KISS 114 by Andre Jute
This text is copyright Andre Jute 1998, 2004 and may not be reproduced
except in the thread KISS xxx on rec.audio.tubes
THE VOLTAGES IN THIS AMP WILL KILL YOU. GET EXPERIENCED SUPERVISION IF
IT IS YOUR FIRST TUBE AMP

MEASURING PROGRESS BEYOND EXCELLENCE

It is no big deal designing a standard good 300B amp. You take two
stages of 6SN7 (we'll come to them) and a 300B, two of the most linear
amplification devices ever made with which only a fool can go wrong,
you surround them with carefully calculated values of the best
components and you present the whole in a tasty but simple chassis to
justify the price you will have to charge for this fine window on the
concert room when the amp is matched with the right speakers. This is
a class act. It is also a standard engineering job, designed,
specified and delimited to work in that part of its operating band and
in an audio chain where distortion will not be objectionable. As an SE
amp it benefits from the well known psychoacoustic fact that second
harmonic distortion is less objectionable than third harmonic
distortion, odd harmonic distortion, higher levels of harmonic
distortion, of all of which nastiness the standard good 300B has only
a very little.

A standard published 6SN7-6SN7-300B design from me (or half a dozen
others) has its little extra justifying its reputation and price
provided by careful parts-matching to our wu or artistic sense. Part
of that artistic sense is ruthless application of the KISS principle
so that the resulting amp has zero negative feedback, no superfluous
filters and other excrescences, and of course the minimum parts count
to color the sound. Build the amp as we tell you and you can reproduce
what we hear, or as near as room effects will allow, without spending
years and several fortunes on wasted parts. Change the parts,
especially the capacitors, and you can tailor the sound to your taste.

Implicit in these statements is the admission that fidelity has not
yet been achieved. There is a school of solid state aficionados who
claim that the battle is over. Their measure of success was once
inaudible total harmonic distortion (THD). Their amps, complete with
provably inaudible THD, still sounded wretched to many of us constant
concert goers. Gradually their belief in inaudible THD morphed, much
like Marxism when it cannot win an argument on its merits, into a
belief that vanishingly small THD is the solution. Many with good ears
and a lot of experience with the finest live performers think that
amplifiers designed to the THD paradigm, however it may now be stated,
still fail to satisfy their perfectly reasonable demand that the
amplifier must faithfully reproduce at the ears the experience in the
concert hall. We listen with our ears, not with instruments, said a
large number of us, including some with decades of engineering
experience in audio and elsewhere plus other skills in psychology
besides.

Meanwhile, dissatisfied audiophiles started returning to tubes because
they seem to offer a better representation of the concert hall
experience, regardless of their measurements. In fact, it soon came to
be believed in a small but influential niche that in tube amps
conformity to the ruling audio engineering paradigm of the day (that
THD predicts which amps are worthwhile) was an inverse indicator of
relative fidelity. For the big powerful, complicated push pull amps
with lots of negative feedback (the solid state cure-all) and
therefore often superb measurements . . . cast the same chill over the
music as silicon amps. By contrast, the worst measuring amps, single
ended zero negative feedback amps, which by all the prevailing wisdom
should sound like **** . . . in fact pleased most of all.

One can always tell when a prevailing fad is past its best. One merely
watches the little men, the hangers-on of the fad, the ones who will
never have a new idea of their own, become shriller and shriller in
their defense of the most extreme positions of the fad. That is what
has happened to THD as the miracle measurement and predictor of sonic
bliss. It turned out to predict only which credit card audiophiles are
most easily parted from their dosh. The big men already slipped away
who the year after next will have the next big idea neatly dressed up
in attractive marketing jargon. (I have left out an intermediate
stage, in which IMD was, to my mind much more creditably, the big
cure-all, and a couple of lesser false starts. If you are reading me,
you are smart enough to get the idea without the iterated boredom.)
The little hangers-on of audio engineering are screeching ever more
hysterically that there is no God but THD, and He is the only God of
every audiophile, and we, His Faithful, will kill all the unbelievers,
infidels!, who dare enjoy themselves.

Meanwhile, it left me with very little to fall back on except my taste
and wu. Having a firm belief in physics as the ultimate explanation of
almost everything, I was not too keen to go down the cablie route of
opinions apparently based on black magic. There had to be an
explanation that could be tested and proven in a laboratory and,
arising from the explanation and tests, an engineering measure of
success which could double as a predictor of further success.

In fact, what I wanted was already to hand in Olson's work, in which
it was found that a higher level of distortion was bearable if it were
second harmonic than if it were third, odd, or higher harmonic. This,
I argued, is not actually about THD but about the distribution of the
harmonics that make up THD, a subtle but very real distinction at your
ears.

It turned out to be the engineering lock to glee, a most pleasing
paradox.

