Originally Posted by stv014
This is the method used in the O2 amplifier. It has several advantages (significantly lower noise, stronger negative feedback in the output stage so reduced distortion, and output op amps with unity gain can be paralleled easily for higher current output and even lower output loading distortion), but it also requires a more careful selection of gain, since a too high gain will always clip, regardless of the volume setting.
No, it is most similar to the "volume before gain" method, and it always outputs the maximum noise level, regardless of the volume. It is very common in PC sound hardware.
Originally Posted by xnor
I don't think that is true. A digitally controlled volume control doesn't mean that the volume has to be attenuated in the digital domain. In fact, many PC audio chips use voltage controlled amplifiers.
you guys are talking about 2 different things; there is Digitally controlled analogue attenuation and there is digital attenuation. the former often uses a buffer opamp on the input, then a sort of internal resistor ladder to control the gain, which outputs through a further buffer opamp. the highest performance of these will drive a 600ohm or higher load directly, but for most headphone loads will require another buffer at a minimum so you cant use it at the output alone, this seems far too much extra components and feedback for a volume control in my book. dont get me wrong, i'm not a no feedback pundit, but there is such a thing as too much, for too little.
I'm not a huge fan of this method as it basically limits and filters the quality of whatever you have previous to this, which may actually be higher performance. above audibility? perhaps, but it just seems inelegant to me. the idea of going to all that work for a clean and linear signal only to do that just to control the volume. …
Digital volume, particularly modern digital volume that is either internal to the DAC (like with AKM, ESS etc which operate at ≥32bit (40 for ESS)) or prior to that at even higher resolution (up to 64bit) has a noise floor that is so low its really not worth worrying about; as should the DAC.
Analogue controls are OK for analogue sources, but for digital sources I dont get it. You may lower the noise from the dac, but then you add your own voltage noise from the resistors, add error due to them not matching and not insignificant expense to do it properly.
Edited by qusp - 6/23/12 at 6:32am