I have had the the Etymotic ER4s for the longest time. Back then, it was considered insane to spend $300+ on headphones. It was before IEM were considered "cool", as most people didn't know what it was. And up till today, they were "good enough" for me. Good enough that I spend my DIY efforts on other things: converting Dynaco ST70s to triode configuration; pondering the virtues of SRPP vs. mu-follower; understanding the principles of screen driven pentodes; making my own tractrix horn expansion profiles for a mid-range horn; figuring out whether such a horn could be mated with a di-pole bass ... I dabbled a bit in headphone stuff, such as comparing a 6AS7G cathode follower vs. LT1010 buffer stage as a headphone amplifier (both sucked), or using buck brigade chips for a cross-feed circuit (never got past the design stage). In the end, the Sennheiser DSP pro that came bundled with the HD580 kept me happy for much longer.
An then, things changed.
No, I didn't hear something way better. I still consider the ER4s "good enough", but
A. I was recommending IEMs to a relative of mine, reading through reviews to predict how it would sound, and most all, trying to see if a particular design really would incorporate all the technological claims into sonic gains. While my brain filled with all this knowledge, equal part started questioning how much of that is for real? It was time to try some of these concepts out for myself.
B. I discovered that BA transducers are now readily available and cheap enough. With all that is in place, it would be a quick foray into this subject, between the "bigger" projects that I am perpetually planning.
So, I decided it was time to try things out and heat up my soldering iron! I quickly defined the scope of the project:
1. According to JH's interview on google talk recently, he discovered the limitation of high frequency extension of a BA transducer to be the rising impedance/inductance of the driver. All drivers suffer from this problem, including loudspeakers, and there are plenty of cross-over solutions to alleviate that. JH's solution in essence is to parallel drivers to lower the inductance relative to the bass drivers. Strength in numbers, that it. But is this better than a properly executed cross-over?
2. Separately, one of JH Audio's CIEM reviews quoted JH saying that the lower distortion figures by using multiple drivers is not just a bonus, but a necessity in achieving good sound. Really? Single BA transducers don't play loud enough? I am curious to find out.
3. One of Etymotics simplicity is the deep insertion, which reduces the ear canal volume, thus raising the resonant frequencies so that is less intrusive. The downside is the discomfort, which is a serious compromise. So, can one design an IEM with shallow insertion such that it becomes a lot more comfortable? The answer is yes of course. But can a DIY effort succeed?
4. The popularity of the Fitear F111 echos the longevity of the single driver solution, and the success of the ER4s in terms of the Knowles ED29689 driver chosen. The variation on the theme by Fitear is the Titanium horn used. I understand the rudimentary issue of impedance matching at the throat and mouth of the horn in loudspeaker drivers, but how much of that translates to BA transducers? Given the limitations of geometry and space, can one really design a properly matched horn? Most horns used in hearing aids are a good 2cm long (such as the Libby horn), yet Fitear's is around 4mm. What gives?
5. Acoustic dampers have become the de facto way of tuning the response of an IEM. This is a brilliant solution and easily tweakable parameter in order to push a given design into the target response. But is there a sonic penalty? Is there a difference when an acoustic filter is used in order to match the impedance when the bore opening changes (suppressing resonant peaks) vs. when it is purely used as an acoustic filter to reduce the output at certain frequencies?
I also made up my mind as to what I wasn't going to fiddle with:
1. Custom molds: this will be an universal insert IEM, there is too much of a learning curve for me to worry about mold casting and re-working the design if things don't pan out.
2. Multi-frequency drivers cross-overs: I am not a bass head, and I always admired the simplicity of a single full frequency drivers, starting with Lowthers, to the next step of co-axial/coincident drivers such as Tannoys and KEFs. Lowthers in the end had too many problems in order to be full frequency to me, but for BA transducers, it should be good enough, as the ER4s proves.
So, this is what's to look forward to in the following chapters:
Warning! The material covered may be considered intermediate/advanced. I would like to apologize in advance if I loose some people by not providing enough background information. Neither is this meant to be a step by step instructable. Rather, it is to convey concepts and principles for correct implementation, to document what I found out to be important for a sound DIY design.
Edited by Aurally - 5/27/14 at 9:35pm