If the IMD and harmonic distortion in the AF stage is so bad, how come it doesn't show in the 2 tone tests?
DSP averaging and error correction. Pure test tones, like those used on two-tone tests, are not like complex waveforms like with voice. The error correction and averaging has "less math" to perform on pure tones. It is easy for error correction to reconstruct a sine wave or other simple waveforms. However on voice, what goes in is not
what always comes out. What comes out is an approximation of what the waveform should have looked like. The more complex the waveform, the more averaging and error correction is performed. In others words low-fidelity. The result on voice is corrected audio, like the difference between a low-grade MP3 file and an original CD or LP recording. This is the brilliance of DSP, you can fudge it, build cheaper, and market around the inherent drawbacks. Like the lack of access or ability to modify in meaningful ways.
Another factor is the quality of the DA and AD converters used in Ham radio. They are ... well ... the one in your basic Soundblaster card or even the audio on a motherboard type computer are of significantly higher quality. However again, a simple waveforms will pass through these unscathed. Again another reason why Flex sound good or most SDR's for that matter. Better AD/DA technology.
Since the manufacturers of these transceivers have for years been selling the idea of "communications quality audio" as a way out, that is all that we expect from them. And since it is obvious most radios are made with contesting in mind, there is no real push for audio fidelity.
For example, in a very well known top of the line transceiver there is a small chip being used to handle the microphone input. The same chip is used in my neighbors Barbie Doll walkie-talkie for the same purpose (I just fixed it for her). Many have wondered why they can't get their audio to sound perfect on their new transceiver. This cost saving design thinking is the primary culprit.
One of the reasons why the FlexRadios sound so good is because they use a more expensive and much better performing audio stage. This is why the Flex sounds the same regardless of mode BTW. Some modes show audio flaws better and some hide them. AM being one that shows the flaws very readily. This is why even the most expensive rice warmers generally never sound good on AM even if operated correctly.
With boat anchors, the audio circuits are just and only that. Audio! NO fudging, no feature sets, just audio. If you look at a push-pull plate modulated boatanchor, the modulator circuit is a basic audio amplifier like that used on a stereo system. The only real difference is the modulation transformer have a different secondary for higher impedance loads. Otherwise they are high-fidelity audio amplifiers. I have used Dynaco amplifiers as modulators. Eico converted one of its monoblock audio amplifiers, by only adding a mic stage and changing the output transformer, to make a modulator called the 730.
New radio's just can't and are not designed to perform on this level insofar as audio goes. The design criteria is for small size and feature sets, and that's it. Everything else is a compromise or even an afterthought.
DSP is a hack that allows a deep feature set that is very inexpensive to produce, nothing more. Furthermore the need for a Class A PA circuit is just a marketing game, not unlike that played on audiophiles for $20,000 audio cables. It's a non-issue issue. It's a "problem" created to satisfy the release of something "new", the ClassA PA. We never knew we needed it until they told us they had it. Does it make the radio perform better? Not really and it runs hotter and produces less output. Sure you can argue the RF is cleaner, but so what? Does it exceed FCC regulations any better. No. Standard outputs do as well in this regard. Does it make the audio better? Certainly not. Now if they used classA circuits throughout the entire analog audio sections then they'd have something. But alas, they use the cheapest crap they can, and talk about features instead. In the audio world we say, "Garbage in, Garbage out."
So endeth the off topic rant! Ha, ya got me started. As if us folk from New York ever have a shortage of opinions nor a willingness to express it!
Anyway, the old boatacnhors do sound great, when restored properly and aligned perfectly
. (A few small mods can help also). Another reason to say a given boatanchor is worth restoring. Some sound better right out-of-the-box than others. Like the B&W 5100b, Collins KWS-1 or KW-1, Hallicrafters HT-32 or 37 or even the HT-20, the Miessner's, Collins 32V and the list goes on. Oddly the only rice warmer that I have ever heard sound like these, and only after being heavily modified is the old FT-101E.