The Solo SRGII headphone amplifier has another trick up its sleeve, that being its headphone impedance sensing output.
Some output impedance is essential because headphone cables are highly capacitive, and without output impedance, amplifier phase margin is seriously compromised.
High impedance headphones with long cables (and there are plenty of them) can lead to phase margin reversal and that’s why many solid-state amps don’t work well with headphone impedances much greater than 30-40 Ω.
Exactly how much output impedance can be tolerated before a headphone stops performing well, will depend on the impedance of the headphone being used – and we can never know what the customer is using.
Headphones have impedance dips and peaks across their audible spectrum, needing more control at some frequencies more than others.
Zero output impedance would be ideal, but screwing down the high frequency performance to preserve phase margin is no good for outright musical performance.
Output impedance cannot therefore be a fixed value that suits all headphones. But, by sensing the headphone current, extra power can be delivered to the headphone to cover for impedance dips, and less power for impedance peaks – thus making for a smoother sound.
The actual output resistance can be kept constant guarding phase margin whilst output impedance will appear to be changing to compensate.
And that’s basically how the Solo Studio Reference Green MkII achieves its really well-balanced sound regardless of headphone impedance.
And importantly for the most sensitive headphones, it doesn’t hiss!