I know you must read the "help me decide what to buy" theads 1000 times, but i hope some of you enjoy talking about it a bit.
i got my paws on a pair of Fischer fa-003s. I just love them except that using the headphone jack on my asus xonar st i get a lot of noise (it seems to be video section noise), I'm looking for a headphone amp with the following:
battery powered (12 hours at least)
usb input (24/96 is awesome but not strictly nessesary)
output to my amp
able to control the volume of the outputs of amp (line out at least) through usb (i'm running linux but that hardly matters)
Nice if if the volume control is *after* the d/a stage so no resolution is lost
What i'm considering is getting a headphone amp with a USB input. I don't *need* 24/96 asychronous input but it would be nice. I'd like to have a line-out on it so i can connect it to my Trends 10.1 class-t amp. I looked at an usb dac/headphone/speaker output amp that uses the same chip as the amp i have now called the Topping Tp30 ( $80) which would do everything i wanted it to (aside that it's 16/44.1) and not battery powered.
The iBasso's look really nice and i'd just hook them up to the Trends i have now, and could take it with me when traveling. Does anyone know which of them are 24/96 and if any of them could be volume controlled through USB? I've looked at some of the reviews here.
Any other suggestions would be appreciated. I'd like to spend less than $200 if possible, but i'd consider spending more.
it is impossible to have the usb control the volume and have the control after the dac, what you want does not and cannot exist as you frame it. it would be possible to use usb to provide a control mechanism to operate a digitally controlled analogue pot, but that is a lot of work/cost, for no benefit imo.
A properly implemented digital volume, that is integrated into the dac chip, creates less noise than adding a pot after a dac, the type of analogue pot that can fit in a portable is by nature inferior in every way. well designed integrated digital volume (one that operates at >32 bits) is imo superior to ANY analogue volume control, no matter the size or cost. if done correctly it will use the same mechanism that us used to create a loud, or soft note in a song,
some of the new breed of dac chips (such as the ESS sabre dacs) have this type of functionality built in, not that many (read none, perhaps crystal semi chip in the jh3a) have trickled through into the portable arena as yet. these use a reference voltage fed through a resistor such as a cheap analogue pot or ladder attenuator to control the internal filter stage. this way you can control stereo balanced volume with a single platter attenuator, making for a cheap pot with perfect channel matching all the way through its range.
other types of digital pot include a ADC stage to convert analogue back to digital, process it and then a DAC stage to convert back to analogue and then amplify the result, i dont like these, as you are then listening to the dac in the pot, anything of higher quality before that is moot.
the misconception is that analogue volume pots do not cause loss of information, or somehow less than digital, this is false, they remove information in the form of noise added into the signal path by the contacts and resistors used, even high grade resistors add noise and contacts have a raft of small issues like capacitance, arcing etc, the low grade resistive elements used in small volume pots create more noise and generally have channel matching problems also, especially at the beginning of the range. most, if not all analogue pots used in portables are series attenuators, so at low volume the audio must pass through more resistive material than high volume, so there is a higher degree of noise at low volume.
anyway as far as your question, i would perhaps recommend the apogee DUET, it has a decent quality headphone amp, high quality DAC, line out and even a pretty cool ADC for recording if you are up for that. it also has a large volume knob that not only controls the units volume, but can also be configured to control the volume (or other functions) in other applications via midi over usb.