That’s definitely not a good combination, assuming those numbers are accurate. The result would be significant rolloff of the deep bass region, and also undesirable phase shifts in that region.
The impedance of a capacitor increases as frequency decreases. At 1 kHz, for example (that being the frequency at which output impedances are often specified), a 0.47 uF cap has an impedance of 339 ohms. But at 20 Hz that impedance is 16,940 ohms. The output impedance of the output stage circuitry which precedes the capacitor will add somewhat to those numbers in determining the overall output impedance of the preamp.
As you may be aware, the rule of thumb guideline to assure impedance compatibility for a line-level interface is that the load impedance should be at least 10x the output impedance, at the frequency within the audible range for which output impedance is highest. Not conforming to that guideline is particularly likely to be a problem when the output impedance has wide variations over the frequency range, as it does in this case.
And for that matter the 80K number doesn’t even conform to that guideline with a 0.47 uF capacitor, although the slightly more than 4:1 ratio in that case might be acceptable if the frequency response of the speakers being used doesn’t extend very far into the bottom octave.
Regards,
-- Al
The impedance of a capacitor increases as frequency decreases. At 1 kHz, for example (that being the frequency at which output impedances are often specified), a 0.47 uF cap has an impedance of 339 ohms. But at 20 Hz that impedance is 16,940 ohms. The output impedance of the output stage circuitry which precedes the capacitor will add somewhat to those numbers in determining the overall output impedance of the preamp.
As you may be aware, the rule of thumb guideline to assure impedance compatibility for a line-level interface is that the load impedance should be at least 10x the output impedance, at the frequency within the audible range for which output impedance is highest. Not conforming to that guideline is particularly likely to be a problem when the output impedance has wide variations over the frequency range, as it does in this case.
And for that matter the 80K number doesn’t even conform to that guideline with a 0.47 uF capacitor, although the slightly more than 4:1 ratio in that case might be acceptable if the frequency response of the speakers being used doesn’t extend very far into the bottom octave.
Regards,
-- Al