FOSI SK01 Review: Headphone amp + preamp with ample tone controls

[Djano]

BATTERY WORKAROUND

The design is flawed: when the battery dies, the device becomes unusable, even in wired mode. There is a workaround, though.

SYMPTOMS​

After two years, the device intermittently stopped working when plugged in USB (it always was).

It could work for a few hours, then did a very quiet "tic tic tic" that could be heard through the speakers.

If set to off, the LED would be red, indicating that it is charging.

It kept getting worse: it stopped faster and faster, and needed more and more time in off mode to recover.

I opened it and tested the 18650 inside: it could only hold 700mAh (standard capacity is about 2500mAh for this size of cell).

INSIGHTS​

So, it now seems clear that even when plugged (as it always was), the device does things with the battery and relies on it (the battery is obviously dead)

If you unplug the battery (disassembly needed), it refuses to start with USB only.

However, if you start it with the battery plugged in, then disconnect the battery (to rely solely on USB), it continues to work for some minutes.

The device seems to check at startup and periodically if the battery is OK.

WHAT DID NOT WORK​

You cannot just unplug the battery and run it on USB.

You cannot send 5V (from USB) to the pins that are meant for the battery. The charging/protection circuit or something else does not like it. You can hear a "real electronical" noise when doing that; I really do NOT recommend it.

I also don't recommend relying on a trick with a switch to satisfy the "battery check" at startup and get rid of it to avoid any problem after the start. Too complicated, and it seems there is a periodical check.

THE SOLUTION​

Obviously, you can buy a new cell (PH 2.0 connector). But I did not want to face the same problem in two years. For a definitive solution, do as follows:
The device should think that there is a battery (3.7-4.2V) with USB only (5V). We will make that happen.

1) Open the SK01​

• Take off the 4 knobs (and nuts/round thing you'll find under). Two pieces of plastic used as levers is far more reassuring than just pulling on them.
• Peel off the rubber adhesives at the back
• Unscrew the 4 screws under the adhesives. Back plate comes off.
• Unscrew the small metal bars that held the screws you just took off (will make next step easier).
• Take the front plate off. There is some adhesive, but it is only adhesive. Don't be afraid to pull, or to use a plastic piece to progressively unglue it everywhere.
• Take the "high plate" (the one with the USB-C port) off. This one will be very hard, but once again, there is only adhesive, don't be afraid to pull hard. Alcohol might help to soften the glue.

You can get a feel of the process with this video. The guy should just insist more on how you should not hesitate to pull hard on some things.
The PCB is now free.

2) Salvage the PH 2.0 battery connector (for our future fake battery)​

Useless if you have one laying around.

• The 18650 protected cell is held with some white flexible silicone glue thing. Use a cutter to free the cell. Be very careful not to damage the cell; this could get dangerous.
• The cell is free. Take its wrap off (careful again).
• The connector is soldered to a round protection circuit. Unsolder the wires from it, or just rip them (keep the wires attached to the connector, of course!).
• Use a lighter on the wires to retract/burn (then clean) the insulation and expose some copper (for future soldering).

You now have a connector that is ready to solder.

3) Buy a voltage step-down circuit​

DO NOT buy a linear one (e.g. LM317), because the dropout is too high. From 5V, you'll get max. 3.3V, which is insufficient. The device won't start. FYI, it already blinks for low battery at 3.7V! If you insist on getting a linear one, the workaround would be to step up the voltage before stepping it down. Then you'll have to deal with heat. I really can't recommend it, even if "linear" seems reassuring for an analog audio device.

INSTEAD, buy a "buck" one, with inductors.
Based on these tests, I was more confident with the MP2307DN circuit.

Do not worry about the audio quality, I will discuss it later (not a lot to say: NOTHING changes).

4) Cable management​

• Cut one connector of a USB cable. Get the right length and the right feel, it will be permanently attached to the SK01.
• Wherever you like on the SK01, drill a hole of the diameter of the USB cable you will use to power the device. I suggest the top plate, with the initial USB-C port.
• Pass the cut end of the USB cable in the hole. Do it! Else, you'll have to unsolder because the step-down board won't fit through the hole...

5) Assembly​

• Solder the USB end you just cut to the IN of the step-down circuit.
• Adjust the voltage. Ideally, it should be 4.2V (fully charged battery). But the highest I could do with mine was 4.1V.
• Solder the salvaged connector to your step-down circuit (OUT).
• Plug to the main board, test if it works.
• Hot glue the step-down circuit where the cell was previously.

6) Reassembly​

Most of it is obvious. Just think of a way to make the cable very secure, in such a way that you won't do anything bad inside if you pull it later (knot, cable tie around the cable to make a diameter bigger than the hole, anything).

Note that the top plate (with the initial USB-C connector) can't be properly reassembled. You'll have to find a trick (black thick tape, strong glue at the seams, hot glue inside, whatever).

RESULTS​

SOUND:

Even though the step-down circuit makes a real electronical noise (like rapid switching) that you can hear from the board, this can't be heard at all when using the device (through speakers, headphones, etc), even without any music and pushing the gain to the maximum on my headphone amp (SK01 is used as preamp)

I did a non-blind test, where I know when I use the "real initial USB" only and when I use my "fake battery" with this dirty circuit. I can't hear ANY difference.
The sound is just as it always was.

The original alimentation circuit already has inductors. So the design accounts for that. Everything is probably filtered in some way.


BATTERY FAKING:

I tried plugging the original USB-C (which is supposed to charge the battery) when my fake battery was plugged. I set it to 3.9V, so charging should occur.

Nothing happened, even though the light is red when the device is shut off (indicating charging).

I recommend just using your new cable, it is useless to plug both. But this did no harm.

 

[Bruce Morgen]

THE SOLUTION

It worked -- thanks for the helpful description of the process. I ran into a few issues that you apparently avoided. The first was that the tiny screws that hold the front panel and case together were apparently torqued down much too tightly -- only two of the four survived the removal process. They are so tiny that even my huge stash of small hardware didn't have any suitable replacements, but the two surviving screws seem to be holding things together adequately. The other is that glue seepage made it a chore to unplug the battery -- I had to carefully scrape and cut away a tough little glue blob while avoiding damage to the wires and connectors. I still haven't glued in the top panel -- I'm considering adhesives (thus far leaning toward some CA glue at the seams*), but for now a rubber band around the case is holding that panel in place.

This is the buck converter board I chose: LM2596 DC-DC Buck Converter 3.2V-35V to 1.25V-35V Power Module, Adjustable Step-Down Voltage Regulator, Wide Input Range Converter for Electronics Projects/DIY Enthusiasts

The chip gets pretty warm, but there's no noise at all coming from the board even with the case wide open.

Thanks again for mapping out a simple, eminently worthwhile project!

* Update: CA glue applied and holding just fine.