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27 December 2007

Two More OCRed Classic Articles

I asked a friend what he thought of my re-created article on
the Brook amplifier, "High-Quality
Audio Amplifier With Automatic Bias Control," that I had OCRed
from scanned images of the original article. He told me that I had surely
messed up and posted the original images, not the text-filled PDF I had
planned on posting. He was wrong; and it took some persuading to get him
to test the PDF’s contents for text that could be copied and pasted. This
is what all art forgers (and plastic surgeons) hope for: a forgery so good
that no one can be convinced that it is a fake. No doubt what fooled my
friend was the use of the Bookman Old-Style typeface.

Scanned Page

OCR-ed
Page

The above images show that the re-created page on the
right is not identical to the original scan on the left, but being
identical isn’t strictly the point. Much like our pursuit of re-creating
acoustic events in our homes, the re-creation need only persuade the
audience that it is real, not match in every detail the actual acoustic
event. With high-quality audio reproduction, the more believable the
better. To my ears, no matter how otherwise besmirched the signal might
be, tube gear always contributes to a more believable musical
presentation.

(Well, that is about as far as I am willing to
subscribe to the proposition that tubes make nice distortion, as I still
adamantly proclaim that I love tubes precisely because of their
low-distortion. Or as Dan Rather might say, It is better to be true but
fake than to be false but authentic.) I have to step carefully here, lest
I trip and fall into epistemological quicksand.

Switching metaphors, Elvis may be alive in our hearts,
but he is certainly dead on our LPs and CDs. If we can resurrect him in
our ears, however, he can live again. Unfortunately, our high-tech
necromancy often creates acoustic Frankensteins, whose scars and sloppy
stitching are all too obvious. So much can (and usually does) go wrong
that it is amazing that we bother to listen to any recorded music. Well,
at least that is my consideration when I listen to the sound from stereos
on display in big-box electronic stores. I can easily see many buying such
sleek solid-state systems; I just cannot imagine anyone actually spending
hours listening to them.

Imagine never having heard a good vacuum-tube-filled amplifier, which
cannot be that difficult as most of humanity has never heard any
vacuum-tube-filled amplifier, as the worldwide median age is under 28
years old (if I remember correctly).

Ah, in days
of old, when tubes glowed bold (Transistors weren't invented)…

Indeed, I happen also to like the 1950s look in magazines, as it seems
much more self-assured and calm. In sharp contrast, the Positive
Feedback and Wired issues from the 1990s gave me a headache.
In case you forget what they looked like, imagine having a new computer
with 800 typefaces on it and the assumed obligation to use all of them at
once, then add conflicting colors or too extreme a contrast between blurry
heavy black text and white paper.

Sonically, I’d say that we are in the 1990s of sonic reproduction,
which implies that in ten years, all of us will be enjoying an
easier-on-the-ears audio reproduction across spectrum of sound reproducing
electronics, from singing greeting cards to iPods to TVs to home stereos.
Today, it’s amazing enough that a greeting card can sing at all; in ten
years, it will be amazing that it can sing so well. Just call me
pessimistically optimistic.

Mercy, that was a long introduction. The two new OCRed Audio
Engineering articles are just a mouse button click away. The first is “The
Sad Tale of a Half-Watt Resistor,” which is short and sweet.
After reading it, see if you don’t recall the scene from the Woody Allen
Sci-Fi movie, Sleeper, where after tasting the food of the
future, Woody proclaims, “This stuff tastes awful. I could have made a
fortune selling it in my health food store.” For the author who wrote,

“For the high fidelity enthusiast
the implications presented by this
investigation are positively frightening. In his eternal
quest for the elusive goal, perfection, he is forever
replacing good components with better components,”

How unlikely would be today’s audio scene, wherein a resistor that
could—all by itself—add 4% of harmonic distortion would sell for at least
at least $20, not the mere 10 cents he paid for it back in the 19950s.
Just call me cheerfully cantankerous.

The second article is “Effects
of the Cathode Capacitor on P-P Output Stage.” I thought that
this article would be worth reading in light of the following
topic—namely, push-pull output stages with sliding idle current.

