As stated before, we have made some mighty interesting discoveries about film capacitors. To begin, we’ll discuss capacitor values in the circuit.
Because a capacitor-resistor network, such as is commonly used in tube amplifiers, is a high-pass filter, it follows that there is a certain minimum capacitor value that can be used before the bass response “rolls off,” as is often stated. Common practice is to size the capacitor to allow for this minimum value. Yet there is more to it than that.
It must be remembered that, first of all, as a capacitor starts attenuating a phase shift develops. When this happens, what is coming out of the coupling capacitor between stages in an amplifier at the lowest of frequencies is not perfectly tracking what is coming in. This is more serious than it may appear at first glance. Since music consists of fundamentals and multiple harmonics of these fundamental frequencies, it follows that the higher harmonics will pass through fine, while the fundamental note lags behind, resulting in poor bass response.
Therefore, the capacitor must be sized larger — much larger — so as to prevent any serious phase shifts from occurring in the audio. Sizing the coupling capacitors so that the transformer is the main weak link as far as frequency response is concerned does indeed have its benefits….
Now for an interesting discovery regarding capacitor brands. Mind this is not an endorsement for any particular brand; it’s just what we have discovered. There seems to be an ideal value of resistor when used in conjunction with a coupling capacitor, and it would appear as though it may vary with brand. For instance, in using the all-around, universal, low-cost, and high-quality coupling capacitor, the MKP Audiophiler, we noticed that with a 1uf capacitor, a 750K resistor is ideal for the best sound. A larger value than that, and the details seem to become slightly muddied; a smaller value than that, and the sound is less full and robust. Experimentation is ideal here to determine the best RC constants.
And now for another discovery: Is there an ideal capacitor brand? Or would a conjunction of brands be better? In our tests, it seems that by itself the new Auriacap XO 1uf capacitor has a detailed top with a rather lame bottom. The Solen 1uf has good lows but not very detailed highs. By putting the Solen in parallel with the Auriacap and adjusting the resistance so as to keep the same RC constant, suddenly we had a perfect bottom end coupled with the details of the Auriacap. Thus, the two capacitor’s virtues added, without apparently detracting from each other. So, our favorite capacitor is now one Auriacap in parallel with one Solen.
To conclude what maybe you have guessed by now: There is no silver bullet, but by experimenting and not staying in the box of conventional circuitry and ideas, one can come up with incredible results.