Headphones and Earphones
- Written by John E. Johnson, Jr.
- Published on 26 September 2012
Design of the HiFiMan HE-500 Over-the-Ear Planar Magnetic Headphones and EF-5 Headphone Amplifier
As mentioned on the previous page, the HE-500's are planar magnetic. The membrane with conductor is disc-shaped, and several small thin permanent magnets are situated vertically, across the area of the circular membrane, and in very close proximity, with a small space in between each magnet for air to pass through to the outside of the headphones. When the music signal passes through the conductor, it creates an electromagnetic field that varies in north-to-south orientation, and intensity with respect to the loudness of the music. This causes the membrane to be pushed away from or pulled towards the permanent magnets, whose north-to-south orientation is constant. The sound created by the moving membrane passes through thin fabric and then to the ears. It also passes between the magnets and out to the room.
The fact that the sound moves un-restricted out to the room makes the design classified as an infinite baffle. In other words, your ears are receiving the front wave of the sound, and the room, as well as openings to other room, and open windows to the outside, are receiving the back wave. This eliminates rear-wave interference that is a problem that speaker manufacturers have to deal with when they put the drivers in a box. When listening to the HE-500's, I could notice a difference in the sound if I passed my hands across the outside of the headphones, even a few inches away from the surface. That produced a rear wave that was reflected back into the open headphone, through the magnets, and affecting the planar membrane.
The cables have silver conductors with a simple braided configuration. They appear to be blue in the photo, which is just the angle of the sunlight that I used to take the picture. Over on the right hand side, you can see the silver in the conductors.
The connectors from the cable to the headphones are very sturdy, and they are threaded, as shown below. The only problem with this, is that the carrying case that comes with the HE-500's does not have the cutout for the connected cables, so you have to unscrew the connectors if you want to store the headphones in the carrying case.
You can, if you wish, just use a box cutter to cut a slot in the carrying case, so that the cable connectors will fit into the case, with the cables folded on top of the headphones. However, I have been using the HE-500's so often, I just store the whole package (HE-500's, EF-5 amplifier, and my iPod) on an end table next to my easy chair.
The EF-5 is a hybrid design, using a 12AU7 dual triode as the input stage, operated in Class A, and two OP275 op-amps as the output stage, operated in Class AB (each op-amp has two channels, which are configured as push-pull for a single channel in the EF-5). It has two chassis, one for the power supply, and one for the gain stages. The rear panel has two RCA single-ended output jacks, an on/off toggle for the power supply, and connectors for a cable that attaches the power supply to the gain stage chassis. The AC receptacle is grounded. There is also a slider switch to select 120 volt or 240 volt operation. Be sure to check this switch and set it to your local voltage. The tube is packed separately, so you simply align the pins in the socket and push firmly, but gently, to insert it.