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Old 16th December 2012, 04:36 PM   #1
Philfr is offline Philfr  France
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Default 24 volt DC chip amp

Hi all,
First, I'am writing from France and my english was not very good , sorry.
I'am looking for a chip amp working on 24 v DC (with two 12 v gel battery)..
I saw the LM1875, cause I want a minimalist schematic and no pcb.
Thank you for your comments.
Phil.
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Old 16th December 2012, 05:18 PM   #2
Mooly is offline Mooly  United Kingdom
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A couple of thoughts...

Some chips (or discrete amplifiers too) react badly when the supplies don't rise and fall together. With batteries and a switch that could happen. You might also need a speaker relay delay.

Gel batteries are great put have huge energy reserves so make sure you fuse each one directly at the terminals of the battery.
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Old 16th December 2012, 06:22 PM   #3
Philfr is offline Philfr  France
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Hi Mooly,
Yes I think add a softstart and speaker protection..
For battery, maybe a big capacitor (1 Farad) solve the problem ?
I have not found articles about battery supply for amp, except on car audio with voltage elevator.
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Old 16th December 2012, 06:26 PM   #4
Mooly is offline Mooly  United Kingdom
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Not sure the big cap is a good idea, not using switches... charge current and all that !

It should be possible to come up with a simple switched rail design perhaps using low Rds FET's (one for each rail) as the switch elements and control both these from a common on/off signal.
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Old 16th December 2012, 06:26 PM   #5
AndrewT is offline AndrewT  Scotland
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Soft start is for starting a mains transformer.
If you power by battery you don't have a mains transformer. You don't need a soft start.
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Old 16th December 2012, 06:55 PM   #6
Philfr is offline Philfr  France
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@ Mooly,
what is "low Rds FET's" ?
Thank's in advance.
@ Andrew,
indeed
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Old 17th December 2012, 03:52 PM   #7
Mooly is offline Mooly  United Kingdom
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Simply put they are FET's that have a low on resistance and so don't heat up when used as a switch. So no heatsink required when used as a switch.
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Old 17th December 2012, 04:34 PM   #8
Philfr is offline Philfr  France
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Hi Mooly,
Thank's, I see clearer..
Maybe TDA7294 was better in my case ?
The goal is to drive a subwoofer until 200 hz.
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Old 17th December 2012, 04:34 PM   #9
Mooly is offline Mooly  United Kingdom
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Something like this. R1 and R3 are the "load" or amplifier. R5 would have a single switch in series with it. Closed for on and open for off. The FET's run cold.

(The IRFP9240 isn't the most suitable but just chosen because its in the standard spice parts library)
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Old 17th December 2012, 04:37 PM   #10
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by Philfr View Post
Hi Mooly,
Thank's, I see clearer..
Maybe TDA7294 was better in my case ?
The goal is to drive a subwoofer until 200 hz.
Any power amp chip will work down to as low a frequency as you want. Will the TDA 7294 work at that voltage... I'd have to check the data sheets.
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