Headphone Amps and DACs

Shortly after I bought the AKG K701 as an upgrade from my dying Sony MDR-V700, I started looking around at ways of improving the audio quality from my laptop.  I had it in my mind that I wanted a new soundcard (which in my case would have been USB since it's a laptop) plus a seperate amp.  Soundcard output levels are much the same, so the K701 would have sounded just as quiet on a top end soundcard as onboard.

I spent some time looking around, trying to find an external soundcard that was at least as good as the X-Fi Elite Pro that I had in my last desktop computer but to no avail.  All the USB soundcards I found were low end.  As I was looking around, I came across threads with posters suggesting DACs instead.  After a bit of research I found the right DAC for me.

I'm lucky enough to hang around with a bunch of cool people that love music and appreciate quality too, so I'm going to explain a little about headphone amplifiers and DACs, and why they are a better alternative than sound cards.

Amplifier
An amplifier simply increases the volume of an input signal.  The features can vary between amps but they all serve the same basic purpose - to take a low power input signal and turn it into something powerful enough to move drivers and in turn, air.  Standalone amplifiers should only be considered if you have a good source, since an amplifier does no decoding of any sort (it just amplifies an analog signal).  In general, you'd only buy a standalone amplifier if you have a seperates hifi system with good quality RCA outputs, or a very high end sound card with RCA out.

The thing to remember is that you get out what you put in.  That means if you connect a low quality laptop audio out to the amp, you will get low quality laptop audio out, but just at higher volumes.  Similarly if you connected a cassette player to your amp, you will get the same hissing, low quality sound.  You may notice a small improvement simply due to the fact the amp has more power, which will usually present itself as slightly fuller bass.

So if your source is high quality and already has a headphone out, why would you buy a seperate amp?  There are a few reasons.  The first and most common reason is that some headphones require more power to drive than others.  This is a result of their resistance (impendance), measured in ohms.  My Sony V700 are 24 ohms and were loud enough for me not to max out the volume on my old laptop.  The AKG K701 on the other hand, are 62 ohms and because of the extra resistance, require more voltage to reach the same listening levels.  With the AKG I could max out the volume level on my laptop and still be left wanting a little more.  The K601 are even more power hungry at 120 ohms and would sound relatively quiet on most headphone out jacks.

The other reason is that headphone outputs in most devices always leave something to be desired.  Portable devices such as the iPod are specifically designed with headphone output, but the limited power of the player and often compromised circuit design due to cost and space means you can get bass rolloff.  Fortunately, you can buy portable headphone amps if it bothers you that much.

Take a look at this image.  This is a 40Hz square wave.  This is a tough test for an amplifier, because it requires it to drive to full voltage, hold it for a short while then back in the opposite direction.

Here we have the iPod 15GB playing back the 40Hz square wave.  On the left is with no headphones connected, and to the right is with the standard Apple headphones

As you can see, unloaded the iPod does a fine job of playing the file accurately, but once load is applied the internal amp runs out of steam and cannot sustain the voltage for the required time.  You have to wonder that if such a difference is seen here, that what kind of difference it will make to your music if signals are being distorted like this?  As mentioned earlier, this specific problem is typically recognised as thin bass.  Now you can see why bass boosting headphones and equalizers are so popular.  They are a workaround for poor amp design.  I can only presume this effect is magnified when using larger, more powerful headphones.

That is the benefit of a dedicated headphone amplifier.  The better design and having its own power supply means it can better sustain the voltages required of it, rather than falling off like weak internal amps.  Even for portable amps there is still a benefit because the amp has it's own battery rather than sharing a power source with the iPod or phone.

Some people use headphone jacks on stereo amplifiers, but since these are designed to drive speakers primarily, the headphone output is usually secondary.  There's nothing to say that the headphone output on a £500 stereo amp is going to be better than that of a £200 headphone amp, in fact it's likely to be the opposite.  As important as good amplification is, you also need a quality source, which brings me on to the next section:

DAC 
A DAC is a Digital to Analog Converter.  Its job is to turn digital signals into analog, which is then passed on to the amplifer and then to the speakers/headphones.  DAC chips can be found in any digital music device and they come in all kinds of sizes and specifications.  DAC chips are found in CD players for example, which is why you can plug those directly into an amp (since the digital signal has already been converted to analog).  A DAC alone does not provide a suitable signal for driving speakers or headphones, so the signal must pass through amplification first.

