MachMat's design philosophies are:

• Don't use too many parts, don't use too less.... Keep the signal line as simple as possible. Remember: designing is the art of making compromises!
• Use parts in places where they perform best.
• Power supplies determine the final result in a great way. Coils are very important, they keep the peakcurrents drawn by the capacitors down. Low resistance together with critical damping makes good supplies. Hf damping is very important, and a very hard job, but a job well done is very rewarding.
• Try to avoid (most) electrolytics, foil capacitors (preferable Paper/oil or silver mica) are much faster and you will hear that!
• Don't use any feedback, it will keep the amplifier from getting alive. Why correct an error if you are not making any?

Some consequences of the above rules, which are incooperated in Machmat's designs (some are quite technical, sorry):

It is impossible to make a design without compromises. Some brands claim this, but they are per definition not realistic. Every part used is a compromise, a speaker or any audio component is a compromise, a cable is a compromise, a CD or LP is a compromise, etc, etc, etc... On a lower level, each extra stage is a compromise, but you need it to get enough output power, a coupling capacitor is a compromise, but you need to get rid of the high voltage, and to keep the circuit simple, a interstage transformer or any transformer is a compromise, etc, etc, etc.... It is the case of choosing your compromises the best way, resulting in the best possible sound reproduction.
• A tube is a very good voltage amplifier. When looking at the small signal scheme almost all distortion will be generated by current swing. It therefore is very important to keep this current swing low. To realise this for example we use constant current source loading on our tubes, using transistors (see next point), resulting in very low distortion and clear and dynamic outputs. In our opinion it for example is a bad mistake building tube OTL amplifiers, since tubes are absolutely not made to deliver currents needed for low ohmic speakers.
• Most transistors have very flat I-V curves. This makes them very suitable for constant current loading on tubes and delivering high currents with low distortion, when used right. For example it is better to use a high voltage transistor than a low voltage transistor for current loading, because the Early voltage will be further away and thus the curves will be flatter, resulting in lower distortion and better power supply damping. Of course also the hfe will be lower, but this is not an important factor in tube loading. Also we use the most simple version of current sources, because they are already good enough, making things technical better and complicated does not help anyone.
• Designing a good sounding circuit is like cooking. You should not use the same parts and tube brands all the way. It is very important to find the right sound mix of silver, carbon, tube brands, wood and steel and other parts and materials. Of course it is important to keep all parts as clean as possible. This is for example (there are many more) why we always try to avoid to use a film resistor in the signal. The way they are made (carved film on glas or ceramic tubes) makes them inductive and also means there always is a relatively long line of material in the signal, adding a lot of sound. Simply part types like bulk foil and carbon composition resistors always sound more open and less distorted.
• The above can be visualised with a stack of windows. Some have colour, some have relief, some are frosted, some are very crystal clear. Suppose you want to get a clear look on what is behind this stack of glass plates. The best way to improve the visual image is to reduce the number of glass plates, next is to make the remaining glass plates as good and clear as possible. If all are perfect, but one, the image will totally be determined by this only one. So all have to be perfect or at least at the same quality. In audio it all is exactly the same.
• Power supplies are like cars. They are a mass and spring system, which need damping. A car without shock absorbers will bounce off the road, a car with to much damping will not follow fast bumps and fly away in curves easy. Imagine what happens to your sound with a poorly designed power supply (and most are). Damping is determined by the capacitance, inductance and resistances in the supply. Higher inductance and lower capacitance and resistance will lower the damping. So using high value inductors (>4H) with low ohmic values (<100 Ohms) and low value capacitors (<100uF) will result in bad sound. Unfortunately many people build such power supplies. We also think this might be the reason tube rectification is very popular. It adds extra damping to the above power supplies, these supplies will perform better as a whole. A well designed solid state power supply will sound really well!
• Many common audio tubes have very nice, but general specs. There are many tubes with really great specs, designed for special purposes which are related to certain circuits in audio. They also are much cheaper because nobody uses them. Most of the time using only one of these tubes (most of the time the high gm tubes, like EC86, D3a, E810F, PC900, 6C45, EC91, A2294, 12B4a, etc, etc) is already enough as a driver or a pre amp stage. No need for an extra buffer or mu stage construction. You will definitely hear this!
• A triode is often used as a cathode follower or as the above tube in a mu stage or SRPP. This is not logical, since the curves of a triode are really bent. This means such a stage will add a lot of extra distortion, since the -Vgk also will vary a lot with the output signal (alomst 1/mu, so the lower the mu, the worse the distortion). It is much more logical to use a penthode or a transistor in this place, since their curves are very flat, resulting in the same -Vgk (Or Vbe or Vgs) over a very wide range of output voltages. Since this stage must buffer the input voltage, this is exactly what you need.
• Film capacitors have a much wider effective frequency range than electrolytic capacitors. This results in better Hf damping and thus a more quiet and relaxed amplifier. If also a lot of care is taken to avoid the production of Hf (with the right diodes, decoupling, the right value capacitor for the current drawn, circuit toplogy and PCB design) amplifier really will perform well. Low ESR electrolytic capacitors also perform remarkably better than normal electrolytic capacitors.
• More to come when there is more time, if you think it is interesting, please check back regularly. If you have comments or suggestions, please always feel free to mail me, I am always willing to learn.