Headphones and Earphones
- Written by John E. Johnson, Jr.
- Published on 21 April 2014
Design, In Use, On the Bench, and Rating
In a planar magnetic driver, a conductor is printed, using electrically conductive ink, onto the surface of the diaphragm, in the shape of a spiral, so that it resembles a conventional cone voice coil, but is flat rather than being wound around a tubular core. Very thin wires are connected to each end of the printed voice coil, which are connected to the headphone's input jacks. A cable is attached to the jacks and the other end has either a 1/4" or 1/8" stereo phone plug that connects to a headphone amplifier, or your portable music players.
The PM-1 has a 7.14 square inch (surface area) oval-shaped diaphragm, and a voice coil is printed on both sides which increases the sensitivity of the headphones. Indeed, I found the PM-1's to play at a satisfactory volume using less power than with other headphones.
Here is a close-up photo of the lambskin headphone ear cups. You can see that the sides of the cups are perforated. This allows for air circulation that keeps your ears cool and prevents perspiration. They are very soft, and coupled with their very light weight (the headphones weigh less than 1 pound), I found them to be extremely comfortable, and I didn't get my usual headache from wearing headphones for extended periods.
The Presentation Case is made from MDF, covered in dark maroon cherry veneer.
Pushing a lock button on the front of the presentation case allows it to be opened, and voila! The black box at the bottom contains one set of cables with a 1/4" phone plug. A second set of cables with a 1/8" phone plug and a foam set of ear cups are contained in a denim pouch that is included for carrying the phones. I attached the cable with the 1/4" phone plug, and when the cable is coiled up, I could put the headphones back in the case with the cable attached. Not all headphones allow you to keep the cable attached when you put the phones in their case. Note that this is not a carrying case. It is a Presentation Case. The denim pouch is for carrying them. You could also use a headphone stand (OPPO has an optional Lucite stand) to put them on, and I store them this way when I am listening on a daily basis, but I like to protect my phones from dust, so in the box they would go if I take off for vacation.
I tested the PM-1's using my iPod and a HiFiMAN EF5 triode headphone amplifier with DY-1 power supply. The connection from the iPod to the amplifier was an analog RCA stereo interconnect. I also used the headphone output from an OPPO BDP-105, configured to play music from a 2 terabyte hard drive connected to the OPPO. All of my music, including 24/192 tracks, are stored on this hard drive. The headphone output of the BDP-105 is driven by two channels of its Sabre 32 DAC and two very high quality op-amps.
The ear cups fit perfectly around my ears (noting that ears are oval shaped, like the PM-1's ear cups), and lambskin is very soft. You will love the comfort.
The sound from the PM-1's was what I have come to expect from top of the line planar magnetic headphones: Superb! There was no audible distortion at any level.
The PM-1's have a very neutral tonality. Headphones are like speakers. They can have the same overall design, same number of drivers, etc., but each speaker model sounds different. Headphones all have their own distinctive sound as well. The Audeze LCD-X phones that I reviewed recently have a midrange that is a bit forward. I liked it very much. The PM-1's are neutral. I liked that presentation also. I haven't listened to HiFiMAN's top headphones yet (planar magnetic), but will do so in the future. I am sure they will sound different than either the Audeze or OPPO phones. Note that the differences are not huge. They are subtle, but noticeable.
I sat for hours at a time, listening to classical and pop music with the PM-1's. Regardless of the genre, the sound was effortless, and the smallest transients were audible. In particular, deep bass sounds fantastic on a top notch set of cans like the PM-1's, because the diaphragm is only moving a millimeter or two, so distortion does not creep into the output. The only way I have been able to get around this problem using speakers, is to have three 18" subwoofers, with each one turned up about 20% of it output capability. This keeps the distortion very low.
I also used the OPPO BDP-105's headphone jack and iPad app to select music from a USB hard drive connected to the 105.
Below are some distortion spectra at 1 kHz, 10 kHz, and 20 kHz. I placed the tip of the microphone about 1 cm from the center of the inside of one of the ear cups. I could not seal the headphone for the tests, so the SPL numbers are probably about 5-10 dB less than what would be the case if the headphones were over your ears.
At 1 kHz, distortion was only 0.5%, with a prominent second harmonic, and a very small third.
At 10 kHz, distortion dropped to 0.2%. All of the three harmonic peaks were very small.
At 20 kHz, distortion was a very low 0.2%.
As I was writing the review, the new OPPO HA-1 fully balanced headphone amplifier (along with the optional OPPO headphone stand) arrived on my doorstep, so I had a chance to listen to it before publishing this review. It has a built-in DAC (the same SABRE DAC as in the BDP-105 universal player), and will decode up to 24/384 PCM as well as both DSD64 and DSD128, so it is fully up to date with emerging high resolution music files. The amplifier is biased into Pure Class A operation.
The sound was satin smooth and had plenty of power such that I only had to turn the volume control up to about the 11 o'clock position for satisfying volume levels, using the 1/4" cable that came with the headphones. OPPO then sent a balanced headphone cable for the PM-1. It is 3 meters in length and will retail for $149. The cable is made from single crystal oxygen free copper.
Below, you can see the standard 1/4" phone plug and the balanced plug. With a balanced amplifier, the - (negative) output legs are separate, and are not grounded, as compared with a non-balanced amplifier, where the - legs are grounded and thus, connected together. Using the balanced plug therefore utilizes both the + and - legs of each channel of the stereo balanced amplifier and delivers twice the voltage of the unbalanced configuration.
When I used the balanced cable, I had to turn the volume down to the 9 o'clock position to get the same volume that I was getting using the 1/4" unbalanced cable and the volume control set to 11 o'clock. I recommend purchasing the optional balanced cable with the PM-1, as this makes full use of all the circuitry in the HA-1 amplifier. A full review of the HA-1 is forthcoming. Notice the cool VU meters on the front panel. You can change the panel to display several things, including the VU meters, and a bar graph of the audio spectrum. At $1,195, it has everything.
Consumer audio continues its love affair with headphones. There are many phones in the hundreds of dollars category. Once you go above $1,000 ($999.95 is included :=>), the consumer has what I would call "Ultra-Phones" available. In the ultra-phone arena, I have not heard any that sound bad. But they all sound different, in a musical kind of way. At $1,099, the PM-1's are not a casual purchase. But if you are a serious listener, and enjoy the special experience that headphones deliver, the PM-1's would be an important and worthwhile investment. Congratulations to OPPO for getting it right on their first attempt!