Hehehe....no, saving those R125 for the J2. However, have been thinking of
some another project as well... 🙂
some another project as well... 🙂
I was thinking about upgrading my F2 to F2J, but that J part...Jesus, it was X-Pen-sive and unobtanium...so I better get used to some 2nd harmonic 🙂6
The Gate leakage figure is something you want to watch with the SS
Jfet parts. Partly it depends on the specific use. In Common Source
applications, it can be more of a problem than in follower use.
The Gate current should be measured in actual operation.
For the SIT amps I tested for 20 uA or less, and a similar figure for J2's and
such. You can live with higher figures, but the design may have to be
adjusted to accommodate them.
Jfet parts. Partly it depends on the specific use. In Common Source
applications, it can be more of a problem than in follower use.
The Gate current should be measured in actual operation.
For the SIT amps I tested for 20 uA or less, and a similar figure for J2's and
such. You can live with higher figures, but the design may have to be
adjusted to accommodate them.
If any of you guys who were thinking ahead, decide at some point that you can part with a pair of the R125s, please let me know. Have 2, 2 to go. Thanks
You can build a perfectly fine J2 with IRFP240's as the current source
with minor adjustments.
Measures and sounds fine.
The only reason I have not done so in production is the desire for
product consistency.
with minor adjustments.
Measures and sounds fine.
The only reason I have not done so in production is the desire for
product consistency.
Wondering about "minor adjustments" here. As a complete electronics idiot, I dont mind smoking an experiment or two in terms of outputs, but I don't want to risk my dwindling stock of precious J74 devices...are the outputs the only thing at risk? Otherwise I will wait until more info is released and those questions more easily answered.
Russellc
No risk, you just have to allow for the Vgs of the Mosfet at about 4V
vs the Jfet at something like 1.2V. In my schematic the resistor which
provides bias current for the current source goes from 18K for the Jfet
to 15K for the Mosfet.
Probably there is no actual performance difference.
vs the Jfet at something like 1.2V. In my schematic the resistor which
provides bias current for the current source goes from 18K for the Jfet
to 15K for the Mosfet.
Probably there is no actual performance difference.
Mr. Pass,
Forgive me if I asked this question before, but is the output stage of the ACA close at all to the J2? Excluding the fact that the ACA uses mosfets, are both circuits Mu type stages?
I'm trying to think if there are other First Watt examples with a differential pair on the inputs and power Jfets on outputs. There's probably more to it than just marrying the two stages, for example how the circuit is biased and distortion figures are set. I thinking- how close to a J2 sound could I get with a power jfet ACA output and a balanced jfet diff. input stage.
The ACA has a special sound, I think, despite being designed for a beginner to construct. It's a really nice, engaging effect. If the J2 sounds completely different from an ACA, well that's what makes it so fascinating. I want to know what people are hearing in the J2, and why it's the Amp of the Year! 😀
Thanks,
Vince
Forgive me if I asked this question before, but is the output stage of the ACA close at all to the J2? Excluding the fact that the ACA uses mosfets, are both circuits Mu type stages?
I'm trying to think if there are other First Watt examples with a differential pair on the inputs and power Jfets on outputs. There's probably more to it than just marrying the two stages, for example how the circuit is biased and distortion figures are set. I thinking- how close to a J2 sound could I get with a power jfet ACA output and a balanced jfet diff. input stage.
The ACA has a special sound, I think, despite being designed for a beginner to construct. It's a really nice, engaging effect. If the J2 sounds completely different from an ACA, well that's what makes it so fascinating. I want to know what people are hearing in the J2, and why it's the Amp of the Year! 😀
Thanks,
Vince
They are similar, as is the case with the Alephs 1, 2, 3, 4, 5, 30, 60,
Aleph J and ACA.
The Alephs with the exception of the 0 used a circuit similar in action
to the mu follower, The J2 has more of a classic mu follower circuit.
Aleph J and ACA.
The Alephs with the exception of the 0 used a circuit similar in action
to the mu follower, The J2 has more of a classic mu follower circuit.
Hi Nelson,
Do you mean the other way around: 15K w/ Jfet, 18K with Mosfet (so that the Mosfet sees a higher voltage drop at its gate)?
Cheers,
Jeff.
I had to change a bit the ratio of the drain resistor of the lower SS and the source resistor of the upper IRF to get similar k2 over k3.
D'oh. If the transfer curves are different, then matching VGS to the same voltage would be no more than an exercise in numerology.
(I have a friend who's an astronomer. Smoke comes out of his ears when someone comes up and says: "I hear you're an astrologist...")
(I have a friend who's an astronomer. Smoke comes out of his ears when someone comes up and says: "I hear you're an astrologist...")
It's not really an issue. The current source bias will work over a fairly
wide range with insignificant difference in performance.
wide range with insignificant difference in performance.