What's the deal with Broskie's iPod buddy?

[leadbelly]

I don't understand the reasoning behind Broskie's "normal" Aikido low-Z headphone amp:



and his "iPod buddy" version:



The latter version features higher gain in the front stage and a feedback loop in order to implement it. Why would you need higher gain when iPod's and MP3 players can already drive headphones directly?

[poobah]

The circuits have much different input impedances.

[moamps]

Originally posted by leadbelly
The latter version features higher gain in the front stage and a feedback loop in order to implement it. Why would you need higher gain when iPod's and MP3 players can already drive headphones directly?

Hi:

The use of higher open loop gain and NFB will result in smaller output impedance than in a non-NFB design, which is important when small-impedance phones need to be driven. Also, in this case, the application of NFB lowers the headphone amplifier gain to an acceptable value.

Regards,
Milan