But before leaving the topic of interesting PDFs, I have a pair of
contemporary PDFs to recommend. The first is by fellow tube-lover Joe
Curcio, who works for National Semiconductor. It's titled "Keeping
Up With the Expanding Demands of High-Performance Audio." In it
you will find many interesting circuits and the last paragraph even
mentions vacuum tubes! The second PDF is shorter, but still makes some
interesting points about the tradeoffs in capacitor design. "Low ESR Capacitors: Fact or Fiction?" by Mark
Gebbia, of Illinois Capacitor, Inc.

Push-Pull Output Stages and Sliding Idle
Current
Last
time, we looked into how a single-ended amplifier could enjoy a
relatively low idle current that would swell with load passages, allowing
more headroom than the low idle current would otherwise imply. This time,
we will look into the opposite scenario, wherein a low-wattage class-A,
push-pull amplifier starts with a relatively high idle current. Then, when
it is provoked by large input signals, a greater negative bias voltage is
applied to the output tubes’ grids, bringing down the idle current, and
shifting the class of operation to a lean class-AB. Yes, this was the
operating principle behind the Brook amplifier, but it used extra tubes
and a complex power supply to achieve this goal. In contrast, I am
interested in exploring how the same task might be performed more simply
and inexpensively.

Long ago, I read a great little book, The Tacit Dimension by
Michael Polanyi, wherein Polanyi describes how a good bit of scientific
knowledge and practice is never mentioned, but nonetheless essential.
Analog electronics also holds its tacit truths and essential practices,
which we all pick up as we do work in analog electronics. Well, I have
several actual circuit-building habits that I seldom, if ever, mention,
but that I always practice. One is to always scrape away oxidation from
component leads before soldering. No big deal, right? I am not so sure, as
I have experienced the odd result of having otherwise identical circuits
sound different. A friend and I build the exact same circuit with the
exact same PCB and parts from the exact same production lot, and my unit
sounds better than his. My only guess is that clean parts sound cleaner
than dirty parts. Of course, this isn’t a big deal with freshly made
parts, but when using 40-yearold NOS parts, it’s essential. (Even as
little as one year is enough to accrue some dull-gray oxidation on
seemingly new parts.)

Another practice of mine is to always place a 10-ohm-based RC filter in
series with the input of my high-voltage regulators. I do this so out of
habit that I don’t even add the RC network to my own schematics. Another
no big deal, right? I would have agreed, just one year ago; but I ran into
an interesting development when modeling a tried and trusted circuit in
SPICE a few months ago. The circuit was a solid-state high-voltage
regulator that I had built many times, with many different components and
never with any problems. In the world of SPICE, however, the regulator
blew up. My first thought was that the ideal parts of SPICE were at fault,
that if I add real-life parasitics, such as ESR and ESL, everything would
work again. It didn’t. Usually, it’s the opposite, a circuit works in
SPICE, but not in reality, because of the non-ideal parts we use in
reality. Well, I added my no-big-deal RC filter—How big a deal can a
10-ohm resistor and a 0.1µF shunting capacitor be after all?—and the
regulator worked in SPICE as well as it worked in reality!

And still yet another practice of mine is to always place a zener in
parallel with a cathode-bias resistor in tube output stages. I do this for
two reasons: I do not want the bypass capacitor to ever see an
over-voltage and I want the limit the climb in cathode-bias voltage during
loud passages, so that the output stage never moves too far off its
correct bias point. An added benefit is that the zener adds a hard stop to
the somewhat spongy and soft cathode resistor and bypass capacitor, just
when the amplifier most needs to work against something solid. Besides,
adding the zener is easy and cheap. Just specify a zener with a
breakdown-voltage above the nominal cathode voltage. For example, if the
cathode resistor sees a voltage drop of 15V at idle, then use an 18V to
22V 5W zener.

By the way, I wrote quite a bit about the topic of automatic bias
circuits and tube output stages a few years ago, so if you are new to
the Tube CAD Journal, reading blog
number 43 is a good refresher on the topic of using a zener clamp on a
cathode-biased output stage.

