Single or dual differential?

[lumanauw]

Mr.John Curl, I agree that anyone cannot take only 1 resource as the only right thing, especially in audio electronics. With more discussion and learning from various experienced people we can get closer to what is the right fact. Actually I always wanted to try to built fet differential, like using k389 or j109. But they are difficult to get here. The best I can get is k30, but that is only 30vmax. Do you have any example of your preferred fet differential input for me to see?

Mr.PRR. I'm interested in using PC's computer card as Harmonic distortion analyzer. In another thread you have mentioned it, but for me it is not clear enough. Can you give me any url on how can I built myself a distortion analyzer with PC's card? Is there any additional software needed?
I'm getting confused when people talks about 2nd order, 3rd, 4th, 5th order harmonics, etc.
Are they (every single harmonics) visible at the pc if we use your arrangement of PC distortion analyzer? Or is it just numbers from calculation?

[Steve Eddy]

Originally posted by john curl
Let me help with this even-odd topic:
'Science and Music' Sir James Jeans 1937 p. 87 This book is available through Dover.
"... The seventh harmonic, however, introduces an element of discord; if the fundamental note is c', its pitch is approximately b (flat) ''', which forms a dissonace with c. The same is true of the ninth, eleventh, thirteenth, and all higher ODD-numbered harmonics; these add dissonance as well as shrillness to the fundamental tone, and so introduce a roughness or harshness into the composite sound. The resultant quality of tone is often described as METALLIC"
Well folks what do you think that this means?

What it doesn't mean is that we're more sensitive to odd-ordered harmonics, which is what Hugh had claimed. Please read what he wrote:

This argument is largely based on the fact that the ear is not sensitive to large levels of H2, up to about 2%, yet hypersensitive to odd, higher orders, such as H7 (around 0.05% according to studies I have seen of the differences between soft and hard trumpet sounds.)

He's not saying that odd-order harmonics sound different compared to even-order harmonics. He's saying that we're more sensitive to odd-order harmonics compared to even-order harmonics.

This is not the first time that I have quoted this passage over the years, but it just gets ignored by those who would not learn from it.

Well I'm ignoring it here because it hasn't anything to do with the issue in question. And the issue isn't whether odd-order harmonics sound different from even-order harmonics, but rather whether odd-order harmonics are more audible compared to even-order harmonics.

SE has told me, in print, to sell my books, since my personally having them is a waste of time and space. Here is one exception to his assertion.

Yes, I'd said that they're a waste of space because of your habit of supporting your arguments by telling people how many books you have on the shelf (i.e. "You're wrong! I've got 200 books on my shelf so I know what I'm talking about!") instead of actually quoting something from those books to support your argument.

So here I'll give you credit for actually quoting something from those books, but unfortunately what you quoted hasn't anything to do with the question at hand.

se

[jcx]

Originally posted by Sawzall

But anyway, accepting the evidence as presented, what effects would the various topology's have on the error spectrum? That really is the underlying question here - and the one this neophyte would like to understand better.

I'm afraid that this question has too many dimensions to be answered in any useful way, I would emphasize PRR's and others comments that implementation "details" totally trump any generalizations drawn from simplistic heuristics and visually pleasing symmetries in the schematic representation

Over the years many designers have made different choices of details in the differential input stage alone; Fet vs Bjt, degeneration, cascodes (Fet, Bjt, bootstrapped, folded), current source vs R bias, current mirror vs resistive load, device part #, manufacturers, matching – any of which could be considered a “detail” of implementation that doesn’t rise to the level of the question of whether to use them in a single or complementary differential input


for a "light" (if not ultimately enlightening) discussion:
Amplifier topology subjective effects

[PRR]

> harmonics created by the machine perform minimal spectral shift on the sounds which are being processed.

Not that simple.

The real harmonics are in the ear. Huge levels of 2nd harmonic. It is irrelevant how much 2nd an amplifier makes. Even on clarinet. And as Steve points out, musical instrument harmonics are not perfect multiples of the fundamental or each other (piano is worst, but even a clarinet has inharmonic overtones).

I don't think that cancelling the even-order harmonics is good or bad in itself. Every-harmonic, or every-other-harmonic, doesn't seem to make a lot of difference. Curl suggests that he used to null even-order, probably because "he could", and has shifted away from that, perhaps because it makes little difference to his designs and his ears, or he finds other benefits in not-nulling the evens.

What does seem to matter is getting the spectrum to slope steeply, and/or get it well below noise level.

