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John
Broskie's Guide to Tube Circuit Analysis & Design

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18 September 2012

Interconnect & Speaker
Cable: Long or Short?
Your speakers stand 20 feet
away from your line stage amplifier, so there are (roughly) three ways
that you can arrange your system: use long 19-foot interconnects to
deliver the line stage's signal to the two monobloc power amplifiers, each
of which sits behind the speaker it drives through an embarrassingly short
1-foot length of speaker cable; or use 1-foot interconnects to the two
power amplifiers and 19-foot lengths of speaker cable; or use 10-foot
interconnects and 10-foot speaker cables, placing the power amplifier in
the middle of the wires. Which is best? It depends is probably the correct
answer.

If I had to choose, I would prefer long interconnects
and short speaker cables. But what would make me happier still would be
long interconnects and a very-hot line stage amplifier, which could put
out 40Vpk of clean signal, more than enough to drive a unity-gain power
buffer situated behind or, perhaps, located within the loudspeaker. Why? A
variety of reasons, such as the pursuit of novelty and simplicity.

Tubes work well with high voltages and can easily
swing big voltages, so using a tube-based line stage makes a lot of sense,
if you desire huge output swings. For example, an Aikido line stage based
on a 12AX7 input tube will provide a gain of about 50 (or +34dB), which is
plenty. (Of course, a flea-power power buffer, say one that put out only
4W, would need only 8Vk of input signal; 50Vpk into an 8-ohm load equals
156W, real watts, not peak-power watts.)

In addition, power amplifiers, be they tube-based or
solid-state-based, have enough work to do by having to deliver high
current into a speaker; adding voltage amplification to its lists of tasks
just complicates its design and construction. And why duplicate functions?
Why cascade two voltage gain stages when one will do? Freed of the job of
voltage amplification, building a solid-state power buffer is easier than
building the equivalent solid-state power voltage amplifier, as the
unity-gain power buffer can consists of only an emitter-follower or
source-follower output stage, only one stage, not the typical three:
voltage-amplification stage (VAS), driver stage, and output stage.

Or, if you fancy MOSFETs over bipolar transistors, then
the LT1166
might be the answer.

Note the 1-ohm load; with an 8-ohm load, much higher
power-supply rails can be used. See Blog
211 for more details.

Even a single stage tube-based unity-gain, power buffer
can be built, but not in a cathode-follower topology, alas. indeed, with
tube power buffers, things get a bit more complicated. For example, the
following tube power buffer uses a grounded-cathode amplifier. Thus,
although the same amount of output voltage develops as input voltage
enters, say 10Vpk in and 10Vpk out, it does so because the output
transformer steps down the plate voltage swings. For example, 10Vpk goes
in and the plate swings 50Vpk, but the output transformer’s 5:1 winding
ratio reduces the 50Vpk to 10Vpk at the secondary.

A closer approximation to a true tube-based power
buffer would require at least one more stage. A tube-based,
negative-feedback-based, unity-gain power buffer might look like the
following.

This circuit appeared in Blog
68, whose topic was tube power buffers. But tube and solid-state
unity-gain, power buffers have been covered here countless times (well, at
least countless times as far as I am willing to count). For example, here
is an interesting balanced power buffer from Blog
190.

Note the pentode topology and the constant-current
source, which auto-biases the output tubes (matched tubes). Normally, a
pentode output stage presents a very high output impedance, which requires
that the input stage apply a big dollop of negative feedback to reduce Zo.
But in this tube power buffer, the feedback is applied directly at the
output tube's cathodes. Also note that the center-tapped secondary sits at
the bias voltage, +36Vdc, not 0Vdc.

What appeals to me most is the simplicity of these
designs. “Simplicity is the ultimate sophistication,” so said Leonardo da
Vinci. Less can be more, although it can just as easily be a bore. Well,
I have been thinking about a stereo system that held a high-gain,
tube-based, line-stage amplifier and long interconnects and monobloc
unity-gain power buffers, with short lengths of speaker cable. Thus, this
type of circuit interested me. And as I can never leave any circuit
unmodified, I have come up with a clever modification of the single-ended
inverted amplifier. As Leonardo also put it so well, “Art is never
finished, only abandoned.”

Auto-Bias Single-Ended Output
Stages
Having just written the above section, I must
quickly announce that the following design idea is not limited to power
buffers; it just that power buffers were what I was examining when the
idea came to me. What is this idea? It is a two-barrel affair that solves
two tube-based amplifier problems, automatically biasing the output tube
and improving the amplifier's PSRR. But first, let's quickly review the
inverted, single-ended output stage. In a conventional single-ended output
stage the output transformer sits atop the tube’s plate, as we saw in the
grounded-cathode amplifier shown before. If we do not “ground” the power
supply, letting it float, then we can move the output transformer to
output tube’s cathode.

