Tag Archives: audio amplifier

Asheville, Walnut Cove, Biltmore Forrest and Western North Carolina’s Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

It’s all in the translation

For most people, amplification of the musical signal means just that. Taking the original signal as generated by the source and making that signal bigger.

Only, that’s not what happens.

In the same way the energy from your leg pressing a gas pedal is not actually amplified in the movement of an automobile, it would be more correct to think of it as being translated rather than amplified.

Why is this important? Because understanding at a fundamental level that an audio amplifier is a power supply whose output valve is controlled by the input signal shines a bright light on the importance of the valve and the power supply rather than focusing on the input signal.

Going back to our car analogy, we shouldn’t care about the quality of the shoe used to control the gas pedal. Instead, we want to focus on how perfectly the translation of our foot’s instructions is carried out by the car’s drive train.

It’s the translator that we should be focusing on as opposed to harboring the notion we’re somehow preserving tiny signals in their original form.

Thus, we designers must pay strict attention to the power supplies that feed downstream stereo equipment and the responsiveness of the valves used to regulate the flow of their power.

It’s all in the translation.

Asheville, Walnut Cove, Biltmore Forrest and Western North Carolina’s Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

Last but not least

Following the past few day’s posts about audio amplifier efficiency, Class AB biasing, and Class A biasing, let’s wrap our little mini-series up with another topology most of us have not heard of, adaptive biasing.

The promise of adaptive biasing is a best of both worlds scenario: the efficiency of a Class AB circuit with the performance of a Class A amp. Sounds too good to be true, right?

The first time I ever heard about an adaptive biasing scheme was way back in the dark ages, the late 1970s. My very dear friend and one of the all-time good guys of audio, Nelson Pass, then of Threshold Corporation, had introduced the idea of what we called a sliding bias scheme, part of what later became known as the Stasis Circuit used in Nakamichi, Threshold and if memory serves correctly, even the Mark Levinson No. 33.

The core of this circuitry is covered in Nelson’s patent from 1975 titled Active bias circuit for operating push-pull amplifiers in class A mode.

Simply put, Nelson’s design raises the level of Class A bias in cadence with the rising input signal.

Recall in our discussion of Class AB design that a small amount of always-on power keeps small signals always on. In other words, we apply Class A (always-on) bias to the first 10% of the amplifiers output signal level, then switch over to the more efficient Class B for higher signals. Compare that to Class A operation which is always-on for any level of signal—always generating a shit-ton worth of heat (recall Class A amps are at their coolest when at full signal out).

What Nelson cleverly suggested was this. Take what we do in an AB amp where the first 10% of the signal is Class A and actively monitor the signal level. When any given input signal starts to exceed our 10% Class A bias, raise the limit from 10% to, say, 20% (or whatever is greater than the signal level), and continue on the path all the way up to 100%. Then back down again tracking the signal. The heat-producing bias is only enough to accommodate the signal, then goes away when it’s not needed.

Thus, we get the benefits of both worlds. Efficient, and sweet-sounding.

Why doesn’t every amp use this even today? (Nelson’s patents ran their course years ago). Well, as with any design there are problems as well as advantages and this post is long enough already. Ain’t nuthin’ perfect. (We used this for several models of amps though their model numbers and dates escape me).

In any case, a juicy piece of history I’d thought I would share.

Asheville, Walnut Cove, Biltmore Forrest and Western North Carolina’s Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

More Class A audio amplifier stuff. Not including sliding bias amps, generally, they are the best sounding of all audio amps!!

If you’ve been keeping up with my rant on amplifier classes you’ll recall that in yesterday’s post I explained class AB operation.

One of the takeaways from that explanation would be that without an input signal, a Class AB amplifier constantly draws a small amount of power out of the wall—enough to warm its heatsinks. Not until there is an input signal does the amplifier start to draw much power.

Remember back to the beginning post on efficiency where I pointed out that in a Class AB amplifier, for 100 watts delivered to the loudspeaker another 100 watts would be converted to heat? The net result of that is an increase in heat for every watt delivered to the speaker. More power delivered to the speaker equals more heat generated by the amplifier. Makes perfect sense, right? The harder the amp works the hotter it gets.

