Pros: Balanced input and output, great sound, smooth but detailed
Cons: Auto output switching occasionally gets it wrong
After spending a lot of time with some high quality tube amplifiers, I thought it was time to revisit the world of solid state (using sound processing chips rather than tubes). Some headphones seem to pair well with tubes while others like solid state so it's always nice to have both options available.
The amp I decided on is the Matrix Quattro amplifier which offers balanced and unbalanced operation (which I'll explain later) and pairs really nicely with the Matrix X-Sabre DAC I bought a little while ago. I was fortunate enough to also be able to buy a second hand Matrix M-Stage (HPA-1), the Quattro's baby brother (although it's been around longer). The M-Stage is renowned as a great bang-for-buck entry level amplifier (around $300) so how would the more expensive Quattro compare at its $450 level? Let's find out...
The Quattro builds on the success of the famously affordable and excellent M-Stage amplifier, but brings improved design and balanced operation. It was the balanced operation that hooked me because I love the idea of it from my days working in car audio where I often designed systems with completely separate left and right channels, but let me explain the concept a bit better.
Every speaker (or driver in a headphone) requires a positive and negative connection or an active signal and a ground connection. In unbalanced systems, there are two active outputs that deliver the stereo signal - one to each driver - and there is a single ground connection that is shared between both drivers. This setup can produce very, very good sound, but there is also the risk of the ground connections causing some leaking of sound between the left and right channels which can result in the sound becoming less defined and less controlled. I'm sure there are much better explanations of these circuits out there if you're interested, but hopefully this paints enough of a picture to say that balanced circuits have the potential to provide cleaner, better defined sound.
Max power: 1 W (balanced mode)
Power per impedance: 800mW @ 60 ohms / 400mW @ 300 ohms (balanced mode)
Signal-to-noise ratio: >98dB via XLR / >95dB via RCA
Just to explain some of these specs, the power of the Quattro in balanced mode is twice that of its unbalanced mode because it's essentially 2 amplifiers working together when running balanced versus just a single amp when running unbalanced. Also, there is just one set of outputs that are used for both balanced and unbalanced operation, but I'll explain that setup a little later.
The specs of the Quattro don't really tell us a lot about its performance and there was nothing in that list which excited me more than any other amplifier on the market, but for the price and with its offering of balanced operation for my beloved T1 headphones, I had to give it a run.
Design & Compatibility
The Quattro is a simple design that's been described as two M-Stage amplifiers sandwiched together. That may or may not be true as there are definite similarities under the hood, but also some differences in terms of the components used.
Perhaps the most obvious differences are the combination of unbalanced and balanced circuits as well as the use of different op amps. Op amps are the chips that amplify the sound and lend the amplifier a significant dose of sound signature (how warm, cool, bright, dark, bassy, or tinny the amp sounds). Where the M-Stage uses the OPA2134 chip, the Quattro uses the OPA604 and OPA2604 chips for its balanced and unbalanced circuits respectively.
Inputs / Outputs
The Quattro has a pair of RCA (unbalanced) inputs and a pair of 3-pin XLR (balanced) inputs. Unfortunately, it doesn't get any line level outputs which is a shame. It's always nice to have the option to use headphone amps as pre-amps or to have a straight pass-through to connect other amps in a chain, but for the average consumer who doesn't stockpile audio gear, the Quattro offers everything you need in terms of these 2 input options.
For output of sound, the Quattro comes with a pair of 6.3mm sockets on the front which are labelled Solo 1 / Balanced R and Solo 2 / Balanced L. It's only as I write this that I realise that the left socket is on the right side and vice versa!? Oh well, doesn't really matter. The point is that this pair of sockets is multi-functional which is both good and bad.
Having 2 jacks means that you can use 2 pairs of headphones simultaneously which is great if you're comparing headphones or if you want to listen with a friend. It 's amazing how often I would like to be able to switch between headphones without having to plug and unplug leads all the time so the Quattro is great in that regard.
Neither good nor bad is the fact that the twin sockets are used for balanced output via normal stereo 6.3mm headphone jacks which are easy enough to buy from most electronics shops if you need to make an adapter. Please be aware though that you specifically need to use stereo jacks. Mono ones won't work with the Quattro's auto-detecting circuit which I'll explain shortly.
In my experience the twin 6.3mm arrangement is one of the least common ways to connect to a balanced output with 4-pin XLR being a much more common choice. That means you're most likely going to need an adapter lead to connect your 4-pin terminated headphones to the Quattro's 6.3mm sockets. It's an easy DIY build if you can be bothered, but you can also buy this type of lead if you prefer so it's not the end of the world, but an XLR socket would have been better I think.
