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Old 8th November 2012, 08:02 AM   #11
AndrewT is offline AndrewT  Scotland
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there are very few chipamps that can work with supply rails @ ~ +-45Vdc.
the lm3886 is limited to <= +-42Vdc absolute maximum when signal is present.

Your mains voltage can rise as high as 254Vac and the PSU will then output ~+-48Vdc. That is what your amplifier must survive !

You will have to use a discrete component amplifier, or one of the range of driver chips with a discrete output stage. Aim for 60W to 80W into 8ohms.
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Old 8th November 2012, 08:32 AM   #12
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Class D needs less heatsinking. The cheapest way could be purchasing a new class D module.
What's the size of your heatsink?
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Old 8th November 2012, 09:39 AM   #13
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TDA 7294 will work. Absolute max is 100volts, i.e. dual 50 volts
TDA 7293 is 120volts.

Gajanan Phadte

Last edited by gmphadte; 8th November 2012 at 09:43 AM.
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Old 9th November 2012, 12:33 PM   #14
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Quote:
Originally Posted by gmphadte View Post
TDA 7294 will work. Absolute max is 100volts, i.e. dual 50 volts
TDA 7293 is 120volts.

Gajanan Phadte
Hmmmm - two quality 10,000uF electrolytics will probably run to more than the cost of a new 225VA toroidal...

I've also read that the TDA 7294 is not suitable for a 4 Ohm load when bridges - is this absolutely true?
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Old 9th November 2012, 02:01 PM   #15
AndrewT is offline AndrewT  Scotland
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The 7294 will see 2r0 when driving a 4r0 test load in bridged arrangement.

Fit a 4ohms speaker and each 7294 sees the equivalent to a 2ohms speaker load.
That is much more severe than a 2r0 test load. It is worse than a 1r0 test load.
Can the 7294 drive a 1r0 test load with the supply rails you plan to use?
If it can that could be a good clue to carry on with your proposal. If it can't then stop and re-evaluate.
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Old 9th November 2012, 02:22 PM   #16
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Quote:
Originally Posted by AndrewT View Post
The 7294 will see 2r0 when driving a 4r0 test load in bridged arrangement.

Fit a 4ohms speaker and each 7294 sees the equivalent to a 2ohms speaker load.
That is much more severe than a 2r0 test load. It is worse than a 1r0 test load.
Can the 7294 drive a 1r0 test load with the supply rails you plan to use?
If it can that could be a good clue to carry on with your proposal. If it can't then stop and re-evaluate.
It is the transformer from the original subwoofer amplifier. This was rated at 175W.

However, +-45V is 90V in total and a 1R0 linear load would draw 90A!

The numbers don't add up. Please you shed some light on this for me as I don't have audio design experience?
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Old 9th November 2012, 02:27 PM   #17
AndrewT is offline AndrewT  Scotland
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No.
The +45V rail will try to push +45Apk into the 1r0 load.
The -45V rail will try to push -45Apk into the 1r0 load.

Not 90Apk.
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Old 9th November 2012, 03:11 PM   #18
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An lme498xx driving triple ouputs will do nicely though demanding one bigger heatsink.
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Old 9th November 2012, 03:26 PM   #19
Boscoe is offline Boscoe  United Kingdom
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Is this 45Vdc after the caps or 45Vac from the secondaries of the transformer?

Cheap and DIY don't always go together. I think this class d amp would be good.

http://connexelectronic.com/product_...n8eg7ckk7rhqi6

Powerful, cheap, good on supply rails and you don't have to worry about heatsinking.
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Old 9th November 2012, 07:26 PM   #20
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Is this 45Vdc after the caps or 45Vac from the secondaries of the transformer?

Cheap and DIY don't always go together. I think this class d amp would be good.

Connexelectronic

Powerful, cheap, good on supply rails and you don't have to worry about heatsinking.
It's 45VAC each side of zero - I thought I'd only lose about 0.7 of a volt per rail through a normal bridge and the whole point of the big caps was to maintain that?

Definitely not the +-62 to 68V specified for that class D amp.
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