A letter to the editor of Stereophile from Charles Hansen...
Editor:
Thank you very much for the insightful review of the PonoPlayer [in April 2015]. I truly believe that you have captured its essence. Specifcally, it is the easiest entry into the world of high-performance audio of which I am aware. The entire goal of high-performance audio is to tear down the walls between the original music performance and the listener.
The barriers of entry to high-performance audio are normally twofold:
1) Price: A high-performance audio system will range in price from several thousand to several hundreds of thousands of dollars. The Pono Player delivers the musical satisfaction of systems costing hundred of times more.
2) Knowledge, skill, and experience: On the Ayre website, we recommend that people purchase Jim Smith’s book, Get Better Sound. The reason is that, in my experience, equipment is at most responsible for only half of the final sound quality of a music-playback system. The other half requires knowledge, skill, and experience in myriad areas—room acoustics, vibration control, EMF fields, RFI, mechanical properties of materials—and, perhaps above all, a commitment of time and a passion for experimentation.
In contrast, with a portable player, simply put it in your pocket, put on your headphones, and you’re all set. The only tweaking even possible is to replace the stock headphone cables with aftermarket upgrades. And as John Atkinson will no doubt write about in a Follow-Up, the Pono Player has a unique balanced mode that raises its performance to an entirely new level. Number one in a field of one.
One point touched on in JA’s “Measurements” section highlights the limitations of working with a portable audio player. Specifically, all of the Pono’s power comes from a single rechargeable 3.7V lithium-ion battery. This leads to two different difficulties during the design phase:
1) With non portable equipment, current draw is rarely a consideration. Sometimes, a large power amplifer must have attention paid to its heat sinking, to ensure reliability as well as nonhazardous temperatures on exposed surfaces. With portable equipment, every single milliamp of current drain shortens the battery life. This puts extremely tight constraints on what can be achieved while still maintaining an acceptable playing time between charges.
2) The de facto standard output level for portable audio players is 1V RMS. Not only is this 6dB lower than specified for home equipment, it is 10–16dB below what is often found in real-world products. Boosting the output level is an easy way to achieve improved signal/noise measurements, and living in the real world of 1V output makes it impossible to match the specifications of stationary equipment
As JA found, at some point the numbers become meaningless. The ambient noise of our listening situation makes the noise floor of the Pono Player a moot point. It still delivers the musical goods: the ability to feel the intent of the performer.
Also, I would like to delve into the topic of output impedance. I would kindly ask Stereophile’s readers to please ignore any “rules of thumb” or other “conventional wisdom” that says that a certain number assigned to the output impedance is “good” or “bad.” Instead, simply let your ears be your guide. The following explains why.
Remember that the audio circuitry of the Pono Player is a completely zero feedback design. Without the artifice of feedback to force the output impedance to a fixed point, it is worth noting the formula for the output impedance of an emitter follower. Very simply, it is equal to 26 divided by the bias current (expressed in milliamps).
The output stage in the Pono Player is a Diamond buffer, essentially taken straight out of Ayre’s KX-R Twenty preamplifier. This circuit in the Pono Player idles at about 4mA and runs in class A-A/B. As there are two complementary output devices per phase, at low levels (class A) the output impedance is roughly 26 ÷ 4mA ÷ 2 emitter followers = 3.25 ohms.
At higher playback levels, the output stage leaves class-A and enters class-A/B. Then, only one of the transistors is on during part of the audio cycle, so the factor of 2 goes away. But here is the beauty of an open-loop design: the more current that is drawn (eg, from a low-impedance load), the more current passes through the emitter follower, which in turn reduces its output impedance.
The bottom line is that a set of high impedance cans will be driven by a relatively low output impedance in the range of 3 ohms or so, which is far more than adequate. And if the user connects lower-impedance headphones and/or drives them at higher and higher levels, the output impedance drops further. It becomes a self-correcting situation so that the Pono Player can drive even the lowest impedance loads without difficulty.
What’s more, connecting to the Pono-Player with balanced cables will connect two separate amplifiers to each transducer, each driven 180° out of phase with the other. This will double the available voltage swing, thus quadrupling the output power (as well as canceling all of the even harmonics and rejecting imperfections in the power supply).
The end result is that the Pono Player is the only portable player to date that will drive virtually any headphone to satisfying levels, regardless of impedance or sensitivity, without the need for an external headphone amplifier, especially when used in balanced mode.
Again, thank you for the efforts expended both in the review as well as in the comprehensive measurement suite that Stereophile has developed (and continues to) over many decades of fine audio reporting.
Charles Hansen
Ayer Acoustics