John Curl's Blowtorch preamplifier part II

[john curl]

Who cares, WHY? What is, and how it performs, is what counts. Talk about REAL engineering.
Why should I replace good bias circuit with an ill performing one? Especially when my peers in Taiwan agree? The chip was initially recommended by ME to save the Taiwanese time and trouble. I don't get paid to FIX the problems, (if possible) of someone else's design.

[Bob Cordell]

Originally posted by john curl
Who cares, WHY? What is, and how it performs, is what counts. Talk about REAL engineering.
Why should I replace good bias circuit with an ill performing one? Especially when my peers in Taiwan agree? The chip was initially recommended by ME to save the Taiwanese time and trouble. I don't get paid to FIX the problems, (if possible) of someone else's design.

The LT1166 performed well for me and others. I really don't care what your peers in Taiwan said. REAL ENGINEERING often requires patience, hard work, and insight. That's what it took for me to harness the 1166. Had you used the 1166, you probably would not have had the bias stability problems you originally encountered in the JC-1 design.

Cheers,
Bob

[syn08]

Originally posted by Bob Cordell
The LT1166 performed well for me and others. I really don't care what your peers in Taiwan said. REAL ENGINEERING often requires patience, hard work, and insight. That's what it took for me to harness the 1166. Had you used the 1166, you probably would not have had the bias stability problems you originally encountered in the JC-1 design.

This rather obscure "Specification Notice" has also a few very interesting ( and pretty well hidden ) ideas for optimizing the LT1166 circuitry.

[scott wurcer]

Everyone knows that SPICE is input polarity agnostic for DC convergence.

[john curl]

In reference to the JC-1 power amp, 9 separate 1166 IC's would be necessary to make the unit more stable, and all of the extra parts to make each one work. WOW! One unit would NOT do anything, but make it easier to initially bias. With the Beta mismatch problems that we found, and the thermal imbalance in the initial heat sink assembly, the amps would have gone into thermal runaway just as fast and easily. Please, engineers, check your topic, before jumping on me. I just might take it personally!

[stinius]

Originally posted by scott wurcer
Inherent in the multi-tone test is the fact that both above and below IM components are present so the distortion products move at different speeds and in different directions. At some point one just makes interesting complicated pictures. I am assuming stinius was suggesting sliding all the tones at once. This would create a signal with a crest factor varying over time (more like music). Does AP do multiple sliding tones at once?

Hi Scott

Yes that was the exactly what I was thinking about.

Sorry for responding so late, but I’m working on a Class D amp, and it is very time-consuming to simulate those things.

Anyway thanks Scott.

Cheers

[myhrrhleine]

Originally posted by scott wurcer


Dick Heyser pioneered the use of the chirp in conjunction with the
Hilbert transform for audio analysis. I don't think THD was ever addressed, it might make for some interesting experiments.

thd was addressed
but for speakers

[stinius]

Originally posted by myhrrhleine



thd was addressed
but for speakers

Yes that’s right thank you Andy_C and Klaus for the links, but that isn’t the same thing.

[Bob Cordell]

Originally posted by john curl
In reference to the JC-1 power amp, 9 separate 1166 IC's would be necessary to make the unit more stable, and all of the extra parts to make each one work. WOW! One unit would NOT do anything, but make it easier to initially bias. With the Beta mismatch problems that we found, and the thermal imbalance in the initial heat sink assembly, the amps would have gone into thermal runaway just as fast and easily. Please, engineers, check your topic, before jumping on me. I just might take it personally!

Hi John,

As I've said many, many times on this forum, the JC-1 is a VERY good amplifier.

I agree with you that using 9 LT1166's would have been a silly solution for an amplifier like the JC-1.

I also agree that use of the LT1166 will not solve every kind of bias thermal stability problem. When a single LT1166 is being used for biasing of multiply-paralleled output stages there is an underlying assumtion that each transistor in the output stage is acting somewhat like the others. My comment completely ignored the possible problems that can originate from current hogging among the output transistors, and I apologize for that.

Let's make this a "teachable moment", as President Obama is fond of saying.

The JC-1 is a very good example of a large amount of paralling of output devices while at the same time employing a fairly agressive choice of emitter resistor (0.1 ohms or 0.15 ohms).

So, it sounds from your description that the stability issue that you experienced was current hogging in the quiescent state. It further sounds like it was a result of mismatches among the betas of the power transistors (among one sex). Was this the case? If so, can you elaborate on the cause-effect that was taking place?

This may be helpful to those considering paralleling many output devices without matching them for beta.

Once again, the LT1166 can greatly improve global thermal stability, but cannot do a lot for local thermal instability that results from a tendency to current hogging among output devices.

Cheers,
Bob

[hitsware]

only lateral mosfets cure all of the above ills ....