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Originally Posted by nikongod /img/forum/go_quote.gif
Im 50/50 on the idea of the "opamp before the volume control" thing. Its not conventional, but has been done before. It solves the problem of noise from the opamp elegantly, BUT the gain stage is prone to overloading with excessive input voltage. This is noted on the schematic - its worth noting if you hook up to pro-level outputs. The fist kid who figures out how to set the gain to 10 (because 10 is more and more is better :facepalm: ) is going to have a wild ride.
The selected gains of 3 and 7 are pretty tasteful. Id lean towards just one. 3 or 4 covers you for 99% of all headphones, people just like switches.
This is a real problem in this design, particularly on battery power. Dual 8.4V batteries are specified, let's say that batteries are charged and does not fall below the rated voltage, we have 16.8V total spread, minus the forward drop of the two 1N5818 schottky diodes (0.5V each), this reduces the available voltage down to 15.8V. And let's assume no further voltage drops in the power supply -- in fact there is some drop across the MOSFETs, but probably small enough to ignore for this discussion. The NJM2608 opamp is not rail-to-rail. It could only swing to about 2V above the negative rail and about 1V below the positive rail. The signal's negative peaks will therefore clip first and effectively we have a maximum output voltage swing of 11.8Vp-p.
The volume pot is not at the input to attenuate the input signal, so the input opamp "sees" the full output voltage from the source. Let's assume a standard Redbook audio CD player's output voltage at 0dBFS of 2Vrms, which is 5.7Vp-p. Even at the lowest gain setting of 2x, the opamp output will be swinging 11.3Vp-p which is right at the verge of clipping. If you switch the gain any higher, it will clip rather severely.
The designer states on the schematic "Input 4V RMS max". If that was true, we'd need to be able to swing 22.6Vp-p at the output of the opamp for a gain of 2x (and more for higher gains). Neither a battery-powered nor a wall-powered version of this amp could do that without clipping.
This is irrespective of the headphone sensitivity or volume pot position. While similar concepts have been used in commercial gear, the input stage in them would be powered by much higher supply voltages in order to avoid clipping, but that is clearly not an option here due to the batteries.