John Curl's Blowtorch preamplifier

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#6,902
john curl said:
PMA and I would prefer to reduce the higher order distortion to virtual unmeasurability, even at the cost of slightly more 2'd or 3'rd harmonic, if necessary.
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Here is my latest phono stage distortion at 5mV input and 500mV output at 1 kHz. It is 3 FETs per channel and a local NFB only. Sounds pretty good too 🙂 .

Alex
 

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#6,912
Bob Cordell said:



Hi Sigurd,

I have not heard the term "telescopic" cascode before, but I have used the term "driven cascode" to mean what I think is the same thing: namely, the bases (or gates) of the cascode are driven with a signal that is a reasonable replica of the common-mode signal applied to the differential pair being cascoded.

I have had very good results in non-inverting power amplifiers by driving the bases of the cascode stage with a replica of the common mode signal derived directly from the output of the amplifier with a divider network that is substantially identical to the feedback network. This yields a very close replica of the actual common mode signal to which the diff pair is subjected without adding and circuitry that might load the input circuits.

The underlying assumption here, of course, is that the differential error input to the LTP being cascoded is small by comparison. In the case of a no-NFB amplifier, this would likely be not true.

Cheers,
Bob
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Hi Bob,

Do you have an example of a practical schematics of such a cascode stage using a biasing signal derived from the output of the amp ? My concern relies in the DC bias of the bases of the devices providing the cascode scheme.

By the way, albeit knowing cascodes for thirty years, I discovered the term "telescopic cascode" only today on a french forum.

Cheers
 
#6,916
Bob Cordell said:



Hi PMA,

Could you please elaborate on why you believe the asymmetrical LTP/VAS I described and use is inferior to the full complementary dual differential pair approach? I believe it is actually superior.

Bear in mind that the approach I am referring to provides full complementary push-pull operation of the VAS (unlike what you may be referring to in referencing Self).

The generic structure I am referring to starts with an N-channel JFET diff pair that is differentially loaded. It feeds a PNP differential pair. One output of that pair drives the top of the bias spreader. The other output drives a current mirror which functions as the NPN part of the VAS, driving the bottom of the bias spreader. The detailed arrangement is documented in my MOSFET power amp paper on my web site at www.cordellaudio.com. That arrangement provides exceptionally low distortion in that amplifier of 0.0006% THD-20 at full power.

Are we talking about the same arrangement?

If we are, could you be more specific about your objections to it?

Thanks,
Bob
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Asymmetrical and symmetrical circuits are just different means to the same end. Your need to constantly reference your 50W amp paper is bordering on the pathological.
There have been a few symmetrical designs presented here that do not lack over your design in terms of linearity and speed and some lesser ones that certainly lack nothing in terms of adequate performance for the intended application. Why not offer some new ideas after ~25 years?
 
#6,917
You are incorrect, Glen. The simulations prove it. It should be obvious from inspection for an experienced engineer, when making the comparison between a single differential pair and a complementary differential pair. Everyone else will just have to accept it, or prove us wrong.
 
#6,918
john curl said:
You are incorrect, Glen. The simulations prove it. It should be obvious from inspection for an experienced engineer, when making the comparison between a single differential pair and a complementary differential pair. Everyone else will just have to accept it, or prove us wrong.
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Huh?
 
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