Review: Red Giant A03 and TDK ST750 headphones Page 5
To measure the performance of the ST750, I used a G.R.A.S. 43AG ear/cheek simulator with otherwise the same gear used to measure the Red Giant A03. Measurements were calibrated for ear reference point (ERP), roughly the point in space where your palm intersects with the axis of your ear canal when you press your hand against your ear. I experimented with the position of the earpads by moving them around slightly on the ear/cheek simulator, and settled on the positions that gave the most representative results and the best bass response.
The ST750's frequency response looks mostly flat overall, with a broad peak between 1.8 and 2.8 kHz and another from 5.5 to 9 kHz. This is stronger treble response than we usually measure. Switching to passive mode reduces output by approximately -8 dB, although this difference will vary depending on the source device. Frequency response in active and passive modes is almost the same, but passive mode has a couple dB less bass. Adding 70 ohms output impedance to the V-Can's 5-ohm output impedance to simulate the effects of using a typical low-quality headphone amp has no effect on frequency response in active mode (not shown) but boosts bass by +2 to +2.5 dB below 300 Hz, which means the ST750 in passive mode might actually sound better driven by a cheap smartphone or laptop.
Total harmonic distortion (THD) at 100 dBA is relatively low: below 150 Hz, typically 2% in active mode and 3% in passive mode. Isolation is decent for an over-ear model: nothing below 1 kHz, maxing out at -30 dB at high frequencies.
Measured impedance in passive mode drops from 72 ohms at 20 Hz to 39 ohms at 6.5 kHz. In active mode, it's flat at around 1,200 ohms. Average sensitivity with a 1 mW signal at 32 ohms rated impedance is 108.1 dB in active mode, 99.3 dB in passive mode, from 300 Hz to 6 kHz.
As a well-crafted, mid-priced, audiophile-oriented headphone, the ST750 is an absolute success. We love it for its treble detail and its tight, well-defined sound. If you want big bass or a full sound, look elsewhere. But if, like that visiting headphone manufacturer mentioned above, you most value detail and precision in a headphone, the ST750 is a great choice at $249.