Bryston Class A power

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werd

Bryston Class A power
« on: 10 Jul 2011, 03:42 pm »
Hi james

Any chance of Bryston building a low power class A in transistor power amp ? Like what we see from Luxman.

stefanovitch

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Re: Bryston Class A power
« Reply #1 on: 10 Jul 2011, 04:11 pm »
The best thing I could recommend is to email Nelson pass for free.
He answers every mail and he can give more likely an advice than any otherone does.

nelson@passlabs.com

James Tanner

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Re: Bryston Class A power
« Reply #2 on: 10 Jul 2011, 04:40 pm »
Hi james

Any chance of Bryston building a low power class A in transistor power amp ? Like what we see from Luxman.

Hi Lorne,

Without referencing some of the white papers we have done Class A is not a direction we want to go given the success we have with our current circuits.

james

Elizabeth

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Re: Bryston Class A power
« Reply #3 on: 10 Jul 2011, 04:48 pm »
James! At what point does Bryston move from class "A" operation?
At 3 watts? ten? just curious.
If you know for the 4B-SST2 that would be nice to know in general.
Also the 7B-SST2 and the 28B-SST2...

James Tanner

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Re: Bryston Class A power
« Reply #4 on: 10 Jul 2011, 04:49 pm »
James! At what point does Byrson move from class "A" operation?
At 3 watts? ten? just curious.
If you know for the 4B-SST2 that would be nice to know in general.
Also the 7B-SST2 and the 28B-SST2...

About 5%.

JAMES

James Tanner

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Re: Bryston Class A power
« Reply #5 on: 10 Jul 2011, 04:50 pm »
Here's our thoughts on it - others may differ :thumb:

It's All A Matter of Class, Eh?

If you have followed power amplifier technology for any length of time, you will have noticed mention of "class", as Class A, Class AB, etc., and perhaps wondered exactly what this nomenclature pertained to. These terms do not refer to quality, but to the operating parameter of the output section. Most power amplifier output stages operate in a push-pull configuration, where the power is delivered from two power supplies on either side of ground, or zero volts. (There are some which do not, but they are relatively non-linear, and need not be considered here).

Operating in push-pull, the output transistors share the load, and are theoretically required to do work only as the signal swings away from ground, in either the positive or negative direction. If the transistors are completely switched off at zero output, and only start conducting when signal is present, this is defined as Class B operation. This is an efficient way of operating the output, and the amplifier runs cool at no signal, but there is one disadvantage; The output devices always have some lag time in their operation, and thus there appears a small but potentially annoying dead zone, called "crossover distortion", at the zero point. Although this crossover nonlinearity does not necessarily add large amounts to the distortion numbers, (0.05% is probably typical), it is easy to is that hear.

Fortunately, crossover distortion can be reduced to negligible proportions by the simple expedient of running the output transistors "biased" slightly "on" at idle, so they start conducting before the output swings through the zero point. When an amplifier runs this biased output mechanism, it is referred to as "Class AB". Moderate amounts of bias are all that is needed, and as it produces only a bit of heat, this type of amp is still reasonably efficient. Crossover distortion has a number of ways to pop up its ugly little head, however, even if there is a fair amount of bias present, so the engineering of this type of amplifier must be very exacting and precise to give the lowest distortion at all frequencies. If done properly, however, there is no more accurate or lower-distortion type of amplifier available; 0.01% is typical, and 0.001% is attainable.

Some engineers prefer not to have to deal with the possibility of crossover distortion in their designs, and they choose another bias system, called "Class A", where the output transistors are biased on so much that they continuously conduct more than the full load current, even at idle. Thus, they never turn "on" or "off', theoretically obviating crossover distortion.

Unfortunately, this operating system has some obvious, (and some not-so-obvious), disadvantages. Running that much current generates a tremendous amount of heat, so the amplifier is not just inefficient, it is large and expensive, due to the huge heat-dissipating mechanisms required. This consequently warms up the whole room as a side-effect. (Nice in the winter, but remember electric heat is the most expensive kind there is).

A not-so-obvious disadvantage with class A designs is that this high idling current has consequences to the distortion levels far beyond the theoretical elimination of crossover artifacts, (which even in itself is debatable). Transistors have numerous types of distortion mechanisms, among which are deviations from linearity under conditions of simultaneous high voltage and high current. These are, of course, the exact parameters necessary to class A operation, and a typical Class A amplifier runs distortion levels at least 10 times, and often over 100 times, as high as a Class AB amplifier of similar power, or around 0.1%. A careful inspection pf the distortion spectrum also reveals that all the'harmonics are increased, including those represented by the crossover distortion at which the class A operation was aimed in the first place!

james

Elizabeth

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Re: Bryston Class A power
« Reply #6 on: 10 Jul 2011, 04:57 pm »
Thanks for the info.
5% hey. Cool!
 :banana piano:

klao

Re: Bryston Class A power
« Reply #7 on: 10 Jul 2011, 07:31 pm »
So for 7B-SST2's, they operate in Class A up to about 30 Watts @ 8Ohms and 45 Watts @ 4 Ohms, before switching to Class AB?  It seems that the reviewer here thought that Bryston amps run at class AB all the time?

