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本帖最後由 Lester 於 2011-12-21 00:41 編輯
想討論下有關pre-amp的問題, 我聽過好多吾同既hifi友講過 analogue pre-amp 係好過digital pre-amp, 因為當較細音量時,digital preamp係會減少resolution (少左bit) , 而analogue pre-amp 就吾會, 不過 John Westlake 就有吾同講法,
以我理解,佢相信好既pre-amp 係應該transparent , 而吾係加味精,所以signal path 愈短, 個pre-amp 對signal 既改變愈少愈好, 佢更加話"no pre-amp is best pre-amp"
另外佢話一隻正確design 既digital preamp 係會用遠多於16bit 既bandwidth來處理訊號,而教音量時只是把那16 bit的訊號在32bit的"空間"內上上落落,其實係可以做到吾影響resolution 既, 反而用analogue pre-amp 教細音量時係會增加訊燥比, 大家有咩睇法?
原文:
Hi Dan,
With any system Digital or analogue, there’s an ultimate limit in resolution – with Digital it’s related to the number of Bits, with Analogue it’s the SNR (Signal to Noise Ratio).
As you adjust the gain control on an analogue system, you ultimately run into lowering the signal into the noise floor. Very few analogue systems even come close to 16Bits Dynamic range (96dB) at normal listening output power levels.
Lets say your replying a 16 Bit audio source (CD), that’s 96dB Dynamic range – your going to need an amplifier with greater then 96dB Dynamic range to reproduce the input source without losing resolution – reducing the signal amplitude (lowering the volume control) reduces the signal level fed into an amplifier with its fixed input noise level, the signal is now partially “Buried” within the limitations of the amplifiers SNR – resulting in loss of resolution.
Very much the same situation exists with a Digital Gain control, reproducing a 16 Bit audio source with a system capable of greater then 32 Bits processing, only moves your “16 Bits” of data up and down your “32 Bit range” – but ultimately you well just encounter the “noise” limitation of analogue path during and after the D/A conversion process. It’s easy to be looking at signals -200dB down on a FFT in the digital domain, but at the very least thermal noise will ultimately limit the resolution in our analogue world… just as with any analogue preamplifier!
Unfortunately, things are not altogether so simple, the human brain can “hear” into the noise floor, and the “noise” spectrum also needs to be understood… where as digital systems have a fixed resolution “limit” in Bits, but it can also be “extended” by adding noise shaping and dithering to extend the “perceived resolution” of a digital source. It’s hard to explain in simplistic terms without taking into consideration many other factors… (and many varying points of view).
I’d just like to add that not all implantations of Digital Gain control are done correctly - Digital Gain control without correct dithering can sound very very bad… internal Data path width's and rounding errors all need to be factored… It's all too easy to get Digital Gain control wrong, where as even budget "Volume Pots" don't sound too bad.
With DQ / CDQ Analogue or Digital Preamplifier mode can be user selected – for the record I prefer the sound of the Digital Preamplifier Mode – with its much simpler analog signal path – no matter how good a Preamplifier is, it can never be better then no Preamplifier…
John |
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發表於 2011-12-21 00:40
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