It may not look right, offending the eye and long-held
custom, but the amplifier functions identically to its more conventional
brother. It is still a grounded-cathode amplifier, not a cathode follower;
it offers the same output impedance and voltage gain; and if we strip away
the two-capacitor Aikido trick from the conventional single-ended output
stage, the same PSRR obtains. What we have acquired from this inversion is
a slightly more efficient use of the available B+ voltage, as the voltage
that the output transformer’s primary will displace, due to its DCR, is no
longer wasted, as it now serves to cathode bias the output tube. But this
advantage is minor and the primary’s DCR is no better than a cathode
resistor in establishing a precise idle current. Achieving a steady fixed
idle current will require a constant-current source.

The constant-current source will automatically adjust
the output tube’s grid-to-cathode voltage to maintain a fixed idle
current. The large-valued electrolytic capacitor that shunts the output
tube and output transformer will allow the output stage to amplify audio
signals. The high-voltage battery is just a placeholder for a more complex
floating power supply. In fact, this entire schematic is rather idealized,
as a truly functional circuit will require more parts and more complexity.
But before moving on, let’s examine how the constant-current source
shields the output stage from power-supply noise.

In this example, as long as the power-supply noise does
not exceed 12Vpk, the constant-current source will vary its voltage drop
with the ripple, so the output stage sees a fixed voltage window to
operate within.

Okay, that’s great, no power-supply noise at idle, but
what about amplifying music? How can the output tube amplify, if its plate
and cathode see a fixed DC voltage? Actually, only the cathode sees a
fixed voltage, as the plate is free to move up and down in voltage as
prompted by the grid signal. For example, a positive input voltage will
provoke an increase in the tube’s current conduction, which will pull down
its plate voltage, just as it would in a conventional single-ended plate
amplifier—the power supply is floating after all. As the plate pulls down
100V, the output transformer’s primary sees an additional 100V voltage
drop. Conversely, as the tube conducts less, its plate voltage will
increase causing the output transformer’s primary to see voltage reversal,
as the electrolytic capacitor pulls it connection to the primary upward.
Because the power supply is floating, each channel will require its own
power supply.

What does the constant-current source see with these
big plate-voltage swings? Essentially, nothing much: just the 12V voltage
drop and the power-supply ripple. Relative to ground, the constant-current
source is swinging up and down with the big voltage swings; but
differentially, the constant-current source sees the same low-voltage
drop. Okay, ideally it sees the same 12V drop. But a speaker cable
breaking contact or shorting can provoke a huge primary voltage that can
overwhelm the constant-current source. The workaround is a 50 cent zener
diode.

Now that it has been fleshed out, I admit that this
schematic looks much more complicated. The battery has been replaced with
an actual high-voltage power supply. Two 270µF capacitors are placed in
series, which halves the capacitance, but doubles the voltage rating. The
two 100k resistors shunting the power supply capacitors work to establish
a true voltage division between power-supply capacitors.

If the power transformer is center tapped, then the two
voltage-divider resistors are not required, as the center tap will halve
the voltages for us.

The LM317 is an adjustable, three-pin, voltage
regulator that has been configured as a constant-current source.
Internally, the LM317 holds a voltage reference, an OpAmp, and a power NPN
transistor. The internal OpAmp constantly monitors the voltage at its
“adjust” pin, comparing it to the voltage reference’s output.

If the adjust pin voltage falls below the reference
voltage, the OpAmp increases the transistor’s current conduction; if the
adjust pin voltage climbs above the reference voltage, the OpAmp decreases
the transistor’s current until the adjust pin voltage falls enough to
match the reference voltage. In other words, the LM317 strives to maintain
a fixed voltage drop across 12.5-ohm resistor. Thus, if a constant voltage
exists across the resistor, a constant current must flow though the
resistor. The maximum voltage the LM317 can withstand is 37V; a
high-voltage version is available, the LM317-HV, which can withstand 57V.
The 30V zener protects the LM317 against inductive kickbacks that can
produce huge voltage swings on the output transformer’s primary and
against the 100µF capacitor's slow charge up at turn on. If the zener sees
more than +30V, it begins conducting, halting the voltage increase to 30V;
if the zener sees a negative voltage, it begins conducting like a normal
rectifier, with a fixed voltage drop of about 0.7V. In other words, the
zener sets an operational voltage window of +30V to -0.7V for the LM317 to
work within.

