Tracing the circuitry

A project log for Digitech GE-4059 hacking

Turning a broken-down el'cheapo USB turntable into a useful USB audio input device

Stuart LonglandStuart Longland 12/29/2017 at 12:090 Comments

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.