While SMPS is better because it can be regulated and it doesn't have hum noise, there are two problems with it that make it sound "thin" compared to LPS:
1) While regulation may hold the output voltage constant (it won't, read later), the caps are still too small to have low impedance in low frequencies. As in most amplifier topologies, for the AC current that goes through the speakers, the electrical circuit is closed through the PSU caps. They are in series to the speakers. They are sized adequately from the vantage point of PSU for smoothing out the HF switching noise but they are not large enough to be in series to the speakers, unless you're leaving it to the amp feedback to recover a bit too early bass roll-off.
2) They are always overrated for the actual power they can supply. For example 300W rated SMPS is more like only 100W continuous power. That is because the manufacturer assumes the crest factor of 6 dB (average music power is 6dB lower than peaks). The problem is that it will hold current and voltage for about 100ms and then the current/voltage limiter will kick in and start reducing pulse width and voltage and current will start drooping as fast as those small output caps are depleted. Not good for long notes in bass.
I have yet to hear a SMPS powered amp (including my beloved Benchmark AHB2) that sounds as full and rich in bass and have a slam as an amp with a beefy LPS. The guys from Esoteric do know why they chose LPS instead of SMPS.
Also, some PSU caps sound much better than other PSU caps and I have yet to see a good sounding caps used in SMPS.
So, my preference goes to a double-power EI transformer (no toroids and no crest factor applied) LPS with 40000-50000uF per rail per channel, preferably with multitude of small Cornell-Dubillier 381LX caps VS. resonant mode zero-crossing switching SMPS, any day of the week.
Besides, for Class D, which is the most dynamic load for PSU of any amplifier class, the best power supply is a battery...No current limit, no noise, no small caps in series to the speaker...
So, if there will be a R-r Class-D headphone amp, it will probably get supplied with gel-acid lead or Li-ion batteries.
Your comments about SMPS are completely misinformed. NCORE Amps ARE DESIGNED TO USE SMPS. THX maps ARE DESIGNED TO USE SMPS. I really hope you guys know A LOT more about ribbon tweeters than you do about amplifiers
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