Supposedly, the EF5 is THE amp for pairing with the HE5/LE. The EF1, which is the predecessor to the EF5, has been known to deliver solid results with low impedance headphones like the Grados. When Hifiman released the HE5 headphone, they also introduced the EF5 as the recommended amplifier for the hard to drive HE5 orthodynamics. I was able to test the EF5 with both the HE5 and the HE5LE headphone, and I can confirm that the EF5 is a very good pairing for either HE5s, besting the synergy of other more expensive amplifiers. This review is written mostly with the HE5LE headphone, as I no longer own the original HE5.
The EF5 is a hybrid amp design that utilizes the popular 12AU7 vacuum tube in the input stage, and a socketed OP275 op-amp on the output stage. Changing the tube will give you a different sound signature, and likewise the op-amp. Of course you can change both the tubes and opamp and get double the fun. The most noticeable change from the EF1 to the EF5 is the DY-1 power supply, in addition to some internal improvements in the amplifier as well as a slightly different top design of the amplifier enclosure.
To operate the amp, first you have to turn on the power on the DY-1 power supply, then the power button on the EF5. The EF5 comes with some sort of a delay the circuit, so you get no sound from the amplifier in the first ~30 seconds of turning it on. On the top panel you can see the gain switch, which you can set to either low or high. The difference between the two gain setting is quite minimal and not as big as I expect it to be. However, the EF5 has quite a high gain to begin with, and I find ~10 O’clock on the low setting is enough to drive the HE5LE to a comfortable listening volume.
The build quality of the amplifier is very nice overall. Good quality aluminum anodized enclosure with a glossy painted front panel. The dimension of the amplifier is also small enough for an office or bedroom side setup. More weight on the amplifier chassis would’ve been good, so that it doesn’t topple when I am plugging a headphone in the socket. The EF5 comes with one set of RCA input. There is also a power connector to hook up the DY-1 power supply to the amplifier chassis.