Also your amp needs time to settle and open up. Keep running in the unit and you will notice changes. I ran mine for 50+ hours before it really gave me that "wow" effect. Keep in mind I wasn't constantly listening to the unit and immediately noticed the difference from brand new. Although the initial improvements over my E7 was enough for me to warrant keeping the unit.
 

It also depends on what you're amping. For instance my Klipsch S4 when amped through the UHA-4 I notice barely any difference but when I amp my Tri-Fi and my X10 I notice a difference. Holy crap, I just realized I still haven't tried my EX600's on Sally yet. Time to change that.

And now we wait.

I'm really interested to hear your thoughts on it, considering how completely differently we heard the C421.

 
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mozu said:

I'm really interested to hear your thoughts on it, considering how completely differently we heard the C421.

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What OP Amp did you get in your unit? To my knowledge Clieos opted for a different OP Amp to change things up a bit.

AD8620.

 
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clieos said:

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Considering when I bought the UHA-4 I was teetering towards the JDS Labs amp I'm really interested in your findings. Sitting here impatiently waiting on your findings ClieOs. Someone pass the salt and butter

This is just a initial impression, as I haven't had the time to volume match and measure them all. But as far as hearing goes, I have to say UHA-4 and C421 are very close in performance. Note, I choose OPA209 in my UHA-4 instead of the default AD8610 as I want to try a new opamp. I already have several AD8620/8610 amp in my collection and OPA209 seems to be very popular and well recommended.

For now, my comparison is limited to UHA-4 vs C421 (both AD8620 and OPA2227 version) and T5. As far as presentation goes, I'll say UHA-4-209 is closer to C421-2227 than it is to C421-AD8620. It shares more of an upfront sound than it is on imaging. However, UHA-4-209 has the smoothness closer to C421-8620. Overall, it seems to be a mix of C421-2227 and C421-8620, but closer to C421-2227. The most noticeable parts for me are two: 1) UHA-4-209 has great bass impact, on par with (if not slightly better than) C421-2227 and definitely more than C421-8620. This could be just because the former two are more upfront. 2) The resolving power and imaging (and thus soundstage wise) of both C421 are ahead of UHA-4-209. Not by much, but it is noticeable. As far as T5 goes, I have to say it is trailing behind the above three closely. The deciding factor for me to say T5 is just behind the three is that it doesn't have overall transparency / openness in soundstage as the other three. For T5, everything is placed closer in. However, one thing I like about T5 that is better than the three is that it has the smoothest presentation of all. I can pair it with any IEM and it will make the IEM sound good. On the other three, they tend to want to push it over-the-top and reveal all the flaws. This kind of smoothness seems to be more common among balanced amp (T5 has BTL-ground, similar to active balanced ground). Regardless, I think T5 is still technically not as strong as UHA-4 or C421, though very close.

Taking O2 as the reference, I'll say, for now, C421-8620 is ahead, follow very closely by UHA-4 and C21-2227, then T5. Of course this might (or might not) change after I have more time with them.

Thx ClieOS.  Do you have any impressions between the 8610 and 8620?  I have some understanding of the 8610 signature in the Leckertons, I'm curious about the 8620.

 
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clieos said:

Taking O2 as the reference...

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ClieOS, what did you mean by that? I'm really looking forward to your take on the O2.

anaxilus said:

Thx ClieOS.  Do you have any impressions between the 8610 and 8620?  I have some understanding of the 8610 signature in the Leckertons, I'm curious about the 8620.

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AD8610 is the mono channel version AD8620, so they should sound identical, or at least very close to identical in a well designed amp. Of course, amp with two single channel opamp generally get better separation in measurement simply because the channel is physically further apart, but whether that is audible or not is another story. At least on the case of C421, I don't notice any disadvantage on using the AD8620 instead of two AD8610.

imackler said:

ClieOS, what did you mean by that? I'm really looking forward to your take on the O2.

