If the ipod had an analog volume control that was controlled by a potentiometer (twisty knob), that knob would be bypassed. But the knob is only changing the Current/Resistance relationship as it the audio goes into the amp from the dac. The DAC chip has line level volume, IE 0 gain applied (positive gain == higher volume than original, negative gain == lower volume than original sound file). Ipod doest have a potentiometer, it uses a click wheel to generate a digital signal, which is sent to the operating system. That signal, after being read, is converted to a negative gain applied to the song DIGITALLY, and then that song's data is sent to the DAC. either that or the signal goes from DAC to another circuit which controls the voltage of the opamp, and thus the op amp's gain.
As I said before, we dont know if the ipod classic actually has a cirrus chip inside. According to Vinnie, ipod classic 6th gen uses an apple proprietary DAC chip. Only ipod touch and iphone are confirmed to have a dac chip.
Here, i managed to do some googling around. I looked for pictures of the board, googled a marking, stumbled across FreeMyIpod, and that actually has all information sitting around right there. Im no good at reading datasheets because I only JUST got past basic analog circuits and am in digital right now. And we arent even going over SR latches or flip flops in this class so it basically doesnt even count as a digital electronics class.
Looks like we're using a cirrus logic chip in this ipod too. Can someone confirm if its the same chip used in itouch/iphone?
Also, considering they have a seperate chip for "usb charging", I can only assume that NXP PCF50635, the power manager, is the op amp. There are no immediately available datasheets for it.
EDIT: Classic 3g is identical in every way to the classic 1g and 2g. This man has measurements for the 6g: http://homepage.mac.com/marc.heijligers/audio/ipod/comparison/measurements/measurements.html
According to him, the ipod 6g is more crisp and detailed, but to him that is not considered an improvement ove the ipod 5g's "acoustic" tendencies. IE Acoustic music doesnt sound like acoustic music.
EDIT 2: despite him being very thorough with his measurements, there is a single sentence that more or less invalidates his entire expirement. He used the line input of his powermac. That is, by no means, a professional sound card of any degree. i cannot trust any of his measurements because of this. I have a dell vostro 1500 and the amount of crap I had to go through to get that little mini sound board working correctly was insane. I would never trust measurements that went into a stock motherboard line-in. Perhaps someone who has a pro-audio device can test with that thing's line input and post the results. He is right by saying that for comparison purposes, the measurements are valid, but I'm more interested in the actual performance than the comparison to ipod 5g since i dont have a 5g, I have a 7g. You can definately take 2 fish out of an aquarium and put them on a race track to see who wins, but even though the comparison is valid it is entirely useless.Edited by shrimants - 11/9/11 at 9:26am