The diyAudio Firstwatt F6

Jump to Latest
#941
I've built the SE BA1 amplifier that is similar to a F4 as both being power buffer amps and the F6. I built the front end of my B1 with the BA3 front end in a separate enclosure as a preamp so the preamp is the same for both the amps. It is as though a curtain has been lifted from the BA1 after listening to the F6. More detail and soundstage with the F6 to my ears. I first commented on the F6 as being a bit bright but my opinion has since changed After experimenting with some changes in the crossovers of my speakers. I think now the F6 showed a problem with my original crossover that I was not hearing with the other 2 Fw amps I have built. This happens all the time when one gets a better piece for their system and sometimes leads one to believe the problem is with the latest purchase but in fact other parts of your system need upgrading as well. That was my case. All of my Pass FW amps are amazing but after many hours listening to the F6 it is by far the best I have listened with. If you have already built a FW amp and have the case, transformer and power supply the cost will be minimal for what you will have when finished, to swap out the boards for the F6 boards.

I would like to get the thought of members on using the LSJ74 C grade jfets with some resistance on the source to bring the idss down to match a 2SK170 B grade jfet that is available from some reliable sources. I believe I have read on this forum of other members doing this or if the idss of the LSJ74C is not too high to parallel 2 2SK170's to match the idss of the LSJ74. Is this viable? Right now the jfets are the sticking part to building this amp.
 
#943
Hi to all, I'm slowly building my F6. I bought 10 pieces of IRFP240 and I measured them; the best matched couple are 4.05-4.05 and 4.22-4.23. Are them okay for this amp? Or the 4.05 is too low? Thanks


Inviato dal mio iPhone utilizzando Tapatalk
 
#947
I finally got around to stuffing my F6 boards today. Does anyone have any pointers for adjusting the 5K pots prior to soldering them in. I seem to remember some recommendations for the Aleph J pots. Originally I was going to put the F6 boards in my Aleph J chassis, but I couldn't bear taking it apart because it sounds so nice, so now I'm going to build a completely separate amp. All parts are ordered and on the way.

PJN
 
#948
Leave them in the middle and place a meter across a source resistor during startup. If the bias is to high, adjust bias pot until a lower value is reached. Once under control, then set bias and adjust offset.

Remember that because of the caps, the adjustments happen in slow-motion. Go slow, be patient, and it will work well. 🙂
 
#954
ammel68 said:
What should the voltage be across the source resistors?

Thanks...
Click to expand...

Well you just use simple ohm's law. E=I*R

In the original article NP mentions a bias of 1.55A, so with that you can figure it out. You can bias it higher than that, it all depends on how much heat your chassis can dissipate. You should be able to hold your hand on the heat sink after it's been operating for a few hours (50-55C max).
 
#955
ammel68 said:
What should the voltage be across the source resistors?

Thanks...
Click to expand...

To start with I set mine at the .47 source resistor for .5 volts. I read somewhere on this forum to start with .5 volts with .47 resistor and .55 volts across .56 resistor so that is what I did. If Nelson did indeed say 1.55 amps then you would want .72 volts across .47 source resistor. I left mine at .5 volts and it sounds great and runs very cool. I had intentions of increasing the bias later but have not done so yet. It may be like the Alephs and would sound better with more bias.
 
#957
wdecho said:
To start with I set mine at the .47 source resistor for .5 volts. I read somewhere on this forum to start with .5 volts with .47 resistor and .55 volts across .56 resistor so that is what I did. If Nelson did indeed say 1.55 amps then you would want .72 volts across .47 source resistor. I left mine at .5 volts and it sounds great and runs very cool. I had intentions of increasing the bias later but have not done so yet. It may be like the Alephs and would sound better with more bias.
Click to expand...

Interesting...I can always start at .5V and see if indeed higher bias sounds better, or not.

Thanks...
 
You must log in or register to reply here.
Top Bottom