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There are certain steps you need to take to keep a given amp/preamp low noise. Let's have a look. The distortion here is 0.01% which is somewhat good.
1. Negative feedback - This is the thing that has the most impact on the performance of an amplifier. The more negative feedbacks you have the better. Local, global, frequency dependant, by AC, by DC, by voltage, by current.... For instance this amp has at least 6 negative feedbacks (by current, by voltage, DC, AC, and frequency dependant). NFB's usually flatten the frequency response, and minimize the amp's response to load variations. In preamps, load variations are not of particular importance, but power amps must handle a very inductive load, where load impedance is not constant. A complex load impedance, has a very profound effect on audio quality, especially in transformer coupled amps.
2. Open Loop Gain - this is the parameter a negative feedback impacts. When a negative feedback is applied a closed loop gain is created. The lower the overall gain, the less times any noise will be amplified, thus the lower the overall noise/distortion. The closed loop gain of the circuit is around 10.
3. Impedance - Another factor with a big effect on distortion is impedance matching. Loading a high impedance amp with a low impedance load will decrease the headroom, and also induce distortion in the wave. Not noise this time, distortion (mostly second order). Proper impedance matching is important for best performance. This is why tube amps have a matching transformer on the output, it matches the high impedance tubes (often kOhms) to the low impedance speakers (often several ohms). Impedance matching ensures maximum power transfer to the load. Keeping away from high impedance nodes is also beneficial because they tend to pick up noise induced from transformers and cables and such.
4. Resistance - Resistance here and there is beneficial in some aspects, but one must know when to substitute a passive resistor for a more linear and constant current source. As you can see the schematic has 2 constant current sources in the collector chains, where they substitute a passive resistor. This is because resistors cannot keep the current constant as they drop different amounts of voltage while operating. Depending on your loading the law of change can shift from linear to logarithmic, and introduce mostly second order distortion. This effect can be partially reduced by choosing such a bias point, that the current change through the resistor is not more than a few percent of the idle current chosen through the resistor. For instance a change of a few hundred uA, through a resistor with idle current of 10mA, will have little impact on performance. A 5mA change will have a more profound impact.
5. Power Supply - There are several requirements from the power supply as well. The higher voltage you can make it the better. This will give you bigger headroom, and even if you use passive resistors as collector loads, the current change through them will remain minimal (depending on bias point). The power supply for preamps is always separate and well filtered.
6. Galvanic Vs AC coupling - Okay this is a very long topic usually, along with impedance matching, but a few quick words can be said. Usually galvanic coupling is better, as it skips some components, which can introduce noise, and makes a DC contact between different stages, which makes a global DC feedback possible. While beneficial, sometimes it's impossible to employ such a coupling (as with some tube stages), so an AC coupling must be employed. With AC coupling a high impedance stage may block, thus appropriate RC chains must be placed. This is a long subject, I won't get into it.
7. Topology Layout - This is somewhat different from the previous steps, because it has more to do with building the amp, rather than designing it. There are certain layout rules, every technician should know. Ground loops are a big no-no. One of the first thing I experienced was a ground loop. Very unpleasant. Also sometimes component placement is important, as high impedance nodes tend to pick up noise from transformers and other leaky components in the circuit. Also especially with preamps quality components must be used. No carbon film resistors and ceramic capacitors, only metal film resistors and polypropylene capacitors or for a lack of components, wirewound resistors and polyester/oil in paper capacitors. Electrolytic capacitors should be used only where there's no other choice. I tend to avoid them, especially in tube amps.
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