My portable mp3 player has a few hundred mV headphone out. Can I use this unit to raise it to a few volts and input that to an amplifier?
According to the manual, when the K1 switch is set to OFF, there are on-board volume control buttons that can vary the gain from −75dB to +23.8dB, in 64 discrete steps. This corresponds to voltage gains ranging from 0.1mV/V to 15.5V/V, so by setting the volume control to the correct setting, this amp would surely be able to provide you with the voltage gain you need.However, this probably wouldn’t be the best method of doing this. For one thing, most amplifiers should be able to output their full power when the input source (your MP3 player) outputs a few hundred mV. So in your case, I don’t see why you’d need a preamp before your amp. Of course some amps have a fairly high sensitivity, so I’m guessing this is the case for you, and the reason why you want this voltage gain.Also, another issue with using this Sure amp in the way you want to (which is pretty much like a preamp) is that although it produces very little distortion at low levels, distortion levels skyrocket at higher levels. For example, when outputting very low levels, such as 0.1W, its distortion will be somewhere around 0.05%, which is totally negligible (I don’t know the exact distortion level at this power output because it’s not listed, but that is a typical distortion level for a class D amp at that output level). When outputting 0.5W into 8 ohms, it produces 0.19% distortion, which is a fairly acceptable level, although not exactly great. This corresponds to an output voltage of 2V. However, when the output level increases to 1.5W into 8 ohms, distortion increases to an incredibly high level of 10%. This corresponds to an output voltage of 3.46V.The distortion level of a class D amp depends solely on the output *voltage* level (and not on the output *power*). So, when this Sure amp is outputting the 3.46V signal, it will have 10% distortion no matter what load it has, whether it’s an 8ohm speaker load, or it’s the input of your amp (which would be something like a 50kohm load). In the case of the 8 ohm load, it will be outputting 1.5W, whereas in the case of your amplifier being the load, it will be outputting an extremely low power level (due to the amp’s high input impedance). BUT, in both cases distortion will be 10%, solely due to the fact that in both cases the Sure amp is outputting a signal of 3.46V.Also note that the maximum output level of this Sure amp will depend on its power supply voltage. You will only be able to get the max 3.46V output (corresponding to 1.5Wx2 in 8ohm loads) when using a 12V supply. The lower the supply voltage, the lower the maximum output level.So, although this amp will be able to provide a voltage gain for your MP3 player, its output will start distorting quite a bit at an output level of 2V (as we saw), and will have incredibly high distortion at its max output of 3.46V. So, as I said above, this setup will work, but it wouldn’t be the best method due to the high distortion levels at the voltages you want.A better method (although, one that would probably cost more) would be to buy a dedicated preamp that is made for this type of application. Most preamps can provide fairly considerable voltage gain and can produce outputs of several volts, and can do so with distortion levels often around 0.001%, even at full output.
Date published: 2011-12-25