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Old 19th March 2009, 08:00 AM   #1
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Default PLH with zero NFB

OK, it is over 20dB local NFB around the input device!

I have always liked the JLH 10W, actually it was one of the first amps I built back in the -60s. For a long time I have also favoured the buffer version of it with a PNP as phase-inverter together with a tube driver.

Got inspired when I saw the simplicity of the PLH. By changing the IRF610 to a p-channel IRF9610(in the schematic a fqp..... as I didnīt have a working model of the IRF) the PLH is now transformed into a unity gain buffer. By adding a BOZ we are done!

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At first I was into a transformer-coupled OCL tubehybrid, with a 6H30 follwed by a Slagle bifilar. But noticing the slight allergy against tubes in this corner of the forum I went for the BOZ.

It sims OK with a nice 2nd harmonic domination and a bandwith to over 100k.
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Old 19th March 2009, 08:44 PM   #2
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So, with the help of a lowdropout regulator we can get rid of the extra +40V. The negative voltage is easier to arrange for, so it stays.

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Old 19th March 2009, 09:07 PM   #3
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That's quite clever.

I suspect, however, that the pot and C10 actually
introduce a small amount of local feedback.

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Old 19th March 2009, 09:15 PM   #4
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Damned, you got me in the bootstrap department !
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Old 19th March 2009, 10:08 PM   #5
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The source of M2 isn't going anywhere.
Current cascodes right thru M2 into R1.
Yeah, I think that ends up being NFB...
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Old 20th March 2009, 05:39 AM   #6
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Kenpeter,

Yeah it might, as it is a concertina.

Still you could also look upon it as M2 being a sourcefollower driving sourcefollower M4 with M3 being a modulated CCS.

No overall feedback, only some local.

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