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It means I actually do use O2 as the reference sound for other amps to measure up to. In other words, it is 'compared to O2's SQ, C421-8620 is the closest, then...'

Here is my initial take on O2 vs. C421 (where Mozu has a different finding from mine), volume matched.

clieos said:

....
However, this is not to say AD8620 crashes OPA2227 in every way. I love the AD8620, but I can understand why someone would find it V-shaped sounding and prefer the OPA2227 instead. It has more to do with the slight difference in presentation between the two opamps (and I had observed the same in cmoyBB as well) - AD8620 has really great soundstage and portrays the image in a very 3D fashion. In a way, it is almost better in imaging (especially in depth) than the O2 and that's quite an amazing feat on its own. OPA2227 on the other hand puts everything upfront with a sense of rich and warmness between the tones. If we were to take O2 as our reference for what 'totally neutral' / standard should be, then AD8620 would be the equivalence of what 3D TV meant to normal TV (where you get the surreal 3D effect) while OPA2227 would be equivalence of sitting close to a HD TV vs. at normal distance (where you get to see the fine detail in individual element but not quite so over the whole picture).

As mainly an IEM user where soundstage is generally more limited, the extra layer of depth that AD8620 brings to the table is a real killer feature to me. It just goes really well with all kind of IEM (*the synergy with Etymotic ER4S is unbelievably good for an IEM that is always considered to be flat sounding). But that same extra layer of depth might not be that appreciated for full sized headphone user (where soundstage is already good) or simply for those who rather prefer an upfront presentation. Instead, I would think they will find AD8620 adds a sense of hollowness to the presentation which might not be to their liking. That's probably why AD8620 is interpreted as V-shaping by some, even though the FR curve is perfectly flat for both opamps.
....

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clieos said:

Compared to the AD860 version, everything is slightly better on the O2, and I do really mean just slightly. If you are not doing a direct ABing, you might not notice any difference at all except that the AD8620's has more depth while O2 has equal amount of depth and width. The OPA2227's version is more noticeably different because it has an upfront presentation and a richer overall tone. That being said, AD8620's sounds closer to O2 than OPA2227's.

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My take on the C421-AD8620 was, to my surprise, almost identical to Headfonias (they did their review about 2 months after I sold my C421).

Off-topic, or maybe on-topic - I'm not sure how, but the UHA-4 is driving HE-400s like a boss (and beautifully) on low gain. I was expecting to get squat out of this thing with the Hifimans.

 
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Thx, that's very interesting and makes some sense to me.  I also find the 8610 'V' shaped and a bit too colored over neutral for my tastes so we are in agreement on hearing the opamp.  I also agree that these opamps do and should measure flat.  Where we differ is the 3D imaging and separation of the 8610 and 209 where I've had better experience in that regard w/ my UHA6S.  To me this likely means we aren't hearing the imaging a separation in the opamps so much as the topology of the circuit.  Explains why the C421 might best the UHA4 and why my UHA6 bests the UHA4 and O2 in that regard.  I actually find the O2's imaging and separation mediocre at best which is being polite tbh.  That's hearing two indentically built but differently sourced O2's AB'd in a 4 way w/ a CMOY and the UHA6S.  
 
Then again, your UHA4 is still fresh out  of the box and maybe phones and sources are playing around with things relative to us users.

 
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mozu said:

My take on the C421-AD8620 was, to my surprise, almost identical to Headfonias (they did their review about 2 months after I sold my C421).
Off-topic, or maybe on-topic - I'm not sure how, but the UHA-4 is driving HE-400s like a boss (and beautifully) on low gain. I was expecting to get squat out of this thing with the Hifimans.

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That's interesting.  Any more impressions would be good if you'd like to compare to the UHA4.  I think you have the 209s as well?
 
I just reread Mike's review and his comments about the 2227s seems to be similar to how the 209 differs from the 8610 as well.  Which also seems to mesh w/ ClieOS' remarks about the 209 and 2227.