The other thing I'm not a total fan of is the auto-detect circuit on the Quattro's outputs. Although the concept of the amp switching modes for you is nice, the reality is that the process isn't seamless and sometimes leaves me with a balanced connection to an unbalanced output. This is because the Quattro detects the status of the connection with the contacts on the 6.3mm plugs. It seems that sometimes, the plugs touch the wrong contacts on the way into the socket and make the amp think it's connected to an unbalanced headphone. The only solution I've discovered is to unplug and reconnect the headphones or to switch the amp off and on again. It's not the end of the world, but I would have been perfectly happy with a third button on the front panel to select my output type.
Other Design Elements
In terms of size, the Quattro is about twice the width of the M-Stage, but is shorter by about 15-20% and a hair lower in height. It's a nice compact size.
On the front right of the unit is a nice large aluminium volume knob with a rough texture around the side. It looks good and feels good and it's attached to a high quality Alps motorised volume pot so you can also use a remote control with the amp if you want. The remote is an optional extra, but it's very well priced.
Also on the front panel are a power button and source button to switch between the RCA and XLR inputs. I was pleased to note that both inputs are able to provide output to balanced and unbalanced phones so you don't have to use a balanced source to enjoy balanced 'phones. Not only that, but the difference between the sound from the balanced / unbalanced input is near enough to identical that it makes no difference which you use.
Finally, on the left side of the amp's fascia are lights indicating mute status (only available using the optional remote control), input in use (RCA / XLR), and output status (balanced / unbalanced).
The Quattro's power output means it will drive most headphones other than difficult-to-drive planar magnetics. I'm hoping to try it with some Audeze LCD 2s soon so I'll update if there's anything notable to share. Unfortunately, the Quattro has an output impedance of around 10 ohms which is quite high and may negatively affect the frequency response and tightness of sound from lower impedance headphones depending on their specific specs. Testing the Quattro with my 32 ohm headphones showed some variation compared to the M-Stage with its 5 ohm output, but it was very much a case-by-case thing. The one thing I can say for sure is that it will not match ideally with low impedance, multi-BA IEMs like the Miracles. The sound is really clean and hum is negligible, but the frequency response is altered slightly by the impedance mismatch.
Other than the minor issues with output impedance - and I do consider it minor because it's an amp designed for desktop headphone listening, not IEMs and portables - the Quattro drives all my phones really nicely and I love the fact that it can run unbalanced / balanced in and unbalanced / balanced out. There's plenty of versatility in how you use the amp and with what.
Having been excited to buy the Quattro, I was initially disappointed with its sound. It was a bit harsh with the T1s and I felt like it wasn't significantly better than the much cheaper M-Stage. The balanced output was definitely an improvement, but I was still left no really enjoying what I heard. Although I'm still quietly skeptical about the effects of burn-in, I decided to leave the Quattro running for a while to see if the sound changed at all. I didn't listen to it during this time so I know I haven't adjusted to its sound, but I can definitely confirm that I now thoroughly enjoy the sounds being produced by the Quattro in both unbalanced and balanced modes.
For the review of the sound, I'm referring to the character of the sound in both balanced and unbalanced mode. The balanced circuit is just 2 of the unbalanced circuits running in parallel so the character of the sound is identical. I'll explain the audible impacts of balanced versus unbalanced output separately.
Treble from the Quattro is present and extended, but smooth. Even with the sometimes edgy Beyer T1s, the Quattro produces very listenable treble that's easily on par with the treble produced by other amps in this price range or slightly above.
The Quattro produces brighter treble than the M-Stage, but manages to do so without getting cold or harsh. The treble can be a little dry, but the overall signature is slightly warm so it balances the treble nicely. Fans of sparkly treble may want to look elsewhere for an amplifier because there's not a lot of sparkle in the Quattro's sound, but don't mistake that for a lack of treble or extension. The treble's there, it's just not enhanced or lively - instead it's smooth and easy to listen to. To my ears, the Quattro strikes a really nice balance by presenting a sound that's not as thick as the M-Stage (which is a great amp) while still keeping the smooth and musical presentation that makes the Matrix gear so enjoyable. I was really impressed with the Quattro's rendition of the detail present in the sound of a solo violin during the listening I did for this review. The Quattro was able to accurately recreate the subtle rasp of the bow being drawn across the strings sounding completely accurate and lifelike. I think that's a benefit of the slightly dry sound - it allows details like this to come through where a lusher, smoother sound might cover them over.
