That should be a lesson to all of us. Well me for sure...I’m guilty of stuffing JFET’s into a board without testing their Idss. Maybe your penance is desoldering yours and checking? Maybe for me someday as well.
Here is some light reading on a biasing technique that will work well for the F6 and similar.
The discussion started when I was looking for a better way to have my F6 reach thermal equilibrium (and best sound) in a reasonable amount of time. The other F6 thread has that discussion.
Dumb Biasing Mod, applicable to F6 and other Papa Amps. Possibly My Dumbest Idea Yet
The discussion started when I was looking for a better way to have my F6 reach thermal equilibrium (and best sound) in a reasonable amount of time. The other F6 thread has that discussion.
Dumb Biasing Mod, applicable to F6 and other Papa Amps. Possibly My Dumbest Idea Yet
Board is not even stuffed yet. Still making sure I understand the ramifications of +-27 volt rails (unloaded of course) before I solder anything.
For the typical* F6 build, a 3U, 400mm deep chassis would be the minimum. A 4U chassis will be better behaved, especially at higher bias currents. I'm using one in between these two, and it takes a long tine to reach a steady temperature with the relatively high bias that I'm using.
* standard PSU with 18V secondary voltage on the power transformer, about 22.5V on the rails, 1.5 Amps Iq
* standard PSU with 18V secondary voltage on the power transformer, about 22.5V on the rails, 1.5 Amps Iq
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I am somewhat of a newbie and I am getting ready to start on an F6 build. I have been buying all the parts over time and I have everything except for the case.
I looked back over everything and noticed that I got an Antek AS-4220 transformer based on a recomendation somewhere in this thread. As I recall there weren't any 18 volt transformers available at the time I ordered it.
Will the 20 volt AS-4220 fry everything?
What adjustments will I have to make?
(Since I'm a newbie I need to be pretty clear about what to do)
I have the Diyaudio complete parts kit - so I have standard parts
Thanks
I looked back over everything and noticed that I got an Antek AS-4220 transformer based on a recomendation somewhere in this thread. As I recall there weren't any 18 volt transformers available at the time I ordered it.
Will the 20 volt AS-4220 fry everything?
What adjustments will I have to make?
(Since I'm a newbie I need to be pretty clear about what to do)
I have the Diyaudio complete parts kit - so I have standard parts
Thanks
If you're worried about the higher rails and want something to do, you could add a cap multiplier to the rails and drop a couple of volts with the juma/First Watt type or a lower volt drop via a simple Darlington transistor, a K-multiplier, or the "Smooth as Butter" type - might improve the sound too!
dumb bias question
Incorporating the Dumb Bias
- 3 Led series into my rebuild. Would really appreciate if anyone can confirm which way these should be oriented in place of the zeners. My read of the schematic is arrow pointing to positive, is that right?
Thanks!
The 3 Led in series are able to squeeze into the zener location, makes install neat!
Incorporating the Dumb Bias
- 3 Led series into my rebuild. Would really appreciate if anyone can confirm which way these should be oriented in place of the zeners. My read of the schematic is arrow pointing to positive, is that right?Thanks!
Well, given that yours dialed in fine with the 10K resistor and 6V zener I went the same route and mine dialed right in also. With 500mv for the bias setting the rails fell from 27v to 25v on my amp. Looking good! Thanks to all for the information provided in this great thread.