Class A amplifiers are the opposite.

In a Class A amplifier, the more watts delivered to the speaker the cooler the amplifier gets! In fact, the point of a Class A amplifier’s maximum power output happens also to be the point of maximum efficiency.

Weird, right?

A 100 watt Class A amplifier with no input signal draws 200 watts out of the AC wall receptacle. All 200 watts are converted to heat. That same 100 watt class A amplifier delivering 100 watts of audio power to the speaker still consumes the exact same 200 watts from the AC wall socket, only this time, half of the 200 watts consumed goes to heat while the other half goes to making music.

Thus, Class A amps are strange beasts indeed.

Asheville, Walnut Cove, Biltmore Forrest and Western North Carolina’s Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

A little dab

In 2008, authors Karl Johan Åström and Richard M.Murray wrote about feedback: “Simple causal reasoning about a feedback system is difficult because the first system influences the second and second system influences the first, leading to a circular argument. This makes reasoning based upon cause and effect tricky, and it is necessary to analyze the system as a whole.”

Now, that’s about as nerdy as one can get but their point is well taken. One must look at the whole.

Feedback is taking the outcome of a past event and, in the future, comparing it to one’s original expectations.

We have many different forms of feedback: from customers, friends, family, our own internal loops.

In circuits there’s also a great variety: loop, local, forward, negative, positive.

In my experience, in the world of stereo, the best use of feedback is to have it do as little as possible. In other words, we shouldn’t rely upon feedback to set our course. Rather, feedback should be the finishing touch.

This applies equally to personal and company feedback as well as circuits. We know that if an audio amplifier’s open-loop performance (operation without feedback) is good, then the addition of feedback generally makes things sound better. We also recognize that the opposite is true. Rely upon feedback for an amplifier’s stable operation and the audible results are not worth your time.

Like all things in life, a little dab’ll do ya’.

Asheville, Walnut Cove, Biltmore Forrest and Western North Carolina’s Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

Closest to accurate

Despite the fact we use some of the most expensive in the world, a microphone’s output isn’t even close to what I hear in person.

And this charade of realism carries forward through our loudspeakers. Different speakers make the microphone’s feed sound different again.

It’s an endless loop.

When I declare a microphone sounding one way what I am really saying is how the combination of microphone, audio amplifier, and speaker sound.

Changing any one element in the chain changes the sound of all three.

Thus, if we are to speak in absolutes the closest to accurate is but a myth.

We could more correctly suggest one chain of equipment or another is closest to accurate—just not a single link within that chain.

Few things stand solidly on their own.

Asheville, Walnut Cove, Biltmore Forrest and Western North Carolina’s Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl

Loudspeakers designed and measured to sound flat in an anechoic chamber will sound terrible in a real life listening room environment. That’s just the way it is.

The price of flat

There’s really no such thing as a flat loudspeaker.

Loudspeakers are such inaccurate mechanical devices that if one were to attempt a flat frequency response, enormous levels of compensation would need to be applied. And, even then, those compensating changes in amplitude would only work in a very small area.

Unlike an audio amplifier whose input vs. output is fairly straightforward, speakers present an enormous challenge—one that depends on so many outside variables (box, baffle, room size) as to make it a non-starter.

The price of flat, as it pertains to loudspeakers, is equivalent to the price of peace.

Unaffordable.

Not to despair. Fortunately, our ear/brain mechanisms are powerful enough to adjust so that when we listen, it sounds “flat”.

And at the proverbial end of the day, if it sounds flat then it is flat.

Problem solved.

 

Asheville, Walnut Cove, Biltmore Forrest and Western North Carolina’s Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

I mostly agree with this and certainly the amount of power an audio amplifier produces doesn’t alway correlate with sound quality, but parts used and poor build quality, can have an effect beyond the designer.

I once imported a high end audio line from Australia and the products were  very well designed and the parts quality was good, but the build quality and packaging were horrible and ended that experiment for me!

