Hi there, Mython. This is actually my first post on head-fi, but I'm very interested in the Calyx M, and have read this thread from first post to the last.
Just to establish my non-credentials, I'm fairly new to the high-end headphone world -- I admit there's a lot I don't know -- and even newer to the idea of high-end DAPs.
That being said, I understand your (and AmberOzL's, among others) issue with the sliding volume control. I don't necessarily share it, though. Calyx seems to have been paying attention to the comments on this thread, and the M now has an SD card slot and external control buttons as a result. The fact that the sliding volume control wasn't deleted hints (to me) that they have addressed the issue. After all, among all the features they've added or deleted to make the device more palatable to the consumer, the sliding volume control is the one feature that might end up being a litigation issue, especially since any good lawyer is going to dig up this thread and say Calyx was warned ahead of time and did it anyway... if it's actually an issue.
How might Calyx have addressed your concerns? I don't know, but I put on my designer hat in thinking about the issue and came up with a few approaches that might work. I'll leave it to those more knowledgable and experienced in the world of audio circuitry to shoot them down or refine them (I don't mind, my ignorance is profound; I'm just a mechanical engineer by profession).
Here are a few ideas about how Calyx might have made it safe.
1. Capacitive touch sensor in the slider (and other external buttons):
What if the volume control simply doesn't work unless it's a human, errr, appendage operating it? True, there could still be a problem if someone has one of those capacitive styli in their pocket, but any other pocket junk won't allow the slider to control the volume.
2. Drop-away slider:
According to my understanding, the slider is held to the device magnetically. If the internals are stiff and the magnetic force is light (and there's no thumb over it to clamp it in place), then a foreign object would just knock the slider off. This could be refined by having the M go silent if the slider isn't in place. Alternatively, a fixed-level alert tone/pattern could play when the slider is missing. (Or, a sexy voice, in the gender of the user's choosing, could offer a "slider missing" message.) Hopefully extra sliders will be provided and/or inexpensive.
3. Volume rate limit:
I'm pretty much set-it-and-forget-it when it comes to the volume of my players, so this might be less bothersome to me than to others. What if the volume can only change at a certain rate by design? Maybe when powered on for the first time, there could be a setup procedure, and the user would be asked to operate the volume at a fairly normal rate a few times. After that, faster changes will be ignored. A reasonable volume rate limit would allow one to at least pull CIEMs out or headphones off if a UVC (Unintended Volume Change) is occurring.
4. Accelerometer:
No volume changes while jostling the device, period. Obviously normal hand tremors would be allowed, but during any accelerations greater than that the volume would not respond. This might be an issue for users who have health issues like MS, though, so I can see why trying to implement this might give Calyx fits. In event of accelerometer failure, volume can only be turned down, or defaults to something like 10% or something.
5. Maximum volume limit:
Another setup procedure. The M would ship with the limit set at an annoyingly low limit and stay there unless the user completes the setup, including setting the max volume limit. Test tones could be provided on-board to facilitate this.
6. Maximum volume limit plus power monitoring:
The problem with item 5, above, is that different headphones and IEM/CIEMs have different sensitivities. I don't know if this is possible, but what if there was some monitoring of the output signal power relative to the volume setting? What I'm getting at is a sort of system that monitors what's going on and adjusts accordingly. For instance, if the max volume limit was set while someone was wearing a pair of MOEs, and they later plug in their CIEMs, the device would notice that more or less current is being drawn for a given volume setting and prompt some action: reset the limit, perhaps? Ideally, of course, it would be transparent (adjust the limit based on the data autonomously), but that might be asking too much.
(Though this doesn't address the slider directly, I think it would be very cool to have a user-curated "library" of headphones available. The maximum volumes, EQ settings [assuming the M has an EQ function], etc. for the users own collection of headphones could be input, stored, and recalled when needed. I'm actually a bit surprised Apple hasn't done something similar already.)
None of these are perfect, and there's room to combine them, I think. As I said, though, I'm no audio circuit expert, and have no idea how any of these would affect the M's sound quality. Response?