External Filter

Technically not part of the chip but to operate properly the 6581 audio output needs a pull-down resistor (1KOhm recommended, not needed on 8580). SID audio output should be AC-coupled to any amplifier with an electrolytic capacitor in the 1-10 uF range.

Additionally, on a standard C=64 motherboard, there is an RC lowpass filter followed by a BJT based common collector acting as a voltage follower. The lowpass filter provides a 3dB cutoff at 16KHz, while the DC-Blocker capacitor acts as a high-pass filter with a cutoff dependent on the attached audio equipment impedance. The C64c board additionally includes a bootstrap capacitor likely to increase the input impedance of the common collector.