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Old 17th February 2012, 12:15 AM   #1
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Default F5 or F5turbo?

Hello guys,

I have a pair of rega rs3 speakers which are 6ohm, 89db sensitive speakers currently by powered by a rega brio 3r amplifier. I am looking to build a better amp and would like to go with a class a design.

I am having some difficulty deciding the amount of power I need for these, the brio is 50w into 8ohm/73 into 4ohm for comparison so it should be something like 60w into the 6ohm.

How much power could I expect from an f5 into 6ohm? Rega says these speakers can work with as little as 30w of power but I'm afraid that would be somewhat lacking.

I am more inclined for the turbo version as it would have some more headroom.

It is my understanding that the turbo version would easily output 50w into 6ohm with appropriate Transformer and biasing, am I correct? I have been reading and I would go with a cascoded input stage, 34v/12A secondaries, 600VA transformer, 4 MOSFET per channel, so a mix of v2 and v3 versions.

Please let me know your opinion.

Manuel
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Old 17th February 2012, 12:23 AM   #2
ra7 is offline ra7  United States
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You should definitely build an F5, but first take this test to determine how much power you need.

You can choose your power level depending on how loud you listen.
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Old 17th February 2012, 12:48 AM   #3
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First,sorry for my english,is so poor!: (
However,i believe that 50w into 6 ohms is really loud,but your loudspeaker dont have sensitivity.What inches are?
Maybe,if you change the speaker,have better results!.
50watt is a good power.Good luck
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Old 17th February 2012, 01:15 AM   #4
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34v transfo way too high for two pairs.
2 x 25v transformer / 33vrails / twin pairs delivers about 75w into 6ohms. Enuff?
Be aware of heatsink size if you choose more.

Last edited by bobodioulasso; 17th February 2012 at 01:34 AM.
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Old 17th February 2012, 01:41 AM   #5
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Quote:
Originally Posted by bobodioulasso View Post
34v transfo way too high for two pairs.
2 x 25v transformer / 33vrails / twin pairs delivers about 75w into 6ohms. Enuff?
Be aware of heatsink size if you choose more.
34v is the secondary voltage
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Old 17th February 2012, 01:47 AM   #6
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That is about 45v rails.
When you say 4 mosfets, is that two pairs or 4 pairs?

Last edited by bobodioulasso; 17th February 2012 at 01:50 AM.
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Old 17th February 2012, 02:20 AM   #7
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2pairs.
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Old 17th February 2012, 08:10 AM   #8
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So, I confirm.
2x 25v Ac transformer gives approx 2x33v Dc (after rectification +smoothing)
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Old 17th February 2012, 09:00 AM   #9
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Tailoring to 6 ohms.

Even a 2 x 22v AC transformer would give about 50w / 6 ohms.
This allowing a deeper bias without excessive dissipation / heatsink size.

That would give about 29v rails.

For 50w full class A you have to bias at 1.2 amp per pair.

dissip per device:1.2 A x 29 v = 34.8 watts.( little bit more than standard F5)
So each channel have to dissipate about 140w.
Which is Already considerable.( about twice standard F5)

If that is too big heatsinks, you could consider biasing less and leave class A sooner, which is not really a problem.

Many dyers, when they come to class A for the first time, do not realise the heatsink size needed.
A 50w class A amplifier is huge and heavy. (and expensive)

Last edited by bobodioulasso; 17th February 2012 at 09:19 AM.
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Old 17th February 2012, 09:38 AM   #10
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Sorry, my math are so bad.
That is 1A and not 1.2A bias
Formula is P = (2xIbias)^ x Rload/2 When Ibias is total bias.
dissip about 30w /device
total 120w

22v transfo is OK

Last edited by bobodioulasso; 17th February 2012 at 09:59 AM.
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