"The weird part is that as long as we have a gain knob, we only need that switch because users are stupid. There's no reason at all that a Line Input couldn't just have an impedance high enough for passive guitars. Then you'd just turn the knob up 9-12db more for the guitar, and you're ready to rock! But the people who buy these things don't understand any of it, so we end up with this sort of thing where the High Z input also automatically comes with that extra gain. So what if I need High Z and that gain is too much?"
That's a pretty kettle of fish with worms in it Ash! First off yes, a line input COULD have an input Z of 1meg but the problem then is noise. The passive guitar only provides a shunt impedance of 10k at the best and much more if pots are adjusted. Yes, 'proper' guitar amps have 1meg input Z but they ARE noisy fekkers! We demand stooodio quiet of our AIs! Then, the AI input amps are not optimized for high impedance. The superbly low noise NE5534 would be rather poor as a HZ amp.
H Z amp with too much gain Ash? They do that ALL the &^%$ing time! Worse case was the very early F'rite 2i2. The problem seems to be that WE all know that even the wimpiest catalogue Strat chucks out enough level to get -18dB on the ole'puter but you might struggle to get it to hit -6dBFS?
Such a low gain instru' input is not acceptable the marketing guys seem to think and the result is a decent humbucker or active cracks the front end up.
Of course the solution is simple? Have a decent amount of gain and then a -20dB pad. Trouble is of course, switches and extra Rs add to sale price in a VERY competitive market! (it is btw very easy to make a ~10dB high Z attenuator, will post pic if anyone wants one)
Am going to try to find some AI schematics but they are pretty secretive!
Dave.