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Old 12th October 2003, 09:44 AM   #21
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class A w/ varying power levels output kinda defeats the point of class A, doesn't it?

Personally, if you're too lazy to actually turn the amp off, I would use a 'sleep mode' that, after a certain time period of zero source power, would keep the PS on, but disconnect the amp from it. Then, when it detects source power again, it would re-power-up the amp section. The caps stay charged up, everything's working except for the actual amplifier section.
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Old 12th October 2003, 01:07 PM   #22
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Default Re: sliding bias control chip

Quote:
Originally posted by smoking-amp
The Linear Technology chip LT1166 is a sliding bias control chip for mosfets, it maintains the product of the upper and lower currents as a constant. Also provides dual slope SOA feature.

the interesting thing about this chip is that it tries to maintain a 20mv voltage drop on the source resistors. I remember someone mentioned a while ago that the optimal "bias" for a class B amp is one that drops about 20mv on the emitter (bjt) or source (mosfet) resistor.

I guess the LT chip confirms that.

unfornately, I don't know why anyone would use such a complicated device to set-up bias in an otherwise very simple amp.
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Old 12th October 2003, 01:41 PM   #23
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Nelson's comments are certainly true of the "before" times.
That is before microprocessor chips and measuring devices
capable of pulling off real time bias adjustment became
available.


You can with some of the new allegromicro hall effect sensors,
some sample and hold circuits and a few a/d converters
do complete real time monitoring of the real bias, and real
output power and then calculate and adjust the bias.
Then you can do peak power detection and sliding bias
all in firmware.

Not trivial to do so. Not so easy to keep the cpu noise out
of the power amp either. Isolation amplifier to drive the
bias recommended.

The krell amps use an inline current transformer and a small
microprocessor to set the bias.
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Old 12th October 2003, 03:18 PM   #24
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There are more subtle problems afoot than the speed with which the bias can be set.
Turn on a class A amplifier and watch the bias settle in. It takes quite some time before the output devices stabilize due to the heating of the devices themselves, the thermal mass of the heatsinks, caps forming, etc. Easily twenty to thirty minutes, and things are still shifting a bit after an hour. All these factors have a non-trivial effect on the behavior of the output devices, hence the sound quality.
And folks expect a dynamic bias circuit to sound like class A? The bias may or may not be able to track, but the devices/heatsinks/etc. scarcely have time to react to the changes.
Now, perhaps if there were some way to keep the devices warmed up, and the heatsinks' thermal mass at equilibrium, and so forth...perhaps then the circuit might stand a chance of sounding like it was class A.
There is.
Dispense with the varying bias and set a nice, constant, fixed high bias. That way you'll be guaranteed that the circuit will be ready when a transient comes along. On paper, bias tracking arrangements look good. Unfortunately, in the real world they don't accomplish what they set out to do. I'm not saying that they don't work--in the sense that music comes out--nor am I aware of any intrinsic reliability problems--but they just don't sound as good as a hot class A amp.
I heard the big Krell (600W?) sliding bias amp a year or two ago out in Santa Fe. Granted it was an unfamiliar system, etc. etc. etc. but I was rather underwhelmed by the sound quality. Not that it was bad, mind you, but for that much money I could easily have bought two or three much better sounding amps and had change left over. Dan D'Agostino would be better served (...I'm sure he's breathlessly waiting for my take on this...) by going back to his earlier design philosophy and building up-to-date KSA-50s or KMA-100s.

Grey
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Old 12th October 2003, 03:35 PM   #25
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Unfortunately no matter how smart your quiescent current controller is you have to remember that when you change the quiescent current you also change output stage transconductance, something that is not a problem in a fixed bias arrangement.

A class-a/b amplifier sacrifices linearity for efficiency but is simple.

A sliding-bias class-a amplifier also sacrifices linearity for efficiency only with considerably more complexity.

An amplifier that can be manually switched between fixed bias class a/b and full class-a offers the best of both modes without the compromise.
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Old 12th October 2003, 04:58 PM   #26
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Amen!
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Old 12th October 2003, 08:03 PM   #27
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Default High efficiency class A

So it seems that a sliding bias class A (you could even consider a non-switching class AB amplifier an extreme case of this) is not the ultimate solution.
But of course you can have a floating very low voltage power supply that delivers a big current for the class A amplifier output stage and have a class AB power stage driven by the same signal that drives the floating power supply. This could even be better than a normal class A since the voltage across the output transistors remains virtually constant, which is good for linearity. Still you get the benefit of a higher efficiency of the class AB amplifier. You only get some additional dissipation from the class A stage but that is relatively small because of the low supply voltage.
This principle has been used by (I think) Technics (New Class A) in the past. A nice example can be found here (from Shinichi Kamijo):
http://www.ne.jp/asahi/evo/amp/J554K2955/index.htm

Steven
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Old 13th October 2003, 08:47 PM   #28
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Personally, I think the QUAD current dumping principle is a more elegant way of combining a low-power class A amplifier and a high power amplifier. At least it doesn't require floating power supplies or anything like that.

I have no personal experience with sliding bias class A, but I guess that until someone invents an electronic version of the crystal ball, it will be very difficult to prevent any sliding bias class A system from temporarily leaving the class A regime when a large and sudden signal peak with a low rise time occurs.

The LT1166 is basically a class AB control loop (non-linear common-mode loop used for class AB control). With class AB control loops, you can make very nice non-switching class AB amplifiers that do not suffer from thermal tracking problems immediately after a change in volume (this is always a problem to some extent in conventional class AB amplifiers). The only thing I don't understand is why LT chose to use a product rule. A harmonic mean rule is much nicer, as it keeps the minimum output device current at half the quiescent current, rather than tending to zero when the current through the other device tends to infinity.
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Old 13th October 2003, 09:11 PM   #29
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I agree with Nelson on this. I tried to introduce the LT1166 into our multichannel amps. We had BIG problems with high frequency distortion and we could not meet THX specs. We had to drop it. Still, in principle, it looks pretty good.
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Old 13th October 2003, 09:19 PM   #30
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Quote:
Originally posted by john curl
I agree with Nelson on this. I tried to introduce the LT1166 into our multichannel amps. We had BIG problems with high frequency distortion and we could not meet THX specs. We had to drop it. Still, in principle, it looks pretty good.
John, how high frequency and how distortion was it (if you may tell)?
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