Pros: Updated DAC; Sound; Pre-amp; Improved cosmetics; Continues HPA-1 synergies with HD800, etc.
Cons: Unexpected change to fixed gain; Buzzing with IEMs; Increased heat output
Matrix M-Stage HPA-1 w/USB DAC
This is the original M-Stage based on the Lehmann Black Cube Linear. It is a far cheaper version while still using high quality internal parts, and shares most of the sonic attributes of the Lehmann. It's become a popular economic amplifier that works well with all headphones, and has synergies with certain headphones that make it punch well above it's price point. As this amplifier is designed to run high impedance loads this comes as no surprise that it would favor some headphones over others. The unusual thing is that even some low impedance headphones seem to work exceptionally well with this unit such as the AKG K701 (and its versions). One of the most surprising and compelling aspects of this amp is it's synergy with the Sennheiser HD800, a $1500 headphone. First, it is to be expected given the HD800 is a 300 ohm headphone, but it goes beyond this. Comparing the HD800 on other amplifiers, both solid state and tube, there seems to be no equal even at 2-3 times the price paid for the HPA-1. There are some that come close, equaling in some aspects, but changing the sound in a way unbecoming of an HD800, such as the Objective 2. The O2 is about half of the price of the M-Stage and does well with the HD800, but lacks in dynamic contrast, leading to a boring experience. There are surely many amps I've not heard with the HD800, but until you reach the $1000 mark, I've not heard an equal. At this $1000 mark we finally have competition with a tubed amplifier, the Decware CSP2+ (now retired) and jumping a little more to $1400, the Bryston BHA-1 again raises the bar slightly. So long as you use the M-Stage with widely recognized strongly synergizing headphones, the value is astounding.
The build quality is reasonable for it's price, but it doesn't feel expensive. It is solid, but it's clear this is an economical device. The stock knob is small and I was annoyed with it, so I switched it easily with a knob I found on eBay for a few dollars. The dip switches for gain are on the bottom of the unit, not an ideal location, but considering they won't be adjusted often, I will give it a pass. I eventually left them on +10dB, a setting that worked well with most of my headphones. One of the great things about this amp was that Matrix socketed the opamp, meaning it was easy for anyone without electronics experience to drop in a different opamp allowing them to slightly change the sound in a way similar to tube rolling. The stock opamp OPA2134 is a good average performer. With HD800s I prefer the LT1364, but have not done extensive comparisons with other opamps.
Where this amplifier with optional DAC falters is the DAC. As it was a $30 option, I won't be too critical of it, but suffice it to say that it is an emergency DAC only. It supports PCM up to 24/96kHz. Highs are etched, scratchy. Soundstage is compressed, lacking layers. Voices are rough and weakly formed. This is the DAC you only use when you have no alternative, but given it only cost you $30, it's hard to blame it too much. Now you may expect me to suggest avoiding the HPA-1 with the DAC, but you would be wrong. The difference between the HPA-1 and HPA-1 w/DAC is merely the loss of 1 of 2 sets of analog inputs (RCA). If you are only amplifying one source there is almost no reason to not get the optional DAC. You never know when you might need a DAC and the added cost is negligible. Sold your main DAC and are upgrading to a new one? Use this while your packages are being shipped.
Even with the optional DAC being poor, I still rate the HPA-1 w/USB DAC 5 stars due to it's total package value.
