Audio amplifier input filters

High frequency amplitude and phase roll-off


Every audio amplifier needs an input filter to keep unwanted frequencies away. Both DC and too high frequency are undesirable to feed into the amplification. Blocking DC is trivial: Just use a large high quality capacitor for blocking DC and controlling low frequency roll-off and the problem is solved. More challenging is the high frequency roll-off and phase issues so I focus on improvement of the low pass filter here. The high frequency suppression is a difficult trade-off. The cut-off frequency should be low in order to keep away high frequency trash that would get inter-modulated with the music signal and form non-harmonic products in the audible range. A low cut-off frequency will cause large phase shift well below the filters cut-off frequency, which is also undesirable.

Filter comparison


For the investigation I compare a first order low-pass filter with a three variants of a second order low-pass filter. I set the roll-of frequency for the first order filter (schematic #1) and the second order filter with high frequency pole first (schematic #2) to roughly 180kHz. The second order filter with low frequency pole first (schematic #3) aims to have the same attenuation at high frequency as the other second order filter and ends up with a roll-off frequency of 250kHz. The second order filter with low capacitance (schematic #4) is optimized to work well with the audio input transformer module that I designed. Capacitor values were optimized on the bench and resulting roll-off frequency is a result thereof.


Both the first order (schematic #1) and the second order filter with high frequency pole first (schematic #2) show 5.5° phase shift at 20kHz. The steeper slope of the second order filter leads to the conclusion that phase shift at 20kHz could be reduced by moving the roll-off frequency higher while maintaining good high frequency suppression. This is what the second order filter with the low frequency pole first (schematic #3) aims for and reaches 4° phase shift at 20kHz.

All second order low-pass filters show significantly better attenuation at higher frequency than the first order filter. This is expected and a huge advantage over the first order filter design.


I find the minor extra complexity of the second order filter worth the dramatic improvement and by tweaking this filter, both attenuation and phase shift can be improved.