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Mixed-Class & Mixed-Topology Amplifiers
If only we could save our cake and eat it at
the same time. The aim of a mixed class
amplifier is to provide the high quality sound of
Class-A operation with the greater efficiency
and power output of Class-B operation. An
additional aim might be to further the listener’s
ability to tune the amplifier’s sound by means of
a single potentiometer...but let’s not get to far
ahead of ourselves.
At first, all new amplifier topologies are hard
to understand. Imagine when all amplifiers were
single-ended how difficult it must have been to
explain push-pull operation. What is “phase”
and why does it need to be split? If the output
transformer isn’t partially magnetized and
doesn't have an air gap, how can it work? These
and other questions would require careful
answering, as push-pull operation also brought
the possibility and the complication of Class-AB
and Class-B operation of the output tubes, which
were not possible in the strictly Class-A world
of single-ended operation. More questions and
more answers would be needed.
Well, now I am asking you to imagine a
mixed-class amplifier, one that is at once both
Class-A and Class-B. No, this is not the same as
the marketing of OTL Class-AB amplifier as
Class-A nor is this along the lines of those
pseudo-Class-A amplifiers from the 70s that
never let the output devices cutoff even though
they made no real contribution to the amplifier’s
output. (Class-A operation is only valuable when
both devices equal work into a load; when one
devices gives up its grip, it may as well not be in
the circuit.)
In other words, like good cop and bad cop,
good-but-weak amplifier is partnered with bad-
but-powerful amplifier. (A better analogy might
be the teaming of Hercules and Iolaus.) If this
arrangement sounds something like the Quad
current dumping amplifier, it should, as the
principles behind both amplifiers are roughly the
same. In Quad’s design, a small Class-A
amplifier was assisted by a Class-B amplifier.
Class-A
Class-B
Just as push-pull operation doubled the single-
ended solo output tube, mixed class operation
doubles push-pull’s double output tubes; thus at
least four output tubes are needed. One pair runs
in Class-A push-pull and the second pair runs in
Class-AB or Class-B (or even Class-C) push-
pull. Thus, the first pair are always conducting,
while the second pair can be completely turned
off (or run at a much lower current) at idle.
As the signal level increases, the second pair
is activated, unburdening the first pair and
greatly increasing the output power. In fact, we
can just as easily mix Class-AB with Class-B or
Class-C, either mix would give even greater
power output.
In fact, we could easily create a three-way
mix of operating classes, say Class-A, Class-AB,
and Class-B, or Class-AB, Class-B, and Class-C.
Of course, at least six output tubes would be
needed. In all the mixes, the goal would be the
same: to create a simple, seamless sounding
push-pull amplifier that uses only a single input
and phase splitting (and driver) stage per
amplifier. This goal can met with an near infinite
mix of output tube types and bias points. One
example is using a pair of EL34s for the Class-A
grouping and KT88s for the Class-B pairing.
Another example might be using a pair of 300Bs
for the Class-AB grouping and 211s for the
Class-C pairing.
How to proceed?
Two separate amplifiers on two separate chassis
with two separate power supplies could be used;
but what a hassle. Using one chassis with one
power supply and one input stage is preferable.
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Alternatively, the Class-A pairing and the
Class-B pairing could share the same type of
tube. Two approaches immediately come to
mind: give the output pairings dissimilar drive
voltages or halve the Class-A pairing’s
transconductance. The first approach requires as
little as using two plate resistors in series. In the
circuit shown below, we see differentially
arranged triodes with series plate resistors. The
bottommost set of outputs go to the Class-B
pairing and the topmost outputs go to the Class-
A pairing. (A further refinement might be to
cross-couple outputs and inputs with small
capacitors to extend the frequency response.)
0v
-15v
-30v
-45v
-60v
-75v
-90v
0v
400v
Class-A bias point and grid swing
+400V
0v
-15v
-30v
-45v
-60v
-75v
-90v
0v
400v
Class-B bias point and grid swing
Given the same output tubes, Class-A and
Class-B push-pull amplifiers require different
drive voltage swings, with Class-B needing
more than Class-A. In fact, the ratio is just about
2 to 1. In the Class-A amplifier, the output tubes
are biased at the midpoint between drawing grid
current and being completely turned off. In the
Class-B amplifier, on the other hand, the output
tubes are biased at the endpoint just above of
being completely turned off. Because a Class-B
push-pull amplifier’s output tubes need twice the
input grid swing to bring the grid to the onset of
conduction as the tubes would in Class-A
operation, a better ordering might be: EL34s for
the for the Class-A grouping and EL84s for the
Class-B pairing (or 300Bs for the for the Class-
A grouping and 6550s for the Class-B pairing).
For example, the EL34 would need to see about
30 volts of peak grid voltage swing in Class-A
and the EL84 would need to see about 30 volts
of peak grid voltage swing in Class-B.
