ER-6 was introduced in 2002, and it was to be an affordable version of ER-4S. According to Donald Wilson, the developer of ER-4 series:
"The new model will use completely different parts than the ER-4. The goal was to produce a device with a response close to the ER-4S with greater sensitivity, at about half the price. Because of price constraints the cable won't be removeable. Even if the response is close there is always the chance that it might not sound as good as the ER-4. I still plan to make a higher quality version of the ER-4 when I have more time." http://www.head-fi.org/t/3090/new-etymotic-headphone/30#post_38588And in 2009, upon the launching of HF-series, the end of ER-6 was finally confirmed by an user at Head-Fi. It was a very sad moment for all of us, the headphiles, and Etymotic enthusiasts.
PRO: The frequency response target is quite on the spot. Etymotic Research claim ER-6's accuracy is 90%, and the data just verify that. Moreover, the impedance is quite high; It is almost free from the portable source impedance-induced damping error, which is a common problem for IEMs available in the market today.
CON: It is a pain in the butt(or fingers) to insert this headphone to the proper insertion depth, not because its form factor is large, but because it is too small & short. It is so small, when it is time to pull the headphone out, there is almost nothing to grab onto. I am not sure why the polarity of this headphone is inverted. Also, the distortion figure is slightly higher than the ER-4 series.
ON SECOND THOUGHT: ER-6 is a team-killer. While the current HF-series closely matches the response of ER-4P, ER-6 matches ER-4S, which is more accurate to the reference diffuse field target. No wonder they discontinued this piece of gem.
Just out of curiosity, I did a simple test with different silicone sleeves:
ON SECOND THOUGHT #2: I completely forgot to mention ER-6 is an OEM product of a Japanese manufacturer, Star Micronics Ltd's PH-001A. In Mead C. Killion's 2003 interview with Audiology Online:
AO/Beck: Can you tell me a little bit about the ER-4 and ER-6?
Killion: Sure. The frequency response of both is very similar to that of the ER-1 from ten years before. Both the ER-4 and ER-6 attenuate outside sounds and mimic the open-ear response so the experience is as close to live music as possible. The ER-4 provides somewhat higher isolation and a slightly more accurate frequency response. We recently found a partner in Japan that had a manufacturing facility in China, which resulted in the lower-cost ER-6 model.
However, Star Micronics' current official website does not have any information on PH-001A. It does not even seem they are manufacturing balanced armatures anymore. Does anybody know what actually happened to them?