F5 power amplifier

[AndrewT]

the F5x operates on +-16Vdc

[jacco vermeulen]

Originally Posted by promitheus

smallest Voltage without loosing quality?

The JFETs require 6 to 7 volts minimum at the drain, so at least 10-11 Volt rails.

[formula 22]

Hi all!
uh ... how long issue?!

[promitheus]

thanks for the information.

I was thinking of making a smaller amplifier to test and for low volume music.

The normal F5 can give 50W peaks and is 25W normal power class A and needs a 300VA transformer.

A 2x12V 100-150VA transformer should give 16V rails.
This should give 9W normal power with 18W peak capability.
I think the correct bias would be 0,75A.
Heat dissipation per channel is 24Watts so the needed heatsink would be 0,84°C/W for 20°C difference.

I hope my thoughts are correct.

[jameshillj]

Are you the same "Promitheus" that has the "promitheus diy" webpage ("mitglied")?

[promitheus]

yes I am but I didn´t know that the website still worked.
I haven´t changed it for like a long time.

[jacco vermeulen]

Originally Posted by promitheus

Heat dissipation per channel is 24Watts

Not a half bad idea actually, for 12W dissipation each, one could even use the TO-220 version of the 19N20C/12P20, an FQP pair does ~€2.25 at DK.

[jameshillj]

Ah, well how about that!

I remember finding your article about "tuning transformers" some years ago and have been using the technique ever since with remarkable results, and I wanted to thank you for that "little gem" - thank you.

Pity the website is plagued by that irritataing advertising.

[Rixsta]

Originally Posted by promitheus

yes I am but I didn´t know that the website still worked.
I haven´t changed it for like a long time.

Sites great! is worth updating!!!

[uvellani]

I am having a problem on one of the channels. This was working fine for over a year when all of a sudden one channel developed this problem. I changed the adjustment pots based on suggestion from this forum, and it worked for a few days of extended testing and listening. But now it's doing it again. The bias voltage on the output resistors dropped from .575 to about .400 at first but now it is all over the place. The other channel is rock-steady.

Do you think the thermistors themselves could be a problem? I know, I know. I could change them and see. But unfortunately, due to the beauty of my chassis layout, I have to take out the power supply and the big caps out first to get at the amp board. So iterative troubleshooting is a bit of a pain. Some pointers would be helpful.

Thanks
UV