The 0.68µF capacitor is needed to prevent the
power-supply noise and the huge signal voltage swings from dragging down
the signal source. The 200k resistor and 0.68µF capacitor define a
low-pass RC filter with transition frequency of about 1Hz. Thus, the DC
voltage gets through as though the capacitor was not there, but AC signals
above 1Hz are greatly attenuated. The effective input impedance is not
300k (100k + 200K) as we might guess, but 100k.

The input coupling capacitor is needed to isolate the
negative-bias voltage from the signal source. The 10k grid-stopper
resistor is essential, as it prevents the output tube from oscillating at
some super-sonic frequency. The 100µF/450V capacitor stores the voltage
charge that the output tube and output transformer will work within; it
can be (and should be) bypassed by a high-quality film or PIO capacitor.
Since the output stage operates in single-ended, class-A, this capacitor
has little work to do, as the output stage draws a fairly constant current
on average.

The output transformer’s primary is part of the
biasing setup, as its DCR helps bias the output tube. With a 100mA of idle
current flow, the 200-ohm DCR will drop 20Vdc, leaving it to the LM317 to
drop the needed 12Vdc to establish the required -32Vdc for the tube’s grid
relative to its cathode.

Pure-Tube or
Nothing
Are you insane? Solid-state rectifiers and voltage
regulators! My tube guru says that just adding an LED power indicator to a
tube amplifier is enough to ruin the tone. It’s pure tube or nothing for
me. Okay, but don’t let me catch you connecting your pure-tube
amplifier to a solid-state DAC or tuner or iPod. Building a more retro
version is easy enough, but we will have to forgo the better auto-bias
control offered by the LM317.

The LM317 has been replaced by an inductor (a choke)
and its DCR combined with the output transformer’s primary’s DCR
effectively create a 320-ohm cathode-bias resistor. An inductor functions
much like a constant-current source to AC signals, as it opposes a change
in current flow through it. The 5AR4 tube rectifier replaces the
solid-state rectifiers and charges up the 47µF power-supply capacitor. Why
so low a capacitor value? Too big a capacitor will damage the tube
rectifier. Remember that the bigger the capacitor, the greater the peak
charging current. The zener is no longer needed, although I would still
use one, just to play it safe. In many ways, this topology is similar to
the old single-ended, para-feed (choke-loaded, transformer-coupled)
topology.

Push-Pull Inverted
Auto-Bias
Yes, it is possible. A push-pull version
will require two isolation transformers and two RC filters, one for each
grid.

The constant-current source sets the idle current, with
each output tube getting half, assuming matched tubes. In addition, the
constant-current source filters away the power-supply noise. Because the
constant-current source limits the maximum current flow, this amplifier
must be run in strict, no nonsense, non-glossy-ad, real-thing, class-A
mode. (The zener might actually help out here, as it could allow the
amplifier to break out of the class-A envelope of operation and intrude
into class-B territory, but at the cost of requiring some tweaking.) The
output stage is configured as a push-pull grounded-cathode amplifier, not
a cathode follower, appearances to the contrary. In other words, the same
output impedance and voltage gain as if the output transformer had been
located at the plates.

The zener offer the same over-voltage protection for
the constant-current source. The 100µF power-supply capacitor holds the
420V charge that defines the B+ voltage for the output stage. The two 300k
grid resistors and the two 1µF decoupling capacitors are needed to provide
a DC path to the -12Vdc and to tightly couple the isolation transformer’s
secondaries to the cathodes. Remember that the output tube will produce
gain, so if 30Vpk of signal goes in the primary, 300Vpk might develop at
the cathode.

One danger with any two-output-tube design, either PP
or parallel SE, is that if one tube is missing from its socket while its
brother is inserted, the engaged tube will conduct twice the idle current,
which could damage the tube. The workaround is to place both output tube
heaters in series and use a 12.6V (or 10V for 300B tubes) heater voltage.
Thus, much like Christmas tree lights, if one tube is missing, the other
will not heat up.

Okay, here is your homework assignment: How could this
circuit be reconfigured as a power cathode follower?*

Next Time

I hope to have some good news about restocked PCBs.

* Answer:

Mind you, the isolation transformers may not be needed,
but truly huge input swings, say +/-300Vpk, will be. How does one get such
a huge differential input signal? One solution would be to build an ES
headphone amplifier, tube-based of course. And, of course, you could
switch between driving ES headphones, such as those made by Stax, or
driving the following cathode-follower power output stage, which in turn
would drive speakers.

//JRB

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