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Old 14th December 2012, 09:53 PM   #1
domolo is offline domolo  Italy
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Default Upgrade the transformer

Hallo,
I got an old Rega Luna (same scheme of the ELEX but rated for 40watt) It has a small PSU. I've upgraded the filter caps with bigger and better ones... From the factory it came with nearly 20000uF filter capacity. At first I brought the value to 60000uF but while the dynamic improved dramatically the bass became blurry and uncontrolled. The transformer can't handle such a large capacity...
I reduce the caps to 40000uF and everything now go fine, the bass is well controlled and the dynamic is far better than it was in the original project.

Now I would like to replace the original trafo with something larger in VA. At the moment the amp is powered by a 24+24 (or 25+25 I can't remember well) that deliver 3,2A. It is rated as 40watt transformer, so I think that it should be something like 150VA or so...

For a no compromise PSU for a 50watt I calculated that I need a 300VA PSU. Can you confirm my idea?
What I have to looking at choosing the new trafo?
Any suggestion will be appreciated!

Do you think go from a 24v to a 25v (easier to find) secondary would give any problem? Clearly I must reset the idling current, but it is just like a 4% of fluctuation in the main AC value....
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Old 15th December 2012, 11:24 AM   #2
AndrewT is offline AndrewT  Scotland
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24+24 @ 3.2A is not 40W !!!!
it is 153.6VA and if 25Vac then 160VA.

A 40W+40W ClassAB amplifier will operate well with a transformer rated from 80VA to 160VA.
I usually recommend 160VA as the minimum for a small Power Amp transformer.

A 300VA transformer would normally be used to power an amplifier with a total maximum output from 150W to 300W.
Your's is only 80W.
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Last edited by AndrewT; 15th December 2012 at 11:29 AM.
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Old 15th December 2012, 11:28 AM   #3
domolo is offline domolo  Italy
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On the trasformer is rated for 40watt, I stated "so I think that it should be something like 150VA"
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Old 15th December 2012, 11:33 AM   #4
AndrewT is offline AndrewT  Scotland
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where did you read 3.2A?
Quote:
................the amp is powered by a 24+24........... that deliver 3,2A..................
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Old 15th December 2012, 11:48 AM   #5
domolo is offline domolo  Italy
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The trafo has its specifics printed on it:
40W 24V+24V @3,2A

Your assumption for 80-160VA is on a 40+40 on a 8ohm quasi-resistive load, easy to drive, I think.
While I have to drive a pair of 4Ohm speaker with a far than linear impedance.

Last edited by domolo; 15th December 2012 at 11:53 AM.
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Old 15th December 2012, 12:05 PM   #6
AndrewT is offline AndrewT  Scotland
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so it is designed for a 40W amplifier and it has a 153VA rating.
The VA rating is just about double the total maximum output power of 40W+40W.

That puts Rega's 153VA transformer selection at the upper end of the range and also meets my own (but not technically supported) 160VA recommendation as a useful minimum.

Is the Rega rated for 4ohms speakers?
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