Adding Bluetooth the M&S N350 Home Intercom/Music System

My Music and Sound N350 Master Station recently stopped working.  It was pretty sweet system... for the 20th century!  AM/FM intercom system.  What about listening to MP3's or Pandora or my favorite Pod-casts?  How about internet radio?  I had some complicated solutions like using a Rasberry Pi to stream music from the web or from iTunes sharing. 

Then I thought about Bluetooth.  With Bluetooth I could stream from any smart phone, tablet, laptop, ipod, etc. I could play internet radio stations from my phone.  Heck, I've got some old smart phones and an iPad 1.  I could  dedicate one of those as the designated streamer.

Some ideas I'm kicking around

  1. Integrate Bluetooth inside the unit
  2. Add 3.5mm jack on edge of the unit for direct connection
  3. Add USB charging port on edge of the unit for charging external players
  4. Add a dock to more permanently attach a smart phone or table
  5. Add a micro to allow remote control of the radio and intercom
  6. Integrate Bluetooth into the individual room speakers for individual listening

 

 

 

In case you are not familiar with it, the N350 "is a state of the art Intercom/Music Center which provides Room to Room communication throughout your house as well as furnishing AM, FM, or Auxiliary source audio."

 M&S N350 Master Station

M&S N350 Master Station

Take out a couple of screws and fold it down to see an amazing rats nest of wires.

 

Behind the pretty panel

Bluetooth streaming is extremely low cost now.  I ordered a few  USB Bluetooth Music Receivers from Ebay for about $7.00 each.  They take power from USB port and have a 3.5mm jack for connection to an audio input. 

The receiver comes apart pretty easily.  It's mostly an off the shelf BT module soldered to a PCB with a few passives and an antenna.  

All I need is to tap power from the master station, add a 5 volt regulator and connect the 3.5mm jack to an audio input.  

 

 That's going to be a problem....

That's going to be a problem....

I'm getting ahead of myself, 1st I have to fix the master station.  Right away I spotted a problem with U6.  Unfortunately, whatever took it out took out a lot more components.  All in all I found several shorted transistors and capacitors.  More on long debug process later.

 Experimenting with mixing resistors

Experimenting with mixing resistors

A couple of quick notes about adding Bluetooth. Since the station is mono input, I needed to add mixing resistors from the BT receiver Left and Right Channels to the station audio input.  I picked 32 ohms to match the typical earbud input impedance.   I also need a 5 volt regulator since the only power source on the station that could be switched on/off was 15 volts.  i put the regulator and mixing resistors on a piece of perf-board and added a 3.5mm jack on the edge of the station in case i wanted to directly plug in a mp3 player or smart phone.  

I'll add some more pics and a schematic later.  This shows the perf-board mounted on the back of the master station.  The master station front plastic (except the speaker grill) is chromed/metalized plastic.  In addition, there is a sheet of steel between the face plate and the pcb.  This is not a good situation for getting blue tooth outside of the  master station.  At first, I mounted the BT receiver so the antenna was right over the  opening for the speaker cone.  This worked great as long as my phone was directly in front of the master station.  I had about 20 feet of range, unless i moved to the side.   

I cut out the steel sheet and the portion of the plastic face plate behind the decorative speaker grill which is larger than the  speaker (thank goodness!).    I now have a 20' range no matter where I am in the room.

 

 M&S 350 Voltage Regulator and Audio Interface for USB Powered Bluetooth Receivers

M&S 350 Voltage Regulator and Audio Interface for USB Powered Bluetooth Receivers

This adapter provides 5 volt regulated USB ports for power/charging and provides Bluetooth and panel mount audio inputs to the 350 intercom.

  • Power Path:
    • P2 provides +15 volt input from the 350 "Phono Enable" line.
    • U1 regulates voltage to +5 volts
    • The J1 USB connector on the adapter provides +5 volts to the Bluetooth module.  No data is supported across the USB port.
    • +5 volts is also provided to P2 through P1 enable jumper for an optional panel mount USB port.
  • Audio Path:
    • To save space, I used jumper wires to connect to the Bluetooth audio output via P4-P6.
    • Bluetooth audio is passed through the  panel mount  stereo jack via P3 connector. 
    • When the panel mount stereo jack is empty, the Bluetooth audio is passed back to the P3 connector.
    • When the panel mount stereo jack has an external source plugged in, the Bluetooth audio circuit is interrupted and the external audio is passed back to P3
    • The audio from P3 is STEREO, but the 350 only supports MONO so I had to mix the left and right channels into a single mono channel.  I used two resistors since the phono input is an op amp.  I started with 33 ohm values since most MP3 headphones input impedance is around 30 ohms.  When I tested the circuit, it sounded good and seemed to match the volume level of the 350, so I stuck with the 33 ohm resistors.
    • The MONO audio is passed to the 350 "Phono Input" via P2.