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01-07-2020, 02:01 PM
Hi makinrose
The digital interface will sound cold mostly because its distortion harmonics are high-order and its IM is not great. It is IM that makes us feel that a sound is good, bad, harsh, cold, etc, much moreso than the THD present. Using tubes to add a veil of low-order THD harmonics and the attendant IM will warm the sound overall.
Electrolytics have leakage currents that are too high to let the cap be effective in this coupling application. The same applies to any cathode-tied cap for gain control. The pot will be scratchy in the latter case and there will be contact noise with the microphone. The DC offset will physically offset the mic diaphragm from its usual resting point, leading to asymmetric distortion of the captured sound.
The other problem with an electrolytic in most signal coupling apps is aging. The cap stays healthy with DC across it, so if there is no DC as in an opamp circuit with split rails, the cap dries out over time and its ESR, DA and DF go crazy contributing distortion to the circuit that will swamp that of the active devices. In the cathode-coupled mic pre, there will at least be a tiny DC bias voltage across the cap to keep it healthy to an extent.
A plastic coupling cap of any type eliminates all of these issues, but you have to use polypropylene to have zero THD from the cap itself.
The digital interface will sound cold mostly because its distortion harmonics are high-order and its IM is not great. It is IM that makes us feel that a sound is good, bad, harsh, cold, etc, much moreso than the THD present. Using tubes to add a veil of low-order THD harmonics and the attendant IM will warm the sound overall.
Electrolytics have leakage currents that are too high to let the cap be effective in this coupling application. The same applies to any cathode-tied cap for gain control. The pot will be scratchy in the latter case and there will be contact noise with the microphone. The DC offset will physically offset the mic diaphragm from its usual resting point, leading to asymmetric distortion of the captured sound.
The other problem with an electrolytic in most signal coupling apps is aging. The cap stays healthy with DC across it, so if there is no DC as in an opamp circuit with split rails, the cap dries out over time and its ESR, DA and DF go crazy contributing distortion to the circuit that will swamp that of the active devices. In the cathode-coupled mic pre, there will at least be a tiny DC bias voltage across the cap to keep it healthy to an extent.
A plastic coupling cap of any type eliminates all of these issues, but you have to use polypropylene to have zero THD from the cap itself.
