BRYSTON CUBED AMPLIFIER PHILOSOPHY

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James Tanner

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BRYSTON CUBED AMPLIFIER PHILOSOPHY
« on: 19 Sep 2019, 03:45 pm »
Hi Folks,

A reviewer asked me recently what differences the Bryston Cubed Amplifiers incorporated (because he was smitten) over previous versions of our amplifiers. So I thought our customers might find the following interesting.

BRYSTON Cubed Amplifier Philosophy

FIRST TO LAST WATT PHILOSOPHY:

A significant part of the design criteria for the new Cubed amplifiers was to develop amplifiers that would maintain an ideal power curve through the 'first and last watt'. Most amplifiers exhibit a power curve whereby the best noise floor, drive capability and distortion are maintained from about 1/3 power and up. The new Bryston Cubed series maintain their ideal power curve right from the first watt to the last watt. Think of it like a torque curve in a car. The sweet spot or the torque curve has been expanded.

Achieving this 'First-to-last-Watt' fidelity and clarity has to do with a number of design approaches:

•   First is complete freedom from low-level crossover, or zero-crossing, artifacts.  This is not as easy as it sounds.  Most class-AB amplifiers have sufficient bias to prevent primary crossover distortion, but there is another type of crossover artifact called 'secondary crossover distortion', caused by insufficient speed in the driver transistors.  We use very fast drivers to prevent this, but more important is Bryston's proprietary Quad Complementary Output design vastly reduces the capacitance 'seen' by the driver transistors, virtually eliminating storage delay in the output stage that could contribute to nonlinearities in the zero-crossing region.

•   Second is Bryston's continuing efforts to reduce low-level noise.  The clarity of Bryston's designs is enhanced at low listening levels by pushing the noise floor far below the signal level, improving the 'silence between the notes' and enhancing the clarity of the music at low power levels.

•   Third is Bryston's concentration on reducing distortion at all levels, and most especially at high frequencies.  Bryston amplifiers are perhaps the only designs to concentrate as much effort at reducing HF distortion artifacts as we do, and the results are remarkably 'flat' THD-with-frequency curves, showing almost no tendency to increase distortion as frequency rises.  This has the effect of reducing overall 'haze', helping to pull the quietest passages out of the background.

•   In addition to the above, the new Cubed Series employs highly effective RF rejection in the power supplies, which traps and eliminates radio-frequency hash from either interfering with or emanating from the amplifier via the power cable or signal cables.

There are other small contributors to this low-level clarity, some having to do with power-supply design for extreme stability, (and in Stereo or multi-channel amps, separated for each channel), which very notably improves the placement-in-space and focus of the sonic 'image'.  We think the overall result is an unprecedented degree of clarity and freedom from artificiality, especially noticeable at lower levels in comparison with other designs, but continuing to even the highest outputs.

Further Info:

A few years ago, we had a review in a Swedish magazine that pronounced the 14B SST2 to be 'perfect'. That was not based on 'he liked it'; it had nothing to do with auditory preference, it had to do with transparency. The reviewer had and still has a setup where he can literally 'bypass' an amplifier to drop it out of the signal path, (dummy speaker load, gain-reduction L-pad, leading to a second amplifier to the speakers). The 14B SST2 turned out to be impossible to detect in the signal path, on any signal they tried, from simple guitar music to full orchestra ffff, to an electronic metronome with instant rise time and no overshoot. And it was the first audio device in his history to do so.

That does not mean the amplifier is actually 'perfect', of course. It just means that it is not contributing, (or subtracting), anything the ear can detect, to or from the signal path in an otherwise exemplary system.

From that result and many others of our own, we concluded that static distortion measurements do not tell the whole story. It would seem that if static THD and IMD are low enough, 0.001% or -100dB, they cease to be relevant factors in the transparency of the device. ‎More important are time smear, deviations in frequency-response, dynamic distortions, noise within the device or from RF interference, etc.

Thus we did not introduce our new 'Salomie' input stage (which we received a patent on) simply to reduce static distortion. Our newest 'Cubed Series' amplifiers do have lower THD and IMD than the 'Squared Series' that preceded them, but not significantly so. They measure out at about 0.001% or a bit below, over almost the entire frequency band. However, and we consider this to be most important, they are quieter and more musically accurate in a number of ways; First, the 20-20K noise is reduced, but the new input stage has far better PSRR, or power-supply rejection ratio. This means hash on the power-supply is rejected, in this case by more than 140dB. The input stage also has much better CMRR, or common-mode rejection. That means it rejects noise coming in on the signal cable much more completely than before. These are dynamic issues that can show up in a real-world system completely apart from static THD and IMD.

