Good headphones call for a good amplifier! We learned this at once as soon as we tried to listen to something in the tested headphones. The Grado’s impedance of 32 Ohms is too much for most operation amplifiers that are installed in the outputs of audio cards and some home appliances. At such a high load the amplifier’s THC increases greatly while the inter-modulation distortion often exceeds 1%. The headphones’ own distortions are much lower, so it is impossible to adequately evaluate their sound quality – the sound acquires noise and loudness that can be easily mistaken for an irregular frequency response. In fact, the Grado SR325i indicate the need for a serious amplifier with their 6.35mm connector and lack of any adapters in the kit. The Sennheiser HD 600 are equipped with a 3.5” connector and come with an adapter for 6.35mm connectors. The impedance of their dynamic heads is 300 Ohms but our measurements proved that it is an improper load for the ordinary line output, too. There appears a hump at the speaker’s resonance frequency, below which the bass is too low. The frequency response is too high at high frequencies as well.
After reading through various reports about headphone amplifiers, we took the HD53R-80 model from the Japanese firm C.E.C. Besides impressive specs, it features two stereo class-A amplifiers with independent volume regulation, which simplifies the test process. The headphones you want to compare are attached to the two inputs of the amplifier, and you only have to put them on one after another.
The amplifier proved to be up to our expectations, working well with every headphones model we had and delivering a very transparent sound. We measured the amplifier’s nonlinear distortion coefficient at an output voltage of 2V (this is the measurement limit for most non-professional audio cards) without load and found it to be 0.02% - the signal spectrum was limited with the third harmonic. When we attached the Grado ST325i, only the third harmonic rose up, and the total nonlinear distortion coefficient grew to 0.028%. Measured for a -1dB level, the amplifier’s frequency range was 30Hz to 30kHz. Thus, the C.E.C. HD53R amplifier suits perfectly for a correct test of top-class headphones.
The sound source was an Auzentech X-Fi Prelude audio card that proved its worth in comparative tests with other audio cards. The sound quality was evaluated using a number of selected recordings of different genres.
Besides, we were also interested to compare the two top-end headphone models with a cheaper product to see what you lose saving some money. We took the Grado SR80 for this test. Designed alike to the SR325i (except for an all-plastic case without anodized aluminum), this model has worse dynamic capsules. Although the difference between the specified parameters of the SR80 and SR325i seems to be small, their prices differ 2.5 times.