What to look for in a power amp?

[Gregadd]

...The fundamental reason why amplifier distortion persists is, of course, because it is a difficult technical problem to solve. A Science proverbially becomes an Art when there are more than seven variables, and since it will emerge that there are seven major distortion mechanisms to the average amplifier, we would seem to be nicely balanced on the boundary of the two cultures. Given so many significant sources of unwanted harmonics, overlaid and sometimes partially cancelling, sorting them out is a nontrivial task.


Make your amplifier as linear as possible before applying NFB has long been a cliche, (one that conveniently ignores the difficulty of running a high gain amp without any feedback) but virtually no dependable advice on how to perform this desirable linearisation has been published. The two factors are the basic linearity of the forward path, and the amount of negative feedback applied to further straighten it out..."

http://www.eetimes.com/design/audio...Part-I-the-sources-of-distortion?pageNumber=0

 

[Gregadd]

Distortion in power amplifiers arises from:


1. Non-linearity in the input stage. If this is a carefully-balanced differential pair then distortion is typically only measurable at HF, rises at 18 dB/octave, and is almost pure third harmonic. If the input pair is unbalanced (which from published circuitry it usually is) then the HF distortion emerges from the noise floor earlier. As frequency increases, it rises at 12 dB/octave as it is mostly second harmonic.


2. Non-linearity in the voltage amplifier stage surprisingly does not always figure in the total distortion. If it does, it remains constant until the dominant-pole frequency P1 is reached, and then rises at 6 dB/octave. With the configurations discussed here, it is always second harmonic.


Usually the level is very low due to linearising negative feedback through the dominant-pole capacitor. Hence if you crank up the local VAS open-loop gain, for example by cascoding or putting more current-gain into the local VAS/Cdom loop, and attend to mechanism four below, you can usually ignore VAS distortion.


3. Non-linearity in the output stage, which is naturally the obvious source. This, in a Class-B amplifier, will be a complex mix of largesignal distortion and crossover effects, the latter generating a spray of high-order harmonics, and in general rising at 6 dB/octave as the amount of negative feedback decreases. Large-signal THD worsens with 4 ? loads and worsens again at 2 ?. The picture is complicated by dilatory switch-off in the relatively slow output devices, ominously signalled by supply current increasing in the top audio octaves.


4. Loading of the VAS by the non-linear input impedance of the output stage. When all other distortion sources have been attended to, this is the limiting distortion factor at LF (say below 2 kHz). It is simply cured by buffering the VAS from the output stage. Magnitude is essentially constant with frequency, though overall effect in a complete amplifier becomes less as frequency rises and feedback through Cdom starts to linearise the VAS.


5. Non-linearity caused by large rail-decoupling capacitors feeding the distorted signals on the supply lines into the signal ground. This seems to be the reason many amplifiers have rising THD at low frequencies. Examining one commercial amplifier kit, I found that rerouting the decoupler ground-return reduced THD at 20 Hz by a factor of three.


6. Non-linearity caused by induction of Class-B supply currents into the output, ground, or negative-feedback lines. This was highlighted by Cherry3 but seems to remain largely unknown; it is an insidious distortion that is hard to remove, though when you know what to look for on the THD residual, it is fairly easy to identify. I suspect that a large number of commercial amplifiers suffer from this to some extent.


7. Non-linearity resulting from taking the NFB feed from slightly the wrong place near where the power-transistor Class-B currents sum to form the output. This may well be another common defect.


Having set down what Mao might have called The Seven Great Distortions " Figure 2 shows the location of these mechanisms diagrammatically " we may pause to put to flight a few Paper Tigers. The first is common-mode distortion in the input stage, a spectre that tends to haunt the correspondence columns.


Since it is fairly easy to make an amplifier with less than < 0.00065% THD (1 kHz) without paying any special attention to this, it cannot be too serious a problem. A more severe test is to apply the full output voltage as a common-mode signal, by running the amplifier as a unity-gain voltage-follower. If this is done using a model (see below for explanation) small-signal version of Figure 1, with suitable attention to compensation, then it yields less than 0.001% at 8 V r.m.s. across the audio band. It therefore appears that the only real precaution required against common-mode distortion is to use a tail current-source for the input pair.

http://www.eetimes.com/design/audio...Part-I-the-sources-of-distortion?pageNumber=2

 

[Gregadd]

...seems surprising that in a world which can build the Space Shuttle and detect the echoes of the birth of the universe, we still have to tolerate distortion in power amplifiers. Leafing through recent reviews and specifications shows claims for full-power total harmonic distortion ranging more than three orders of magnitude between individual designs, a wider range than any other parameter.


