I just tried something a little different. Hooked up a Little Dot headphone amp in between the Oppo and S300, and it sounds extra sweet! (Using Voskhod missile guidance tubes shipped directly from Russia. Even better than GE joint army navy tubes.) Nice blend of Class D’s clean bountiful power with tube sweetness.
This is an inexpensive way to hear what a little tubeness does for the signal. Makes perfect sense to me that the BHK amps have tubes in the input stage. (Looking for an excuse to visit Colorado so I can audition the BHK amps with the IRS speakers.)
PS: Here is an Art Dudley column that distills his theory behind tube’s euphonic appeal:
The electronic distortion products added to a music signal by a simple tube amplifier tend overwhelmingly to be the second harmonics of the affected frequencies. Add global feedback to the circuit and, despite a reduction in the amount of second-harmonic products, the far more objectionable odd-order harmonic products are not only left behind like audible driftwood but can actually increase in amplitude. Change the active devices from tubes to transistors and the problem gets even worse. Nice.
Reorganized on the noise floor
Why do harmonic products become more objectionable as they rise in order? (footnote 3) To begin, consider that the first harmonic (1xF, where F stands for frequency) is the fundamental itself. The second harmonic (2xF) is the octave of the fundamental, and the third harmonic (3xF) is the octave-plus-a-fifth of the fundamental. In other words, if the pure tone being amplified is A=440Hz, the wonky amplifier would superimpose over that note an E.
But I just told a pretty lie: That third harmonic is measured as 1320Hz, yet the musical fifth of A=440Hz is actually 1318.5Hz. That’s because, in well-tempered Western music, an octave is divided into a 12-tone chromatic scale (footnote 4), the 12 notes of which are not precisely in tune with the harmonic series. The third harmonic added by an amplifier to a fundamental will thus be very slightly off—a problem that worsens the higher one goes (again, except for the octaves). If you think third-harmonic distortion is a bitch, just get a load of seventh-harmonic distortion—which, for A=440Hz, is a 3080Hz tone. Your perfect A is now overlaid with a tone partway between F# and G. Ouch.