/sony_ep30_bluetooth

hacking a Sony EP30 compact stereo combo to support bluetooth audio

sony_ep30_bluetooth

hacking a Sony EP30 compact stereo combo to support bluetooth

parts

  • Sony EP30 compact stereo
  • KRC-86B bluetooth module
  • blue LED (included)
  • 47uF cap (included)
  • LM7805
  • 0.33uF foil cap (or ceramic)
  • 0.1uF foil cap (or ceramic)
  • some piece of prototyping board

resources

goals

  • bluetooth audio playing as long as bluetooth is connected
  • else: tape audio passthrough
  • bluetooth module is powered only powered if the system is powered (using internal power supply)

build log

stage 1: audio test with external supply voltage

  • supply via 5V breadboard supply + 9V wall wart
  • simply soldering the L/R/GND to PL/PR/GND on the back side of the tape connector (labeled CASSETE BOARD/CN201)
  • audio does only play while the hardware tape play button is pressed
  • caveats: tape motor is running

stage 2: using the CD audio input

  • soldering the L/R/GND to back side of the CD connector (labeled CN203)
  • could not get the audio to play at all
  • I think the output is muted (CN302/MUTE) as long as no CD data is read

stage 3: back to tape

  • soldering the L/R/GND to back side of the tape connector (labeled CASSETE BOARD/CN201)
  • unplugging the tape play switch and tape motor (labeled CN202)
  • shorting the S201 un-muted the output, bluetooth audio is playing

stage 4: power supply

  • LM7805 constant voltage supply circuit is shown in the datasheet
  • input voltage is +9V, soldered to the back side of the CN202 connector (V+)
  • ground soldered to CN202 (M-)
  • shorted S201
  • +5V output is connected to KRC-86B VIN
  • GND is connected to KRC-86B GND
  • bt-audio is working but there is noise, KRC-86B powers on when the audio system is turned on
  • putting everything back together

stage 5: audio troubleshooting

resources:

there are two types of noise present:

  • a solid lower frequency humming, by comparison it sound like 50Hz triangle, according to the fourier transform of the triangle wave it contains 50Hz*(2n+1)
  • high pitched noise if bluetooth is in pairing mode

solutions:

  • removing KRC-86B AGND -> decreasing low-frequency humming volume, but still present without sound playing
  • the system uses a common ground for both audio and V+, so disconnecting AGND removed a grounding loop

final thoughts for this stage of the project:

  • moving the grounding to the central GND point of the systems power board could prove beneficial in a star grounding ansatz
  • the high-frequency noise could be induced by the unshielded bluetooth circuit, but since it is all on one board, putting the circuit into a shielding box would not work
  • I would like to look at the L/R and especially GND line with an oscilloscope and maybe spectrum analyzer (kinda overkill here) to perform a frequency analysis
  • regaining tape functionality could be achieved by breaking the solder bridge (made by me) on S201 and plugging the connector CN202 back in
  • closing SN201 sets COTR on CN201 high (+9V) (traces to the muting switch control), so VIN and GND should be moved to CN201 and a transistor could pull COTR to +9V if EN on KRC-86B is high (+3.3V) (high if bluetooth device is paired); no damage should be done since the IC seems to be protected by a diode from the COTR signal