syn08 said:
Hi Syn08,
My intent was not to lecture you; however, my style is to be more complete in explaining things so that others less experienced reading the thread can gain some understanding. You obviously know a lot more about the LT1166 than most others here. It also sounds like you have found a workable solution to using the LT1166, but one that is different than the one I was pursuing about three years ago.
In my experience, there are two major issues to tackle with the LT1166. Bear in mind that this is in the context of my goal to use the 1166 in the way the designers presumably intended.
The LT1166 implements a common-mode bias spread control loop, where what we are calling the common mode is the bias spread. This loop forces the product of the currents of the output transistors to be constant (at a given LT1166 temperature). The stability of this loop is a big concern and is the biggest reason I had to do a lot of reverse engineering of the LT1166. In many situations, the internal compensation of the LT1166 does not keep the gain crossover frequency of this common mode loop to a low enough frequency. Parasitic oscillations thus can result. I recall that compensating this loop can be tricky. Your solution of placing a 1 uF cap across the bias spread terminals of the LT1166 solves this problem.
The other big issue with the 1166 is the way in which the conventional global feedback compensation is applied. This again largely becomes a non-issue when the 1 uF capacitor solution is used.
When the 1166 is being used as a dynamic shunt bias spreader, being fed with VAS current sources with signal (as we drive a conventional Vbe multiplier), the bias spread changes with the magnitude of the current being sourced to, or sunk from, the load. This is a necessary behavior if neither of the pair of output transistors is to be turned off, as required by the translinear operating law of the LT1166. This in turn means that the common mode spreading voltage is a highly nonlinear version of the signal. If any of this gets into the signal path in such a way that it is not canceled out, distortion will result.
This means that if conventional Miller compensation is used, and the Miller capacitor is tied to one end of the LT1166 bias spreader, considerable distortion will result. This was why I was asking about how the overall amplifier compensation was implemented. If instead, two Miller capacitors of equal value are tapped from both ends of the LT1166 bias spreader, this distortion will be greatly reduced.
If other types of non-Miller compensation are used to stabilize the global feedback loop, this may not be an issue.
Cheers,
Bob
Bob Cordell said:
This is correct. The bottom line is that LT1166 can barely be used with good results in non-symmetrical designs.
What I eventually found as an optimal is a lead-lag compensation from each the 1 and 4 terminals to the ground. Usually 47p in series with 50-100ohm will do very well.
janneman said:
The chirp is a popular stimulus for the acoustic testing. It can be used with FFT and monitoring the source to get gain (response), phase and with some more math HD and IMD. Praxis has a good implementation of it. It is a very transient signal and depending on the low frequency limit and the frequency resolution is possible to get good input in 100 mS. If I wasn't swamped with other work I would try it on some amps to see if there is a difference in phase response at different power levels for different sweep times.
john curl said:Didn't I say that in the first place?
No.
You said the LT1166 was incapable of high performance without explaining why.
Both syn08 and I have discovered and explained why, and have pointed out how to avoid these shortcomings. Syn08 described using twin lead-lag networks hanging off of either end of the LT1166 bias spreader to avoid distortion in the case where the bias spreading voltage is allowed to be a function of signal. I described using twin Miller compensation capacitors hanging off the ends of the LT1166 bias spreader.
Cheers,
Bob
Yes, that's a good one, pretty complete.andy_c said:
Nice stuff here from A.Farina himself
http://pcfarina.eng.unipr.it/Public/Papers/list_pub.htm
http://pcfarina.eng.unipr.it/Public/Papers/134-AES00.PDF The original Paper
http://pcfarina.eng.unipr.it/Public/Papers/238-NordicSound2007.pdf Detailing improvements to the above
http://pcfarina.eng.unipr.it/Public/Papers/246-AES126.pdf The silence-sweep method
Some time ago I mentioned that I wanted to try swept multione (two-tone, non-identical signal levels) testing with the farina method, which I did, as a basic concept (and it works). Only that it's hard to figure out how to exactly interpret the results and what the requirements are for the best suited stimulus....
- Klaus
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.
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.
john curl said:
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
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!
john curl said: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
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