http://www.10audio.com/bryston_3bsst2_4bsst2_7bsst2.htm

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Re: Bryston Class A power
« Reply #8 on: 10 Jul 2011, 07:37 pm »
So for 7B-SST2's, they operate in Class A up to about 30 Watts @ 8Ohms and 45 Watts @ 4 Ohms, before switching to Class AB?  It seems that the reviewer here thought that Bryston amps run at class AB all the time?

http://www.10audio.com/bryston_3bsst2_4bsst2_7bsst2.htm
P

Yes I think there is a lot of misunderstandings out there on how the details work.

James

bummrush

Re: Bryston Class A power
« Reply #9 on: 10 Jul 2011, 07:42 pm »
Also myself included miscinceptions big time about feedback applications

Elizabeth

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Re: Bryston Class A power
« Reply #10 on: 10 Jul 2011, 07:49 pm »
error error... :duh:
if you must read it, James has quoted my comment.

James Tanner

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Re: Bryston Class A power
« Reply #11 on: 10 Jul 2011, 07:52 pm »
I would say it is in class "A" OR class "B" operation. The "AB" is a use of nomenclature for amps that swing from part class "A" at low power to class "B" at higher power.
But the amp is actually operating in class "A" OR operrating in class "B"
clear?

Hi Liz

No usually they operate Class AB over most of their power range but for a few watts some Class AB amps run Class A.

James

klao

Re: Bryston Class A power
« Reply #12 on: 10 Jul 2011, 07:52 pm »
Thanks, James and Eliazbeth.   :thumb:

Elizabeth

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Re: Bryston Class A power
« Reply #13 on: 10 Jul 2011, 08:07 pm »
From Wiki:

"Class A 100% of the input signal is used (conduction angle Θ = 360° or 2π); i.e., the active element remains conducting[6] (works in its "linear" range) all of the time. Where efficiency is not a consideration, most small signal linear amplifiers are designed as class A. Class A amplifiers are typically more linear and less complex than other types, but are very inefficient. This type of amplifier is most commonly used in small-signal stages or for low-power applications (such as driving headphones)."
 
Class AB
"Class AB Here the two active elements conduct more than half of the time as a means to reduce the cross-over distortions of class B amplifiers. In the example of the complementary emitter followers a bias network allows for more or less quiescent current thus providing an operating point somewhere between class A and class B. Sometimes a figure is added (e.g., AB1 or AB2) for vacuum tube stages where the grid voltage is always negative with respect to the cathode (class AB1) or may be slightly positive (hence drawing grid current, adding more distortion, but giving slightly higher output power) on signal peaks (class AB2). Solid state class AB amplifier circuits are one of the most popular amplifier topologies used today"

My bad sorry, OK. got it.

NOT:
ClassB:
 50% of the input signal is used (Θ = 180° or π; i.e., the active element works in its linear range half of the time and is more or less turned off for the other half). In most class B, there are two output devices (or sets of output devices), each of which conducts alternately (push–pull) for exactly 180° (or half cycle) of the input signal; selective RF amplifiers can also be implemented using a single active element.
 
These amplifiers are subject to crossover distortion if the transition from one active element to the other is not perfect, as when two complementary transistors (i.e., one PNP, one NPN) are connected as two emitter followers with their base and emitter terminals in common, requiring the base voltage to slew across the region where both devices are turned off.[

tdinut

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Re: Bryston Class A power
« Reply #14 on: 25 Apr 2017, 02:12 pm »
I am sorry for bringing up an old thread but would this still be true for the Cubed series of amps?

Appx. 5% of power from Class A?


James! At what point does Bryston move from class "A" operation?
At 3 watts? ten? just curious.
If you know for the 4B-SST2 that would be nice to know in general.
Also the 7B-SST2 and the 28B-SST2...


About 5%.

JAMES

srb

Re: Bryston Class A power
« Reply #15 on: 25 Apr 2017, 04:19 pm »
Yes Class A for about 1% of the power rating for amps.

?

werd

Re: Bryston Class A power
« Reply #16 on: 25 Apr 2017, 04:28 pm »
?

Not only that, is it rated 20-20khz or at 1khz?

Elizabeth

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Re: Bryston Class A power
« Reply #17 on: 25 Apr 2017, 05:45 pm »
For the quotes about 5%, 1% and then the ?

Back in 2011 the 4B-SST² was 'biased' to class A about 5%.
In 2017 the Bryston amps are biased to 1% class A

There is no ? confusion.
If you read the copy of the reprint article (in blue) in the following (where the 1% was mentioned)

Here is the full quote of the quote!!
JAMES TANNER quoted in full:

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
JAMES TANNER:
""Hi Folks

I was asked by a magazine about Class AB amplifier design and thought it might be of interest to some on the forum.