Admittedly the higher end of this range is represented by subjectivist equipment that displays dire linearity, presumably with the intention of implying that other nameless audio properties have been given priority over the mundane business of getting the signal from input to output without bending it. (Note: we subjectivists blame the the objectivist for claiming amplifiers, if not already perfect, were certainly good enough.)

Given the juggernaut rate of progress in most branches of electronics this seems to me anomalous, and especially notable in view of the many advanced analogue techniques used in op-amp design; after all power amps are only op-amps with boots on. One conclusion seems inescapable: a lot of power amplifiers generate much more distortion than they need to.


This series attempts to show exactly why amplifiers distort, and how to stop them doing it, culminating in a practical design for an ultra-linear amplifier. It should perhaps be said at the outset that none of this depends on excessively high levels of negative feedback. Many of the techniques described here are also entirely applicable to discrete op-amps, headphone drivers, and similar circuit blocks. Since we are almost in the twenty-first century I have ignored valve amplifiers.


Since mis-statements and confusions are endemic to audio, I have based these articles almost entirely on my own experimental work backed up with spice circuit simulation; much of the material relates specifically to bipolar transistor output stages, though a good deal is also relevant to mosfet amplifiers. Some of the statements made may seem controversial, but I believe they are all correct. If you think not, please tell me, but only if you have some real evidence to offer...
Sounds familiar.
http://www.eetimes.com/design/audio-...n?pageNumber=0

 

[Gregadd]

I think the problem with choosing components is not the fear of measurements. The problem is their is almost no neutral advice out there. Almost everyone is trying to sell us something. Either a product, a magazine, or an idea. If you look deeply those who are trying to sell us an idea (okay scientific principle) are being financed by someone who is trying to sell us a product or magazine. Despite the fact that everyone concedes financial motive is a serious conflict of interest, they all maintain it's a conflict for the other guy. I'm just applying science and am completely unbiased. When I encounter an expert in court, the first thing I want to know is who is backing your research and who is paying you to be here.
What about science ? One would think that science is neutral and could tell us what we want to know. Evening assuming honesty and competence, a huge act of faith, we find no agreement by scientists on what the perfect amp is or how to evalaute it. They argue and call each other names just as we do on the audio forums.

What is amplifier design? Art or science? The author above desperately wants it to be all science. If you allow too much art, along comes the valve amps and those pesky subjectivists with thier "listening evaluations." He can't quite pull it off however and concedes amplifier design is part Art. That means even after you do all the science you have to make some subjective calls to finish the job. The perfect window of opportunity for the "anti-measurement subjectivists." The subjectivist dig thier heels in. This is what I hear and while misprints are nice, I'm never going to allow measurements to convince me I'm not hearing it. The objectivist cling to thier measurements. If i can't measure it, I can't hear it. Nothing is going to convince me otherwise. Both sides feign tolerance of the other sides position. Phrases like go ahead measure all you want. Spend your money any way you like. If pushed their contempt for the other sides position emerges.

It would be nice if every manufacture had to submit his product to an independent lab for testing. That of course would involve money, politics, and a scientific consensus of what test should be used and what significance to place on the results. Good luck with that.

From a marketing perspective their is nothing more attractive than an ill -informed market acting on thier personal preferences. OTOH there is nothing more frustrating than the markets blind allegiance to some audio guru. A review still carries more weight than all the measurements and DBT the objectivists can muster.
Reviews have let us down. Measurements have let us down. Measurement intensive reviews have failed us.

I recall this young married guy I and were having a beer, he related he really loved his wife but she was driving him crazy. They argued about everything. I told him there is only one way to be happy. You must accept the fact that your wife is always right. Even when it appears she is completely insane.

So it is for any audiophile when he relates that listening or measurements have failed him. Or that the corrupt, incompetent reviewer does not not know what he is talking about. The true believers reassure her that it will work. You just have not fully committed to the ideology( okay for you measurement guys science) or not executed it properly.

I think I'll stop here for now. Please remember I write this solely for my amusement. If you found it entertaining more's the better

 

[Gregadd]

How Much Distortion is Audible?

