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Old 5th May 2010, 03:25 PM   #1
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Default Help with power supply trafo

Hi all,

I need to calculate the power for a trafo substitution. Here I put the schematics from the psu:

I don't remember how to calculate what's the watts power and how much AC volts have to be the trafo. The amplifier has an output of 150W 4 ohms, and my intention is to use a toroidal trafo. Please can you help me?

Grateful!!!
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Old 5th May 2010, 04:18 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
for +-50Vc you need a dual secondary 35+35Vac, or a centre tapped 70Vac.
For a 150W amplifier you need 150VA to 300VA.

If your design can only produce 150W from +-50V there is something wrong with it.
A BJT output stage should get around 200W into 4r0 and a FET output stage a bit lower, probably around 170W into 4r0.

Maybe your design is really a 110W into 8r0 amplifier that can drive a resistor load of 4r0 but cannot drive a reactive 4ohm speaker adequately.
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Old 5th May 2010, 09:32 PM   #3
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Thank you so much Andrew,

The schematics is from a Gallien Krueger bass amp. The power supply that originaly was designed by the manufacturer probably has to be work a little at the limit because this psu was changed in the last models and the manufacturer has to do changes and modifications to support the old model I have.

The old psu was burned and the trafo was discontinued and the power supply was replaced for the schema that I added.

Here I let you the complete schematics.

Click the image to open in full size.

Thank you so much.
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Old 5th May 2010, 10:16 PM   #4
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For some reason the link don't work. Here is the good: 4shared.com - document sharing - download MBE series.pdf
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Old 6th May 2010, 10:40 AM   #5
AndrewT is offline AndrewT  Scotland
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A 2pair output stage with a triple EF. looks OK.
It is a 150W into 4r0 from +-45Vdc not 50V.
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Old 6th May 2010, 12:46 PM   #6
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Thank you again Andrew, but can you explain me a little more? I don't understand so well.
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Old 12th May 2010, 05:53 PM   #7
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It will stick 150w into 4 ohms, with 45 volt rails (after rectifying etc).

Assume 50% efficiency, you need at least 300VA, but...
I'd say than you'll need more than 300VA. When in use, it only supplies power in very short bursts. This means that, instead of something that will do 300VA continuous, you'll need more like 500VA, because that's what it will supply in those short bursts.

For example, I used a 150VA transformer for my little 30W guitar combo. Doing so means it will push 60W into 4 ohm, but it's happy supplying those high peak power requirements.

Chris

Edit - it also means that your PSU won't be the limiting factor...
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Old 12th May 2010, 08:44 PM   #8
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Quote:
Originally Posted by chris661 View Post
It will stick 150w into 4 ohms, with 45 volt rails (after rectifying etc).

Assume 50% efficiency, you need at least 300VA, but...
I'd say than you'll need more than 300VA. When in use, it only supplies power in very short bursts. This means that, instead of something that will do 300VA continuous, you'll need more like 500VA, because that's what it will supply in those short bursts.
That's a LOT of pointless overkill (and a big waste of money and weight). Short bursts of power are supplied by the capacitors, and transistor amps are far more efficient than 50% - 300W would be fine, and 225W would as well.

Try looking what's used commercially.
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Old 12th May 2010, 09:34 PM   #9
AndrewT is offline AndrewT  Scotland
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Nigel,
well said.
A 150W amplifier gives good value for money when paired with a transformer between 150VA and 300VA. Anything more is extra money for a small benefit.

However, it's going to be expensive getting a custom wound 33+33Vac transformer.
30+30Vac and 35+35Vac are easy to source.

If 35Vac is adopted them max power should be around 200W into 4ohms and the matching transformer becomes 200VA to 400VA. But only if the 2pair output stage can survive the extra voltage and current that 200W imposes.
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Old 13th May 2010, 02:03 PM   #10
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Thanks to all.

This is what I found following your advice. Toroid encapsulated, 230Vac, 2x35Vac, 300VA, 46,98. Have a very good aspect and a very good price too, I think.

http://docs-europe.origin.electrocom...6b800284bc.pdf
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