More power equalled worse sound

One of my readers, Daniel, was surprised to find that a smaller power amplifier sounded better than a bigger, more powerful version. He wondered how that could be given how many times I have waxed enthusiastically about the benefits of headroom and power.

Of course, the answer lies not in the power differences but the skills of the designer.

It is often tempting to focus on one area of performance as the key indicator of how a piece of equipment will sound in our systems. Unfortunately, it’s never quite that easy. The number of variables determining sound quality is so many as to make one’s head spin like Regan MacNeil.

Which is why we listen.

Asheville, Walnut Cove, Biltmore Forrest and Western North Carolina’s Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

OK, but not sure about this.

Watts watts?

A watt’s a watt, or is it? The technical definition of a watt is a unit of power, equivalent to one joule per second, corresponding to the power in an electric circuit in which the potential difference is one volt and the current one ampere.

To get a watt out of our audio amplifier we need voltage and current. Once we have that unit of power the magnetic motor system in the drivers converts that energy into movement that compresses the air and we hear sound. A watt is a watt, yet not all stereo amplifiers at the same wattage sound the same. So what’s going on?

Amps with identical wattage ratings sound different because moving watts are different than steady-state watts.

If we were instead looking at wattage steadily powering a lightbulb we’d not really see any difference if that watt was produced by hydro or steam power. But in audio, we don’t listen to steady-state tones. Instead, we’re listening to rapidly moving air pressure. How that air moves defines what we hear.

Thus, we’d be more accurate to suggest that the motion of watts is what matters, not the watt itself.

 

Asheville, Walnut Cove, Biltmore Forrest and Western North Carolina’s Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

Measurements can explain certain things regarding how an audio component might sound, but to design just around measurements is usually a bad idea. Paul goes into this a bit more deeply.

Beware the measurements

For readers of this blog, it will come as no surprise my thoughts and attitudes on measurements of audio equipment. Yes, measurements are important, but only when in concert with listening—and then too only if you look deep into those measurements to find out how/why what you measure sounds the way it does.

If you’re interested in diving deeper into this subject there’s perhaps no better way than to sit down with an experienced designer and have them take the time to explain how and why measurements simply do not tell you the whole story.

Take for example global feedback in an audio amplifier circuit. If we have an amp circuit with relatively high levels of distortion, we can simply wrap the output back to the circuit’s input and voila! lower distortion. The feedback is a fix-it mechanism correcting for the failings of an amplifier’s design. But at what cost? Nothing in engineering is free. Loads of negative feedback offer low distortion figures but at the same time, harden and constrain the sound. Knowing this is invaluable to a designer who can then ask the question, “what if we don’t have distortion in the first place?”

If you’re interested in diving deep with a designer, here’s your chance. Our own Darren Myers, one of the brightest and best engineers in the industry, spent a good hour and a half with John Darko in a great podcast titled, Beware the Measurements.

Have a listen and enjoy.

 

Asheville, Walnut Cove, Biltmore Forrest and Western North Carolina’s Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

The sound of class A

I am often asked what’s the difference in sound between audio amplifier bias schemes like Class A and Class AB?

I understand the motivation of placing labels on circuits when making a product choice. Like tubes vs. solid state, or analog vs. digital. We like to characterize the qualities of technologies so we might put them in neat little category boxes.

The problem is, of course, that Class A and Class AB are biasing schemes that alone haven’t as much to do about how circuits sound as you might imagine.

Certainly, we could say that many Class A topologies generally are sweeter and without the character-changes-with-level some AB circuits have—and we would not be too far off the mark—but consider that many of us have never actually heard a pure Class A power amplifier.

In fact, what we think of as Class A is generally found in a small signal device like a preamplifier or output stage of a DAC, and that most power amplifiers are Class AB.

So, for most of us, our experience with the various classes of biasing schemes aren’t apples to apples, they are more like peaches to plums.

Methinks, in this case, most of our opinions are formed on the basis of what the audiophile myth-making machine would like us to believe.

From where I stand, biasing is but a very small factor in a complex world where everything has an impact on sound quality.