this amp ?
The diyAudio Firstwatt F6
R3 isn't the speaker load - this means for me - at least at first glance - voltage control.
But in series to the feedback network (to the sources of input jFETs) is to find Jensen's JT123 - go to
https://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-123-flpch.pdf
to get a frequency dependent mixture between voltage and current drive.
The diyAudio Firstwatt F6
R3 isn't the speaker load - this means for me - at least at first glance - voltage control.
But in series to the feedback network (to the sources of input jFETs) is to find Jensen's JT123 - go to
https://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-123-flpch.pdf
to get a frequency dependent mixture between voltage and current drive.
What I mean is: take out R3 100 ohms al together and you have a very high output impedance. Reach total current drive nirvana. 100V into 50 ohm at the bass side for instance. Will it give deep bass control? It will be different 
.
If you like, you can also short the 18 ohm R4 and leave R3 in place - then the output impedance is somewhat tamed or damped, the amp now really driving a load of the [6-50 ohms] speaker//R3.
You can do this "f6" too with the newer JFETs such as the Unisick UJ3N120065K3S or UJ3N065080K3S or even with SITs, but then you gotta change the bias in a negative way and maybe forget about the degeneration for most part. . . . It needs the smart guy
to find the sweet spot of course.
It is great on a Lowther by the way.
.If you like, you can also short the 18 ohm R4 and leave R3 in place - then the output impedance is somewhat tamed or damped, the amp now really driving a load of the [6-50 ohms] speaker//R3.
You can do this "f6" too with the newer JFETs such as the Unisick UJ3N120065K3S or UJ3N065080K3S or even with SITs, but then you gotta change the bias in a negative way and maybe forget about the degeneration for most part. . . . It needs the smart guy
to find the sweet spot of course.It is great on a Lowther by the way.
- And you can take out R3 or do the R4 shortcut with a switch of your liking, add a blue signaling LED of course.
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Maybe someone should point out to them that this is wrong, equal Zout and Zload does not give max power transfer.
Which is easy to prove: Say you have an amp with 8R Zout and 8R Zload, measure the outputs power with some input signal. Now don't change anything except cut Zout to zero and presto! your power has quadrupled!
Jan
The hypothesis is equal Zout and Zload so:
Zout =0
Zload =0
Which is the output power?
Maximum power transfer theorem:
Maximum power transfer theorem - Wikipedia
Of course the amp will clip at Vb. And 30 amps will not go. The drive is pure on current, in sofar as the mosfet is -theoretically- a pure current-drive source. So 30A with a suitable modern device in 0,5 ohm - why not drive a maggie HF section directly?? But it wil distort, count on that.
I tried the non-FB version somewhere halfway the nineties. It was interesting: very pure output. Almost magic, such a high amplification.
[My problem was a telco-transformer that did not go beyond 15 kHz, so I dropped this nice 'vingeroefening' into the bin.
]
I tried the non-FB version somewhere halfway the nineties. It was interesting: very pure output. Almost magic, such a high amplification.
[My problem was a telco-transformer that did not go beyond 15 kHz, so I dropped this nice 'vingeroefening' into the bin.
]