[1] The mistake you're making is not understanding that human hearing together with the human brain in and of itself constitutes an extremely sensitive and useful piece of audio measuring gear.
[2] A little context: I've been a music producer and engineer professionally for over 30 years. I have owned several full-blown recording studios and obviously all manner of audio gear, from $3000 single-channel vintage tube compressors to $20 border microphones from Radio Shack.
[3] In the recording audio world, there is ridiculous hype around certain pieces of gear and every manufacturer and dealer wants to convince you that their extremely expensive unit is worth all the money. This has been the world I've lived in for all this time, so few people are more experienced with sellers trying to "hype" things with high prices and fancy tech-talk about this or that than I (and people in this field) are. If I couldn't see through all that nonsense, I'd be broke.
[4] And the way to see through all that is simply by listening.
[4a] Every different piece of gear sounds different. Obviously. They have to: they're made of different stuff. To argue the contrary is either to say A+1=A, or to admit to not being able to hear the difference. Which is fine. There are some occasions where I can't hear the difference. But differences there are.
[4b] I can use a $20 Radio Shack border microphone to get a certain useful sound, and a $6000 Manley tube microphone to get another.
[5] The exact same thing applies to different DACs. Different one simply sound different, as they must by definition.
1. The mistake YOU seem to be making is believing that statement is always true. Sometimes it fairly true, other times it's completely false. Firstly, as an engineer/producer for 30 years, I assume you must have used measuring equipment on occasion (spectograms, voltage and all manner of other meters). Compared to what we can measure, human hearing is in fact extremely insensitive. Digital meters and analysers can produce results in thousandths of a dB (or billionths of a second or a frequency cycle) and are many, many times more sensitive than human hearing/perception. Secondly, even relatively new/inexperienced engineers quickly learn how easy it is to fool the perception of hearing. In fact, to a very large extent music composition and production depends on that fact. For example, we don't want listeners to actually hear the fact that the orchestra was recorded from 30 different positions/locations or that in fact that rock performance was never a performance but a number of different, individual performances edited and mixed together.
2. Me too, although in my case, just slightly under 30 years. I've also owned 3 "full-blown" recording studios and have been lucky enough to work in several of the world's top studios. I could go on but this isn't the place to post my CV. Like you I've also used a vast range of audio gear, from a factory matched pair of M50's (valued at over $100,000) to the same Radio Shack boundary mic (although I modded mine for phantom power).
3. I would somewhat disagree with the "ridiculous hype" statement, at least when compared to the audiophile world. While there is a lot of hype around studio gear, it's generally nowhere near as ridiculous as that found in the audiophile world, as many of the most ridiculous audiophile claims simply defy logic, the science and engineering facts.
4. Not always, which is why we have meters and measuring equipment in studios! Educated and experienced engineers know full well the ears can be fooled and know when to measure the performance of equipment to see exactly what's going on.
4a. No, you cannot say "Every different piece of gear sounds different", many do sound the same and that should be "obvious"! Using your own analogy, do we get a different result when calculating A+1 depending on whether we use a calculator, an abacus or a pencil and paper? They are all clearly very "different stuff" but the result is exactly the same!
4b. I've used both of those mics too and they do sound substantially different, which is because they are substantially different. A spectogram clearly reveals frequency and other performance differences which are well within the range of audibility, as in fact do pretty much all transducers.
5. No, the same thing does not apply to DACs or in fact to most digital gear! Digital gear, by definition, operates on math, not sound and therefore by definition it must sound the same unless it is deliberately designed not to. Math is Math, "A+1" is always equal to "A+1", whatever gear you use! There was a big thing in the industry about 15-20 years ago about differences between summing busses (digital mixers and various DAWs). However, those apparently obvious differences couldn't in fact be identified in blind testing and furthermore, it was pointed out by the system designers that summing is in fact relatively simple mathematically and all the summing busses tested actually produced exactly the same mathematical result. This and various other examples (for example the EQ plugin comparison and others) was a wake-up call for many old time engineers, who'd grown-up in the analogue era, took for granted that different gear almost always sounded at least somewhat different (to the trained, experienced ear). Many heeded that wake-up call, made the effort to learn and understand digital audio theory and backed that up with testing (both with objective tests such as measurements and null tests and with blind/double blind listening tests) but not all! Even today, I still come across the occasional old time producer who clearly doesn't know/understand even the basics of digital audio.
Obviously a DAC has an analogue section as well as a digital section and with analogue there is always a measurable difference and then the question becomes; is that difference audible? If the difference is (for example) essentially thermal noise or some other random artefact down at say the -120dBFS level, then obviously that difference is inaudible at any reasonable (or even unreasonable) listening level. That's the issue we're often facing today, as even cheap DACs can now attain those levels of performance.
[1] But I'm well aware of what an acoustic guitar sounds like in acoustic space.
[2] And I can tell with 100% certainty that a recording of an acoustic guitar sounds more like a real acoustic guitar coming out of a Hugo2 (or a Benchmark or an Apogee DA16x) than it does coming out of an iPod.
1. I'm also aware of what an acoustic guitar sounds like in an acoustic space and as an experienced engineer I'm also well aware of the fact that it will sound very significantly different depending on numerous factors: The exact manufacture of the guitar, how it's tuned and played, differences between different acoustic spaces and where the musician is positioned within those acoustic spaces, what mics we use and where we position them relative to both the musician and the acoustic space/boundaries. All of these factors cause very significant differences in what we record and no two guitar recordings will sound identical, even if almost all of those factors remain unchanged. However, the vast majority of DACs do not produce differences anywhere near the magnitude of ANY ONE of these other factors, let alone all of them.
2. That maybe true under certain circumstances, for example an iPhone output has relatively low power and high impedance, so using cans which are sensitive to those issues will reveal obvious differences. However, if we use higher impendance/less power hungry cans and carefully volume match, then in a controlled test the differences are inaudible, as indeed the measurements indicate, because there's only about a 0.2dB freq variation throughout the entire spectrum (with an iPhone). However, we're now talking about different transducers rather than the DACs themselves. BTW, the Apogee unit you mentioned (the 1U purple unit if I remember correctly) is only a DAC, it doesn't have a built-in HP or speaker amp.
The music producer's role is (or can be) effectively entirely artistic and the role of the engineer is largely artistic but if we're going to call ourselves a music/sound engineer, then we must actually have a decent understanding of the engineering technicalities, principles and practicalities of how digital audio (+ acoustic and analogue audio) actually work.
G
