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The Sulzer Regulator Click on schematic, or here, for full-version The above circuit is the Sulzer regulator, a very high performance, low noise, linear regulator based around an op-amp. Analysis of the circuit is interesting, in that it addresses some of the points raised in the previous page that can affect performance. The ReferenceThe reference chosen is an LM329 sub-surface (or buried) zener reference (D1). Whilst the circuit symbol is that of a zener, it's actually an active device, which results in much lower dynamic impedance than a normal zener, of around 1ohm. It also has low noise relative to it's output voltage (6.95V nom.), due to the sub-surface zener used. The reference circuit addresses some of the issues raised previously - as can be seen the LM329 is biased (via R5) from the regulator output. In effect the reference gets its own super-quiet regulated supply. Additionally the reference is then low-pass filtered, via R1 / C1, to
further reduce the noise present on the reference device. f3dB = 1/(2*pi*C*R) f3dB = 1 / (2 * 3.14 * 1e-6 * 51e3) = Approx 3Hz. This very quiet DC reference is then fed to the non-inverting input of a low noise op-amp, in this case an NE5534. The Error AmplifierThe NE5534 used in the Sulzer regulator is a precise, low noise operational amplifier. It compares the ultra quiet filtered reference, with a fraction of the regulator output voltage (via R3 / R4). A further enhancement performance is made via the capacitor C3, in parallel with R4. As can be seen, as frequency rises the impedance of C3 drops, in accordance with the reactance of the capacitor (Xc = 1/(2*pi*f*C). This in effect lowers the impedance of the feedback resistor R4 as frequency rises, which in turn reduces the gain of the error amplifier. This simple trick reduces the noise gain of the amplifier, and hence prevents amplification of any residual noise present at the error amplifiers inputs. It has a further benefit in that it reduces the output impedance of the regulator by a significant margin, further reducing load-related dynamic noise at the regulator output. Some caveats must be mentioned here though, in respect of error amplifier choice. The amp chosen has to be unity gain stable, or have careful attention to compensation as it's the noise gain not the DC gain that is the relevant parameter for Bode / Nyquist stability of the circuit. Interestingly the Sulzer regulator is internally compensated to be stable at gains of three or above, but overall stability has been brought about by careful component choice elsewhere, despite the noise gain reduction in the circuit. Be wary though that component substitution may not be simple. A further enhancement has been made by the addition of an RC network at the op-amps supply pin. This has the effect of adding a low-pass filter to the supply, further enhancing line rejection of the circuit by filtering noise from the raw incoming supply. The effect of this filter is to reduce supply noise as the op-amps inherent PSRR degrades with increasing frequency (by 6dB / octave). The ResultThe above circuit offers excellent performance in all of the major areas relevant to a regulators operating parameters, it has very low output impedance, very low noise both static and under load, and good line rejection. All of these parameters are maintained over a far wider bandwidth than any readily-available monolithic IC regulator. The sonic benefits are huge, offering quite stunning improvements to audio circuit performance, when implemented with care. So is that it? Well no, as always there are people out there driven by a desire to do even better. Once the benefits of such improvements are heard (and they are not subtle, going right to the heart of music) it's a natural human desire to see if even better performance can be achieved, and find out if there is a point beyond which things do not make a difference. The following circuit presents what is probably the current pinnacle of what has been achieved, in easy-to-build form, and contains all of the above circuit improvements, plus a few more, that really push the envelope a lot further. Inevitably the gains may look small on paper, but it would seem that the better the power supplies get, the better a system becomes at resolving smaller differences, to the point where the sonic gains are not small at all. Click here for the conclusion.
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