All FET folded cascode driver for "JLH"-buffer with no GNFB - diyAudio
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Old 28th January 2010, 05:52 PM   #1
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Default All FET folded cascode driver for "JLH"-buffer with no GNFB

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.
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Last edited by revintage; 28th January 2010 at 05:56 PM.
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Old 28th January 2010, 06:05 PM   #2
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Hi Lars; is your servo stable enough?
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Old 28th January 2010, 06:26 PM   #3
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Hey Anatoliy,
Donīt know, just drew a std configuration. Any suggestions how to do a stable one? Will anyway try the circuit without servo at first.

Last edited by revintage; 28th January 2010 at 06:30 PM.
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Old 28th January 2010, 06:32 PM   #4
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You have 3 poles, and at least 2 of RC networks have the same time constants.

I'm afraid it will oscillate.
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Old 28th January 2010, 07:49 PM   #5
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OK, then we skip R13/C3 to get only two poles. Also we make C1 bigger, like 2,2u. Will that be good enough?

Or letīs go crazy and dump the servo.
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Last edited by revintage; 28th January 2010 at 08:06 PM.
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Old 28th January 2010, 07:57 PM   #6
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I would make it smaller
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Old 28th January 2010, 08:11 PM   #7
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Yeah, but at some point it begins to impair lowend response/phase. No problem going to .47u or .33u anyway.

Last edited by revintage; 28th January 2010 at 08:14 PM.
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Old 28th January 2010, 09:50 PM   #8
Bigun is offline Bigun  Canada
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Would there be some benefit if impedance of feedback could be lower, say 2k series and 220R shunt ?
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Old 28th January 2010, 10:00 PM   #9
juma is offline juma  Germany
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Lars, that looks like a nice gain stage
I see you love the gyrator - I never tested one (except long time ago in parametric EQ) in preamp/gain stage - what sonic benefits you experinced with it?
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Old 29th January 2010, 12:13 AM   #10
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It may sound gorgeous, when properly built.
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