Hey Andrew,
Is this something you have found out empirical as this info is nowhere else to find? Toshibas datasheet only states 400mW at 25 degrees. Found another typical JFET where dissipation derating above 25degrees was close to 3mW/degree.
Though my main concern was running B+ of two times max Vd, .
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It's very easy to find out - eventual sacrifice of one JFET will not cost much and you'll know for sure.
OTOH, cascode is an easy way around the Pd problem (with additional benefit of characteristic's linearization and larger swing capability).
Hey Juma,
Good idea, this is cheap stuff.....
Maybe one should try a MOSFET capacitance-multiplier with long time-constant to get around the evntual start up problem and also get better PSRR.
Will go singlestage Zen-style(not BOZ), cascode is two stage
.About linearity and swing this one with a single 2SK170 will put out ca 25Vrms into 10k(Zout below 500ohm) at 11dB gain with below 0,1% THD. This as shunt feedback is a lot more effective than current feedback commonly used here.
Question though is if it will sound as good as the figures indicate
?
Valid concern IMHO. FWIW 2sk117 gives you another 10V to max Vds. Another way to deal with this would be to go the cascode way, by using a jfet with larger Vgs(off) on top. I use a 2n3819, but many others would qualify. My 2c.
No, it's still one gain stage with two active devicesHey Juma,
...Will go singlestage Zen-style(not BOZ), cascode is two stage
So, you want that poor JFET's Drain to sit at 40V and swing 35V (peak). It means that you'll have Vds=75V during the positive peak. Doesn't seem viable to me.
Zout will be Rd (6k8) || RLoad.
THD figure is unrealistic too.
Hey,
Checked 2SK117 and unfortunately it had 25% lower dissipation. Otherwise it look good with much lower Ciss.
About cascode this isn´t an option as I told Juma above that I don´t want to go two-stage
. Will anyway sim a cascode side-by-side.
Now you are unrealistic, as you haven´t seen the circuit
. What I showed was only the DC part. I told you in a previous post that it wasn´t a current feedback but shunt feedback circuit. So Zout will still be below 500ohm out and now below 0,1%THD and 7Vrms before clipping when going the B+ 40V route.
Unfortunately that isn´t good enough, so gyrator -oad should be better.
About the Vds it was what I initally asked about. If we take a triode it can swing above its Uamax without problems. In other words you can´t go over Vds AC-wise? Ie never above 40V B+?
Patrick,
Checked 2SK223. Looks promising with low Ciss and descent Gm. Is it easy to find?
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I thought we are discussing the circuit in post #1
OK, let's see the real circuit before we reach a deeper level of missunderstanding
Generally, tubes don't break easily. Semiconductors do
AC is AC only through time domain. In certain moment (which, roughly said, lasts differently for different frequencies) there is a difference of potentials (voltage) between Drain and Source that supercedes max. allowed value for that device. The device might endure that, or it might not ? Empirical test, executed through longer period of time, will show
Hey,
Checked 2SK117 and unfortunately it had 25% lower dissipation. Otherwise it look good with much lower Ciss.
About cascode this isn´t an option as I told Juma above that I don´t want to go two-stage
Why not just use a higher resistor between G,S and moderate the current lower so to avoid much dissipation. Me when I use it for Led Vrefs if the voltage across is low, I leave it run full idss so not to up the intrinsic noise, and grab leds harder, but when near Vmax I cut it down to about 30% for current with an 82-100R for typical 8-9mA BL. Thanks to syn08 in a discussion we learned that higher pinch off voltage is what really makes a quieter CCS with a Jfet driving some impedance on its tail, so I am not that happy when I have to utilize 2SKs and not some higher Vt 2Nxxxxs just because some I have are just 25V. Cascode is the better option, and it needs about double the Vt at least for the higher Jfet and lower than higher's Idss in the lower one. About 30% of the free Idss that the lower one can achieve by cutting down with a resistor, if I remember well, but Iko maybe knows more so to confirm or not because he uses them a lot more in practice.
You may find that cascoding the JFET is a simple and
effective solution. Typically it also improves the performance.
Hi Nelson
Can we expect such a pair to really withstand the total Vmax of the two JFETs added up as straight datasheet values, or we lose some due to the way they must be interconnected?
With 40V B+ at hand the resistor version has to be dumped as the small resistor value loads the circuit to much.
With a gyrator(AC-CCS) that also controls Vds we have an possible solution.
Off course frequency response is affected by feedback so this one should go well beyond 200kHz -3db and can bettered by lower valued Rfb´s.
Nelson, you have seen this idea before when we discussed it in the Schade-thread where I showed it simplified with a CCS(error) on top.
With the shunt feedback we make the JFET act triodelike instead of its initial pentode characteristics.
Expected and realistic ballpark figures, RMS values into 10kohm:
Well below 0,05% THD at 1V in, 4V out.
0,1% THD just above 3V in, 12V out.
Zout below 600ohm.
Will build two, one with this gyrator, the other with the real thing, a 100H choke as I have a pair lying around that Jack Elliano wound for me.
OK, all talk about cascodes. Pls show me a good one so I have something to compare with.
With a gyrator(AC-CCS) that also controls Vds we have an possible solution.
Off course frequency response is affected by feedback so this one should go well beyond 200kHz -3db and can bettered by lower valued Rfb´s.
Nelson, you have seen this idea before when we discussed it in the Schade-thread where I showed it simplified with a CCS(error
) on top.With the shunt feedback we make the JFET act triodelike instead of its initial pentode characteristics.
Expected and realistic ballpark figures, RMS values into 10kohm:
Well below 0,05% THD at 1V in, 4V out.
0,1% THD just above 3V in, 12V out.
Zout below 600ohm.
Will build two, one with this gyrator, the other with the real thing, a 100H choke as I have a pair lying around that Jack Elliano wound for me.
OK, all talk about cascodes. Pls show me a good one so I have something to compare with
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