Another question: Since I'm using a bit of DIY with my project, what would be better:
a) Using a stepped attenuator/pot to attenuate the line level signal.
b) Using a pot to adjust my amplifier power output according to my needs.
I'm asking that because the second option would make no changes to the original signal and is something that I don't really see very often as a way to adjust an equipment volume level. Does this makes sense that this way would avoid harmonic distortion, giving I can keep both channels with the same volume or perhaps it makes sense but is way more difficult to keep both channels with the same output?
Thanks!
What OJNeg said. If the pot is on the output of an amp, it makes no sense because you would have no control over the clipping point of the amplifier. It could be into full clipping mode and all you'd be doing is attenuating the clipped and distorted output.
Plus, if you are talking about the pot ahead of the amplifer in (b), but after the signal stage, that has risks as well. The O2 did it, but it's the only headphone amplifier I'm aware of that did. Just about everyone in the DIY community recommended against it.
Typically, the power output amplifier section of an amplifier string is distinguished from the signal stage in that it only produces current to supply the load. The real amplification takes place in the signal stage. Placing the pot between the two is only possible in a more sophisticated amplifier that has two stages. In the case of a receiver or separate speaker components, this would be the pre-amp and the amplifier. Note that volume controls are always on the pre-amp unless the component is a combined, integrated amp or receiver. In either case, the separate components are inside the same chassis in the receiver/integrated amp, but they're still there.
In the case of a sophisticated headphone amp (not a CMoy), there is typically a signal stage and an output buffer. It is possible to place the pot between the two as the O2 did, but you run into the same issue as placing it on the output of the entire amp - your source could have an amplitude high enough to cause the signal stage to clip, but the pot will have no means of controlling it. It will simply pass on an attenuated, clipped/distorted signal to the output buffer.
I believe NWAVGUY made extensive notes about source selection and determining whether outputs were too strong for the O2. In any other headphone amp, that's controlled automatically by placing the pot at the input.