Matrix M-Stage HPA-2 w/USB DAC
Starting with the DNA from the HPA-1, the weaknesses that needed work, IMO, were the optional DAC, volume knob (bigger), and the front facia looked a little too 1990s. Matrix fixed two of these, leaving the one that is easily user-changeable. The front facia is no longer gold on black and there is a scoop where the LEDs, input select switch and headphone jack are located. The stock HPA-2 knob is an attractive silver thing that is slightly recessed into the face. It's a nice touch, but the part you hold is still small and harder to manipulate. I had an additional replacement knob from when I changed the HPA-1 so I fit it to the HPA-2 and I am happy. The HPA-1 with this knob had a slight gap between knob and face, but the HPA-2, with it's recess, has no gap there. The replacement knob I used does have a small ring gap around the circumference that I didn't like at first, but now I actually do like it. The hex screws were changed from 2.0mm to 2.5mm and from black to chrome giving the face some much needed contrast. With it's modern face, the non-audio updates were more or less complete. Another physical change concerns the sides of the unit. They are more attractive and are supposed to have increased cooling capacity. Something that may have been necessary due to the HPA-2 running noticeably hotter.
So far I've not gotten specs I can fully trust. For a long time the HPA-1 was listed as having a 5 ohm output impedance on it's headphone jack. When I got the HPA-2 I was told output impedance went from 15 to 10 ohms and that doing so has increased the power output. Power output for both units is still listed the same, however. I don't know what to believe.
One thing I can believe is the HPA-2s updated DAC is truly updated, and in a big way. This update comes in the form of a Burr Brown PCM1793 DAC chip giving us expanded sample rates up to 192kHz. What used to be an emergency-only DAC in the HPA-1 is now one of the best affordable DACs I've come across. Gone are the scratchy highs, non-existent soundstage and grainy voices. It's not all rosy, though. The soundstage is still somewhat compressed, but in a way a $500-1000 DAC would be. The highs still sound slightly artificial, but they are pleasant, not grating. Voices are smoother and more realistic. The optional DAC here is fantastic value. Combined with the already high value amplifier, this is now a usable all in one affordable solution for all headphones, especially the K701 and HD800 and others that share synergy with the M-Stage. This unit redefines my conception of an entry high quality headphone rig. Pair the HD800 ($1500) and the HPA-2 w/USB DAC ($320) and you have a $1820 rig that is hard to beat even with a budget of $3000-4000.
If you enjoy the sound of the HD800 and are planning upward moves for DAC and amp, I would highly suggest the HPA-2 as a stand-in while you save money or wait out production times as I have with my incoming Headamp GS-X mk2. I currently use the Matrix X-Sabre DAC into the HPA-2 which will be replaced by the GS-X when it arrives. The included DAC on the HPA-2 is surprisingly good and should not be underestimated, but compared to the X-Sabre it's no contest.
Try as I might, I never could seem to find reliable differences between the amp sections of the HPA-1 and HPA-2. There are probably some slight adjustments and perhaps improvements in parts quality, but the end result is too close to call. If you own an HPA-1 amp only, I don't see a compelling reason to upgrade to the HPA-2 amp only. If you are an HPA-1 user looking for a DAC but are tight on space, the HPA-2 with DAC gives you this in the same space.
Bringing up the rear are areas where I find the HPA-2 lacking. With the amp circuitry largely unchanged I didn't expect the buzzing with IEMs from the HPA-1 to go away, and indeed it didn't. Matrix has been clear about this amp being targeted towards high impedance headphones, though, so I will let this pass. The only other area of disappointment was the unexpected change to gain. The HPA-1 had dip switches allowing gains of +0dB, +10dB, +18dB, and +20dB. The HPA-2 no longer has the switches and the gain is fixed to +15dB. This shouldn't affect most headphones that are high impedance or are inefficient, but unlike the IEM buzzing issue, using even full size efficient headphones with the unit may be problematic due to reduced volume adjustment range. This makes the HPA-2 less versatile since it can't be used with as wide of an array of headphones as the HPA-1. Although I will dock it Design marks on this review, since it doesn't affect the majority of it's targeted headphones I will not remove stars or partial stars. A questionable design decision to be sure.
As I rate the original HPA-1 w/ USB DAC 5 stars, it's hardly surprising, after reading my HPA-2 comments, that the updated version is also 5 stars. The HPA-2 is clearly superior, but as the HPA-1 is being phased out I see no reason why the can't both be rated 5 stars.