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John
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19 May 2012

Aikido Split-Load Phase
Splitter
Although I have covered the Aikido
split-load phase splitter before, here is a quick recap. A
grounded-cathode amplifier stage (not an SRPP stage), made up from two
triodes and two cathode resistors, directly couples to a split-load phase
splitter, which delivers two anti-phase signals to the push-pull output
stage. The Aikido aspect of this configuration lies in the phase
splitter's two outputs holding identical amounts of B+ noise, which the
output stage, being a difference amplifier, largely ignores, resulting in
a noise-free output. The split-load phase splitter normally has terrible
PSRR (close to zero) at the plate output and a fine PSRR at its cathode
output (roughly 1/mu). The configuration shown below is quite simple, but
powerful. Since the two identical input triodes share the same valued
cathode resistors, they define equal resistances and, thus, equally split
both the B+ DC voltage and B+ AC ripple. (I am duplicating names here,
sorry. The fancy Aikido split-load phase splitter that I created back in
blog 196 is an
altogether different animal, as it uses two triodes in a totem-pole
configuration and it delivers two phases of output signal with no
power-supply noise, amazingly enough.)

The configuration shown above yields a signal gain
roughly equal to one half the mu of the input triodes used. For example, a
12AX7, with a mu of 100, would develop a gain of 50, or +34dB. Note that
the split-load phase splitter's output signals, although out of phase with
each other, are equal in magnitude. Such a pair of balanced output signals
is perfect for the following push-pull output stage, as its signal
reference is ground.

On the other hand, a totem-pole configuration of
push-pull output tubes requires asymmetric balanced input signals, as the
bottom triode signal reference is ground, but the top triode's reference
is the output stage's output.

For proper push-pull operation of the output tubes,
wherein both tubes undergo equal but anti-phase current swings, both
triodes must see equal cathode-to-grid input signals. With one small
modification to the Aikido split-load phase splitter, the totem-pole
output stage gets just the set of balanced input signals it needs. Note
that the bottom 10k resistor is no longer "grounded" via the capacitor;
instead, it attaches to the output stage's output (usually via a large
valued capacitor).

The asymmetry between output signals bothers many
electronic newbies but the asymmetry only exists if we view the signals
from the perspective of ground. If we examine the cathode-to-grid voltage
relationships within the output stage triodes, we see that both top and
bottom triodes see an equal signal.

Okay, now we can move on to power-supply noise
elimination from the output stage's output signal. Remember that a
push-pull output stage is a differential amplifier, which treats balanced
AC voltages as input signals, but largely ignores in-phase input AC
signals. A signal transformer is a good example of a device that functions
differentially.

If both of the primary leads are tied together and
attached to a signal source, no matter how high the signal's amplitude,
the secondary sees nothing, as both of the primary leads saw the same
signal. On the other hand, if one of the primary leads is grounded and the
other lead is attached to a signal source, the secondary sees an AC signal
across its leads.

In short, if we want a noise free output signal from
the push-pull output stage, we must either feed it a noise-free balanced
input signal, which is difficult, or a noise-ladened balanced input
signal, whose noise is equal in magnitude and in phase.

The input stage splits the power-supply noise and
delivers it to the split-load phase splitter. This noise is then passed on
in phase by the split-load phase splitter's cathode output. Since the 10k
cathode resistor sees this AC noise signal superimposed across its leads,
the current flowing the resistor varies accordingly. Since the plate
resistor falls in the same current path, it also sees the current
variations, creating an anti-phase signal at the plate; in other words,
negative noise. Since the negative noise signal is half the magnitude as
that found at the B+ connection, the amount of noise at the plate output
is equal to 1 - 0.5, or 0.5. Thus, each output from the split-load phase
splitter holds the same amount of in-phase power-supply noise.

Aikido SLPS Headphone
Amplifier
Now we can combine the Aikido split-load
phase splitter with a totem-pole output stage to create a fine tube-based
headphone amplifier. (The very second some see the word "headphone" they
tune out, hating headphones and all things headphone related. Big mistake.
The following circuit would also make a fine linestage amplifier that
could work well extremely low-impedance loads, such as Zen power
amplifiers or 600-ohm input impedance amplifiers.)

