Most PWM amplifiers today are self oscillating which means that the amplitude of the ANALOG input signal decides the switch frequency. This means that the switch frequency is high (> 300k Hz) when the audio input signal level is low, and that the switch frequency is low (= moving towards the audible frequency range) when the audio input level is high = when the output from the amplifier is high.The advantage of this topology is that it is cheap and simple to design since it uses well proven feedback from the analog output to the analog input partly to create the oscillation and partly to create a low THD amplifier. The disadvantages are that it requires an analog input signal (= it is really an analog amplifier!). So, the PCM (Pulse Code Modulated) signal from your CD player is converted to analog and then the analog signal is converted to the PWM signal. Also, the feedback loop typically isn't linear which is why distortion often increases towards higher frequencies.
還沒開始認真看Class D的設計，不確定是否如此...
他家的機器則是採用固定400KHz切換頻率，沒有任何feedback，因此是純數位的
So, the Lyngdorf true digital amplifiers are open loop amplifiers - no feedback used at all. It is quite obvious that developing such an amplifier is a much more complex process. It is simply more expensive since it requires very stable and ripple free power supplies and other special solutions such as the Equibit for the PCM to PWM conversion and extremely linear design of both power supply, output stage and reconstruction filter.
祕密就在這裡囉...
這篇談的是音量調整 http://www.lyngdorf.com/index.php?option=com_content&task=view&id=65&Itemid=35
做法與Sony相同，以調整功率輸出級的電源電壓方式運作。
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