The audible improvements we realized with these approaches are rather subtle. They would have to be with amps that started out 'perfect' in at least one reviewer's description, but they show up from very low listening levels, almost whisper-quiet, where the silences are more inky-black than before. It's almost an unconscious realization. They amps also cope better with dirt and hash of all kinds on the power cord, from distorted 60Hz Voltage to RF. Thus the quiet middle-of-the-night listening is even more relaxing and emotionally accessible. The depth of the soundstage and the precision of instrument placement is closer to reality than ever before. ‎
Of course, that's what it really boils down to; recreating the original musical experience in your home. The 'you are there' feeling. It's what we have been pursuing from day one, and are still focused on. The Cubed Series is closer than ever.

james
« Last Edit: 19 Sep 2019, 04:57 pm by James Tanner »

Grant Hill

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Re: BRYSTON CUBED AMPLIFIER PHILOSOPHY
« Reply #1 on: 20 Sep 2019, 11:36 am »
thanks James, very interesting.

If it's not out of topic, what do you guys at Bryston think about class A amplifiers?

gberger

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Re: BRYSTON CUBED AMPLIFIER PHILOSOPHY
« Reply #2 on: 22 Sep 2019, 04:01 pm »
James,

IMO, you should print this as a "Whte Paper" and include it in every item you sell.  Also, have it available as a handout at the various exhibits where Bryston equipment is featured.

Just a thought

George

James Tanner

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Re: BRYSTON CUBED AMPLIFIER PHILOSOPHY
« Reply #3 on: 22 Sep 2019, 05:07 pm »
thanks James, very interesting.

If it's not out of topic, what do you guys at Bryston think about class A amplifiers?

Hi Grant

Here is something I wrote a number of years ago but I think basically still holds true today.

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 so 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 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 artefacts, (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!

Going in the other direction, Class D offers high efficiency through a very different approach to output operation. Class D, often erroneously thought of as "digital amplification", is actually an analog system which varies the width of the top-versus-bottom duty-cycle of a square wave carrier frequency. The amplifier still traverses from negative to positive voltages and back again, but does so continuously, at a high frequency of perhaps 500 kHz. The time it spends at one extreme or the other is proportional to the locus, or exact voltage-time relationship, of the desired signal at that moment. Since the output devices spend almost all their time at either full-on or full-off, (areas of absolute minimal dissipation), efficiency is very high, from 80 to 90%. Thus, these amplifiers produce very little heat, and do not have to be as heavy or as large as typical class AB amplifiers, (to say nothing of the class A monsters)! There are naturally disadvantages as well. Class D, by definition, uses very large RF signals, and must be shielded and well-filtered to prevent interference and speaker damaging outputs. This in turn harms overall linearity, as well as adding to the cost, thus this is not an inexpensive technology. The overall distortion is usually on a par with Class A amplification; good but not great, at around 0.1% or so. If efficiency is your requirement, though, this is the way to go.

Next time, we will look into some other classes of amplification, such as class H, or variable power supply, as well as some interesting "combination" classes, to see if some of them might have merit


schmidtmike76

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Re: BRYSTON CUBED AMPLIFIER PHILOSOPHY
« Reply #4 on: 30 Sep 2019, 12:13 pm »
James not sure if this belongs on this thread.  Any news on the BP26(3) if I remember correctly fall release? 

gberger

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Re: BRYSTON CUBED AMPLIFIER PHILOSOPHY
« Reply #5 on: 30 Sep 2019, 12:51 pm »
James,

Also wondering about the new multi-purpose "preamp."

Is the new DAC-3.1 the answer?

BTW: The 17 cubed is awesome. I've had two friends say they didn't know I was using a preamp.

George

James Tanner

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Re: BRYSTON CUBED AMPLIFIER PHILOSOPHY
« Reply #6 on: 30 Sep 2019, 03:38 pm »
James not sure if this belongs on this thread.  Any news on the BP26(3) if I remember correctly fall release?

Hi Mike,

No work being done on the 26(3) at this point.

We are looking at a preamp that would combined the BP17 with the BDA3 DAC first.

james

Phil A

Re: BRYSTON CUBED AMPLIFIER PHILOSOPHY
« Reply #7 on: 30 Sep 2019, 03:47 pm »
There's a whole (old) thread here about the Swedish Magazine Review - https://www.audiocircle.com/index.php?topic=44020.0

schmidtmike76

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Re: BRYSTON CUBED AMPLIFIER PHILOSOPHY
« Reply #8 on: 30 Sep 2019, 05:11 pm »
Hi Mike,

No work being done on the 26(3) at this point.

We are looking at a preamp that would combined the BP17 with the BDA3 DAC first.

james
thanks James