DACs That Don't Sound Harsh
by Graham Slee
With DACs it's often a choice between one with a harsh but full frequency sound, or one that isn't as harsh but simply sounds false.
Just what causes such harshness?
Basically it's the same as trying to squeeze a quart into a pint pot - something has to give.
DAC chips output 1 volt or more but "low-noise" analogue output filter-stages saturate above 20mV (rms). The signal is 50 times too big.
So why isn't there gross distortion? The answer is negative feedback - "the cure for all ills" - except, in this case it doesn't effect a full cure. It fixes the measured distortion but doesn't get rid of the perceived harshness.
Faced with this, manufacturers try all sorts of things to remove the harshness, but none of them do it without changing the sound.
The problem they face is the mixing of interfering frequencies (and there are plenty in a DAC) with the high frequency harmonic distortion - the harshness.
The intermodulation artefacts this produces are actually "alien sounds" which weren't present in the original recording.
Some people like that sort of thing - they think it sounds better because they hear more noises - but most people don't.
For those who want the most natural sounding music, our output filter stages use a different approach.
As always, concentrating on the lowest possible noise it's the music which suffers, so let the academic pursuit of low-noise hang and let's use "music-friendly" techniques.
Low-noise designers wouldn't dream of using our inverting FET filters but the linearity and greater slew-rate (the output's ability to keep up with its input) results in a far more natural sound.
So is it going to be noisy? If you go on your own hearing you'll hear no difference at all, but the harshness... is gone!