F4 power amplifier

Vix said:


Someone on diyaudio once said "Stop pushing my Woo-woo button!"



Push mine, instead...
I have a nearly infinite capacity for 'woo.'

This sort of circuit would work well for headphones, but you could use something like IRF610/IRF9610s for the outputs and lower the bias. Smaller heatsinks, smaller power supply, as well.
Bypassing the input buffer stage will work, but it's hardly going to be a groundbreaking circuit, as it will be pretty nearly the Zen follower circuit. At least this way Nelson is able to publish specs for the circuit. If it did not have the buffer, the performance would be highly variable, depending on the Zout, current capabilities, etc. of the source. Being DIY, we don't really have to worry about such things. We have more control over those variables than the average consumer.
Patrick,
If I remember correctly, the 2SK170/2SJ74 do not have the same voltage ratings, which always struck me as odd, given that they're marketed as complements. At any rate, whichever is lower will be the weak link in the chain. Everything else will be tethered to that one part. I'd love to find 100V JFETs...or even 75V, for that matter...
Of course, I'd like to find a large sack of unmarked $100 bills, too, but that's not going to happen.

Grey
 
Tube frontend maybe ?

Voltage is almost same as this linestage.
On one PCB maybe ?
 

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If it did not have the buffer, the performance would be highly variable, depending on the Zout, current capabilities, etc. of the source
And what might that look like -- perhaps a phase lag in one channel?

I like the unity gain amp concept--at low listening levels, the output from my preamp has to be attenuated to ridiculously low levels before being fed to my amp.....
This amp lets you bypasses that whole process.

Good ol' P5 in the BZLS might actually work as a good volume control.

JJ
 
The Gate capacitance of the output MOSFETs is the fly in the ointment. It's big, it's unruly, and it's nonlinear. Quite an ugly beast. By isolating that capacitance from the outside world, the performance of the amp becomes much more predictable. Yes, the input JFETs have Gate capacitance, but an order of magnitude (actually probably more like two orders of magnitude) less. Quite tame by comparison.
If you leave off the JFET buffer, be sure to go for as low a source impedance as you can get and provide as much current as possible.
Basically at that point you're building a whole 'nother circuit, as the output stage is an entirely conventional compementary follower--pretty much an industry standard sort of thing. It will become whatever you use for a front end; an A-75, for instance.
Yes, I'm overlooking the bias dingus. On purpose. You could build this amp with a conventional Vgs multiplier, or a Vbe multiplier, or use diodes, or a CCS and variable resistor, or...
Really, it's the JFET follower front end that's different.

Grey
 
There is something a little confusing here though Grey:confused: The old Class A peak output current rule... That you can get double the queicent current out. Nelson gets more than that???
Dosen't this thing idle at 1.5A or so??? Gets 20V peak to the load??? O.K. thats good for 25Watts out...
But when you drive a 4 ohm load... If you can drive 20Volts out, thats 50 watts like he says but thats 5A pk??? Well beyond the double the idle current addage of old. Does this thing do Class AB for that power level???
Did I miss something???
 
Formerly "jh6you". R.I.P.
Joined 2006
Like a mad dog, RRRRRRRed to Reno Hi-Fi, but no F4 there.
RRRRrrrrRRR . . . torn off the drawer and squeezzed into it.
RRrrrrRRRrrr . . . found all from TL431 to the blue LED piece by piece.
RRrrRRRRRR . . . but, missing matched 240/9240s and jfets.
RRrrrrrrrrrrrr . . .

Do you know who has them (matched mosfets and jfets)?
Will give him one spicy bite.

Thanks for info.



:darkside:
 
juma said:
In that case the input JFET pair could be left out of the circuit.

Without the JFET input it would look like a PP output stage for a hybrid tube-MOSFET power amp, such as the one on the picture.
I've tested that output stage, connected to a pre-amp before the tube front end was hooked up.
Adding a buffer and then running the circuit balanced is a really cool idea. Wish i thought of that, as usual.
 

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flg said:
There is something a little confusing here though Grey:confused: The old Class A peak output current rule... That you can get double the queicent current out. Nelson gets more than that???
Dosen't this thing idle at 1.5A or so??? Gets 20V peak to the load??? O.K. thats good for 25Watts out...
But when you drive a 4 ohm load... If you can drive 20Volts out, thats 50 watts like he says but thats 5A pk??? Well beyond the double the idle current addage of old. Does this thing do Class AB for that power level???
Did I miss something???


Yes,

AB it is.

William
 
Official Court Jester
Joined 2003
Paid Member
steenoe said:

Me too:D I can be woo-woo'ed all day with no ill effect, at all. I think it was Metalman who said the "woo-woo- button" thing:)
F4; another genious act of Mr. Pass. Did anyone build this thing yet?
Being a study in simplicity, it shouldn't take long to weld one together;)

Steen:cool:


who can be faster than you in this?

hehe
you even have pcb pdfs.........
 
The one and only
Joined 2001
Paid Member
Commenting on the particular mis-match of the complementary
types, notice that the distortion is primarily second harmonic
on the amplifier and that the distortion vs power curve looks
just like a classic single-ended amp.

This imparts a particular sonic signature different from the
usual push-pull arrangement where the character is 3rd harmonic.

:cool: