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Old 22nd November 2004, 06:46 AM   #1
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Default Optionally boosted LM3886

Hello,

I would like to have some comments from these schematics in made some day. Mostly because of the possible mistakes these may have . And i need some value suggestions to R1-R6 in the booster sch, and other values too, do they look good? And the most important, do these work at all .
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Old 22nd November 2004, 10:08 AM   #2
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In my opinion you should:

1) Ditch the .33 supply series resistors. They increase the impedance and do not really work as filters. Perhaps you could try an RC filter, putting the resistor BEFORE the cap, but be ready to listen to it with and without the resistor.

2) If you are going to boost current, use a smaller signal IC instead of the LM3886, which has limited bandwidth and other limitations.


Carlos
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Old 22nd November 2004, 03:02 PM   #3
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carlmart you where too quick. I ment to post the booster circuit option straight away, but i'm new here, so i'm still under moderation and my posts don't apear at once into forum.

carlmart the .33 resistors was ment to be shorted if not use the booster option.

Here is the booster circuit:
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Old 22nd November 2004, 04:34 PM   #4
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Yes, I am quick in the draw. Just like Shane...

What I said about the series resistors after the regulator still goes, even more if the IC is a small signal type. Keeping the impedance low is adamant.

So cut the resistors and send it straight through.


Carlos (The fast hand)
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Old 22nd November 2004, 07:18 PM   #5
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Yes, .33 resistors should be shorted if using only 3886, but when you want to drive under 4ohm loads you could connect the booster transistors, then you have to have those resistors to drive transistors.
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Old 22nd November 2004, 07:19 PM   #6
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Do these layouts look proper to use?
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Old 22nd November 2004, 10:04 PM   #7
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Quote:
Originally posted by savenger
Yes, .33 resistors should be shorted if using only 3886, but when you want to drive under 4ohm loads you could connect the booster transistors, then you have to have those resistors to drive transistors.
Why? Where did you get that information?


Carlos
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Old 23rd November 2004, 12:35 AM   #8
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Don't know if we are arguing even the same thing, but here is explanation picture. How you suppose to drive these BD transistors if there is no base emitter resistors, if they are shorted, there is no need for transistors, but my seek for the holy grail was those extra power transistors for even say 2ohm loads, or bridging 2 of these to 400w sub amp.

Edit: I choose LM3886 because no need to biasing the transistors and to keep the circuit simple.
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Old 23rd November 2004, 12:54 PM   #9
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Quote:
Originally posted by savenger
Don't know if we are arguing even the same thing, but here is explanation picture. How you suppose to drive these BD transistors if there is no base emitter resistors, if they are shorted, there is no need for transistors, but my seek for the holy grail was those extra power transistors for even say 2ohm loads, or bridging 2 of these to 400w sub amp.

Edit: I choose LM3886 because no need to biasing the transistors and to keep the circuit simple.
To start with you should use separate connections to the supply for powering the chip and biasing your transistors. They should be separated, as there's no feedback acting there to justify not to be.

Second you should use separate supplies for the chip and the output. The first stage should be quite well regulated, and voltage could probably be smaller even.

Third you are using CFP output stages, which may not be as stable as EF stages.

Why don't do something like this:

http://www.zero-distortion.com/tests...oosting_04.htm


Carlos
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Old 23rd November 2004, 07:52 PM   #10
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Now i see where you were getting to, but that is almoust a hole another circuit. I've seen that circuit before, but i forgot it's existence, thanks for remainding me of that Carlos. Maybe i start looking into that.

Any other good and simple circuit suggestions? Transformer is 2x26VAC and will be driving subwoofer (hard impedance, may go as low as 3ohms). Little too hard for one LM3886, two in parallel would to the trick, but in my opinion it needs dc offset and gain matching.
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