History and Iterations - HSA-1a vs. HSA-1b
The HSA-1b is a revision to, and upgrade of, RAAL-requisite’s prior, and somewhat short-lived, HSA-1a amplifier - of which there have been two iterations. The initial release of the HSA-1a had two 4-pin male XLR connections for driving two pairs of RAAL-requisite’s “SR1a” ribbon-driver EarField ™ monitors:
A subsequent revision, an optional conversion to which was available to existing HSA-1a owners, swapped one of the SR1a outputs for a conventional, balanced, 4-pin XLR headphone output. This also included an adapter cable for driving inefficient/power-hungry conventional headphones, such as the HiFiMan Susvara and HE6/HE6-SE etc., from the SR1a output:
The HSA-1b refines the implementation of the HSA-1a, retaining both SR1a (ribbon driver) and conventional 4-pin balanced XLR outputs, adds a 1/4” TRS connection, moves all the switches to the front of the unit, replaces the conventional, potentiometer-based, volume control with a 24 position stepped-attenuator (for perfect channel balance) and adds configurable input level attenuation. The adapter cable for using the “SR1a” output with hard-to-drive dynamic/planar cans is still included.
I have heard all three of these models, at length, in my own system. The first HSA-1a was on loan. On the basis of its performance, I bought one of the second revision units, and have since replaced it with the HSA-1b.