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When it works, it sounds good. Internally, however, the quality of the workmanship is very poor, and the design is faulty to the point of being dangerous. The high voltage AC leads from the transformer are only tack-soldered to the board. When I plugged in a pair of Bose Quiet Comfort 15 headphones, the full wave bridge rectifier partially burned out and the resultant overcurrent caused one high voltage AC lead to spontaneously desolder itself and ground out to the case, blowing the fuse. Apparently, it has to have a resistive load on the headphone jack or it blows up. This is not mentioned in the documentation I received. QC15 headphones have essentially no DC impedance. The amp sees that as an open circuit, and that causes an overcurrent of sufficient magnitude to destroy its rectifier. In a couple of decades of using and repairing tube equipment, I have NEVER seen any audio tube circuit designed in a such a way that having an open on the output could cause damage. Shorts circuits, sure. Open circuits, never.
The manufacturer has disappeared from the Internet, is not answering any emails to his eBay seller account (DavidZheZhe), and has disabled new user registration and all email functionality on his website (littledot-dot-net). There is no way to contact him, there is no way to obtain support, and therefore no warranty. The product has obviously never been evaluated by UL or TUV as there is no way it could past muster from a product safety standpoint.
chindokaeExcruciating, but thanks for the warning. It's unfortunate the US is flooded with so much Chinese junk like this.
chindokaeThe Bose QuietComfort headphones have a built-in amplifier. So, what you effectively did was connect the outputs of one amplifier to the inputs of another. No surprise at all something fried.
LOL.. you can't use Bose NC headphones with a headphone amp without the headphone amp burning up? That's hilarious.. of course you can, that is unless the amp is junk. Read the original post again for clarity.
BlueSkiesI understood the original post perfectly clearly. And, with both electrical engineering and audio engineering degrees, I stand by my post. The input to the Bose QC is essentially an input to an amplifier. The load of the driver is not presented to the external amp. If your external amplifier's impedance and power characteristics are such that it will deliver more power into the input of the Bose's amplifier (which, without looking at the schematics, I would expect with an OTL tube amp), then it is very likely you will fry either the input to the Bose amp or, by way of excess current, the external amplifier. Now, technically, if for some reason you wanted to feed the Bose cans a signal from an external amp, if the aforementioned point isn't a problem in your case, then you could, but you'd get virtually no benefit. This is because the signal is going to be attenuated and re-amplified by the Bose's built in amp. now, some of the later model Bose QC cans allowed bypassing the internal amp so they can still be played when the battery dies..in this case it may be no problem, but the Bose cans rely on active EQ within the internal amp to make the mediocre driver in the sub-optimal enclosure sound passable. As such, they sound like fecal matter without the built in amp. Not that they sound all that much better with it.
A headphone amp should never burn itself up simply by plugging in a pair of headphones - under any circumstances. It's indicative of shoddy, unsafe, and unprofessional engineering. But when it comes to this cheap Chinese audio gear, one gets what one pays for.
BlueSkiesI'm not trying to take sides in this discussion, but think it mainly reveals the evolution and family tree that's grown from the ancient 1/4" and 1/8" headphone interface.
Dedicated headphone amps are a relatively small audio niche.
Massive tube-based headphone amps are a generally tiny audio niche.
Having evolved quite independently, noise cancelling products are a radically different technology and a large market niche.
Not to mention the absolute chaos resulting from the shift from 1/8" TRS to TRRS jacks. Just a few months ago, my new $400 headphones (TRRS) refused to play at all with a 5 year old Macintosh (TRS). I was forced to buy an adapter (or an external DAC/amp) for that machine.
So, we are now in a world where a plug is not a plug and an amp is not an amp anymore. I'm starting to understand why Apple dropped the 1/8" jack, even though I still want it.
Nope, not guessing. I know for a fact that older models of the Bose QC headphones have a non-bypassable headphone amp. Hence, they are unusable when the battery is depleted. This was a common complaint about those models. So, on some subsequent QC models, Bose added the ability to bypass the amp to run passively when the battery is dead. But, the sound in this mode was degraded, as Bose used active EQ in their internal amp to compensate for the poor audio quality of the driver or driver/enclosure pairing. The only thing I'm speculating about is which model the OP owns, and the particular design of that model, as I'm not familiar with the more recent Bose QC cans. But it is a scientific fact based on specific knowledge that if you feed the output of one amplifier directly into the input of the Bose cans, without bypassing the internal amp, you will likely fry it. It is exactly analogous to connecting the speaker output of a power amplifier to a line-level input, just on a different scale.
But I'm always fascinated by how the uninformed fabricate erroneous opinions out of thin air, completely without basis.
Perhaps you should re-read again. I didn't claim to have communicated the science behind my statements. I simply said that my assertions we're "based upon" said knowledge. I don't have time to write a dissertation nor to teach you basic electronics. Now why don't you quit trolling and go contribute to our economy? And, when you do have time for recreation, I suggest that you expend some of that time discovering the truth for yourself. It might be a good learning experience for you to take a couple of your expensive amplifiers, connect the output of one to the input of the other, fire 'em up, and see what happens. It would bring joy to my heart to know that you embarked upon this educational endeavor and gleaned the valuable resultant lessons. Now, I am done replying to this thread. Adieu.
chindokaeListen to him... Every word above he says is 100%. I bought this not too long ago from eBay. From their official seller. It was all good until I realized there is a low motorized kind of noise happening in the background. My HD6XX didn't pick it up until I put it to Gain Level 2, 3, and 4. All other small IEM's do including venture electronics Monk Plus (68 Ohms to 70 Ohms). I still can't figure out where it is coming and I tried to contact little dot people, Their forum is locked, and after many months a dude called "Sword_Yang" replied to me saying I should use to the HD6XX on lower gain mode because it is super sensitive. Well I know it is all bullshit because even at Gain 1 (Lowest) every other headphone picks this up...! I changed tubes, Isolated from Ground loop, Done everything and yet the noise is there probably due to faulty equipment. I live in a wasteland of electronics where no place to get any of these repair equipment. If someone can pinpoint the noise for me, I might able to fix it but without Little Dot's help, I can't do it.
So beware of products like. For many people 160$ is something you can throw away...! but for me, it is a lot when converted to local currency. I could live for a month with that money with all my food and expenses paid. Here are the recorded noise files I am sharing with you.
Amplified audio file https://voca.ro/nzwrqmzR05l
Unamplified audio file https://voca.ro/f3dJACghizH
I really regret buying this.
nofacemonsterSame problem with my unit. Noise in right channel. Tubes swap doesn't help, noise remains in right channel.
RealGredI got rid of this problem by building an artificial central tap using some resistors. it worked, an electronic also recommended me to make a simple attenuator which could get rid of it forever. But the central tap is good enough for me on high gain settings, there is no buzzing or hum.
nofacemonsterCould you please attach photo of your modification and/or scheme?