John Curl's Blowtorch preamplifier part II - Page 2204 - diyAudio
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Old 4th April 2012, 08:12 PM   #22031
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The output transistors would need small emitter resistors for this reason.
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Old 4th April 2012, 08:21 PM   #22032
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Quote:
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The output transistors would need small emitter resistors for this reason.
The common mode current in the output is still undefined. I posted a circuit over a year (2?) ago that overcomes both problems, I could post it again tonight. I built it and tested it to show that as claimed the input offset and Aol could be independently trimmed to 0 and infinite respectively, I think I even posted scope plots.
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Old 4th April 2012, 08:57 PM   #22033
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Borbely's last power amp used matched and parralled 2sk170 and 2sj74 jfets cascoded with 2SK2013 and 2SJ313 Fets as a 16ma VAS configured as John's Parrasound 3000 VAS. The input dual differential was cascoded 2sk389/2SJ109.

Last edited by ticknpop; 4th April 2012 at 09:00 PM.
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Old 4th April 2012, 09:00 PM   #22034
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John,

Going back to your subcircuit #3 and using Mosfets in 2nd stage. Can one assume that, all else being equal, despite their higher capacitances the sonic advantages (less high order distortion components) of using them outweight the loss of the circuit's slew rate?

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Old 4th April 2012, 09:04 PM   #22035
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The mosfets that we would normally use for the second stage would be 1/2A-1A devices, and would not have too much capacitance to work. Just look at the JC-1 power amp that uses this topology.
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Old 4th April 2012, 09:20 PM   #22036
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I might clarify what I mean by 'noise gain' or whatever it is called formally.
The first estimated example of this, I learned from a classroom handout of the UA741 op amp that had been analyzed by the best SPICE emulation at the time. It was shown that the bipolar active loads actually ADDED noise to the op amp, because they were not degenerated enough to kill their noise gain. This was a revelation to me, in 1971. I would have never thought it would be important, until it was discussed in class.
You see, we normally think of the input device(s) as the primary noise generator, because we normally use a resistive load. However, when we use an active load, then the load itself can amplify its own self noise, and sometimes overwhelm the input noise. The easiest solution is to add emitter resistors in order to reduce the intrinsic gain of the active load. However, too much emitter resistance will cause voltage drop problems that could effect the common mode range, or the maximum output swing of the circuit. This is where 'finesse' is required, to keep the potential problems almost negligible.
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Old 4th April 2012, 11:35 PM   #22037
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Quote:
Originally Posted by scott wurcer View Post
The common mode current in the output is still undefined. I posted a circuit over a year (2?) ago that overcomes both problems, I could post it again tonight. I built it and tested it to show that as claimed the input offset and Aol could be independently trimmed to 0 and infinite respectively, I think I even posted scope plots.
Scott I'd be interested. Can you post your circuit again?
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Old 4th April 2012, 11:53 PM   #22038
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Scott I'd be interested. Can you post your circuit again?
Sure, I need to rerun some sims for a simplified version.
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Old 5th April 2012, 04:57 AM   #22039
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Thanks - look forward to your input.
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Old 5th April 2012, 05:00 AM   #22040
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Sorry, OT I know - but re '2012, our time is running out', did you see the article on NatGeo website about rogue planets that have been ejected from the core of the Milky Way at >40million Kms/Hour. One of those through our neck of the woods . . .
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