Audio electronics question…

[Jacques]

Ok, so I have a conundrum…

I have a pair of Sony WH-1000mx4 bluetooth headphones, they have a built in DAC and amp but can also run ‘corded’ with a 3.5mm stereo cable. The built in amp works while connected to the stereo cable, but the bluetooth and other features ‘noise cancelling’ turn off automatically. You can also ‘power off’ the headphones and use them as a standard pair of non-powered connected headphones (which is my preference when listening to music).

When I use them as bluetooth headphones, the battery will last about 30-35hrs, pretty good.

However, when I plug in a source stereo cable and use them non-powered, the battery drains in about 6hrs. Heck, I could just leave them plugged in and turned off and 2 days later they will be flat. Not great, even though I don’t need the battery it should not drain that quickly.

Is it possible for the battery to drain via the stereo cable? Seems odd!

 

[stormy]

I don't understand, why do they need power when the audio source has been plugged in? Especially as you mention all the noise cancelling etc features become disabled? Very strange.

All I can imagine is that the analogue input is very much an 'after-thought' for the engineering of the product and perhaps they honestly don't know why, or even expect a person to use that feature - but for whatever reason they were asked to include it.

Using that hypothesis as a basis, all I can think is this: There is no analogue by-pass circuitry at all, it doesn't simply take in the analogue audio and pipe it to the headphone outputs. Instead it probably does a very complex thing - it probably runs the input through an ADC (analogue to digital converter) into the digital processing hardware (even though that hardwares features are disabled?! So why did they bother?!) Then it goes through a DAC that converts that digital signal back to analogue for the output. This extra stage of processing (analogue > digital > analogue) would most likely be the drain on the battery, where as in bluetooth mode it is simply pure digital > analogue.

This is all theory, if my theory is correct though I'd say you'd be bonkers to use an analogue input on that pair of headphones, as these multiple conversion stages aren't going to preserve the signal quality of the source material. Just use a good pair of analogue headphones instead, like Sennheisers etc.

 

[Jacques]

Well, the input option is a nice to have as bluetooth on gaming has a delay which is pants, but for general music listening to or meetings they are great. And they are sold as for use with and without a stereo cable. They even include a cable with the headset.

I have a suspicion that they forgot to put some form of isolation layer between the analogue and digital, meaning the battery drains via the stereo cable as most amps, even those on pc motherboards, expect current to only flow in one direction - source -> output. I’ve heard talk on audio forums of amps with and without decoupling capacitors on the output stage, maybe it’s related? I have no idea!!

Edit - i don’t the dac or power unit is involved when you plug a stereo cable in and power off the headphones, as the sound comes through as per the source. It sounds the same as a normal pair of headphones. But it still drains the battery…

 

[Elf]

That is odd! I do think the ADC -> DAC theory is plausible (it wouldn't necessarily alter the sound), and I would guess that there is at least some sort of internal amplifier in the path.

However the theory about draining the battery via the 3.5mm jack could be tested: take a multimeter in current mode (milliamps) and connect it between the jack sleeve (base of the plug, on the cable) and the tip or ring portion of the jack. You should be able to measure current flow.

A lot of those jacks internally (on the headphone side) contain a switch that can open or close contacts to turn the device on or off as well, so leaving the cable plugged in may be activating the device or some other part of it? However if the other side is unplugged I don't think there would be a current path through the cable. Audio amplifiers with no decoupling capacitor or DC servo circuit are capable of having a DC bias on their output that could move current, but, if there's nothing on the other end of the cable it shouldn't be draining anything, and the multimeter / ammeter can tell in any case . And, it should be the input to something rather than the output, and so higher impedance and not sourcing current.

My suspicion though is just that putting it in headphone jack mode just turns on some extra circuitry or puts it into a different mode of operation that wasn't as well energy optimized as the rest, whether that's an ADC or something else. It's also worth considering a lot of bluetooth audio devices do have something of a power saving mode when no actual audio data is being transmitted. It's possible whatever microcontroller, radio, and amplifier circuitry is inside takes advantage of that whereas if the headphone jack is in it doesn't really get to decide if it can go to sleep or not.