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Old 22nd January 2007, 07:14 PM   #1
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Default Gainclone for subwoofer, need help

Hello,

It’s my first word in English on a forum, so please forgive my poor vocabulary…

I read a lot of post but I can’t find the solution of my problem :
I would like to build an amp with the lm3886 chip for my subwoofer.
I need about 200 to 250 watts on 4 ohms.
My questions :
- is it possible ?
- how many chip 1, 2, 3 or 4 (I think 2 or 4), parallel or bridge ?
- where can I find a very simple design of a parallel or bridge gainclone ?

Thank you very much,

À bientôt,

Stephan
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Old 22nd January 2007, 07:34 PM   #2
sek is offline sek  Germany
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Welcome, Stephan,

please have a look into National Semiconductor's Application note AN-1192. The manufacturers themselves describe such a project in detail.

You can find a lot of discussion and project material on this forum just by searching for threads and posts with "AN-1192" in it.

Cheers,
Sebastian.
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Old 24th January 2007, 12:47 AM   #3
Leolabs is offline Leolabs  Malaysia
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or "BPA".
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Old 24th January 2007, 04:57 PM   #4
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Thank you very much,

i read the documents and the topics, so i conclue it's possible to have 200 watts on 4 ohms with a parallel/bridge BPA-200 composed with 4 LM 4886.

So three questions again :

1- there is two schematics of BPA-200, a detailed (7.2.1) and a basic (7.2.4), which is better for an amp for subwoofer ?

2- I don't know what kind of transformer is necessary, i read a 385 VA toroidal transformer, but the voltage must be 42 V before or after the power supply circuit ?

3- Is it possible to use the same transformer for the power supply circuit of the LF411ACN and LF412ACN component ?

Thank you very much
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Old 24th January 2007, 10:42 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
building a bridged 200W into 4ohm load is the same as building two amps each capable of 100W into 2ohm loading.

A pair of parallel chipamps with the correct supply voltage can meet this requirement i.e. 50W into 4ohm // 50Winto 4ohm.

But there is a problem.
One of the criteria to make an effective 4ohm capable amplifier is the ability to drive a resistive load of about half the target load impedance.
Effectively this is the same as building a pair of 90W into 2r0 amps connected in parallel to give 180W into 1r0. The only saving is that this does not need to be continuous, so it's not a heatsink limitation but it is an SOA limitation.

The proponents of BPA do not consider this as a design criteria and whereas a discrete amp designer would ensure that something along these lines was possible for reliability the chipamp designers seem to ignore this.

The data for designing into 8ohm and 4r is available in the datasheets. But the data for designing into 4ohm and 2r is not sufficiently detailed.

You have to decide if that is the way to go.
Personally, I would go discrete and design the 200W properly, it only needs one PSU whereas your BPA will require four PSUs, that's some expense alone.

If ICs really appeal then look at lm4702 with a 2pair + driver output stage running on +-52Vdc supply rails. That is a fairly easy way to 200W and is detailed in the datasheets.
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Old 25th January 2007, 01:37 AM   #6
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also if your sub uses 2 drivers you divide the load between 2 smaller amps.
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