Super Symmetric Amplification
Super Symmetric Amplification
(c) 1998 Nelson Pass, Pass Labs
U.S. Patent # 5376899 describes a new amplifying circuit topology that
takes advantage of the character of special matched balanced amplifiers
that are cross-coupled to provide cancellation of distortion and noise.
The result provides high performance with very simple linear circuits,
better than previous efforts by an order of magnitude. We have dubbed
the approach Super-Symmetry (Su-Sy), an homage to particle physics.
Super-Symmetry works by exploiting the complementary characteristics
of precision matched balanced circuits to differentially reject distortion
and noise, and extends this symmetry to make the distortion and noise
virtually identical on each half of a balanced amplifying circuit. This gives
as much as a 100:1 reduction in unwanted signal components without
requiring the equivalent amount of negative feedback. It is simply much
easier to tweak the two halves of the circuit into symmetry than to
eliminate the distortion in each half of the circuit.
To best understand the Su-Sy circuit, we start by considering previous
approaches to balanced amplifier design. All three types are composed
of a pair of operational amplifiers, where the operational amplifier might
be small or large, made of integrated circuits or discrete components,
but always having two oppositely phased inputs and and one output. In
the first type, two operational amplifiers have their outputs bridged
across the load, each has its own independent feedback loop and its
own input. Each also has its own distortion characteristic independent of
the other amplifier.
In the second type, the negative inputs of the two operational amplifiers
share a feedback resistor to form a balanced circuit. This circuit has an
advantage in that it can accept balanced and unbalanced inputs, but
suffers from the greater distortion and noise since each amplifier reamplifies the distortion and noise of the other. In the third type, the
output of a first amplifier is fed back into the negative input of a second
amplifier. This circuit does not require and cannot use a balanced input
and it suffers from the same increase in distortion as the second type.
However, it is commonly employed in conventional amplifiers as a
bridging circuit.
The first type gives the best distortion characteristic, being no worse
than the performance of a stand-alone amplifier channel. None of these
approaches takes any advantage of balanced operation to
fundamentally improve the performance of the amplifier itself.
identically and cancels.
The diagram on the patent cover sheet shows this topology in its
simplest form. Each of the two input devices 20, 21 are driven by an
input signal, and their outputs run through a folded cascode formed by
devices 30, 31 to develop voltages across current sources 34, 35. The
sources 20, 21 are coupled through resistor 40 which is the sole
connection between the two halves and which also sets the gain of the
circuit.
The gates of the input devices 20, 21 are virtual grounds, and ideally
would be at absolutely zero voltage. However, as the gain stage is not
perfect, finite distortion and noise voltages appear at these points.
These are fed to the other side through resistor 40, and appear in phase
at the output of the other half of the system, where they match the
distortion and noise of the first half.
By actual measurement, this circuit does very little to reduce the
distortion and noise of each half. Distortion curves before and after the
application of Su-Sy are nearly identical. The distortion curves for the
circuit shown in the patent cover sheet are: (A) the intrinsic distortion of
each half of the real example circuit, (B) the distortion of the differential
output lowered due to the intrinsic matching between the circuits, (C) the
distortion of each half with Su-Sy applied, and (D) the differential
distortion with Su-Sy applied.
With curve (B) we can clearly see that intrinsic symmetry due to the
matching of the two halves reduces the distortion by a factor of about 10.
Application of Su-Sy (D) creates a more perfect match, and results in an
additional reduction by a factor of 10. However there is essentially no
difference in the distortion figures at the output (C) of each half of the
circuit considered alone. Su-Sy does not work by reducing the distortion
per se, rather it works to precisely match the two halves of the circuit and
lets the balanced output ignore the unwanted components. As long as
the two halves are matched, this performance tends to be frequency
independent, and does not deteriorate appreciably over the audio band.
With mid-level distortion figures on the order of .002%, this is very high
performance for a single balanced gain stage.
By contrast, the Super-Symmetry topology does not use operational
amplifiers as building blocks. It has two negative inputs and two positive
outputs and consists of two matched gain blocks coupled at one central
point where the voltage is ideally zero. The topology is unique in that at
this point, the distortion contributed by each half appears out of phase
with the signal, and we use this to reinforce the desired signal and
cancel noise and distortion. This occurs mutually between the two
halves of the circuit, and the result is signal symmetry with respect to
both the voltage and current axis, and anti-symmetry for distortion and
noise. This means that the distortion and noise of each half appears
Pass Labs: Articles: Super Symmetric Amplification
page 1
Su-Sy is an approach that takes advantage of balanced operation like
no other design, and requires a balanced input to retain the precisely
matched behaviour. You can drive one input alone, and the circuit will
amplify reasonably well, but it will not exhibit the very low distortion
character of fully balanced operation.
Su-Sy is ideally used to obtain high quality performance from very
simple circuit topologies, avoiding the high order distortion character and
feedback instabilities of complex circuits. A single gain stage amplifier
using this approach can perform as well as a two or even three gain
stage design, and a two gain stage version of this topology can
outperform the four or five stages of a conventional amplifier.
The first power amplifier implementing Super-Symmetry is the Pass
Labs X1000. It is rated at 1000 watts into 8 ohms. It delivers high quality
performance with a single stage Su-Sy front end driving 80 Mosfet
power devices used as voltage followers and biased at 600 watts idle
dissipation. Early data shows distortion as low as .002% under normal
listening levels, and about .1% at 1000 watts. The damping factor is
about 1000.
This performance is accomplished with only the two gain stages, an
approach previously reserved for low power single-ended Class A
designs. By contrast, other high power amplifier offerings in the market
use as many as nine gain stages, and still offer less power than the
X1000.
Why is this approach important? Because there is a different character
of sound attributed to simple linear circuit topologies versus the complex
circuits that obtain their performance through the generous use of
feedback. It is difficult to produce high power amplifiers with simple
circuits, and previous efforts have often been described as powerful and
dynamic but musically sterile. Super-Symmetry makes it easy to
produce high power amplifiers with the musical characteristic of simple
low power amplifiers. It allows elimination of frequency compensation
and blocking capacitors and DC servos and regulated supplies and
other design band-aids.
Where a thousand watts is not enough, the superb stability of the Su-Sy
circuit allows direct coupled output operation in series/parallel arrays up
into the territory of 16,000 watts and beyond, retaining the sonic
character of a single amplifier, but with massively more voltage and
current.
Pass Labs: Articles: Super Symmetric Amplification
page 2
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