Mids The Quattro's midrange is accurate and clean. It's got good presence in the overall presentation and strikes a nice balance between musicality and accuracy. Once again it's drier and cooler sounding than the M-Stage, but still enjoyable and slightly smooth. I really like the presence of the midrange created by the Quattro. Even in busy, active tracks, the vocals are always clean and separate, but not enhanced or pushed in your face in any way. THey are slightly forward compared to the rest of the spectrum, but only to a degree that you notice if you're listening for it.
The Quattro produces bass that is largely accurate, but possibly a bit behind the rest of the spectrum. I wouldn't say it's lean sounding at all, but the mids and treble overshadow the bass ever-so-slightly. The quality of the bass though is very good. There is texture and impact as well as body on sustained bass notes - it's just that all of this happens slightly behind the rest of the spectrum. Extension of the bass is really good all the way down and is probably more balanced in the lower ranges than around the mid-bass.
It's hard to be sure because of the time taken to switch, but I think bass impact and present improves slightly when running in balanced mode. This could simply be a reflection of the power output doubling and therefore having more energy to apply to bass notes.
The Quattro's bass presentation pairs really nicely with bass-tilted phones like the Fischer FA-011 LEs and I imagine it would also really suit some of the Sennheiser models with a bit of extra mid-bass warmth.
I don't normally do this, but I wanted to clarify the descriptions I've provided of the bass, mids and treble. I found myself struggling to describe the Quattro's sound in this review and I think it's because the Quattro is largely neutral and accurate to the source with a slightly dry, but musical presentation. The comments above reflect very subtle impressions only and should not discourage anyone from considering this amp because it's very, very good for its price and the evidence of that fact is the difficulty I had trying to really dissect its sound signature.
General Presentation and Staging
In unbalanced mode the Quattro produces a good-sized soundstage (or headstage) with decent width and depth, although I would say it is wider than it is deep. There's not a lot of vertical layering or vertical space so the result is that all of the sound seems to be placed across a narrow stage extending side to side. The stage sounds open and in no way congested, but it's not expansive and spacious like some amps I've heard, although those amps also cost significantly more (e.g. $200+ more). Staging is definitely more open and more accurate than the M-Stage so the Quattro is a definite upgrade.
Beware: moving to balanced mode makes a significant difference to the soundstage! Switching over to use the balanced outputs of the Quattro lifts the sound to a whole other level. The overall signature is unchanged, but staging resolution is dramatically increased. Everything becomes more sharply defined, gains extra presence and weight and just generally gets better. This is why you buy a Quattro!
In balanced mode the stage is deeper and taller while retaining the same side-to-side space. The accuracy of placement is improved and the ability to hear vertical layers in the sound is improved as well (i.e. vocals now sound higher than a guitar being played by the singer). In balanced mode, the Quattro's stage and presentation is outstanding and highly enjoyable. It's still not expansive and massive, but strikes a good balance by placing the listener "a few rows back" from the stage while keeping the music close enough to be engaging.
For its $450 price tag, the Quattro is definitely an amplifier worth considering if you're running balanced headphones (or if you can get a cable to make them balanced).
I probably wouldn't specifically recommend the Quattro if you're only using unbalanced headphone connections even though it's a good amplifier even in unbalanced mode. With the new M-Stage (HPA-2) out now, I would expect it to match the Quattro for unbalanced performance and you can always change op amps (see below) to upgrade the sound of either amp so I'd probably choose the M-Stage plus an upgraded op amp for unbalanced use, but it's a no-brainer for balanced headphones - the Quattro is excellent!!
Just like its older brother, the original M-Stage, the Quattro offers outstanding value for money with good power, an enjoyable sound that's more neutral and detailed than the older M-Stage, and the opportunity to wring every last bit of performance out of your headphones with balanced cables.
A Quick Note on Op Amps
I'm new to op amps, but having come from recent experiments with tubes in amplifiers, I look at these little "chips" kind of like digital tubes. What I mean by that is that you can tweak the sound of your amplifier by using different op amps. In the image to the right, the op amps are the black, rectangular things - there are 4 along the top and one in the bottom left corner.
The Quattro uses a single, stereo op amp for unbalanced operation and uses 4 mono op amps for balanced operation. This can make it an expensive prospect to upgrade because you need to buy 5-6 op amps depending on the versions you use, but it can also provide some relatively cheap fun because you can pick up decent op amps from as little as $5-6. You can also spend up to nearly $100 each if you want to spend that much, but it's not necessary. You do need to make sure the op amps you buy are suitable for your device, but it's not too hard to work out with a bit of online reading and asking on forums.