F5 Turbo Builders Thread

oh. i see there is been some layout changes.
terminal blocks changed.
is C1 and C2 correct on the output boards? i can not tell from the pcb pic in the store.
also. i can not read the markings on the sec pair of J-fets on the front end board.
 

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Source resistors issue with 2SJ/2SK

Hi all,

getting closer and closer to starting the build of the F5T as I have almost all parts on the desk.

Unfortunately I purchased a parts kit organized buy a forum member, buzzforb not to name him (excellent kit btw) without checking the value of source resistors needed for my specific build.

Thing is I am building it with SJ/SK and remember having read the need of 0.25 Ohm instead of 0.5 Ohm (Walter W. as well as EUVL, iirc).

As I am getting tired of sourcing parts I was wondering what drawbacks I should expect when using the SJ/SK outputs with 0.5 Ohm source resistance.

Any help is highly appreciated.

Cheers,

Max
 
I was wondering what drawbacks I should expect when using the SJ/SK outputs with 0.5 Ohm source resistance.

Any help is highly appreciated.

Cheers,

Max

None at all.

The value of the source resistors simply determines the bias at which the fets will operate. In an ideal world, you would have no source resistors and the diodes would be unnecesasary. Unfortunately, fets without Rs tend to be unstable with nothing to temper their response. This is the main purpose of the resistors.As a side note, they do help linearize the response with a bit of local feedback. What you give up with source resistors is gain. They eat part of it. The use if diodes is a clever trick to get some of this back. As they slowly turn on, they act as a bypass for current. This increase in current results in an increase in gain. The increase in gain means more feedback and as a result, better performance in terms of distortion. Also with the increase in current, you get a turbocharged output, capable of delievering more current to the load. Kinda of a win/win situtation. Really wonderful amp for low Z loads, which is exactly what it was developed for.

Your .5r worth of resistance means you bias will be between .8 and .9a fet. Lowering this simply to .25 would simply mean a doubling in bias per fet, nothing else. I personally think that is too much heat per fet, but i am a rookie in these things. Euvl runs his at a higher bias in the f5x, but he give some of the heat back in the form of lower rail voltages. He gains this voltage back in the form of a bridged amp.

Audio is about tradeoffs.
 
Higher rail voltages tend to improve distortion as it increases Yfs/gain. I do not know the sweet spot of these fets in that regard. From what little i know(second hand knowledge), Nelson uses up to 32v rails in his big amps, but then again, they are bridged, giving higher swing capability into the load. It is easy enough to try. Give it a shot.