No-feedback triodes have a steep slope of distortion spectrum. It roughly parallels the ear's own distortion and tends to be masked.

High-feedback amps (any technology and topology that allows really high feedback) can get distortion well below system noise where it vanishes. (It has to be well below system noise; we hear tones 10dB or so below broadband noise.) That works well enough in "small" amps which can be run well below clipping. In large power amps, transient overload non-ideal behavior may color the sound even when "perfect on test tones".

> 40dB per octave

That's a steeper slope than I think is needed. I'm not sure the masking threshold is the right guide. But for comparision: simple THD measurements apply NO weighting and are well-known to be inadequate. IMD applies a first-order weighting and often compares better to "sound". Olsen's tests suggest more like a second-order weighting (12dB/8ve, 40dB/decade), and RDF 4th cites a study where this is shown to corellate well, but this seems to be forgotten and certainly needs re-examination on current "very clever" amplifers.

As to whether hollow-state, BJT, FET, etc is "best": aside from some real-life issues (you can't take huge feedback around a tube with the transformer it often wants), I think very-good designs are possible many ways. John argues well for FETs; a hard-core BJT designer could argue against his points. I'm inclined to think that details are more important than devices. And also: that many existing designs have flawed details. Case in point: the many straight and elaborated "Classic BJT power amplifiers" that don't work as well as Doug Self's designs. There may be better amps than Self's, but there are certainly a lot of worse ones out there.

Bootstrapping an AB output.... shudder. I think it was whatzhisname at SWTP who first woke me up to this folly. I mentioned the 1972 Fisher because it was a clean example of the post-bootstrap era.

> what effects would the various topologies have on the error spectrum?

Less effect than the detailing. If a BJT diff-pair is not kept perfectly balanced, distortion won't cancel. Not keeping enough effective current gain does nasty things to BJTs. And while Class AB output stages are a mess, there are a lot of bad Class A designs too. Some of these ills are "easier to avoid" with FETs, but you can't just stick some FETs in a "topology" and get great results.

As for "getting stuck in Doug Self"... While I only discovered Self a few years ago, I've been mucking-around with The Classic Topology for (hmmm...) 30 years, and still find tidbits to chew on in his writings. More brilliant designers have noted all this before and moved on, but for many of us a few years of pondering Doug Self is a good foundation for design in any device or topology.

> 'Science and Music' Sir James Jeans 1937.... "... The seventh harmonic, however,..."

Same thing pointed out by Helmholtz in On the Sensations of Tone written in 1875. Before there were amplifiers and harmonic meters. It is amazing how much he demonstrated, measured and extrapolated without electricity. It is heavy reading, and some of the terminology has changed since then, but if you really are interested in what and how we hear you must start reading it.

Another classic, mostly for musicians, is Benade's Fundamentals of Musical Acoustics.

All these books are shockingly cheap via Dover's reprints. (I don't want to know how much my near-mint 1882 copy of Helmholtz is worth...)

> some folks should just be ignored

At least not over-reacted-to.

[Fred Dieckmann]

"Less effect than the detailing"

What an excellent post. I may nominate it for post of the week. There so many considerations in designing a good amp that the choice of the front end topology is a small part of the equation. I have heard several really nice sounding amps that had very different front end topologies. I am building an amp (a friends design) with a very simple and classic topology but with great attention to details. I am very curious what an "oldy but goody" design sounds like with attention to details.....

[Steve Eddy]

Originally posted by PRR
High-feedback amps (any technology and topology that allows really high feedback) can get distortion well below system noise where it vanishes. (It has to be well below system noise; we hear tones 10dB or so below broadband noise.)

Yes, we can hear pure tones a good way down below broadband noise. But the music itself is rather broadband and will be dramatically greater than the system noise so the question becomes how deep into the music can we hear under practical conditions?

Under practical conditions we have not only system noise, but the "noise" of the music itself, as well as the ambient noise in the listening room which even in a recording studio is typically around 30dB SPL, not to mention the fact that the typical listening room is anything but anechoic so you also have a rather reverberant acoustic environent to contend with.

And then there are our ears themselves which when exposed to sound have an autonomic "clinching" response to sound such that the very act of listening to music at typical listening levels dramatically reduces their sensitivity which is already reduced due to the ambient noise levels we're constantly exposed to.

For example, entering an anechoic chamber from even a relatively quiet ambient environment, it takes some time before our ears "relax" and fully acclimate to the reduced noise level before you can start hearing all those things you don't otherwise hear.