At this stage I had already by experimenting, listening and conducting
double blind tests with real live classical musicians, discovered that
it was not SE that held the magic, though it was more obvious in SE
and easier to achieve, but Class A1 operation, and ZNFB or very little
negative feedback where absolutely essential. The pointers to Class A1
were in fact pointers to a pleasing harmonic distribution because that
is what Class A1 excels at, or at least it is easier to arrange in
Class A1.

In search of ever smaller THD I had also by this stage developed my
HIGH concept, which was simply high voltage, high current, high
impedance, all of which singly, and more so in combination, ensure
that the signal running along the loadline crosses the transfer curves
at the most advantageous angle, often perpendicular to the transfer
curves. This is what we shorthand as a flat response. Another
successful investigations used shunts of all kinds (silicon, wound,
tube) to stabilize every current to as near constant as possible. I
ended up with a bigger tube to shunt the current beside every tube
carrying the signal, and everywhere else. A venture in heavy current
ITs, leading to all-transformer coupled amps, was also successful.
Costs and weight spiraled through the ceiling. Experiments in
achieving the same effect without the penalties were partially
successful and I came away from them with a liking for the sound of
the SRPP (which is not, repeat not, a constant current topology).

At this point statistical analysis of my measurements persuaded me
that what matters is not how distorted the output is but how the
distortion is distributed. Do not go overboard here. My amps were
already far less distorted (THD) than those of every other designer
because I was and am willing to pay a higher price for silence than
anyone else. An example is The KISS Amp, the overarching subject of
this major series, where I have deliberately insisted that those who
build it must have speakers of high sensitivity, and where you will
shortly see me limit output to less than half of what most engineers
would say should be available from the amp.

THD must be under the audible limit already before any other benefit
of these principles can be realized.

In KISS 115 I shall publish instructions for a more detailed
calculation of distortion than the easy way I recommended before. Do
at least read it and make one or two stabs at a working up an example,
just to be able to say you did it. Those who are mathematically minded
can find in the sources (the RDH describes a few) even more
complicated and detailed methods but there won't be much to calculate
or measure of odd or higher distortions in The KISS Amp, so for this
amp those methods are of academic interest. If you have the math and
you adapt The KISS Amp to other tubes than those under consideration
by me, you may find more detailed methods of calculating distortion of
use.

THE INCOMPETENT AND THE SUBLIME

All of these considerations, experiments, measurements and analyses
conducted over a period of a decade came to a head when a hanger-on of
a none-too-competent winder, whose wares I refused to specify,
challenged me to a design contest. The contest broke down in a flame
war after the power stages had been published so the driver stages
were never published, which was probably just as well because I have
little taste for kicking a dog when he is down.

The challenger published the power tube stage of his own amp, designed
by himself and his wannabe-winder buddy or more likely ignorantly
adapted by them from a perfectly competent if dull design by an
established professional. To make the two designs directly comparable,
I detuned the design I was working on to use the same load impedance
as the challenger used in his design (but not a transformer from the
wretched winder!). The implication is that my amp in real life, when I
would load the plate with an even higher impedance, was even further
ahead of his amp.

Here is a comparison of the two power stages calculated not by me but
by that distinguished RAT, John Byrns, who also gave the two amps
their descriptive names. I had no need to calculate because I already
knew what a comparison would show.

Table © 1998 John Byrns
(Let us hope you can make a table by setting your viewer to 10pt
Courier.)
Bob Cs "Bubbaland "Andre's "Hedonist"


Grid Plate Grid Plate

Voltage Current (mA) Voltage Current (mA)

0.0 102 0.0 136

-22.3 87 -23.4 119

-76.0 50 -80.0 80

-129.7 18 -136.6 44

-152.0 6 -160.0 30


Load Resistance 5000 Load Resistance 5000

Supply Voltage 350 Supply Voltage 390

Plate Dissipation 17.5 Plate Dissipation 31.2

Power Output 5.9 Power Output 7.0

2nd Harmonic 4.13 2nd Harmonic 2.83

3rd Harmonic -0.82 3rd Harmonic -0.03



Table © 1998 John Byrns. If it dinna work above, copy the text below
and use a word processor to make a table, setting 4 columns, 16
rows, comma separator.

