Do you think that I should ditch it and find a new one or will it work out okay?

Any suggestions for how I should go about this? Are their better solutions then the lm3886?
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Old 5th February 2007, 04:44 PM   #2
BWRX is offline BWRX  United States
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You may want to have a look at some class d modules.
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Old 6th February 2007, 01:59 AM   #3
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Wouldn't 33VAC secondaries give something like +/- 46VDC when rectified? And isn't that 4v over the max of the 3886/4780?
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Old 6th February 2007, 09:51 AM   #4
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Hi

if your woofer has min 8Ohm, you could parallel 2 TDA7293.
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Old 6th February 2007, 09:56 AM   #5
sek is offline sek  Germany
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I'd recommend bridge-tied loading or parallelling of LM3886s according to National's AN-1192.
Or buy a UCD module. BPAs are cheaper, though.
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Old 6th February 2007, 10:05 AM   #6
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IMO for just 100 watts class-ab is fine and will be cheaper and DIY. Could you fit something like a P3a in there?
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Old 6th February 2007, 10:18 AM   #7
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To support richie00boy's comments - I have a paralleled 4780 feeding a cheap 12" 4ohm driver - and at 75% of the output volume from the pre-amp it sits about right with the rest of the speakers in the room. The sub isn't in a box yet - nor does it have the Linkwitz transform I intend to use, so that could all change.

The LM4780 is running +/- 24.5VDC supply which the Overture Design Guide Spreadsheet tells me should give about 105W output power.

With +/- 34VDC the output power (for a parallel setup) raises to 205W - and the size of the heatsink starts to get big too.
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Old 6th February 2007, 12:37 PM   #8
BWRX is offline BWRX  United States
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Quote:
Originally posted by richie00boy
IMO for just 100 watts class-ab is fine and will be cheaper and DIY. Could you fit something like a P3a in there?
Class ab would be fine but it may not be cheaper or better than other solutions. http://www.41hz.com/ has some nice low cost kits that could probably be used. UcD/other premade modules would be more expensive but are more or less a drop in solution.
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Old 6th February 2007, 12:39 PM   #9
BrianGT is offline BrianGT  United States
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I measured around 80wrms in lab with my lm4780, with a 2x25vac transformer putting out +/- ~34vdc, with a 4 ohm resistive load box.

If you need much higher power for a sub, I would try again to search for a cheap plate amp.

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Old 6th February 2007, 04:51 PM   #10
StalfoS is offline StalfoS  Canada
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Thanks for the responses guys!

I will look into some class-d solutions.. but I don't think it needs to be too powerful. It is 8-inch driver and I have a small appartment. Furthermore the box is about about a 1.5x1.5x1.5 cube -- not too big.

Ideally I would use the existing transformer because it is allready mounted to the inside quite nicely.. but as raromachine pointed out, +/- 46VDC will be a bit much for the 3886/4780's. Also it is not that big/heavy so I worry that the VA rating is not very high. I guess I will have to go transformer shopping.

I figure something in the 70-100 W area would work quite nicely. I don't really have the resources to make PCB's so that's why I was thinking part to part gainclone, but if there are kits available that would be cool too.

What do you suggest??
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