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Old 14th January 2006, 09:15 PM   #21
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I think you should lower your power supply and you'll be fine. If it stays +/-30V, then you'll get more then 60W of dissipation on each chip, because in bridge configuration each chip sees half of loudspeaker impedance (in this case 3ohms). The chips will be very hot. I would provide them about +/-25V maximum to safely drive your speakers. According to LM3886 datasheet, with +/-25V you should get about 140W of undistorted output in bridge mode, and the chips wouldn't get hot.

If you use basic schematic from National's AN-1192 pdf, then you can easily connect everything p2p because this schematic doesn't have too many components.

I hope this helped a little.
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Old 15th January 2006, 06:47 AM   #22
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ok,i`ll lower the voltage.
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Old 18th January 2006, 07:29 AM   #23
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i`ve decided to go with this schematic,is it ok?
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Old 18th January 2006, 10:13 AM   #24
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costy_ro - Not much wrong with that schematic but I have a few questions about it:

Why take the trouble to match the resistors in the power stage ? Seems all they do is ensure the two halves have exactly the same gain but who cares if one side has 1% more gain than the other. Not like it would create a DC offset or distortion. Besides the resitors in the input stage are not matched so the two gains are not garanteed to be the same anyway.

Why have the 270 ohm resistors on the outputs of the input stage when we have resistors on the inputs to the 3886s ? I'd skip them.

There are too many capacitors. I would skip the caps on the input stage inputs and allow it to have a little DC offset which is then removed by the caps on th inputs to the 3886s.

As this is a pair of normal non-inverting amps on the output I would build them as per the carlosfm schematic Version 4 as seen here Improving the Non-Inverting chipamp
I have build a couple of these and they are fine.

I always add an inductor wound as a few turns of wire around the ouput resistors Ro as per Rod Eliots chip amp seen here http://sound.westhost.com/project19.htm

Again keep the voltages down.
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Old 18th January 2006, 10:19 AM   #25
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Sorry I get it now. That schema is only one side of of a bridge and has two chips in parallel hence the need for resistor matching. 4 chips total.

For 100 watts I would build two amps as per carlosfm schema (very similar to the ones shown) and drive them from the inverting stage. Surely two chips is enough and no matching problems.
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Old 28th January 2006, 09:43 PM   #26
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i`ve made the pcb(after the app note sch).
I`ve got some questions:
1 do i need to el. isolate the chips from the radiator(i`ll put 2 bridged chips on one radiator)?
2 is this a good radiator for 2 ic`s?
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Old 29th January 2006, 03:32 PM   #27
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which schematic?

I'm trying to learn about bridging. The schematic in post23 is for bridge/parallel is it not?

How many of those heatsinks will you use?
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Old 29th January 2006, 04:34 PM   #28
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i was hoping to use only 2 radiators(1 for 2 chips) but i am wayting for answer from those who already made an amp like this,btw the power supply will be +/-25v@400VA.
if you want to make an amp(whatever the configuration) i suggest that u should make it after the app note sch. or after a sch that has been tested.
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Old 29th January 2006, 05:18 PM   #29
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Quote:
Originally posted by costy_ro

I`ve got some questions:
1 do i need to el. isolate the chips from the radiator(i`ll put 2 bridged chips on one radiator)?
If you use LM3886T

Quote:
Originally posted by costy_ro
2 is this a good radiator for 2 ic`s?
If its 32mm x300 x 75mm,then it is almost the size of the heatsink that national suggest for bridged parallel or 4 chips,you should have no problems with 2 ics
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Old 29th January 2006, 05:30 PM   #30
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Quote:
Originally posted by jaudio


If you use LM3886T



someone said that it is not necessary if i`ll power them from the same transformer
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