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Hikari1 14th August 2013 10:28 PM

How hot can I bias my F5?
I've built a F5 using the diyaudio 5U case (BIG heatsinks).

It's seen a couple different mods...went from Fairchild Mosfets to IRF's, rewired the internals, power cords, etc. All these things brought subtle changes.

So I decide to go from 1.3A (.6v) bias to 1.5A (.7v) bias.

WOW. That was a big change. Amp has more punch, drive and basically more "wow". Soundstage is bigger and more forward. By far a bigger difference than any of the above mentioned changes.

Bias seems to have a dramatic effect on my F5.

How far can I push it? 1.7A (.8v)? My heatsinks are 45C now measured with a digital meat thermometer. I am not crazy enough to touch the Mosfet with the metal meat thermometer so I don't know the Mosfets temp. I am using K-10 silpads.

Is 1.7A safe? Even if the heatsinks can take it that doesn't mean the source resistors can. Anybody experimenting with higher bias on the F5?

Nelson Pass 14th August 2013 10:43 PM

Technically, the junctions of the devices are rated at 150 deg C. If you are
running as high as 50 watts per device, then you will want to measure well less
than 100 deg C on the case (you can use a cheap Radio Shack IR thermometer).

Don't expect them to have a long life span at those figures, but I have seen
reasonable reliability at 25 deg less junction temp.


Hikari1 15th August 2013 01:50 AM

Thanks for the answer NP. I'll measure the mosfet temp and push the bias some more.

Thank you even more for the amp, it's a goodie. But your biggest gift is what I have learned while building the amp. That's priceless.

AndrewT 15th August 2013 09:27 AM

Well done to you Hikari.
There are not many that learn as they progress through their DIY.

Magura 17th August 2013 08:03 PM

I've played around with loading IRF240/IRF9240 hard some years ago.

You can easily run them at 50W, but as Nelson wrote, the device should be kept at a reasonable temperature.
As I recall, I had mine around 85C case temp, and the heat sink was like 70C or so.
That worked for approx. 1000hrs., till the amp was taken apart, to make room for a new amp.
What I did learn also in that process, is that 50W+ is possible as well, if the heat sinks are big enough, and you are able to mount the device directly on the heat sink, with just thermal compound, and no insulator.
In that very same process, I too found the limit, which caused some rather interesting explosion.
The reason I mention this, is that at those power levels, an IRFP240 simply goes BANG! when it gives up the ghost.
Take that into account when working on the amp (safety glasses), and make sure that the box is closed when the amp is in use, and the vent holes are small enough to keep shrapnel in.
When they blow under that sort of load, all that's left on the heat sink, is the metal tab. :D

Magura :)

boatbuilder 18th August 2013 09:31 AM

that answers a question i was pondering on while building my aleph 5.
what will happen if i blow the irf240's.
i am happy nothing went wrong.
still have to play around with them to get them to run a bit warmer, still just at 10 degrees C above ambient. no idea of the output power.

AndrewT 18th August 2013 09:36 AM

Use a SOAR modeling programme and "see" what difference to device stress is created by operating at the two different temperatures.

Magura 18th August 2013 01:10 PM


Originally Posted by boatbuilder (
still have to play around with them to get them to run a bit warmer, still just at 10 degrees C above ambient. no idea of the output power.

Then you got a large margin to play with.

Magura :)

fpitas 18th August 2013 02:48 PM

If it were me, I'd add some small CPU-sized fans mounted on the ends of the heatsinks, and spin them very slow so the noise is minimal. You would still have the junction-to-case and case-to-heatsink rises to reckon with, but at least the heatsink would stay cooler.

Magura 18th August 2013 03:05 PM

In my experience, active cooling for stuff that needs to be in the listening room, is something invented by the devil in anger.
All of a sudden, things gets expensive and complicated real quick, if it has to be safe, silent, and reliable.
Especially if you run your fets on the edge.
I've seen a few do it, and none that worked in the long haul.

Magura :)

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