A guide to building the Pass F4 amplifier

[Giovanni1968]

Hello, how you guys doing?

I had to dismantle my stereo setup and left all as it is, the BA3 and F4 working but somehow feeling not complete. With enough time to think about it the idea is to make the BA3 balanced (there already is a second PSU), I got a board and will only need to populate it and then add balanced outputs.

On the other hand my doubt is about how to make mono-bridge the F4 and make it balanced so that I can then have another one done and pull out 50W each.

My F4 was built dual mono so it has two transformers, if I recall correct 230VA each, if the project sees the light I'd want both F4 to be exactly the same, what is your suggestion, provided it is feasible, make the second F4 as the first, in dual mono fashion with twin transformers or swap the two for one and make things easier (but more expensive by swapping)?

Thank you

 

[Zen Mod]

what's wrong with reading F4 Article?

everything is explained

 

[triode_al]

Yes, I am using a fairly large CRC supply with (8) 33,000 uF caps. I’ve also used CLC. Pretty beefy for a brute force analog supply.

I put a scope differentially across the jfet drains and indeed see the power supply ripple (~4mv) at idle as expected. DC voltage about 27 from drain to drain. All as expected. With music playing the power supply noise signal becomes mixed with audio signal as expected with the passive mixer of bias and bootstrap signals. Ideally for lowest distortion and noise from a source follower circuit I’d like to see constant voltage gate to drain. I tried adding a 100 uF cap jfet drain to drain and it worked well. The drain to drain DC voltage stayed fairly constant even with the amp driven hard. I could no longer see any ripple voltage drain to drain with my oscilloscope. I don’t have any great tools to measure PSRR or distortion to verify any signal improvements, but from a jfet power supply noise perspective this looks like an improvement. Super easy to implement without removing boards, just tack solder cap from top of R6 to top of R7.

As a side note I also am using Exicon laterals with no source resistors and a LMV431 in place of the TL431 to enable lower bias voltage adjustment range for the laterals. If there is interest, I’ll post schematic and photos. Not sure if it belongs here or the F4 forum for comments.

I am working on an F4 + thinking of Exicon-double die ECW's ( 'nice' propietary pin layout - argh) + SMPS 10A + 4700µF + smallR + 56000µF +5mH + 2x56000µF = too crazy, too big, won't fit even in the box, the lid would not fit and no place for the driver. And I don't want to think out of the box. Instead. I'll have to go to the weight watchers.

• How do the laterals behave? How many do you have? How easy to set bias with the TL431? (difficult @ +400mV bias for 500mA/each side; I expect even a red LED with a resistor array works like the Dutch audiophile teams showed in the eighties)

I'm sure that 6L6 thinks: "however, after building pretty much all of the firstwatt amps, I know that they sound best with the recommended components"

 

[thimios]

Please help me to understand.
1)Can I use the BA3 to drive the F4 for full output?
2)Is it possible to eliminate the fets from the F4 input and use those for the BA3?
Thanks in advance!

 

[Zen Mod]

1. yes; BA3 amp is practically shown FE mated with OS functionally identical to F4; well, by memory there are 2 iterations of BA3 OS, and I'm speaking about complementary follower stage

2. yes, search for "crippled F4"

 

[thimios]

Thanks Zen Mod!

 

[thimios]

I hope that I found it!
Crippled F4.

 

[Zen Mod]

if using original pcbs/circuit ....

 

[thimios]

Thanks again!

 

[thimios]

Salas DCG3+Elliott P37+ PassF4