USB input for audio DAC runs hot (130F / 54C)

[13hm13]

The following "Mini ES9038Q2M DAC" $60 Chinese DAC is widely available on eBay and Ali.




Note that the 12v input power has been updated by manuf. to 8.4vdc. The unit comes with a cheap 8.4v adapter.
The adapter power (8.4v) seems to only power the analog section (output opamp).
The USB 5.0v powers, I think, the digital section (dac chip, XMOS chip, logic chips) . The toggle switch powers on/off the 8.4 v. But the LED next to "POWER" label lights up when the  USB is plugged in (regardless of 8.4 on or plugged in) -- a confusing scheme!

Here is a top view of the inside:


I was operating the unit with the cover removed and noted a soft buzzing noise. The noise was active ONLY when the USB cable was plugged in. Removing the 8.4v supply made no difference.
Finger testing heat, I noted the components in the USB plug area (top right corner of PCB) getting quite warm AFTER about two min. of operation.
I dug out my IR thermometer and measured 130F (54C) at the USB input coil choke and/or the resistor and/or the inductor next to it. The XMOS chip was about 100F. Everything else was a few degrees above room temp.
Sorry, I don't have a FLIR (yet!).

While I have not performed any objective tests, subjectively the unit functions and sounds just fine.

I think I have noted hot-running USB input sections before in other devices, but can't confirm.
Is this USB input power section behaving normally?
Any tests that can conform/deny defect or long-term reliability of associated components?
Are there mods that may "quiet" or cool things down?
Thx!

NOTE:
I was summing the currents eaten up by the USB power input:
XMOS (45ma) https://www.xmos.ai/download/XU208-256-QF48-Datasheet(1.16).pdf
LEF (20mA)
 .....what else?

[13hm13]

I measured the current draw of the DAC via USB... 152 mA @ 5.1v.
Pretty sure all is "normal". But that small choke (with fine strands of wire), getting 130F is not reassuring.

[Buriedcode]

On the PCB it notes a MP2122 ( https://www.monolithicpower.com/en/mp2122.html ) which appears to be a dual sync buck regulator, using the two SMD inductors either side of it.  I suspect that third choke is for power supply filtering, to filter the USB power (and perhaps to reduce the noise coming from the unit, back onto the USB supply).
You could measure the voltage across it to calculate its power dissipation (you already measured the current going through it as thats the current draw from USB). 

As for the buzzing, the MP2122 datasheet states that both the converters inside the device operate from a 1MHz common oscillator - so I doubt its a beat frequency between two converters - but perhaps in lighter loads it skips pulses. 

If the buzzing is bothersome, then I guess the only way to know what is making the sound - one of the MLCCs or the SMD inductors would be to remove them and test.  But if the unit otherwise works fine and you can't hear the buzzing with the enclosure closed, I wouldn't' bother.  Those two SMD inductors look poorly soldered - and frankly undersized given the application and PCB footprint. Handy that it states their value on the silkscreen, so you can replace them if needs be.

[BrianHG]

Probably, your DC12v supply has poor isolation from the mains AC in, or, a really high value Y cap in it, or the Y cap is on the hot pin.  This will usually create a buzzing and even with an added hum.  If you can test the device with a 12v battery and see if the buzz is still there.  If not, then a quality linear, regulated 12v wall wart will erase your problem.

This problem may also happen if the polarity of your AC outlet is wired backwards for any equipment in the chain of the audio hardware all the way up to the PC.  If this is the route of the problem, a ground-loop isolator or mains isolation transformer will also fix this.

Do you get the buzz with headphones on the output and nothing else connected?
Do you get the buzz if your audio amp is directly connected to your PC motherboard audio output?

Note that the 12v input power has been updated by manuf. to 8.4vdc. The unit comes with a cheap 8.4v adapter.
The adapter power (8.4v) seems to only power the analog section (output opamp).

What does this mean?  I've never seen an 8.4v power supply unless it is un-regulated transformer, or a cheap un-filtered noisy switcher which is intended to be filtered and re-regulated after the fact.

[amyk]

A 54C operating temperature is not "hot" for the majority of electronic components; electrolytic capacitors being one notable exception.