What capacitor should I use as input coupling capacitors? Polypropylene, polyester, silver-mica......??? Or even electrolytic? Too much types and brand: Rubycon, Elna, Nichicon.....
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Polypropylene by far. They can be hellish expensive if you go for the true audiophile types, like £50 for just one 1uF capacitor. But Wima do a range which are used universally in decent amps and they don't cost the earth. I think they are Wima FKP4 polycarb film type:
https://www.wima.com/EN/WIMA_FKP_4.pdf
If you need a capacitor with a value over about 1u5F you will need to go for either aluminum or certain tantalums. Bead tantalums sound OK but don't like much current. Rubicon aluminum electrolytics are generally favored for audio work, and if you shop around they are not too expensive, but never cheap. One dodge with electrolytics is to put a pollyprop or similar in parallel. If possible try to keep electrolytics out of the audio signal path and especially out of the feedback network.
On the transitor equivalent thread I wrote a bit about components which is reproduced here:
General
People often think that a resistor is a resistor and a capacitor is a capacitor, and all the talk about differnt sounds is an illusion. As an engineer that was exactly my view, but I had a rude awakening when I built my first serious audio amp using ceramic capacitors. The next rude awakening was when I heard a good commercial amp with matching quality speakers and audio source.
The other thing is that, if you have ordinary speakers and average signal source you won't notice the different sounds of components. But if, for example, you have a pair of speakers that are revealing analytical and dynamic you certainly will hear a difference. Sometimes it is not imediately obvious but you just get the vague feeling that something is not right. The other point is that you can get listener fatiuge after a couple of hours of continuous listening- not so if you listen to the same music live.
Resistors
Metal foil resistors are the best type for high quality audio work. (inductance and capacitance are not too important). Metal film are the next best and far less expensive than metal foil.
The next quality resistor is tin oxide, but they do not sound as good as metal foil and film. Tin oxide are very stable and rugged and are best for general electronic projects. Because they are so reliable and stable, they are the military's preferred type- they are all I used at work.
Carbon composition resistors are liked by the valve boys, but I have not found them to be brilliant for solid state amps. I think it is because valve circuits have low currents and high voltage, while solid state is the other way around. Carbon film have very little going for them and are best avoided for all work.
There are also wire wound and ceramic composition power resistors (1W upwards). Special low inductance wire wounds will be required for the two low value resistors in the output transistor emitters.
Capacitors
Yes, polypropylene caps are what you want for the solid caps. Polycarbonate are the best, but they are not made anymore. In practical terms polypropylene are just as good
Electrolytic capacitors should be a good quality aluminium type, established as sounding nice, Rubicon and Alps for example.
Cetain tantalum capacitors don't sound bad but, for some reason, not the latest generation of high-value digital decoupling tants. Bead tantalums are quite good and have a sweet pleasing sound, but not as pure as polypropolene.
Don't put ceramic capacitors anywhere near the audio signal path or you will get a brittle disturbing sound. The first half-decent amp I built had ceramics- it sounded awful.
Having said that, there is absolutely no reason why you could not use ceramics to get the amps going. Later you can fit what you want.
Polystyrene also sound good, but you only get them in low capacitance values. Silver mica are very nice, but expensive and also not available in high values
Most run-of-the-mill solid capacitors are polyester. They are cheap and easy to get. They do not sound too bad either. My advice would be to use those if you find polyprops are too expensive or hard to get. Later you can upgrade. Polyester caps are not good for decoupling though.
In the dual-supply version of the amp, only the input capacitor is directly in the audio signal path, so that is the capacitor to concentrate on the most.