Katz's Corner: The Great Headphone Shootout Part 2
The Headphone Slutz group meeting was fast approaching, and the demo cans would soon arrive from the Cable Company. I was excited! In a few weeks I would be able to hear and compare four of the world's best headphones, in my own room with the finest associated gear and music sources, and share that experience with seven friends.
I knew I would need a real good dynamic headphone amplifier, with a quality that could hold its own against my KGSS Stax amp. So after discussing headphone amps that I could afford with my good friend (and fellow columnist) Steve Guttenburg, I dropped my dimes on a new Burson Soloist headphone amplifier sold by a private party at a discount I luckily found on EBay. At the time the Burson arrived, the best headphone I could throw on it for auditioning was my Sennheiser HD 600, which I am sure is not in the same league as the phones I would soon be testing and hopefully buying. I listened briefly to the Sennheiser on the Burson, and reached no more conclusion than that it produced music. I longed for the robust quality of the Audeze's that were coming.
The build quality of the all-discrete Burson Soloist is impeccable: it's solid, impressive looking, and its interior layout is very clean. Before measurement, as is my wont, I cleaned, preserved, and enhanced all internal and external contacts with Stabilant 22. The Soloist can be used as a three-input unbalanced line preamp with three output gain settings; when the phones are plugged in, the line output is defeated. Its custom-built switched analog attenuator has a solid feel. There's something about a stepped pure-analog attenuator that feels assuring, and I believe has less distortion than traditional potentiometers and digitally-controlled analog ICs (such as MDACs).
The published specs of the Burson are skimpy, to say the least. The only specification they give is the wattage capability with a "1 kHz line level input" (whatever that means) with "30 ohm output impedance" (whatever that means). Off the bat this is two mistakes: the first is not to specify the source voltage and the second is to confuse "output impedance" with "load impedance". Of course they meant "load impedance" but still they lose points for imprecision of language. Without knowing these, the purchaser is in the darkall he has is a vague idea of the power output capability of the amplifier. Actual output impedance is not given, nor is the voltage gain. Three wattage levels are specified: 0.18 W, 0.7 W, and 2 W at the three different gain settings, but without giving a source voltage or the position of the volume control these specifications are specious and incomplete. A knowledgeable engineer could produce any of those wattages or any variation in between at any of the gain settings by using a different source level, assuming the amplifier doesn't go into clipping. So every amplifier manufacturer should specify the voltage gains, as well as the output capability, at a specified distortion.
So, I knew almost nothing about the technical capabilities of this amplifier except that it could probably produce 2 watts, probably into a 30 ohm load. While waiting for the Audeze phones to arrive and to ensure that all was in order with this new amplifier I performed measurements on the Burson with a 20 ohm, 110 ohm load, and high impedance, representing a range of typical headphones. (Please let me know in the comments if you'd like to see even more comprehensive technical measurements in my column. I can perform them, but the demand from the public for equipment measurements seems to be very low.) In general I've noticed a lack of published specifications in audiophile gear with some exceptions. Even though a specification such as THD does not seem to correlate with sound quality once it is below a certain per cent, at least we can observe the voltage level where the device clips or begins to produce a high THD, assess a headphone amp's output impedance, and measure frequency response to confirm the most basic performance parameters.
Burson Soloist Measurements
First measurement was the steps and tracking of the 24-step attenuator (volume control). Burson has constructed a rugged rotary switch with carefully-selected metal-film resistors. The measurements show diligent resistor matching, with a remarkable 0.1 dB or less difference between channels at most levels of attenuation. The steps (changes from position to position) have been sensibly chosen, with the highest volume steps being 0.3 dB, 0.3, 0.55, 0.65, and gradually growing as volume reduces to between 2 and 3 dB per step. Larger steps are only employed at the lowest volume control positions. Voltage gain to the line outputs is the same as to the headphones. At the top of the volume control (0 dB attenuation), the maximum voltage gain is 11.55 dB, lowering to 7.3 dB and 1.3 dB at the other two gain positions (measured at 1 kHz). This is enough gain to drive any current dynamic headphone to deafening SPLs with any typical source. I advise listeners to stick with the lower gain unless necessary, and start with the volume control turned down and carefully turn them up "enough to satisfy".
Next, I measured the clipping point of the Burson at 1 kHz using an oscilloscope. Visually assessing clipping is a rough, subjective interpretation, but it gives a quick idea that the amplifier is working and roughly producing its specified output. Into an open circuit, the amp clips symmetrically at about 9.49 volts RMS, which demonstrates an excellent power supply voltage, no skimping there. Into 110 ohms (the load of an LCD-3) the clipping point is the same, equal to 819 mW or 0.8 watts. Into 20 ohms (the load of an LCD-X) it clips around 8.00 volts, which is a hefty 3.2 watts. On the scope the waveform seems to soft clip, which I think is a good sign. As you will see, I should have looked for clipping at low frequencies as well. Output impedance (calculated with a load drop) at 1 kHz is less than 1 ohm, another good sign.
Long before visual scope clipping, distortion components rise as the level approaches clipping. I like to calibrate the vertical scale of a harmonic distortion graph of a headphone amp in equivalent SPL, which gives us a tangible idea of the amplifier's real-world capabilities. Using the LCD-X as an example, its rated load impedance at 1 kHz (not measured) is 20 ohm, and its sensitivity is rated as 103 dB SPL at 1 mW, which is only 141 mV. I like my forte passages to be about 83 dB average SPL per channel; leaving 20 dB (or more) headroom for short term peaks (transients), that would come out to 103 dB on peaks. On occasion, with large orchestral or epic rock pieces I may try out a headphone circa 90 dB average on forte passages, but only for very brief periods, or I would go deaf. Again, leaving 20 dB amplifier headroom for occasional peaks, this means that to get 110 dB peak SPL, the maximum amplifier voltage needed for the LCD-X would be 317 mV, or only 5 mW into 20 ohms. So why this craze for super headphone amplifier power?
