I'm developing a custom earphone and I'm having some issues with producing a clean sound from my balanced armature (BA) driver (Sonion 31D005/8). I've come up with a pretty standard circuit (see below) that receives a sine wave at three possible frequencies (227,660,1000) from a 12-bit DAC.
When I probe the output of capacitor C2 without the speaker connected, I see a clean wave with a zero volt average, however as soon as I plug in the BA the peak of the negative portion is truncated, and the voltage (p-p) is reduced (see below). This outcome is more prominent for the lower frequencies, ultimately reducing the loudness of my speaker for lower frequencies, contrary to what's shown on the datasheet (should remain relatively constant).
Another important factor is that the BA's impedance decreases for lower frequencies (500Hz:38 ; 1000Hz:66) so I expected the opposite effect, however I'm assuming there's some time constant effect or an inductive feedback that is distorting the waveform. I've confirmed with a microphone that the received waveform is quite distorted in a similar way, and the lower frequencies do not sound very clear. Am I missing a passive component or something that should be considered when designing headphone (high impedance) as opposed to speaker (4-8 ohm) circuits? Perhaps a different amplifier (tried the LT1010 with worse performance)?