Unlike an SRPP or White cathode follower or SRCFPP
headphone amplifier, all of which require a known, fixed load impedance,
the Aikido SLPS headphone amplifier can work with a wide range of load
impedances, as the Aikido split-load phase splitter within it
automatically adjusts the balanced output signals for each load impedance
attached to the output.

Before anyone asks if other tubes could be used, let me
say that I just pulled these tube types out of my hat. Topology is what is
important here, not the tube types. Huge 300W radio transmitting triodes
could be used as the input tubes and tiny pencil tubes or nuvistors could
be used as output tubes. Different tube do not make different topologies.
A cube is cube whether its made of gold or carbon or wood or chocolate; as
long as it is a regular solid having six congruent square faces, it is a
cube. Having said that, I quite like the tubes shown above, but I would
also like to use an ECC99 or 5687 or 6H30 or 6BX7 as the output tube. If
the lowest noise was my goal, I would use a 6N1P as the input tube. If I
planned on using a negative feedback loop to lower the output impedance,
then I would use a 12AX7 or 5751 as the input tube.

Now let's look into the Aikido-like elimination of power-supply noise
from the amplifier's output. The schematic below shows the noise
relationships within the amplifier. The input stage halves the
power-supply noise; the split-load phase splitter passes this noise to
both output tubes; and the two output tubes see the same noise signal, so
they both vary their current conduction equally as result; thus, there is
no difference in conduction between the output tubes, so there is no
difference, no delta to pass on to the external load impedance.

Very cool indeed. I should mention that the wonderful
power-supply-noise null only occurs while both output triodes are
conducting, i.e. class-A operation. Once one output tube is cutoff, the
noise null no longer obtains. When does class-A operation end? When the
current output exceeds twice the idle current through the output stage.
For example, 20mA, with an idle current of 10mA, which into a 300-ohm
load, would equal an output voltage swing of 6Vpeak—plenty loud.

But what about output impedance? The best way to
examine this aspect of the headphone amplifier is to work backwards by
imagining a positive-going pulse applied to the output (by a buffed
solid-state power amplifier or a strong battery being quickly attached and
then unattached tot he output).

The positive pulse is relayed to the split-load phase
splitter via the 10µF capacitor, which then forces the split-load phase
splitter's output at its plate produce the positive pulse, causing the
bottom output tube's grid to go positive, increasing its conduction,
thereby pulling the amplifier's output down. The top output tube will see
its cathode being pushed up in voltage, but its grid remains fixed, so the
top tube decreases its conduction, causing the amplifier's output to fall.
In other words, both output tubes work equally to counter the positive
pulse.

What happens if a negative-going pulse is applied
instead? Everything is much the same, but this time the top output tube
increases its conduction, pulling the amplifier's output up; while the
bottom tube decreases it conduction, letting the output climb. Once again,
both output tubes work equally to counter the pulse.

The key point here is that both output tubes function
as cathode followers, in spite of the bottom triode looking like it is
configured in a grounded-cathode amplifier topology—it is not. Thus, the
output impedance is roughly equal to

Zo = [ rp/(mu + 1) + Rk]/2

With 6DJ8/6922 output tubes, the output impedance would
be roughly 100 ohms. If the two cathode resistors were bypassed by a large
capacitor, the Zo would roughly equal 50 ohms. My preference would be to
live with the higher Zo and not use bypass capacitors, as they are very
much in the signal path. If a lower output impedance were absolutely
required, then a negative feedback loop could be employed.

This tube headphone amplifier is inverting, so the feedback
loop must attach to the bottom input tube's grid, not its cathode. Note
the greatly increased coupling capacitance at the output. Why? I assume
that low-impedance headphones, say 32 or 50 ohms must be driven, so a
larger coupling capacitor value is needed. In Addition, with the negative
feedback loop in place, we do not want excessive phase shifts within the
audio band of frequencies, as they can turn an amplifier into an
oscillator.

Next Time

Wow, that was a shockingly brief post by me. I hope to return with a
hybrid variation of the above circuit.

//JRB

Kit User Guide PDFs
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