The above schematic shows a push-pull output stage with cathode bias
and the zener clamp. At idle, the zener is effectively not in the circuit,
as it conducts so little. But once its breaking voltage is reached, it
conducts admirably, effectively shunting the cathode resistor and bypass
capacitor. Now this same set of three components, cathode resistor, bypass
capacitor, and zener diode, can be used on a single output tube, which
will improve the idle-current matching between output tubes. (It’s a fun
mental problem to work out why that would be so.)

Now, a zener isn’t the only component that can be used to limit the
voltage climb at the cathode. For example, a power transistor and two
resistors can be used to make a pseudo zener.

In the above schematic, we see the single cathode resistor replaced by
two in series. At idle, the voltage presented to the power transistor’s
base is only 0.6V, which is not enough to turn on the transistor, so the
transistor is effectively out of the circuit. But under the bigger current
swings that loud music provokes, the base will see its turn-on voltage,
usually 0.7V, and then it will conduct and strive to maintain a constant
40V at the cathode. This is a simple DC feedback loop at work. If the
cathode voltage exceeds 40V, the transistor increases its conduction,
pulling the cathode voltage down back to 40V. If the cathode voltage drops
below 40V, the transistor ceases its conduction, allowing the cathode to
fall back to its nominal 34V. In AC terms, the bypass capacitor passes
much of the variation in current flow into the power tubes; thus, using
high-quality capacitor in this position is essential, but using the bypass
capacitor is not. We can bypass the bypass capacitor and leave it out of
the circuit altogether. Or, we can bypass only one of the two cathode
resistors.

So, why use a transistor and an extra cathode resistor? The advantage
power transistors hold is that they are cheap and easily heatsinked, which
can allow for some intense heat dissipation. An added advantage is that
the transistor-based voltage reference can make use of a potentiometer,
which would allow adjusting the break voltage; very cool indeed. The
downside to the transistor is that it is not likely to be as accurate as a
voltage reference as the zener would be (although a power transistor and a
low-voltage zener would probably be much more accurate a voltage
reference) and it will creep out many tube fanciers to no end to see a
TO-3 power transistor sitting in a tube power amplifier.

Now, let see what someone else has done with transistor-based voltage
references, thanks to Neal, a kind TCJer who sent me the information. In
the following schematic, we see each output tube getting its own special
cathode-bias circuit.

Click on image to visit the
Turner Audio website

This schematic is from the Turner Audio website and I
highly recommend that you read the long and detailed explanation of the
circuit at the website.

One problem I see with any cathode resistor setup is
that the resistor is not a constant-current source, as it conducts
linearly in relation to the voltage drop across its leads. This means that
the resistor will fight the voltage climb at the cathode, as the amplifier
is pressed into service beyond its class-A boundary. On the other hand, a
true constant-current source would at least apply a consistent drag
against the bypass capacitor’s climbing voltage.

In the above circuit, we see the cathode resistor replaced by a
constant-current source. This current source can be made up out of
three-pin voltage regulators or many discrete components. (The famous
LM317 is available in a high-voltage version.) But even the
constant-current source will subtract from the available charging current
that the bypass capacitor will see, slowing the voltage buildup.

The circuit below makes use of a transistor-based constant-current
source with a twist.

The twist is found in the 1kµF capacitor. This capacitor dynamically
reduces the constant-current source’s nominally specified current. How?
When the 30-ohm cathode resistor experiences increased current conduction
beyond the class-A ceiling, the 1kµF capacitor tugs up on the power
transistor’s base, thereby reducing the current conduction through the
transistor, which will allow the cathode voltage to climb more rapidly.
What happens if the capacitor pushes down on the base? Nothing much, as
the two diodes in series effectively limit the base’s negative travel. (By
the way, the voltages shown are no longer related to the previous
schematics, as I took them from SPICE simulations I had run on the
circuit, with 300B output tubes.)

Next Time
My biggest hope
is to announce the arrival of the new batch of PCBs. In addition, I will
cover push-pull, fixed-bias, sliding idle current circuits.

//JRB

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