Amplifier reproduction of instrument Timbre

Drew -- Yes, lack of phase coherence and time alignment between the drivers of multi-way speakers is also a frequent cause of problems relating to clean transient response. But excessive negative feedback in amplifiers is a separate and comparably significant cause of similar symptoms. Just Google "transient intermodulation distortion" and you'll find lots of interesting reading.

I'm not particularly aware of specific dac implementations being prone to TIM problems -- perhaps others will comment.

And yes, the character of harmonic distortion in tube designs is widely regarded as consisting of low order even harmonics, while solid state harmonic distortion, at least at high power levels, tends toward being primarily odd harmonics. Perhaps in turn that leads many solid state designers to apply negative feedback too liberally, reducing harmonic distortion but leading to TIM and problems handling transients cleanly.

Regards,
-- Al

I couldn't find the comments about transient issues in DAC design, however, I did find a lot of information on TIM. I thought this comment was particularly interesting given the constant debate about measurable characteristics of amplifiers, I will leave the company unnamed,

"We have developed a high quality, precision audio hi-fi amplifier designed with a completely different set of criteria from conventional commercial hi-fi amps. As most naive hi-fi buyers compare harmonic distortion specs between amplifiers and influence their choice on little else, manufacturers compete by using large amounts of negative feedback to reduce their total harmonic distortion (THD) to absurdly low levels, typically less than 0,05%, sacrificing transient response. This is actually ridiculous, as the ear finds it impossible to hear harmonic distortion below about 5%, and in fact creates significant (and measurable) internal harmonic distortion of its own. Even excellent loudspeakers produce 1 to 5% THD at low frequencies! The result of all this excessive negative feedback is that when a transient signal with a short rise-time is applied to the amplifier, the input stage(s) overload for a brief period of time, until the transient arrives at the output and the correction signal is fed back to the input. For a simple transient, such as a step function, the result is merely a slowing down of the step at the output. If, however, the input consists of a continuous tone, plus a transient, then the momentary overload will cause a loss of the continuous tone during the overload period and the creation of instantaneous dynamic intermodulation products. These specs are never specified in commercial hi-fi amplifiers, as they are difficult to measure, usually very poor, and sound terrible.. Some designers now hold the view that in current amplifier designs, harmonic and intermodulation distortion levels are so low that transient effects are the main cause of audible differences between designs, and the area in which the most improvement can be made. "

Drew -- I think that all sounds right, but when expressed to that extreme is probably applicable mainly to mid-fi and lo-fi mass market products. I think that at this point most high-end designers are aware of these issues, and it's more a matter of the degree to which the particular design can minimize the use of feedback without unreasonably degrading the parameters that are helped by feedback.

Regards,
-- Al