Power supply options.. bigger trafo with less caps or medium trafo with heavy caps? - diyAudio
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Old 22nd October 2007, 12:08 AM   #1
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Default Power supply options.. bigger trafo with less caps or medium trafo with heavy caps?

Dear friends,
I want to build a 3 way active system which needs arround 8 chips per channel as two LM3886 for tweeter and 2 for mids and 4 for 10 inch woofer....

But the complication is its costing alot for the toroidal trafo to get 800VA since its expensive. I want to opt for an alternate option that is..

using a 200VA but using 60,000uf of caps per rail... which will cost me less...

I believe the voltage will be constant but we need more current so I want to use the capacitors as reserved power for the chip amps.. but no idea how it works... In theory it works fine but in practice Im a bit doubtful.... can anybody clarify my doubt please....

thanks,
Ken
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Old 22nd October 2007, 12:24 AM   #2
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200va will not be enough. I used 8 lm3886 and a 330va transformer with 68,000 per rail, the voltage drop like a rock! Large capacitance is good for short burst but nothing continuous. You will have to increase VA
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Old 22nd October 2007, 01:38 AM   #3
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hi jaudio thanks for the reply... but i have a doubt that if I use such heavy trafo then do i need to use softstart circuit?

I believe even Jeff rowland concentra doesn`t uses such soft start ( slow start) ... do i need?
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Old 22nd October 2007, 08:06 AM   #4
Nuuk is offline Nuuk  United Kingdom
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Why not use dual mono PSUs and switch them on and off separately? Mono PSUs will give you better sound as well.

For a three-way system, I would be looking to use large caps for the woofers, but keeping to the 1000-2200 uF per rail for the mids and tweeters!

BTW, here is some food for thought!
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Old 22nd October 2007, 08:22 AM   #5
Nordic is offline Nordic  South Africa
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Re slow start, On that chip all you need is a resistor and capacitor on the mute pin, nothing fancy... look at the datasheet for instructions. It is not realy a slow start but a delayed de-mute.

I get fairly stable voltages from my 250VA IE core transformer, used for 2 channels. It was even easier to not have ground loops than my attempts with 2 transformers, with only 10000uf per rail.
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Old 22nd October 2007, 02:51 PM   #6
sangram is offline sangram  India
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I use 220 VA for 2 channels, and find it does fine with 10,000uF per rail - almost exactly Nordic's setup, down to the fact that my transformers are EI. So no soft-start issues.

If you are in Mumbai or Calcutta send me an email and I'll give you a contact for transformer manufacturers who'll wind up some good stuff for you. I get my 22V x 2, 5 amp and 10 amp transformers wound locally with inter-winding shields for not too much money.

I prefer large transformers, FWIW.

I arrived at a decent compromise with 440VA transformers in dual-mono for 2 LM4780 (4 channels), and 4700+1500 uF per rail.

All of them EI.

If you want to stay with toroid, I doubt you'll need a soft start at that VA unless you're loading up the secondary with lots of capacitance - and then you've to worry about the rectifiers too.

The soft-start will also help you prevent frequent primary fuse blowouts if you size that fuse small.
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Old 22nd October 2007, 02:51 PM   #7
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I made a mistake. It was 2 68,000uf per rail. It was for a bpa with 8 chip per channel not for a set up like yours. I still think 200va wont be enough.

Quote:
Originally posted by rhythmdiy
hi jaudio thanks for the reply... but i have a doubt that if I use such heavy trafo then do i need to use softstart circuit?

I believe even Jeff rowland concentra doesn`t uses such soft start ( slow start) ... do i need?
I used an iron core transformer but with toroid transformers,you will need a soft start.


Quote:
Originally posted by Nordic
Re slow start, On that chip all you need is a resistor and capacitor on the mute pin, nothing fancy... look at the datasheet for instructions. It is not realy a slow start but a delayed de-mute.

I get fairly stable voltages from my 250VA IE core transformer, used for 2 channels. It was even easier to not have ground loops than my attempts with 2 transformers, with only 10000uf per rail.
How many chip per channel?
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Old 22nd October 2007, 05:55 PM   #8
Nordic is offline Nordic  South Africa
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I have to say sorry, but I didn't see he wanted to use so many chips off the transformer... Certainly he would need much more than 250VA for that...
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Old 22nd October 2007, 11:39 PM   #9
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I think what I would do is:
Design the amplifier enclosure for the larger transformer.
But get the smaller transformer for now.
Use the preferred capacitors for the final version.
It will take months to get the crossovers tuned.
This way you have an operating system.
At a later date get the large transformer.
And use the small transformer in another project.
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Old 23rd October 2007, 12:30 AM   #10
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You're missing the point of the filter caps. They are there to provide power to the amp when the voltage from the transformer drops low enough for the rectifiers to turn off. The transformer should be spec'd to provide adequate power to the load when the voltage is at its peaks. Adequate power means big enough to power the amp LOUDLY, AND charge the filter caps so the amp can keep working when the voltage drops in about 1/120 sec. All the capacitance in the world won't help you if you use an inadequate transformer because the caps won't get fully charged.

Likewise, the caps have to maintain adequate voltage to keep the amp from clipping due to supply voltage drop and the filter caps are discharged. Forget about PSRR specs. That is a small signal number. The misunderstanding of that is what leads some to the silliness of using only 500-1000 uF caps in their gainclown power supplies. You have to size the caps based on the load and power line frequency.

If you aren't up to the calculations, just use a 20k uF cap per rail per amp chip. That's adequate for at least 100W out. It's a good idea to bypass those big caps with some smaller caps.

Use a big transformer AND big caps.

I_F
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