DAC IV stages
Reply #61 – 2013-02-22 21:21:24
The fixed noise floor kills you. So while analog gain is better then digital, it doesn't save you running into the noise floor at low volumes. So SNR does decrease, although slower then 6dB/bit of digital. The flip side of this though is that going to more effective bits doesn't really help you (unless you do it by reducing the device's analog noise floor). My point, therefore, was that since we operate under these constraints at all but the very highend, worrying about DAC design is not very productive. You need better analog amplifiers after the DAC or its all for nothing. I agree, and yes amplifiers are sometimes a limiting factor. If digital gain control is used, the quality of the converter must increase to maintain the same level of performance that could be achieved with an analog gain control following the converter. Every 6.02 dB of gain reduction that is used in normal listening will require 1 additional bit of effective resolution. If we start with 129 dB SNR (as in the DAC2), then we can use generous amounts of digital volume control without impacting the overall performance of our playback system (the power amplifier almost always becomes the limiting factor). But if we start with a laptop that has a 95 dB SNR and use the digital volume control on iTunes or Windows Media Player or the OS then the situation is very different. iTunes and WMP do not control the gain of an amplifier following the internal DAC, they control the gain in software (digital gain control). These players and most other computer audio applications place significant demands upon the performance of the built-in DACs. In a typical media server system, (line out to amplifier, or headphone jack to headphones) the overall performance will suffer if significant use of the software volume control is necessary to achieve a normal listening level. You can't start with a 95 dB DAC and apply 20 dB attenuation and expect 16-bit performance. In a practical system, few users want to operate at 100% volume to reach a normal listening level (even though this would give the best SNR). The sensitivity of the amplifier or headphones demand that adjustments are made. Many users are perfectly satisfied with the resulting (95-20)=75 dB effective SNR. However, it is not unreasonable to expect that people will notice a difference when connecting a 129 dB converter to the laptop media server and amplifier. The differences should be detectable in ABX tests. I posted ABX test results showing the effects of truncation (different issue), but similar tests could and should be run on consumer products operating at typical volume settings.