If it is 300uF it is likely an extremely old capacitor. The construction of good audio capacitors has advanced a fair bit in those decades. Panasonic FM is an example of a respected and high quality part. If you are interested in audio from either a serious hobby or professional viewpoint you should invest in quality components. (And buy from authorized distributors due to the serious problem with fake components especially transistors and capacitors.)
Besides changes in capacitance and ESR there are other issues that can occur too. You might want to check to see if the capacitor is leaky after such a long time. Leakage current creates noise and is like a parallel resistance. That varies when capacitors are substantially aged. It also varies between capacitors from different vendors, formulations and quality levels. I think many perhaps do not consider that. Very old capacitors can also be unreliable and I replace them in my equipment such as generators and scopes. (Many people have dealt with damaged oscilloscope PCBs due to capacitor failure. Don't forget that sometimes component failure results in much greater damage to the rest of the circuit and/or connected equipment. Replacing one capacitor is much better than fixing etched away traces and/or blown-up equipment. In power amplifiers failures can easily cascade.)
Besides changes in capacitance and ESR there are other issues that can occur too. You might want to check to see if the capacitor is leaky after such a long time. Leakage current creates noise and is like a parallel resistance. That varies when capacitors are substantially aged. It also varies between capacitors from different vendors, formulations and quality levels. I think many perhaps do not consider that. Very old capacitors can also be unreliable and I replace them in my equipment such as generators and scopes. (Many people have dealt with damaged oscilloscope PCBs due to capacitor failure. Don't forget that sometimes component failure results in much greater damage to the rest of the circuit and/or connected equipment. Replacing one capacitor is much better than fixing etched away traces and/or blown-up equipment. In power amplifiers failures can easily cascade.)
I think it would improve the very low frequency performance (distortion) by maintaining a more constant current at LF into the bias network for the PNP driver but given you are talking about extremely low frequencies it is immaterial and would offer no practical benefit.
Nothing like that. All the cap does is attempt to maintain a constant voltage across the 560? ohm feeding the driver (haven't got the circuit in front of me). 68, 100, 220uF any of those are fine. Fwiw the 47uF is perfectly adequate for the precisely defined response engineered into the Quad.
A lot of reports on this thread that the finished quad 405 sounds good. But how good? Could you put a monetary value on it.
I mean, does it compare well against a decent budget amp, something like the topping PA5, or does it compare well against a mid level amp, or even a high end amp like bryston
I mean, does it compare well against a decent budget amp, something like the topping PA5, or does it compare well against a mid level amp, or even a high end amp like bryston