-100V
Dual outputs for Class-A & Class-B output pairs
This same multi-tapping of a split-load phase
splitter is easy to construct. However, one
liability stands out: the PSRR is substantially
worse from the plate as from the cathode. This
means that the noise that would normally be
cancelled out in the push-pull output stage
becomes amplified when the power supply noise
presented to the output stage’s grids is not equal
in amplitude and in phase. Even my trick of
giving this phase splitter half of the power
supply noise to bring the dissimilar PSRRs into
alignment fails when multi-tapped, as the
midpoint between plate and B+ has 75% of the
power supply noise, whereas the midpoint
between cathode and ground only contains 25%
of power supply noise. In other words, this
phase splitter can be used, only if an extremely
well filtered (or regulated) power supply is used.
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For example, when the plate load equals the
r
p
of the triode, its transconductance is halved, as
G
m
is equal to mu / (r
p
+R
a
). Adding an
unbypassed resistance in series with the cathode
also decreases the triode’s transconductance.
When the cathode sees a resistance equal to r
p
/
(mu + 1), the effective transconductance is
halved. For a 300B, this resistance would equal
143 ohms. Notice that resistance is too low to
correctly bias the 300B. Thus slight modification
is required and is shown below.
+400V
+350V
+300V
+100V
+50V
10k
Split-load phase splitter with multi-taps
300k
100%
75%
300k
50%
10k
50%
50%
Cathode bias with transconductance halving
25%
The problem with cathode bias is that while it
works beautifully with Class-A amplifiers,
whether they be single-ended or push-pull, it
does not work well with Class-B amplifiers. The
reason is easy to discern: in the Class-A
amplifier the idle current is equal to the average
current through the output tube even when the
amplifier is putting out its full output. In
contrast, the Class-B amplifier’s idle current is
but a small fraction of its conduction at full
output, making its average conduction roughly
half of its peak. In other words, cathode bias
would results in the Class-B amplifier trying to
turn its self off during heavy use, creating a
good amount of distortion in the process.
The solution is to use only fixed bias for the
Class-B pairing.
Split-load phase splitter’s PSRR per tap when fed
an input signal with 50% of power supply noise
The other approach is to give all output tubes
the same drive signal, but halve the Class-A
pairing’s transconductance, which would require
an effective doubling of its drive requirements.
How can a triode’s transconductance be
decreased? Well, just placing a plate resistor in
series with the triode will reduce its
transconductance, as a triode is sensitive to its
plate voltage, which this resistor will alter.
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60mA
100
10mA
70mA
10k
EL34
10k
100
EL34
120
300k
300k
10
+400V
-50V
220
10
300k
120
300k
EL34
100
10k
EL34
10k
10mA
70mA
100
60mA
Mixed topology amplifiers
The amplifier shown above not only mixes
classes of operation, but also topologies. The
first pair of EL34s work as Class-A triodes,
while the second pair works as Class-B ultra-
linear pentodes. This amplifier would put out 60
watts of power, with 10 watts coming from the
Class-A stage and 50 watts coming from the
Class-B stage. The Class-B pair could just as
easily been configured as pure pentodes, which
might yield some benefit from its increased
output impedance. In other words, having the
Class-B stage produce a high output impedance
allows the low-output-impedance triode pair to
dominate the output, as the triode’s low r
p
will
buck any extraneous contribution from the
pentode pair by increasing or decreasing the
triode’s conduction in response. (As an aside,
even within one output stage made up of only
two output tubes, mixed mode configurations are
possible. For example, in the amplifier below,
we see an amplifier that is configured as an
ultra-linear at low frequencies, but becomes a
triode configured amplifier at high frequencies.)
Push-pull mixed mode output stage
10k
2k
EL34
300k
10
+400V
-V
10
300k
EL34
2k
10k
Mixed mode within one output stage
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+400V
30k
10k
.33µF
+291V
10k
100
KT88
+100V
300k
10
300
5687
.33µF
-45V
+400V
10k
+109V
10
300k
300B
KT88
100
5V
5687
10k
100k
.33µF
392
300k
10k
Truly mixed modes/topologies
The amplifier shown above combines Class-
A and Class-B with single-ended and push-pull.
Two output transformers are required, but only a
single power supply and input circuit is needed.
This amplifier relies on the 300B to deliver the
majority of that all important first watt of power
and on the 6550s to deliver the wallop missing
from so many single-ended amplifiers.
I would love to see some research on the
Class-A-SE/Class-AB-PP amplifier that Nelson
Pass has made famous in solid-state circles. In
past issues we have covered possible tube
implementations of this style of amplifier. The
attempts relied on using a pair of output tubes
and controlling the grid signals to these tubes.
An alternative approach would to add a third
tube to an existing push-pull output stage. In the
schematic to the right, we see a constant current
source loading half the primary with a current
equal to that being drawn by the bottom EL34.
This balanced current ensures that the
transformer will not saturate at idle. In the
absence of the top EL34, the amplifier would be
purely single-ended, but the top EL34 allows the
to break out single-ended operation.
Class-A/Class-AB SE/PP amplifier
60mA
0mA
100
10k
EL34
300k
10
-56V
+400V
-28V
10
300k
EL34
10k
60mA
100
Class-A-SE/Class-AB-PP
This amplifier works as single-ended until the
top EL34 turns on, creating a mixed mode push-
pull amplifier, as the top EL34 works in Class-
B; the bottom EL34, in Class-AB.
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