One of my goals as an amplifier designer is to provide high-quality audio at a reasonable price. Unfortunately, I can no longer keep my prices as low as I would like while maintaining a viable business. The recent increase in the price of my products is necessary in order to offset some of my rising manufacturing costs. I understand that price plays an important role in the buying decision, and I feel that my products will continue to provide a great value. My customers should also continue to expect the level of quality and service they have experienced in the past.

Thank you for your support and understanding.

Nick
Leckerton Audio, Inc.

The UHA-6 and 6S series amplifiers are designed to provide a low-distortion and low-noise output into all types of headphone loads. The graphs below demonstrate this by showing harmonic distortion across the audio frequency band of 20 Hz to 20 kHz. In order to see how the UHA-6S stacks up against the competition, I also took some comparison data on a popular DAC/Amp from a competitor. The competitor shall remain nameless, but the model I’m using for this comparison has a similar feature set, size and price point as the UHA-6S. The UHA-6S outperforms the competitor amp in all cases I tested.

My Test Setup

My audio analyzer is a Prism Sound dScope III. The DAC/Amps under test were driven by the dScope digital optical output, using a 48-kHz sample rate and word length of 24 bits. I used a single-tone stepped sweep at a level of -1 dB full-scale. The amplifier output was connected via a length of three-conductor headphone cable to a dummy load consisting of 1/4-watt resistors (same load on each channel). The analog input on the analyzer was connected across the dummy load on one channel. (I’m assuming the left and right channels have nearly identical performance.) The amps were set to low-gain mode except in the cases where I needed the extra gain to get the appropriate output level. The gain knobs were dialed to give the desired power across the load.

I decided to test at two different output powers: 1 mW and 10 mW. That may not sound like much, but even 1 mW is enough to produce uncomfortably high sound pressure levels on many headphones and earphones. (My Sennheiser HD 600, for example, has a sensitivity of 97 dBSPL/mW, which means 1 mW of applied power produces 97 dB SPL – about as loud as a “typical” car stereo at max volume. Some earphones have sensitivities greater than 115 dBSPL/mW – at 1 mW, that’s approaching the threshold of pain!) An increase from 1 mW to 10 mW is roughly equivalent to a doubling of subjective loudness. In any case, 1 mW of output power should not be beyond the expected capability of any headphone amplifier.

The graphs show distortion as a percentage of the desired signal, and the curves are in groupings of three:

1. THD+N (Total Harmonic Distortion Plus Noise) – this is a measure of everything added by the amplifier, including noise.
2. 2HD (Second Harmonic Distortion) – this is the level of the second harmonic of the signal (the first overtone).
3. 3HD (Third Harmonic Distortion) – this is the level of the third harmonic of the signal (the second overtone).

The measurement bandwidth used was DC to 80 kHz, and no weighting filters were applied.

The Results

Let’s start with a “typical” load impedance of 32 ohms, at 1 mW of power dissipation (click the thumbnail to enlarge):

Notice how the curves tend to rise at high frequency. This is typical for most types of amplifiers designs, including high-end power amplifiers for driving speaker loads. The THD+N curve is naturally higher than the 2HD and 3HD curves because it includes all harmonics and the noise floor.

Now let’s take a look at how the competitor amp does with the same load conditions:

The scale has changed, in order to try to fit the competitor’s THD+N and 3HD curves, which have a significant rise above 1 kHz. Comparing THD+N at 1 kHz, the competitor is about five times higher than the UHA-6S.

To understand what’s going on, it sometimes helps to look at the actual spectrum of the signal. On the left is the UHA-6S output spectrum for a 1 kHz signal at 1 mW into 32 ohms, and on the right is the competitor:

These are graphs of actual signal voltage level (expressed in dBV) versus frequency. They are 128k-point FFTs with 4 averages. In both graphs you can see the 1 kHz fundamental and its harmonics. Notice how the harmonics from the competitor amp are higher in level and extend all the way to 20 kHz. High-order harmonics such as this are usually considered to be an especially unpleasant form of distortion. A few other characteristics also come to light: the UHA-6S has a lower noise floor, and the competitor amp has some noticeable low-level spreading of the 1 kHz fundamental, possibly due to clock jitter on the DAC.

Now let’s step up the power to 10 mW, still into a 32 ohm load:

Note that the THD+N curve from the UHA-6S is actually lower when compared with the 1 mW curve. This is because the 10 mW signal is higher above the noise floor than the 1 mW signal (and remember that THD+N includes a measure of the total noise).

The competitor amp doesn’t fare so well:

Notice the scale on the distortion axis. It’s difficult to even show these two sets of curves on the same graph. The competitor amp reaches a THD+N of 0.22% at low frequency. The distortion harmonics of a signal at 20 Hz are only 53 dB below the fundamental signal. Surprisingly, the manufacturer claims this amp can deliver up to 100 mW into 32 ohms. I was only able to get it to 50 mW before severe clipping set in.

All other things being equal, a lower impedance presents a heavier load to an amplifier than a higher impedance. The heavier load requires more current, and not all amplifiers handle this well. Here we have the curves for 1 mW into a 16 ohm load:

The UHA-6S THD+N at 1 kHz is 0.006%, compared to 0.005% for the same power into a 32 ohm load. At 1 mW into 16 ohms, the competitor shows a THD+N at 1 kHz of 0.048%, eight times higher than the UHA-6S. The low-frequency bumps in the competitor curves indicate multiple distortion mechanisms are at work.