The best answer I can give is that superior headroom seems to translate into better sound. In other words, although in reality we aren't requiring that amount of peak power, it seems that the capability to do it is necessary for good sound quality. There are other considerations, such as using well-designed Class A discrete amplifier stages as opposed to integrated circuits, power supply design, topology differences, but even then, as you will see, when my listening panel compared the Burson to other amplifiers which also use discrete components and have high headroom, sonically the Burson came out on top.
Back to the Burson measurements. I used Spectrafoo and Room EQ Wizard to generate the graphs, calibrating SPL to the published rating of an LCD-X at 1 kHz. All SPL readings below are the equivalent of the measured voltage, no headphones were driven or harmed for these amplifier measurements! My Prism USB interface with an external resistive attenuator permitted measuring up to a maximum voltage equivalent to 136.2 dB SPL (6.48 V, 2 W in 20 ohm) before overloading the Prism's ADC. Crazy high and unrealistic! Although I quickly checked the performance at 1 kHz at this extreme, I wasn't surprised to start seeing the harmonic content go to unrealistic levels into a tough load like 20 ohms. Anyway, let's consider 110 dB SPL to be the fair maximum level for harmonic distortion and frequency response measurements.
Most of the visible spikes are the residual noise of the Burson, which are all components of the 60 Hz power line frequency. There is a very small trace of second harmonic at a level equal to 5 dB SPL, so it's surely inaudible, and an even smaller trace of the third harmonic. So for all intents and purposes there is no audible harmonic distortion at 110 dB SPL at 1 kHz, excellent performance.
With this signal there may be a trace of second harmonic, but in reality the only visible spikes are components of the power line. You may ask if I confirmed this measured noise is not a problem with my interface or interfacing (grounding). I doubt it, as this graph, Fig. 3, shows.
With the sensitive Audeze X phones the Burson has a completely silent noise floor, with no hiss or hum to be heard. Measurements show, in red, the noise floor with the cables connected and the Burson switched off, in blue with it switched on. There are only four spikes in red which could be due to interfacing or grounding issues in my measurement setup, the rest is the noise of the amplifier itself. But be aware the highest spike in blue is 120 Hz at only 14 dB SPL, that is the residual hum from the Burson's power supply, about 6 dB below the ear's threshold of hearing at that frequency. The 180 Hz spike in blue is caused by induction, probably from the power transformer, again close to the the ear's threshold in a quiet room. During listening I never heard anything but a completely silent noise floor from the Burson, so if these noise components are there, they are way below normal hearing even in a quiet room. And headphone listening is far more sensitive than loudspeaker listening to any hum or noise because of the proximity of the drivers.
I did discover an anomaly that is a bit disconcerting: The Burson Soloist unit I purchased clips (severely distorts) at low frequencies (below 50 Hz) at levels above 126 dB SPL (above about 2 volts) into 20 ohms, or 200 mW, about 6 volts below its clipping point at higher frequencies. The low frequency response starts dropping radically at this level and the THD goes through the roof, a definite misbehavior. Other headphone amps I measured did not exhibit this anomaly so my test gear was not the culprit. This issue manifested itself identically in both channels. It was also the case into higher impedance loads. I'll be happy to check another unit to confirm this is not an anomaly. But to put this into perspective, 126 dB SPL is a deafening level, so I think we are safe to say that in the real world, no one will operate near this level and the Burson will probably perform very well!
At 126 dB SPL frequency response is ruler flat from 10 Hz to 20 kHz, with total harmonic distortion at 33 dB SPL for the majority of the spectrum, equivalent to 0.002%.
So we can conclude that below its low frequency clipping point, the Burson performs impeccably, although, at least in the unit which I own, it exhibits measurable problems at high levels and low frequencies. This could be a power supply-related issue or a lack of negative feedback below a certain frequency, I cannot say for sure.
Benchmark and Prism Headphone Amplifier Measurements
I measured two other dynamic headphone amplifiers. First, the one built into a Benchmark DAC-1, which has always been a respectable, solid DAC for me. In fact, I was one of the first to discover the Benchmark DAC-1's virtues and wrote an early review in Pro Audio Review magazine which single-handedly provoked the Benchmark craze amongst audiophiles and professionals. Never having given its headphone amp much thought, I was quite surprised to find measurement-wise the Benchmark smokes the Burson.
At an equivalent 127 dB SPL I could not see any anomalies in the Benchmark's readings. Benchmark considers its HPA-2 discrete headphone driver to be a "premium" amplifierin professional circles it's rare to find such an amp built into a DAC, though in the audiophile world we expect to find one in any DAC equipped with a headphone jack. I measured about 0.002% THD for the majority of the frequency spectrum at an equivalent of 125 dB SPL for the LCD-X with a 20 ohm load. I measured a near zero output impedance so it can drive most any headphone you can throw at it.
Surprisingly, I cannot say the same for the headphone jack of the best-sounding DAC that I own, the Prism Lyra-2, a rival for the best DAC in the world. But I would not consider its headphone amplifier to be "premium" based on output impedance alone. Its output impedance is about 80 ohms, so I doubt it will damp dynamic headphones as well as amplifiers with a lower drive impedance. At maximum headphone gain into 20 ohms with a -20 dBFS RMS 1 kHz sine wave the Prism can "only" produce 165 mV, equivalent to 1.3 mW or about 104 dB SPL, which is still very loud. Its THD measures below a very low 0.001% for the majority of the spectrum. We'll see how it performs in listening tests at the Headphone Slutz get-together, described in parts 3 and 4 of this series.