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Old 23rd April 2008, 06:17 AM   #1
zdr is offline zdr  Belgium
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Default LM4780 on 40V rails

I am looking for some experiences with 4780 running on +-40V (2x30VAC) toroid. I am aware of the datasheet supply voltage limits, just could use some real life experience. The idea is to get close to 100W RMS out regardless of the load, in parallel config.
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Old 23rd April 2008, 06:48 AM   #2
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If you can choose, choose 2 x 24-25VAC and max 2 x 28 VAC. You won't get many dB's extra risking the IC. 2 x 30VAC will produce 44-45 VDC and the marginal is zero. the difference between 50 W and 100 W is only 3 dB which is not much. If 50 W is too little you must go to 150-200 W at least in order to perceive any difference.
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Old 23rd April 2008, 06:56 AM   #3
zdr is offline zdr  Belgium
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True, but I don't think I will get as much as 44-45VDC with 30VAC. I am running 25VAC now and getting 34,4VDC out. Assuming the same losses and filtering setup with 30VAC, I should get around 41VDC.
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Old 23rd April 2008, 07:46 AM   #4
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If if put my professional hat on I'd say: 2 x 24-25 VAC if you want to use LM4780. If you want substantially more power, look for a discrete power amp like Lynx or paX.
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Old 23rd April 2008, 08:37 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
you are getting 34.4Vdc from 25Vac.
What is the mains input voltage?
What is the mains tolerance?
What is the ripple on the PSU?
What is the maximum output voltage taking account of ripple?
What is the range of voltages from your existing PSU for the full range of input voltages and output loadings?

Now, redo the calculations for a 30Vac transformer but include regulation and tolerance as well as loading.
In the UK using 5% regulation and 6% tolerance I reckon the maximum voltage from 30Vac is ~46.7Vdc. If the transformer is specified as 30Vac from 230Vac then the output is even higher.
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Old 23rd April 2008, 08:55 AM   #6
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Good point Andrew. 24-25 VAC will create no worries. 30 VAC is unsuitable according to me.
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Old 23rd April 2008, 09:01 AM   #7
AndrewT is offline AndrewT  Scotland
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Hi Peranders,
If one cannot, or will not, follow all those stages in checking design output voltages of a simple PSU then the designer is incompetent.
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Old 23rd April 2008, 10:38 AM   #8
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Incompetent, maybe so but this is DIY, a different thing. Some people don't want to listen to facts.
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Old 23rd April 2008, 12:00 PM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by peranders
Incompetent, maybe so but this is DIY, a different thing. Some people don't want to listen to facts.
You're absolutely right.
Incompetent DIY is = Incompetent DIY and that is completely different to DIY.
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Old 23rd April 2008, 12:53 PM   #10
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I think we must create a sticky thread called:

"2 x 25V toroidal for 8 ohm loads
2 x 20V toroidal for 4 ohm loads (2 x 18V is more common)
You could get by with 2 x 22V for both loads"


Oh yes.

and,

"50 watts is actually MUCH louder than you think"



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