Yes - I didn’t know about the BA300S 
Still doesn’t fit the niche that I was looking to fill with the C9, unfortunately. Namely, I was looking for the dual-timbre capabilities since I have some music I absolutely love on tubes and other music that is better on solid-state. And I’d prefer not to have two devices that I have to switch back and forth between, so the C9 is pretty much my only option, as far as my research has been able to find.
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Fair enough. I just went from knowing like 6 portable amps existed to being given a list of like 40 of them a couple weeks ago myself 
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Now that sounds like a useful list!
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Ill hunt it down in an hour or two for you. FWIW, it was twoards the end of the L&P P6 Pro thread on hifiguides
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After significant searching, I found this entry from a Cayin rep in the Head-Fi forums that I think does a much better job of talking about the grounding issue.
In short:
- Always charge the device with a separate USB charger.
- Do not charge and playback both C9 and the connected source (let’s assume that is a DAP) at the same time.
- You can charge and playback C9 at the same time before the batteries is fully charged.
- If you charge and playback C9 at the same time and your charging rate is slower than discharge rate, your battery will consume slowly. At certain point, the batteries will drop below 3V and triggered the protection circuit to power off the C9.
- Please do not use USB charger from Huawei mobile device to charge up C9.
But I’d still like clarification on bulleit #2. Does this mean we can’t use a DAC that is connected via USB? Does this only apply to DAPs? I just do not understand this requirement.
Is there anyone from Cayin in this forum that can clarify?
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As above just get a separate charger ie like the one I have when I used to vape and use some Sony VTC5A’s etc and not worry about charging while the amp is in use 
PS nice review @eru_illuvitar thank you 
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Really great writeup @eru_illuvitar. Very thorough impressions. It really does seem like a nice bit of tech though I’m with @Torq on this one the power issues make it a non starter until these issues are taken care of.
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Does changing out the battery resolve that? In particular, does it resolve the “Don’t charge both the C9 and its source at the same time” restriction?
I think @LittleCans is talking about using an external 18650 charger with an additional set of cells*. Then charging the spares while listening to the amp, and swapping them over as necessary. In which case, the amp is never plugged in to power.
A cleaner way would be to just buy an additional C9 battery box and put the extra cells in that, since the extra battery boxes can be used as chargers also (per Cayin, at least). Then all you’d be doing is swapping the battery box, and you’d still charge it via a USB charger.
If I was looking at the C9 for home use, that’d “work” … it’d be a pain in the arse (stopping playback, fiddling with screws, swapping cells between amp and charger), but it would work.
I don’t need the C9 for home use though; I have a proper tube amplifier for that.
For my transportable use, carrying an extra charger, or even an extra battery box, with exposed/separate lithium cells is a non-starter.
To begin with, I want to travel as lightly and compactly as possible. I do a lot of it, and a lot of it is very involved. Every ounce and square inch counts**. I carry a single multi-port USB charger when I travel. It’s compact, works everywhere, and will charge everything I need to carry - including my laptop if it is coming with me, and only needs one socket in the hotel or lounge or on a plane etc.
While a few cells and a charger don’t seem like much to most people, they’re more than I am willing to add to work around the C9’s charging architecture. It’s easier to just not bother with the entire thing, especially on TOP of the C9’s size and weight.
Beyond that, carrying open lithium cells (i.e. not sealed within an appropriate device) is an issue when flying. You can’t check them, so they have to go in your carry on. If you have to gate-check your carry-on, which happens a lot on smaller aircraft, especially outside the US, and that was your only bag (common for me), now you have to have them in a pocket.
At the end of the day, if I can’t charge and play both source and amp simultaneously, from a common USB charger, it isn’t the transportable solution for me. Others will have different requirements.
*I have dozens of 18650s and several chargers (my favorite is the MC3000, but it’s huge and overkill for just 18650s) for other purposes … dive lights, RC gear, flash and tactical/shooting lights, sterilizers and so on).
**To the point that I have three different sized laptops, and two different sized tablets purely so I can take the smallest and lightest stuff I can get away with for any given trip.
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Then that’s gonna be a Dap like a P6 pro etc…there’s only so much you can pack into a flight bag lol.
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Yeah, my intended use-case is a little different (but that might be changing soon). I need it to be transportable, not necessarily portable. My goal for this is to be able to easily listen to headphones from either my couch or my bed (my primary listening area is my work desk which has dedicated gear) and to be able to listen from my in-office desk or a coffee shop (whenever that happens again), so it only needs to be able to fit in a backpack for transport between my home and office. If I was intending to fly with it, I probably would’ve gone for the a tube DAP like the N8 instead. When I do fly, the flights are rarely more than 3-4 hours long and fairly infrequent, so that’s not a big use-case for me.
I’d say that for your use case, the C9 is probably not the right device (and not just because of the power weirdness). It’s pretty hefty and the fact that it needs a separate device (DAC/DAP) to source from makes it, in my estimation, an even worse candidate for your use-case. If you find anything that really meets your needs, I’d love to hear about it! There’s a non-zero chance (as of Friday) that my work may be requiring me to fly a lot more in the coming months, so your use-case might very well become mine in the not-too-distant future.