"Hi James

I'm going to be writing up an editorial for April 1 and I wanted to focus on amplifier development/evolution, particularly as it relates to class-AB. It seems, as a non-engineer reviewer, that class-AB architecture has been taken about as far as it can go in the past 10-15 years. Obviously it has some great advantages, too, but I'm curious what your/Bryston's views on it are."

BRYSTON

Bryston has used Class AB output stage architecture for many years because it provides an excellent platform for one of the things we find most important in music reproduction: Linearity. There are numerous ways to implement a high-current output stage for an amplifier meant to drive speakers. There is Class A where all output devices carry signal current at all times; Class B where the individual output devices carry signal current 50% of the time, each on one polarity of the signal; class AB where each polarity of output device carries signal current more than 50%, but less than 100% of the waveform cycle.

There is also Class D, where the signal is converted to a varying pulse-width square-wave on a supersonic carrier, but that is beyond the scope of these comments. Suffice it to say that this can be a good, accurate way to amplify a signal, but the overall linearity is dependent on the accuracy of the conversion to an HF carrier, modulated square-wave. It also presents challenges with excluding the carrier itself from the final, delivered signal to the speakers. It can represent many hundreds of watts of RF energy, and the necessary filtering has a tendency to affect audio frequencies to some extent.

Once we have chosen maximum linearity as one of the most important parameters of an output stage, we can look at the characteristics of the power devices to see where they perform best, and where they may have problems.   Power transistors have a rather broad linear region in their transfer function, from just above their 'on' threshold, up to the higher current region closer to saturation, or fully 'on'. At very low current near threshold, and at very high current near saturation, the transfer function bends toward an overall 'S' shape. Thus, it becomes obvious that for the best linearity we want to keep the output devices operating within the optimal linear region of their transfer function. We can do that by operating the output transistors as opposite polarity pairs, switching from one polarity to the other at the zero-crossing region. Class B will inevitably show a discontinuity in the zero-crossing region, since as we saw above, the transistors are not linear at those low-Voltage, low current parts of the signal. Placing a known continuous biasing current on the transistors prevents them from entering that nonlinear part of their curve, and smoothes the zero-crossing region. There may still be a small discontinuity in the zero-crossing region if the upper and lower output devices do not have exactly equal and opposite characteristics at the crossover point. Unfortunately, opposite polarity devices have similar, but not exactly equal, characteristics.

Attempting to overcome the small remaining zero-crossing discontinuity by increasing the bias current to a level that it keeps the output devices 'on' for 100% of the waveform cycle does nothing but push the devices into the nonlinear region at the high-current end of their transfer function. Distortion actually goes up, not down, all else being equivalent.

Thus, Bryston began exploring the possibility of giving the two polarities of output transistor exactly 'equal and opposite' characteristics. In that way the zero-crossing region can remain linear.  The way we found was to incorporate our 'Quad Complementary' output section, discussed in the white paper sent to you by James Tanner earlier.  That output configuration goes a long way toward virtually eliminating zero-crossing artifacts from the signal, and sound quality is very improved.

In the future, it remains to be seen whether Class D can achieve the overall musicality that a correctly designed Class AB amplifier can achieve. It is not there yet, but we feel it holds promise.

In today's noisy world, there are challenges remaining in terms of an amplifier's immunity to RF and other noise signals coming in on the power-supplies and input cables. Our latest 'Cubed' models have demonstrated that it is quite important to take that into account as well as overall linearity of a test signal. That's also outside of this discussion, of course, but it matters.

I hope this above is helpful, but please let me know if you have questions or comments."" JAMES TANNER quoted
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

me (Elizabeth) again:
You can also get to the article by clicking on the header over this quote: "Yes Class A for about 1% of the power rating for amps." in the post by srb a few posts prior to this one.

you can figure out that to drop distortion and sound better the new cubed amps have a bias of around 1% instead of the squared series 5% ....... most likey done to sound better. But also I bet the amp, if left on 24/7 like I do, will save money on electricity

You can thank me later, I just hope this clarifies the confusion of thinking a post from 2011 and 2017 can be conflated..

ADDED: one other reason for the move to 1% class A may be energy compliance. Rules abound which address energy savings. and the 1% may also be to meet rules from the Canadian government, or even EU or US rules. And those may also be why the squared models just cannot be upgraded. As James has mentioned, they would have to pass current rules.. And maybe they cannot make it work well.
« Last Edit: 25 Apr 2017, 07:06 pm by Elizabeth »

tdinut

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Re: Bryston Class A power
« Reply #18 on: 26 Apr 2017, 12:20 am »
Thank you Elizabeth, I appreciate the clarification.

Ralf

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Re: Bryston Class A power
« Reply #19 on: 27 Apr 2017, 09:25 am »
I think that the BHA is a pure Class-A amp?
I love the sound of my BHA!