Weird that the Atrium Open is listed as non-compatible? Unless the current output is significantly different from that of the HPA-21, I find the pairing fantastic and among the best I’ve heard! It certainly doesn’t have bloated bass with the HPA-21.
Sorry if this is common knowledge, but is this solely due to that the current output has a high output impedance?
The voltage output is great with IEMs. It's extremely quiet. The current output is incompatible, essentially, with IEMs.
I have a few dynamic-driver IEMs. With the exception of Zero Red whose FR is designed to interact with the output impedance, everything else works with the current output. The improvement/change going from voltage output to current output on my E500 is quite staggering, in a good way. The imaging improves vastly and the treble is more refined, all without changing the FR. The midrange sounds completely different from what I'm used to hearing from the E500 on different setups, and I don't think the word "imaging" captures well the differences I hear. But it is a favorable change nonetheless. I posted some measurement here.
The voltage output and current output have an identical FR, so I'd say it was "compatible". The change going from Hugo2 to Hugo2+23RM voltage was a <5% improvement in soundstage, imaging and depth. But the overall presentation of the music is still the same and familiar. The change going from Hugo2+23RM voltage to Hugo2+23RM current is hard to quantify. The treble is straight up better. But the rest is almost like someone did a remastering on the recording. If you are familiar with audiophile remastering, it sounds like an AP remaster instead of a MFSL remaster. Very hard to describe... With the voltage output, the music sounds more "whole", you know, like how it is supposed to be. I hear the soundstage as outlined by the decay of instruments. With the current output, the music doesn't sound "whole" anymore; instead, I hear individual instruments playing, somewhat disconnected from each other. The decays are still audible but more localized. I "hear" less of the soundstage. Perhaps "dry" (as in reverberation) is the word I'm looking for...3. E500 driven by
- iPhone SE 2016
- Hugo 2 green filter
- Hugo 2 white filter
- Hugo 2 green filter + HPA-23RM voltage output
- Hugo 2 green filter + HPA-23RM current output
Hugo 2's line out is 3Vrms, very high. At 3Vrms I can hear clipping from the 23RM, so I have to connect the Hugo 2 via pre out. I set the volume on the Hugo 2 to about 1/3 of the way (not sure what Vrms it corresponds to), set the volume on 23RM on low gain to about 11, and listen to the E500. I adjust the volume on Hugo 2 instead of the 23RM because 23RM's volume step is larger.