For the most part, when I refer to a DAC, I'm talking about an external device that pretty much does the same job as a soundcard.  They are usually available with USB connections, require no special drivers and work perfectly fine on Windows, Mac and Linux.  All that's required is that you change your audio output in the control panel to the DAC.

The vast majority of USB DACs have a built in amplifier and headphone output, but it is possible to buy standalone DACs in which case a standalone amp must also be used.

Using an external/USB DAC bypasses any existing audio hardware, so it's an ideal solution for getting away from poor quality and noisy internal DACs.  Another advantage is since they are seperate from the computer, they are less likely to pick up interference from power supplies the other circuitry (eg WLAN card).  The DAC chips used in the external DAC device are generally a lot better quality than those found in sound cards too.

If you are reading this post, then it's probably because you want to buy some decent quality headphones or upgrade your computer audio.  Although the most common usage for a DAC is to use as an external soundcard via USB, most of them also come equipped with Coax, Optical and AES inputs which means you can use them in a variety of other ways.  Combine that with the RCA outputs and you've got a device with a lot of potential in a seperates system.

A DAC can be used in such a way to turn a cheap DVD player into an audiophile grade CD player.  How you ask?  The trick is in using the optical output.  By using the optical out, you bypass the DAC stage in the DVD player, and the digital audio is sent to your external DAC and converted to analog.  Connect the RCA outputs to a speaker or headphone amp and enjoy the quality.  You can also use an external DAC to play computer audio through your hifi; simply connect the PC to the DAC with USB or optical, and the RCA out to your stereo amp.

When buying a DAC, you should aim for the highest quality you can afford.  The amplification can come later, or on most models, there is a built in headphone amp.  Good quality source is important, and as I explained earlier, there's no point cutting corners getting a low spec DAC that will show noise at high volumes.  A source should also be completely neutral and transparent.

With regard to connecting the DAC to your computer, you usually have a choice of USB or optical (at least on Vaio and Macbook laptops).  Optical is almost always the better connection, and I'd suggest you go with that.  My experience with Windows 7 is that I could not choose 16 bit output to the DAC (16 bit being the bit depth CD audio uses), so the OS only gave me the choice of 24 bit at 44.1, 48 and 96kHz, which meant the bit depth was upsampled before it was sent to the DAC.  That's not a problem in itself, but most DACs upsample the source anyway, so it's just a pointless process which can be avoided with using optical since you can set 16 bit, 44.1kHz.

Another advantage with optical is that there is no electrical connection between the computer and the DAC.  This means less chance of interference.  I found that when using the D100 and A100 with my laptop plugged into the mains that I would get a lot of interference, and it was really quite noticable.  I ordered an optical cable and just like that it cured the problem.  I'm not sure how interference was transmitted via USB, but according to my experience, it can happen.

The only downside to using optical is if you switch between speakers and the DAC a lot.  Generally it means going into the control panel and setting the optical or speakers as the default device.
Protip: Right click the speaker tray icon and choose playback devices

This can get quite annoying.  During my time using USB, I set the DAC as the default device so it would take precedence when connected, but when disconnected Windows would fall back and use the speakers.  Unfortunately it seems that the OS has no knowledge of devices being connected to the optical, but that can also work in your favour since it means you can disconnect it without your media player exploding or that annoying hardware disconnected sound/notification.

Buying an Amplifier or DAC
Now that you have an idea of what amplifiers and DACs do, it's time to look at some of the options.  As mentioned in my previous post, I own the Yulong A100 amp and D100 DAC, so I'm happy to vouch for those and answer any questions if you are interested.  Here's a short list of gear that's worth looking into.

Stand Alone Amplifiers
These have no DAC section and are intended to boost the level of good quality sources.  These should be used in combination with a DAC, or some other high quality source.

