matching amps input voltage to source output volta

So how much power is the amp actually capable of. Is it for example only 25 watts max ?( a quarter) or does it not work that way.

It doesn't work that way. The amp's output voltage (not power) is proportional to its input voltage. Per Ohm's Law and the definition of power:

Power = (Voltage squared)/Resistance.

Therefore the 100 watt specification (presumably into an 8 ohm resistive load) for a 2 volt input would become only (100/(4 squared)) = 100/16 = 6.25 watts for a 0.5 volt input.

So which on paper at least is the better option.

I doubt that it's possible to determine the best combination of settings other than experimentally. Among many relevant variables is the dynamic range of the music that you tend to listen to. Well recorded classical symphonic music, for example, will often have an enormous difference between the average power that is required and the power that is required for brief peaks, while for most rock music that difference will be very small in comparison. On the other hand, you will most likely listen to rock music at a higher average volume than the average volume at which you would listen to symphonic music. And the information loss resulting from the digital attenuator will vary depending on the internal design of the player. So it's all hard to predict without actually trying the various settings.

That said, it appears that your player is specified as using 24 bit internal processing, and having "21 bit resolution." And its digital volume control provides 100 steps of 0.5 db each. Since the material being played is only 16 bits, that suggests that you might not get significant information loss until the volume control is turned down below the half-way point.

Regards,
-- Al

Pcoombs, since you are apparently using a balanced connection between the Wadia player and the Nagra power amp, an additional point that occurs to me is that on occasion I've seen inconsistencies between different manufacturers with respect to how balanced signal amplitudes are defined. For instance, if the output of the Wadia is defined based on the maximum voltage of EACH signal in the balanced signal pair, and if the input sensitivity of the Nagra is defined based on the instantaneous DIFFERENCE between the voltages of the two signals (which is what the receiving circuitry in the amp responds to), there would be a 6 db (four-fold) increase in the power capability that the numbers would seem to suggest. So it is conceivable that you might presently have 25W capability.

Re the 75% recommendation, that MIGHT be overly conservative, given that the player's internal resolution is 21 bits, and the resolution of the source material is 16 bits. Each bit contributes 6 db of resolution, so the 5 bit margin corresponds to 30 db. Turning the control down to 75%, i.e., by 25 steps of 0.5 db each, results in attenuation of 12.5 db, which means that only about 2 bits out of the 5 bit margin are discarded. Although other factors in the design might result in a greater sonic impact than those theoretically-based numbers would suggest.

Also, if you haven't already, see this Wadia paper.

Regards,
-- Al

Hi Philip,

Whatever power capability you presently have, whether it is 6.25W or 25W, would increase by a factor of 4 if you changed the Nagra's sensitivity from 2 volts to 1. Doing that would double the output voltage of the amplifier for a given input, and power is proportional to voltage squared.

Best regards,
-- Al