Pass Labs X1000 monoblock power amplifier Supersymmetry Explained
No negative feedback circuits might be more accurately described, in most cases, as local feedback only designs. Some might argue that local feedback is still feedback, but according to Nelson Pass, "there is always some amount of feedback locally around any gain device by the nature of the device." He thinks that "no feedback" describes a circuit in which negative feedback doesn't extend further than a single stage or gain device.
Conventional negative feedback, local or otherwise, is used to make the circuit's output look more like its input. In Supersymmetry, feedback isn't used in the conventional sense, but rather works to make two halves of an already symmetric balanced circuit behave identically with respect to distortion and noise. According to Pass, "This dramatically lowers the differential distortion and noise, but not in each half of the circuit considered by itself. Supersymmetric topology doesn't use operational amplifiers as building blocks, nor can it be represented with operational amplifiers. 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 identically and cancels.
"If you build such a symmetric (balanced) circuit, you get much of this effect already. If you drive a matched differential pair of transistors without feedback with a balanced signal, you will see a balanced output whose distortion and noise are typically a tenth that of either device alone, purely out of cancellation. With Supersymmetry, the same differential pair's characteristic can be made so identical that the differential output will have only 1/100 the distortion and noise of either device alone....
"Supersymmetry does not reduce the distortion and noise present in either half of the output of the balanced circuit. It is the balanced differential characteristic that improves dramatically, and that leads to one singular requirement of Supersymmetric operation: it must be driven by a balanced input signal and produce a balanced output signal. Supersymmetry makes the two halves of the balanced circuit behave absolutely identically. Constructing the two halves of the circuit with identical topologies and matching the components precisely achieves a 20dB or so reduction in distortion and noise, and a modest amount of local feedback achieves another 20dB. This is easily accomplished with only one gain stage instead of the multiple stages required by conventional designs, so it results in only one 'pole' of high-frequency characteristic that is unconditionally stable without compensation. In fact, if you build a Supersymmetric circuit with multiple gain stages, it doesn't work as well. Ironically, the Supersymmetry concept not only allows for very simple gain circuits, but actually requires them for good performance.
"'Balanced single-ended' is a phrase I use to refer to differential use of two single-ended class-A gain devices. The classic differential pair of transistors (or tubes, for that matter) is just such a topology. Balanced single-ended is an oxymoron in the sense that most single-ended enthusiasts believe that the most desirable characteristic of single-ended circuits is their generation of even-order distortion components by virtue of their asymmetry. Purists will point out that a balanced version of a single-ended circuit will experience cancellation of noise and even-order components. Just so. Interestingly, the single-ended nature of each half of the balanced circuit doesn't give rise to much in the way of odd-order distortion, and when the even-order components and noise are canceled, there isn't much distortion and noise left. In any case, 'balanced single-ended' is a phrase that accurately describes the circuit."
And there you have it.—Nelson Pass via Jonathan Scull