This might be useful:

http://www.silcom.com/~aludwig/Amplifier_distortion.htm


http://www.axiomaudio.com/distortion.html

 

[Gregadd]

Should I go balanced or single ended?

http://www.walrus.co.uk/tech/bal.htm

 

[DonH50]

One of the issues with balanced circuits is that they are not always truly differential, and quasi-differential as often used in consumer (and some pro) gear is not as robust. Better than SE, if done right, but you don't get the full benefits of balanced without everything also being differential. Unfortunately, I realize that explaining the difference could take some time and effort... Suffice to say that most gear that is truly differential will say so, or perhaps include something like "fully-balanced throughout" in the product sheet. - Don

 

[Orb]

One of the issues with balanced circuits is that they are not always truly differential, and quasi-differential as often used in consumer (and some pro) gear is not as robust. Better than SE, if done right, but you don't get the full benefits of balanced without everything also being differential. Unfortunately, I realize that explaining the difference could take some time and effort... Suffice to say that most gear that is truly differential will say so, or perhaps include something like "fully-balanced throughout" in the product sheet. - Don

Also I do wonder how many products implement signal/chassis ground correctly (I have the Rane papers but is that even a standard) or even how they mix unbalanced/balanced circuits (appreciate this can be a nononono depending on consideration with isolation).

Cheers
Orb

 

[DonH50]

I have no way of knowing but it's a good question. In the RF world it is always in our minds that the ground is just the other side of the signal path, and the ground is as critical as the signal (it's the same current). As a data-converter designer, it is even more critical, if possible, since I also have to worry about coupling across the A/D boundary. High-gain and digital audio have the same issues. I have seen very good and very poor ground schemes through the years in consumer and pro equipment. The better the equipment, generally, the more attention has been paid to every detail, including proper current flow. - Don

 

[muralman1]

I finally took the time to read through this amp topic. Class D was mentioned only once, and that was by Greg. It was a favorable report on the efficiency of Class D. My case is a very special case. At 1 ohm, and 76 db, my speakers are probably the least efficient speakers extant. Putting their specs together, the match between class D and an inefficient speaker is a no-brainer. At only 250 watts per channel, this 93% efficient amp can drive the 1 ohm speaker to deafening levels. That is the definition of efficiency.

Some disparity was thrown on feeding caps. The amp I am using has huge caps bridged with .1 caps. There is a bank of small caps that efficiently feed the quick transients. I am not a techy, so that class D tutorial will have to suffice for now.

There was a lot of characterizing of tube amps vs. solid state and how one would do better on bass than the other. Feedback was brought into the discussion too. What the varying topologies did was carve their house sound.

The initial question was what should be an amp's requirement.

Class D, the ones I am using, has no sound I can hear. What a great class D amp will do, is spotlight what the owner's other components really sound like. The overwhelming majority of components sound pretty bad in a class D system. That accounts for the general dismissal by audiophiles. The fact is there are components that work fabulously with class D, making class D the preeminent amp type IMHO.

This is the reason why I keep droning on about the superiority of NOS sources. There is nothing so important when harnessing the power of the class D amp. There basically is no good solid state preamp that I know of, save one. Lots of class D users use tubed preamps. That is a bandage solution. It is there to ameliorate the bad behavior of over sampling sources, and sinful cables.

The one solid state preamp that is needed for full system performance just happens to be the one I am using. It was designed with class D's abysmal low impedance in mind.

So, it is my contention that a great class D amp has no particular sound. Cabling will throw the sound wildly from coarse to pristine. When you have faith, and work diligently to find the best cabling, source, and preamp class D is unrivaled when pushing an inefficient speaker.

 

[DonH50]

I would think that a great amp of any class should have no particular sound. The issues with class D amps have historically been getting their clock rate high enough (BTW, class D amps fit the "oversampling" label), making them stable under various loads, and providing sufficiently low wideband output impedance given the need to filter their HF noise.

As far as I know there are no class D tube amps -- is that still true?

The first class D amp I have had in ages is in my system now, in the outputs of my Pioneer receiver. It sounded fine to me on my old Infinity speakers, and much better than the Sony it replaced (on the same speakers -- Infinity Beta something or other). I am only driving rear channels with it at the moment; plan to someday use the main L/R and compare to my Emotiva amp but have not felt like taking the time (especially since I finally got the console back against the wall -- no room to get behind and swap wires around now!)

