Re: Re: Cascoded jfet buffer
I found link to this very old thread (with 1900 views and no reply)
at Pedja Rogic gainclone page.
http://users.verat.net/~pedjarogic/audio/gainclone/gainclone.htm
He refers his solution to this thread
and also to a Nelson Pass paper:
Cascode Amp Design: by Nelson Pass
Steve Eddy said:
amplifierguru said:
An externally hosted image should be here but it was not working when we last tested it.
I found link to this very old thread (with 1900 views and no reply)
at Pedja Rogic gainclone page.
http://users.verat.net/~pedjarogic/audio/gainclone/gainclone.htm
He refers his solution to this thread
and also to a Nelson Pass paper:
Cascode Amp Design: by Nelson Pass
Pedja Rogic
also recommends a more usual and simple alternative to buffer LM3875:
also recommends a more usual and simple alternative to buffer LM3875:
An externally hosted image should be here but it was not working when we last tested it.
amplifierguru said:
In the circuit in post 1:
The input FET should have some resistance to 0V reference or "ground" of the +/-18V supply, 1M to 10M is fine. No "ground" point is shown in the schematic, a glaring omission.
C1 provides low AC impedance from T4 gate to output, for current to drive T4 Cgd, just what you would want for a bootstrap cascode with highish Cgd. With 10mA sink current I would choose a JFET with high Idss and Vp instead of a power MOSFET, something like PN4391, connect R4 to output and dispense with C1,R6,R7. Yes there should be power supply bypass caps but in this schematic there is no 0V to connect to.
I have to wonder how stable the T1/T2/T3 compound feedback triple is. I've had enough trouble with oscillation in CFPs. Has anyone actually built this?
Post 1 is about 31 months old but rbroer has posted recently so a reply may come.
The link in post 7 doesn't offer any clear explanation. I agree with this, but given 1/gm=33 ohms the -3db of C2 and 33 ohms is about 150kHz, well outside the audio range, so the variability might not matter. I would check for any signs of oscillation on the 2SK170 source.
Yes, this would be fine. I also have seen he is around.
Regarding C2 in the gainclone circuit, I also find difficult to see the reason.
I dont know if Nelson Pass paper, PDF link above, on Cascoding will have something on this.
Joe Rasmussen does not use such a capacitor in his tube buffered gainclone, JLTi MK2.
http://www.customanalogue.com/
PMA said:have you tried to compare driving a high impedance input from conventional medium Zout and from low Zout high current capability signal source?
Why yes. Just take a look at my preamp.
An externally hosted image should be here but it was not working when we last tested it.
se
Wow,
I might put my stereo Volume/FET cascode buffer which measures
2" x 1" x 1" -
http://www.diyaudio.com/forums/showthread.php?s=&threadid=70276
into a metre square block of machined rhodium with $$$ 4 zeroes price tag for Hi End street cred! Castors extra.
Cheers,
Greg
I might put my stereo Volume/FET cascode buffer which measures
2" x 1" x 1" -
http://www.diyaudio.com/forums/showthread.php?s=&threadid=70276
into a metre square block of machined rhodium with $$$ 4 zeroes price tag for Hi End street cred! Castors extra.
Cheers,
Greg
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