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Most of the time it’s not even a DAP. But for when it is, I have that side of things covered (SP2000 and N8). The C9 was something I was interested in as a transportable amp, for when I have the space, that retained solid-state and “tube” outputs, while being usable with a natively-streaming source.
Not something I’d use while moving, only at the destinations or hotels on route to final.
It’s not a big deal, and not a problem I have to solve … it was a situational “want”. On it’s own, while a bit of a push, I felt the C9 would (depending on how it sounded) be a way to fill that want. The charging arrangement makes it too much of push for me. A pity, but again not a big deal.
I only really commented in the first place because it really stood out that you can’t play and charge both amp and source at the same time and I think that’s a common enough expectation that’ll it is going to catch some people off guard.
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I understand that living on a boat, power and space restrictions apply.
Hopefully Tesla can get into audio? lol
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Aaah, that makes sense. Sorry for misunderstanding!
Yeah, I’m still hoping to get some clarification on the charging situation, namely because my primary source for it is going to be my Q5S which I use via USB power, so it’s basically constantly charging while in-use and it’s unclear to me if that’s a potentially damaging situation or not. So far, I haven’t had it enter safety mode because of it, but I’m concerned it could still damage the device.
It’s not even that, since I still have a house as my base in each country. Audio on the boat is a solved problem since we decided to upgrade as that’ll be a proper full-size rig. But it’s a problem no one else needs to worry about, as long as they don’t need to play and charge both devices (for whatever reason).
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That’ll come down to what size boat/yacht you have?..pretty sure most big yacht’s can run 250w Krell mono blocks so if that’s the case then plain sailing 
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The old boat was big enough to do that on. The new one is 50% larger.
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At a certain point, isn’t a string quartet just part of the crew?
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That was 3 upgrades ago. He’s got a 16 piece on staff now
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Alright, so I finally got clarification via Facebook on charging both the C9 and its source at the same time from their post on the Head-Fi forums.
It seems that you just can’t charge and playback simultaneously from the same DC power source. For example, if you used a battery pack or something like that to charge either the source or the C9, that’s perfectly OK. It’s just charging both the source and the C9 from the same battery pack or wall wart that would be problematic.
Thus, for my scenario of using the Q5S as the source, I can both charge and playback simultaneously so long as I do so from different DC sources (eg, my laptop and a wall wart or two separate wall warts).
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I have a question for you about the C9. In particular, in this post - https://www.head-fi.org/threads/cayin-c9-dual-nutube-fully-discrete-fully-balanced-class-a-ab-portable-headphone-amplifier.943135/page-48#post-16152883 - on Head-Fi, note #4 states “Do not charge and playback both C9 and the connected source (let’s assume that is a DAP) at the same time.” Does this mean I can’t use a device like the Q5S as a DAC since that could potentially create its own ground loop?
You sent February 22 at 11:02 AM
Specifically that I can’t use it in wired mode rather than as a bluetooth source?
Mon 8:21 PM
Cayin sent February 22 at 8:21 PM
The C9 is a pure analogue portable headphone amplifier, it does not have Bluetooth or digital audio function, only accept analogue input and headphone output.
You sent February 22 at 8:39 PM
Yes, I understand that. My question is about the C9’s grounding loop issues when used with a source - in particular a dac - where the dac is wired to a pc via usb. Can I charge the C9 and use a dac wired to a PC at the same time?
Cayin sent February 22 at 8:42 PM
You can charge the C9 with a wall charger, and connect to your PC via a USB DAC. You cannot charge the C9 from the USB out of your PC at this connection, this will create a ground loop as mentioned in the post you previously quoted.
You sent February 22 at 9:05 PM
So what makes that situation different from using a DAP instead, like item #4 in that post states?
Cayin sent February 22 at 9:17 PM
Actually, even if you were using a DAP, you still can run into similar situation, that is if you connect your DAP to C9, charge both device with the same USB charger, and playback during charging
Cayin sent February 22 at 9:17 PM
The situation only happens when you charge and playback at the same time.
You sent February 22 at 9:20 PM
Is it only when you charge and playback whole charging both the C9 and the source from the same charger? Or does it not matter if they’re from the same charger?
Cayin sent February 22 at 9:31 PM
Yes, the ground loop problem only happens when you charge and player both C9 and connected source from the same charging device
You sent February 22 at 9:32 PM
Great! Thank you so much for clarifying!
Cayin sent February 22 at 9:35 PM
For example, if you are charging your C9 with a USB battery pack, and your source with a wall charger, then the ground loop situation will not kick in
Cayin sent February 22 at 9:35 PM
But you are not recommend to charge the C9 continuously, it is not designed as a desktop replacement
Cayin sent February 22 at 9:35 PM
When your 18650 lithium battery is fully charge and you continue to charge your device, the 18650 protection circuit will put the battery into safe mode in order to avoid over charging
Cayin sent February 22 at 9:35 PM
18650 lithium battery has very strict over-charge and over-discharge protection when compare to 3.7V lithium battery used in your mobile phone
Cayin sent February 22 at 9:38 PM
So back to the previous scenario, if you are charging your C9 with a USB battery pack, and your source with a wall charger, the ground loop situation will not kick in, but C9 will likely auto shutdown after the batteries has been 100% charged and you charge and playback at the same time
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