diyAudio F5 Build Guide

[Dennis Hui]

I too suggest keeping to approx 24V rails as designed. A good 300VA
transformer will do nicely but you can obviously go higher. (The commercial
F5 used a 300VA 2x18VAC Plitron.)

At 1.3A bias, each channel should draw less than 70W.

Good heatsinking, proper mounting/interface of the transistors and
a reasonable dissipation for each transistor should ensure a long service life.

[goodguys]

Good Point.

Is it 70 watts for these particular boards? I thought it was 125. Even more confused now.
My suggestion is if the power consumption is 70 watt per board then a 200 va should be enough. Why should i spend extra money on a 400va, difficult to see any sonic benefits.

[6L6]

Transformer power in VA should be 6X the output power of the amplifier in W.

That’s the rule of thumb that Nelson uses. As I said, everybody likes to oversize the PSU because it seems like it’s a clear benefit. (It likely gains nothing, but that’s not going to stop anybody... )

BUT, you not want to go much smaller than recommended because the transformer will be in a steady-state load and that’s what needs the extra capacity.

Tl;dr, 300VA is ideal. Smaller isn’t recommended, bigger makes the amplifier heavier.

[goodguys]

Sorry, not trying to be a nit picker, probably am being stupid, isn't 25 x 6 = 150


Dennis Hui, are you saying you modded the amp to get the power consumption down to 70 watts

[astromo]

Originally Posted by goodguys

Sorry, not trying to be a nit picker, probably am being stupid, isn't 25 x 6 = 150

Yep .. 150 W per channel .. multiply by 2 = 300 VA

[goodguys]

Oh right sorry my bad i thought you meant 300va per channel


So why is dennis hui saying less than 70 watts per channel

[Dennis Hui]

The power draw is mostly coming from the output stage.
Standard bias on the output stage is 1.3A, running on +/-24V rails.
So that's about 2 x 1.3 * 24 ~63W for the output stage per channel.

[goodguys]

Thanks Dennis. With the fullest of reeespect to yourself, how is such a high efficiency possible. I thought class-a's were 80% heat and 20% music.
Is it something about the design of this amp

[JeffYoung]

I think your 20% efficiency is a bit off. I've always heard class A falls in the 25% - 50% range.

Something like the Aleph or J2 might do even better as they have active current sources which follow the load to some extent.

Beyond that, is efficiency somewhat dependant on how close to the rails the amp can swing the output? Is the F5 reasonable good at this?