Bob Cs "Bubbaland, , "Andre's "Hedonist", ,

Grid, Plate, Grid, Plate,
Voltage, Current (mA), Voltage, Current (mA),
0.0, 102, 0.0, 136,
-22.3, 87, -23.4, 119,
-76.0, 50, -80.0, 80,
-129.7, 18, -136.6, 44,
-152.0, 6, -160.0, 30,

Load Resistance, 5000, Load Resistance, 5000,
Supply Voltage, 350, Supply Voltage, 390,
Plate Dissipation, 17.5, Plate Dissipation, 31.2,
Power Output, 5.9, Power Output, 7.0,
2nd Harmonic, 4.13, 2nd Harmonic, 2.83,
3rd Harmonic, -0.82, 3rd Harmonic, -0.03,

Note first of all that my Hedonist (thanks, John) though deliberately
run high and hot, is still within 80 per cent of maximum dissipation,
still a conservative amp. Next note that the power output of my
Hedonist is higher than that of the Bubbaland, while distortion in my
amp at full power is 2.86 per cent and in the Bubbaland at its lower
full power distortion is at least 4.95 per cent and probably well over
5 per cent when the probably substantial higher harmonics are taken
into account. The incompetence of the Bubbaland as an SE amp is
announced by a third harmonic distortion that reaches for 1 per cent!
As a contest this was already such a massacre of the challenger that
no one bothered to calculate how much my Hedonist would be ahead at
the 1W which is probably beyond the maximum it would ever see in
everyday use.

The excuse of the incompetent so-called designers of the Bubbaland
300B was that they took their operating point directly off the tables
Western Electric published. There is of course nothing wrong with
those tables if you understand the assumptions on which they are
given. These two jokers did not. How they could not have heard that
their amp was grossly distorted baffled everyone. Perhaps they
listened to it only with instruments, perhaps they really have cloth
ears.

That is all very entertaining, if you like schadenfreude, but what is
the real nugget of truth here, the main beef, the fillet for the
bourguignonne? Actually, it is a pity that the headline number of
second harmonic distortion in the challenger's amp was so bad at 4.13
per cent because it masks the real triumph of the Hedonist, that the
distortion for practical purposes consist only of pleasing second
harmonic, with third harmonic being simply inaudible at 0.03 per cent,
a silicon dreamland number.

It is that tiny little distinction which makes my amps sound so much
better.

It is the engineering face of musical ecstasy, both its measure of
success when measured and its predictor of future success when
calculated on the transfer curves.

In the larger scheme of things then, THD is useless to measure
anything you really want to measure because it doesn't predict
anything.

What you really want to know from any tube amp with ultra-fi
pretensions (DHT SE ZNFB) is:

1. THD at 1 or 2 watt is below audibility which I arbitrarily set at
0.75 per cent because that is where distortion begins to be noticed by
the most critical listeners.

2. At any power up to twice or thrice the maximum recommended draw
with sensitive speakers, second harmonic should be in excess of 90 per
cent of all distortion. A really good amp does much better than the
bare minimum on this parameter. (Mine in the contest above managed 99
per cent at full power and would have done better still with the 5K6
tranny I specced for the real amp.)

3. Even at full power third harmonic distortion is tiny, many, many
decibels below the fundamental, below second harmonic, below
audibility. In other words, the designer disappeared it.

Those in the habit of making semantic arguments will notice that the
third point, the absence of third harmonic distortion, is a corollary
of the second point, that virtually all residual distortion should be
second harmonic. I think it is worth making both checks, writing down
both numbers on a sheet of paper and to sit studying them for a while.
I usually play one of the more triumphant Bach cantatas or soaring
masses while I do it.

ooOoo

Now, having put our minds in gear to discover what really matters, we
are in a position to return to The KISS Amp, where we left the power
stage at 320V operating, -65V negative bias, 65mA on the plate, and a
3K primary impedance hookup on the OPT loading the plate.

If you're looking hungrily at the 80mA, -80V and 390 operating volts
of my Hedonist above, don't get carried away. I have no intention of
running my last pair of gennie Western Electric 300B that high and if
you're a newbie you might want to cool your ardour until you have more
experience. That amp was designed for a fellow who will never again
have to count the cost of a pair of WE300B.

We cannot do anything about the quiescent current without changing our
power supply transformer or our desired supply filter principle, both
of which I am loath to do. We will return to the power supply after
designing the driver stage of The KISS Amp.

The only other element still within our control in the power stage is
the OPT, which I remind you is a Lundahl 1623-SE 90mA. This very
sophisticated cut-C-core tranny is however so segmented that its
secondary can be configured to reflect an impedance of 5K6 onto the
primary and thus the plate of the 300B, instead of the 3K with which
we have been working until now. We make new links on the transformer
already in place and voila! 5K6 will flatten the transfer function
much more than adequately to give us a very quiet amp with an
agreeable harmonic distribution. The price in power may bother
engineers (it may bother them a lot if they know the dollar price of
300B!) but we don't care. The 3.8W The KISS Amp when finished will
produce is still higher than the rather generous outermost maximum of
3W we calculated.

With our 100dB horn that is enough drive for 106dB or, more sensibly,
to allow us to trade in some of the signal swing for silence in the
driver stage, which in turn will also linearize the 300B further.

NEXT: SEVERAL INTERESTING SECTIONS OF EXPLANATORY MATERIAL ABOUT
QUESTIONS RAISED SO FAR
FOLLOWED BY: A DRIVER FOR THE KISS AMP