Turning a broken-down el'cheapo USB turntable into a useful USB audio input device
So last Wednesday night, I was over at HSBNE… I deliberately brought this little device, my laptop, my turntable and a couple of records.
First step was to remove C16 and C29. The hot air re-work station did a good job of that. I plugged the slightly modified device into the turntable and laptop, fired up Audacity and ran a test. Yep, still getting audio through. So off came the remaining parts.
I'm still undecided as to whether to expose the GPIOs or not. It'd be a pain to solder to those pins on the chip and to be honest, I'm not sure if they'll be useful or not. Most software I'd use with this device can't make use of the GPIOs directly.
Having removed the remaining parts, I was able to digitise one of my records, stashing the recording for later listening. The removal of the pre-amp means it's no longer there to create noise. I took the now useless 2.54mm JSTs off, since I have the mating connectors for those, they can go into the junk box for another project now, as have the two electrolytic capacitors.
I'll have to find a suitable case to mount it all in, but in all, this is practically done.
One artefact of my current turntable I'll have to ponder is some speed correction. The player has no speed adjust beyond a selector switch for 33⅓ and 45 RPM. Unfortunately, what it calls 33⅓ is more like 34⅘ RPM. For now, putting up with it playing a little fast during the recording session and doing a speed correction in Audacity works, but long term, I'd like to fix that on the turntable. But that'll be another project.
I haven't yet modded the module… the amplifier is powered by an on-board linear power supply (the rectifier bridge and filter capacitor is seen bottom right of PCB).
I really should remove those capacitors, but figured I'd do a test with the board as-is, since in theory it should work. The turntable I have (a Kenwood P-110) features a pre-amp in it already. Turntables that don't, probably will need to hook up to that JST connector instead.
As can be seen here, a bit of level adjustment and I was seeing the waveform in Audacity. As a bonus, I don't have the tinny audio problem that the now dud turntable gave me (RIAA filtering? What's that?).
So I think a bit of careful SMD part removal, and I should have exactly what I'm after.
So, last time I tried using this record player… that's when I discovered the unit had shifted off its mortal coil… at least all the mechanical bits. At that point there was a record I wanted to capture on the computer, and I would up dragging out my ageing MacBook to record from a purely analogue turntable.
That piece of Cupertino Engineering a month or two back, lost its WiFi module; it was there, then next minute Linux was having trouble connecting. A reboot later, and the WiFi was gone: never to be seen on the PCIe bus again. Even MacOS X reported "No AirPort card installed".
Good riddence to that Broadcom-based pile of excreta I guess. I can use Ethernet with it of course, it's old enough to have one of those, but given the battery decided it would bloat out like a puffer fish about 6 months before… that tells me the machine is on death row. Time to get something else working.
Sadly, my main laptop does not feature a line-in port. Panasonic thought of everything else, including RS-232, Cardbus and even a heater for the hard drive (fantastic with a SSD in Queensland's summer conditions!)… but not a line-level stereo audio input.
Yesterday I bought yet another record, and I want to listen to that whilst riding home on the bike… so I need to figure out how to get audio from vinyl to Vorbis.
It's projects like this when you realise an audio signal generator is a really handy thing. Never mind, yes, I can build one with a 555 for this purpose. Yes, I should. But for now, I had a crack at just buzzing it out.
To say the audio path takes lots of twisty turns is an understatement, but after a close examination, I think I've worked out its various bits:
So, from the 5-pin JST connector (not sure which exact one), bottom left… the audio passes through a low-pass filter (blue) then a high-pass filter (cyan) before being fed into the amplifier (green). This appears to be a simple class-A one-transistor amp, not sure if it's BJT or MOSFET based.
From there, the audio passes through some AC coupling capacitors (yellow).
At this point, the audio branches out. In one direction, it passes through another DC blocking capacitor (red) before finally reaching the RCA jacks.
In the other, it passes through a filter network (magenta) for the ADCs.
It would appear that based on this, removing C16 and C29 (in yellow) will disconnect this now superfluous amplifier and link the RCA jacks directly with the input filter network to the ADCs.
I can probably dispense with the two capacitors I stood up before too, which should reduce the size somewhat.
So, having stood the two electrolytic capacitors up… I see a bit more detail that was hidden beneath them.
I have no idea what the transformer ratio is, but I don't see any regulation on this power supply. There's a full-wave bridge rectifier (D1, D2, D3 and D4), then it goes into C36 which is that monstrous 2.2mF (25V) capacitor. The two extra pins next to the audio input are power outputs, as suspected.
In the above photo, the index mark of the main IC is the top-left. If you look closely, you can identify the various GPIO pins are not connected:
Making use of those will require either very careful soldering, or liberating the part from the board.
So the module itself is on a separate board that is held in to the back of the turntable by a single screw (which screws into the dual-RCA jacks) and a fair amount of hot glue. To get inside, you'll need to undo the four screws around the back and sides… the top of the turntable lifts up. (Make sure the tone-arm is secured.)
The 2-pin JST connector in the corner is where the low-voltage AC power supply comes from the transformer to power the audio circuitry, and over on the far side, is a 5-pin JST which carries audio and, I think, DC power.
On the other side of the board are the RCA sockets, which provide line-level audio outputs for connecting the turntable to an amplifier, and of course, the USB connection.
As mentioned in the description, the business end of the module is a C-Media CM6317 USB-Audio chip, which features dual ADCs for sampling microphones.
I note the presence of footprints for LEDs and the current-limiting resistors to go with them, the CM6317 also mentions having GPIOs on it. My first change though, I think, will be to stand those capacitors up properly.
The traces to the RCA sockets' centre pins appear to go direct to two capacitors, so I suspect I can convert this to an input by removing those capacitors and tacking wires on to the old footprints, possibly via an attenuator pad to the 5-pin JST. Exposing the GPIOs will be a stretch goal.