confused about -3.5db @20k on nos dac build

[Bernhard]

You could measure 5 k, k2 10k, k3 15k, k4 20k. Or 4k up to k5 20k.

 

[skibum]

Why not? But not with the traditional THD meter that filters the fundamental and measures the rest. You have to use an FFT analyzer, measure the 2nd, 3rd, etc components and calculate. In your case measure the 20 kHz, 30 kHz, 40 kHz components (not above) and calculate the THD. But since the fundamental is a first-order hold (~ square) signal, it has natural harmonics. Separating these high amplitude natural harmonics from true distortion components will be impossible IMHO.

I am not sure if that is correct. If you look at a 10KHz sine wave output of a NOS DAC it doesn't look ANYTHING like a 10KHz sinewave. Sure I can see 20Khz, 30Khz, etc components on my scope but are they not due to a large part by this horrendous signal that I see? I am thinking that THD at higher frequencies is a meaningless thing for NOS DAC's. From my measurements the 2nd, 3rd, 4th order harmonics that I am seeing are massive and seem to have no correlation with the sound quality that I am hearing.

 

[DF96]

Most of them are not harmonics, but images. As I said, THD at higher frequencies is meaningless for any DAC, or anything else with limited bandwidth. The difference with a DAC is that instead of outputting nothing above the normal bandwidth it can output images (if it has no reconstruction filter), some of which can be quite strong.

Remember, by definition a harmonic must be at an integer multiple of the fundamental frequency. Not a multiple, not a harmonic.