 

[muralman1]

I would think that a great amp of any class should have no particular sound. The issues with class D amps have historically been getting their clock rate high enough (BTW, class D amps fit the "oversampling" label), making them stable under various loads, and providing sufficiently low wideband output impedance given the need to filter their HF noise.

As far as I know there are no class D tube amps -- is that still true?

The first class D amp I have had in ages is in my system now, in the outputs of my Pioneer receiver. It sounded fine to me on my old Infinity speakers, and much better than the Sony it replaced (on the same speakers -- Infinity Beta something or other). I am only driving rear channels with it at the moment; plan to someday use the main L/R and compare to my Emotiva amp but have not felt like taking the time (especially since I finally got the console back against the wall -- no room to get behind and swap wires around now!)

There are no class D tube preamps that I know of, and very few class D preamps at all. My preamp is a class A preamp. It was devised with class D in mind, but folks who love using it in solid state applications too.

The 999 Sony SACD Modwrite failed miserably on my system. The fellow who brought it over had the same amps as I. He sold the Sony and bought a NOS optional player.

You brought up a very good point. Yes, Class D does sample rate at much higher levels than CDs do. It is just possible the digital filter of the oversampler CD players interact poorly with the class D module.

However, I switched to NOS players when I was driving these speakers with Pass Labs X 600 monos. It just sounded more true to me then. Solid state amps produce a lot of haze when pushing 1 ohm. That is self noise.

 

[Gregadd]

1 ohm, and 76 db

That better be a damn good speaker.

 

[DonH50]

I'm still confused, sorry. This thread is about power amps, not preamps; are you saying your power amp is class D and it uses tubes in the output stage? What kind is it?

For that matter, what kind of speakers? The only thing I have seen in the past with that combination of low impedance and low efficiency are big (full-range) ribbons...

Curious, thanks - Don

 

[muralman1]

The tubes are in my DAC. The preamp is Class A. The amps have 4 giant ELCO caps bridged with .1µF Sonicap Platinum caps. There is a bank of fast firing caps too. The power supplies are responsible for this abysmally inefficient speaker to attain high spls, and turn on a dime during those quick changing dynamic scores.

My speakers are the venerable Apogee Scintilla.

 

[Gregadd]

My speakers are the venerable Apogee Scintilla.

Well worth the effort of finding a suitable amp.

 

[muralman1]

If you're still using these ice module amps: www.iceh2oaudio.com, the Sonicaps are being used as bypass caps.

I meant bypass..... Like I said, I am no techy, I just prove techys are wrong.

 

[DonH50]

Not to quibble, but I was right about the type of speakers... Techies have ears too, and some of us are musicians on the side. - Don

 

[Gregadd]

Premium parts in amplifiers

Steve's Comment on the Spectral DMA 260 brought my attention to this next discussion of premium parts. In looking for our amp we have opened it up. G I am not sure there is any real purpose to point to point wiring anymore, Given the excellent circuit boards.
Premium parts

I'll probably need help here. I'm sure we have heard of Vishay or Caddock resistors, black gate or MIT caps. It is remarkable so many megabuck amps sem to utilize off the shelf parts.

If you knw of any premium parts that make a difference tell us.

Spectral claims the DMA260 benefits from the availability of new parts. Check it for yourself. http://www.spectralaudio.com/bulletins/0410.pdf

 

[DonH50]

Interesting article (if a little vague, but it's not an IEEE paper for goodness' sake). Since audio circuits have very high gain and wide dynamic range, proper signal isolation and grounding is incredibly difficult. I am not surprised the article dwells a bit on that; most folk probably do not realize just how challenging it is to build a quiet, (relatively) broadband, high-gain amplifier chain.

A lot of audiophiles do not realize that some premium parts provide no audible benefit and in fact are worse in audio circuits. A prime example is metal-film resistors: they are noisier than other film resistors and have higher self-capacitance. Vishay (have not checked Caddock lately) has articles on this, but it is well-known to *ahem* techies in the design arena and has been for decades. They are great for stable bias circuits, properly decoupled of course. Certain types of film capacitors are better than others, etc. The differences are measurable and often audible. As usual, finding the right components requires a blend of science (engineering) and art (plus listening), with a deep understanding of not only the technical parameeters but how they translate to audibility.

And, of course, there are always cost considerations...