This idea is with some unusal takes. Feedback is local and only over the cascode as the buffer is enough feedbacked in itself. Also the locally feedbacked cascode will act very much like a triode.
Output stage is a JLH with a PNP driver that makes it a pushpull follower with unity gain, low distortion and low Zout.
RDC is for balancing initial offset of the buffer and only works if Q4 has lower Vbe than than Q1.
The DC-servo is integrated with the voltage-controlled gyrator( a CCS will not work against a JFET) at the bottom.
With Q1, Q3, Q4, Q5 and Q6 on the heatsink thermal-drift will be quite low even without the servo.
The Q5/Q6 CCS is fed by 20V to allow Q1 to swing up to 15V-Vbe.
Tried sims with a BJT in place of J1 but got indications of a less pleasant overtone structure.
Output stage is a JLH with a PNP driver that makes it a pushpull follower with unity gain, low distortion and low Zout.
RDC is for balancing initial offset of the buffer and only works if Q4 has lower Vbe than than Q1.
The DC-servo is integrated with the voltage-controlled gyrator( a CCS will not work against a JFET) at the bottom.
With Q1, Q3, Q4, Q5 and Q6 on the heatsink thermal-drift will be quite low even without the servo.
The Q5/Q6 CCS is fed by 20V to allow Q1 to swing up to 15V-Vbe.
Tried sims with a BJT in place of J1 but got indications of a less pleasant overtone structure.
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Have not experienced any as I haven´t tried it with sand-amps yet. Works better than CCS together with tubes.
Anatoliy might know more as I believe he has used them longer than me.
Bigun,
Don´t think that lower value feedback resistors would benefit this circuit. Might load down the cascode to much.
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