I’ve managed several hours listening with the HSA-1a since it returned. After a quick validation that it’s performance with the SR1a is the same as the original unit I’ve had, I focused the rest of my time so far on using it with conventional headphones (quick “with my morning coffee” picture for provenance in the early Seattle morning).
Initial impressions … (again, driving conventional headphones form the normal 4-pin XLR output with step-baffle compensation turned OFF - since you don’t need it for normal cans)*:
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Overall signature is slightly warm with some added tonal richness.
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Tone/weight reminds me of the EC Black Widow, while other technicalities bring the Violectric V281 to mind (but without the dark presentation).
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Punchy delivery with impressive, and visceral, slam/impact - think Schiit Mjolnir 2 or Phonitor X (or good speaker amps feeding an HE6).
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Low-end is capable of excellent growl and rumble, but remains tuneful and tight.
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Very smooth overall presentation.
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No noise/hiss nor hum, even with my lowest-impedance, most sensitive, conventional headphones (dynamic and planar)**. Being solid-state, you wouldn’t generally expect any different, but it is very common to encounter such when driving headphones with speaker amplifiers, which is what the HSA-1a began life as.
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Some very minor low-volume channel imbalance (not heard via the SR1a at all) with very efficient cans. It’s completely gone well before reaching anything like a listenable volume … and before the volume dial passes the “10” (1 for other amps) position. But you’re probably not going to want to use IEMs with this.
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Audible click when switching between XLR/RCA inputs***. Louder in the right channel, but not loud enough to be worrisome in regards to hearing nor, probably, headphones, but I would personally be inclined to disconnect them before switching inputs.
*Despite effectively being out of time to do more in the online audio world, I am still planning on reviewing this unit, and probably finishing up a couple of other pending reviews.
**One exception; with both XLR and RCA connections in use, some very low-level hiss (about -90 dB) was audible using the Focal Stellia when the volume was turned up to maximum and no music was playing. Back the volume off even a fraction, and this disappeared completely.
***Very minor compared to the transients you get from, say, either version of the RNHP or the Fidelice Precision DAC.