An excerpt from Everest's Master Handbook of Acoustics illustrates this well:

The delicate and sensitive nature of our hearing can be underscored dramatically by a little experiment. A bulky door of an anechoic chamber is slowly opened, revealing extremely thick walls, and three-foot wedges of glass fiber, points inward, lining all walls, ceiling, and what coul dbe called a floor, except that you walk on an open steel grillwork.

A chair is brought in, and you sit down. This experiment takes time, and as a result of prior briefing, you lean back, patiently counting the glass fiber wedges to pass the time. It is very eerie in here. The sea of sound and noises of life and activity in which we are normally immersed and of which we are ordinarily scarcely conscious is now conspicuous by its absence.

The silence presses down on you in the tomblike silence, 10 minutes, then a half hour pass. New sounds are discovered, sounds that come from within your own body. First, the loud pounding of your heart, still recovering from the novelty of the situation. An hour goes by. The blood coursing through the vessels becomes audible. At last, if your ears are keen, your patience is rewarded by a strange hissing sound between the "ker-bumps" of the heart and the slushing of blood. What is it? It is the sound of air particles pounding against your eardrums.

So now how sensitive are our ears after being exposed for a time to music at average levels on the order of 80-90dB SPL, on top of the ambient noise, and in a reverberant environment?

In any case, I don't believe there is any singular "best" way to go about designing an amplifier. At least in the practical sense, outside of purely technical arguments. Most every approach has its adherents and the subjective tastes and preference of listeners varies considerably which is why everything from highly non-linear single-ended tube amps and highly linear solid state amps such as the Halcros are able to thrive in the same market.

I think it ultimately just boils down to trying things for yourself and decide which you ultimately prefer.

se

[john curl]

How about a harmonic weighting factor of: A=(n-1)!/2
This will make higher order harmonics important very quickly. This was found in a '72-73 'Wireless World' article by Bob Stewart, now of Meridian. Works for me!

[sully]

Originally posted by Steve Eddy


The silence presses down on you in the tomblike silence, 10 minutes, then a half hour pass. New sounds are discovered, sounds that come from within your own body. First, the loud pounding of your heart, still recovering from the novelty of the situation. An hour goes by. The blood coursing through the vessels becomes audible. At last, if your ears are keen, your patience is rewarded by a strange hissing sound between the "ker-bumps" of the heart and the slushing of blood. What is it? It is the sound of air particles pounding against your eardrums.[/i]

Did some speaker work in an anechoic chamber undergraduate..

That account is accurate, from my experience.. I didn't wait long enough for the hissing sound. But, boy the blood sound is really neat..

Cheers, John

[PRR]

> I'm interested in using PC's computer card as Harmonic distortion analyzer. Can you give me any url on how can I built myself a distortion analyzer with PC's card?

RMAA 5.1 release with manual on "Basics of Audio Measurements"

> 2nd order, 3rd, 4th, 5th order harmonics, etc. Are they (every single harmonics) visible at the pc...?

Yes, and audible to the ear. Get a sine wave generator. Put it through an amplifier. At normal levels the output sounds a lot like the input. When you crank it "too high", it sounds like another higher-pitched instrument has joined-in. If that instrument is playing an octave higher (2X frequency) than the fundamental, it is Second Harmonic. If it sounds like 1.5 octaves higher (3X frequency), it is Third Harmonic. Two octaves (4X freq) is Fourth Harmonic.

(Note that Physicists count the Fundamental as "First", while musicians often speak of "partials" counting the octave-up overtone as "first". Most technical literature uses the Physicists' numbering, but you may see it otherwise in musician's and instrument-makers' writings.)

In a single-ended tube amplifier, you typically hear the 2nd come up first, then the 3rd, and if you listen close you can identify 4th, 5th, and higher harmonics. Some push-pull or heavy-feedback amplifiers won't make the even-numbered harmonics no matter how hard you smack them. BUt many-many "push pull" amplifiers have non-push-pull driver stages. Curl cites this as a reason to go all push-pull. If you use the simple push-pull tube amplifier with voltage-amp, split-load phase inverter, and P-P output, with triodes, the driver stage is often making lots of 2nd harmonic while the output's 3rd harmonic is still small.

While it is easiest to hear these harmonics when you smack an amp into clipping, lesser amounts of harmonics are present all the time, and you hear them even if you can't pick them out. Once you hear them "big", it is easier to identify small amounts of distortion.

[PMA]

I would prefer the complementary differential topology. With added current mirrors and cascodes.