Matrix M-Stage
Price: $220 excluding shipping
Input: 2x Stereo RCA
Output: 1x Stereo RCA, 1x 6.3mm Headphone
Where to buy: Shenzhen Audio Store (official outlet)
Review: Project86 at head-fi
The Matrix M-Stage is a great amp for the money.  It was hard for me to choose between the M-Stage and the Yulong A100, but the neutrality of the A100 was what won me over.  Project86 has described the M-Stage as the darker sounding of the two and comments that the M-Stage has a little more low end.  This amp is quite popular with fellow K701 owners for that fact.  Despite its low price, it's essentially a clone of the Lehman Black Cube Linear, which retails at £600 ($985).  This amp has a very good reputation and I'd be happy to suggest it. 

Yulong A100
Price: $330 excluding shipping
Input: 1x Stereo RCA
Output: 2x 6.3mm Headphone
Where to buy: Shenzhen Audio Store (official outlet)
Review: Project86 at head-fi
The Yulong A100 isn't cheap, but it features top of the range opamps.  The only thing I feel lets it down is the lack of inputs.  Sound quality is excellent.  An ideal amp if you have a stand alone DAC, but if you already have something like the Yulong D100, the amp in that is good enough for most people not to notice much difference.  It largely depends on your headphones, so bear that in mind.  See this post for more of my impressions.

Combined Amplifier/DAC
These are the best way to get started in high quality audio, and some of them are good enough that you will not even feel the need to upgrade.  If you want to jump straight in to a seperate amp/DAC system, look at standalone DACs so you are not paying for a headphone amp section that won't even be used.

FiiO E7 (Portable Amp/DAC)
Price: $100 excluding shipping
Input: 1x USB, 1x 3.5mm Line In
Output: 2x 3.5mm Headphone
Where to buy: FiiO
Review: Misc E7 Reviews
This is a great little portable unit.  Not only will it amplify the output from your phone or portable audio player, but it also has 2 headphone out jacks meaning you can share with a friend.  What's more is that it also doubles up as a DAC.  Obviously, the DAC section isn't going to be as good quality as the rest of the choices here, and neither is the amplifier, but at $100 can you really complain?  Decent signal to noise ratio (>100db) but probably not enough to notice a difference from a decent onboard soundcard. 

Maverick Audio D1
Price: $200 excluding shipping
Input: 1x USB, 1x Coax, 1x Optical, 1x Stereo RCA, 1x 6.3mm Line In
Output: 2x Stereo RC, 1x 6.3mm Headphone
Where to buy: Maverick-Audio
Review: Maverick D1 Condensed FAQ
This unit is the definition of bang for buck.  At only $200 it has more inputs and outputs than more expensive models, and features a very powerful amplifier which can deliver up to 300mw at 600ohms - more than enough to ruin your headphones and your hearing.  You can also upgrade it by replacing the  opamps, which is a feature you rarely see.  Be sure to check the review link above for tons of information.

Matrix Cube DAC
Price: $300 excluding shipping
Input: 1x USB, 1x Optical, 1x Stereo RCA, 1x BNC
Output: 1x Stereo RC, 1x Coax
Where to buy: Tam's Audio
Review: Project86 at head-fi
The Matrix Cube was one of my first choices for DAC, before I learned of the Yulong D100.  My intention was to buy the Matrix Cube and pair it with the Matrix M-Stage.  While not as fully featured as the Maverick D1, it's assumed that with less features and a higher cost, that it has been invested in the audio quality, which is why we are here to begin with.  If this is in your budget, be sure to check out Project86's excellent reviews.  The cube has a decent SNR at 109db and a wide soundstage.

Yulong D100
Price: $450 excluding shipping
Input: 1x USB, 1x Optical, 1x Coax, 1x AES
Output: 1x Stereo RCA, 2 x XLR
Where to buy: Shenzhen Audio Store (official)
Review: Project86 at head-fi
I eventually decided to go for the D100 based on the strength of Project86's review.  The most expensive amp/DAC combo here, but also has the highest SNR at over 120db.  An excellent all in one unit.  The DAC section is highly praised and the internal amp is comparable (but slightly lacking) to the Matrix M-Stage.
To quote Project86, "I find the Yulong amp to be very neutral and transparent sounding. I've owned various models in the $1,000+ range including Benchmark DAC1, Grace Design M902, and Lavry DA10. This Yulong competes well with those units and is even superior to some of them in certain aspects. The headphone amp, while not quite on the level of the DAC section, is still quite good, and you'd need to spend a significant amount of money on a standalone amp to get much improvement."