At 10 mW into 16 ohms, the UHA-6S is still performing quite admirably:

As with the 32 ohm load, notice that THD+N is lower compared with the 1 mW case, due to the signal level being higher above the noise floor.

At 1 mW into 100 ohms, the UHA-6S second and third harmonic distortion is down in the noise floor of the measurement instrument. Compare this to the competitor:

At 10 mW into 100 ohms, the competitor distortion is off the scale (0.7% THD+N at 1 kHz), but the UHA-6S is still looking very good:

At a power of 1 mW into a load of 300 ohms, the UHA-6S distortion is exceedingly low, but the competitor amp has all but given up:

Finally, at 10 mW into 300 ohms, the UHA-6S performance shines. The competitor amp went into clipping at 6 mW and hence is not shown:

When it comes to low-distortion performance, not all headphone amplifiers are created equal. Choosing a low-distortion amplifier means you are less likely to experience unwanted and unpleasant coloration of the sound. The Leckerton Audio UHA-6S and UHA-6 amps deliver a highly linear output into all types of headphone loads.

It didn’t take long for folks to figure out that the Apple iPad camera adapter could be used for more than just connecting cameras. It can also be used to connect a myriad of USB audio devices such as headsets and headphone amps. This includes the Leckerton UHA-6S and UHA-6 USB DAC/Amp models.

Compatibility with the iPad requires a special factory modification of the circuitry in the UHA-6S (or UHA-6). This modification allows the amplifier to go into a “low-power” USB mode when the amplifier charging switch is set to the Disable position. As of iOS version 4.2 (the latest version as of this post), only low-power USB devices are allowed to connect to the iPad. This means that the iPad cannot be used to charge the battery in the amplifier, but charging proceeds as normal when the amplifier is connected to any standard high-power USB port on a desktop or notebook computer (with the charging switch set to the Enable position, of course).

The necessary factory modification is provided free of charge to all new and existing UHA-6S and UHA-6 customers. If you currently own an amp and would like to have the modification added, please contact us. If you are purchasing a new amp and would like to have the modification included, simply let us know in an email when you make your purchase.

First of all, what is crossfeed? When we listen to a recording with headphones or earphones, the right ear gets only the right channel, and the left ear gets only the left channel. If a sound occurs only in the left channel, it’s heard only in the left ear. The right ear gets nothing. This almost never occurs naturally due to a principle called diffraction. Diffraction is essentially the ability of a sound wave to bend around an obstacle. For example, when you are sitting in front of a stereo pair of loudspeakers, there may be no direct path from the left speaker to your right ear (because your head is in the way), but some of the sound from the left speaker is able to bend around your head and reach your right ear. The lower the frequency of the sound (compared to your head), the better it is able to diffract around your head. Really high frequency sounds mostly get absorbed by your head, but the low-frequency stuff just goes right around as if your head weren’t there. As the sound is bending around your head to get to the opposite ear, it has to travel a longer distance, so there is also a slight delay of the sound to that opposite ear. Headphone crossfeed aims to simulate these three effects: diffraction, head absorption, and time delay. This is why it is sometimes referred to as an acoustic simulator. It simulates the acoustics of loudspeaker listening.

The idea of a simple, passive crossfeed circuit has been around for quite a long time. Siegfried Linkwitz described a circuit back in 1971. In fact, the “Linkwitz” crossfeed circuit is the basis for many crossfeed designs still in use today, including the UHA-4. The version used in the UHA-4 has a modified configuration which allows true bypass when the switch is off. The circuit is isolated between two active stages, which means it cannot be affected by the source impedance or the load impedance. In other words, its sound doesn’t change with different headphones or sound sources.

If you’d like to read more about crossfeed in general, I’ve seen plenty of great explanations and illustrations out there. Chu Moy wrote an excellent article which I often see referenced. It’s available here:
http://gilmore2.chem.northwestern.edu/projects/cmoy1_prj.htm

Why use crossfeed? Quite simply, it can make headphone listening more natural and less fatiguing, especially over long periods of time. If you are sometimes bothered by that “inside-your-head” sound which headphones can give, crossfeed can reduce that.

How does the UHA-4 crossfeed sound? It is designed to have minimal impact on overall frequency response. The effect depends on the nature of the recording. With a very wide sound stage or hard-panned instruments, the crossfeed moves the image forwards and away from your ears. On a recording which has a narrower soundstage to begin with, the effect is more subtle. Like I mentioned, I don’t use it often, but I’m glad I have it available when I need it.

The UHA-4 also features a simple crossfeed circuit to reduce the unnatural, “inside-your-head” sound which headphones can create. The crossfeed circuit is based on the popular Linkwitz design. The circuit essentially feeds a delayed and filtered version of the sound to the opposite ear, mimicking the way our ears work in a real acoustic environment. When switched off, the crossfeed is in true bypass mode.

The UHA-4 output stage uses the AD8610 op-amp, same as the UHA-6S and UHA-6 amps. This op-amp excels at driving a wide variety of headphone loads.

The UHA-4 is available now for $199. For more information, visit the UHA-4 product page.