Modding wireless RF headphones to Bluetooth headphones

Thread Starter

AliPaca

Joined Nov 18, 2020
2
Hi guys,

I have been using a pair of Sennheiser headphones for quite some time now (HDR 118). They offer good sound and sit comfortably, however they do work based on RF signals (863 - 865 MHz) and have been catching a lot of interference as of late.

Instead of just getting a new pair of headphones I decided to go ahead and try to mod this pair to work with Bluetooth instead of RF to get rid of the interference and lose the dependency of the RF transmission station which sends the signal to the headphones.

After taking them apart, I have identified one chip as FM/AM stereo radio chip (CXA1838N) and a stereo headphone opamp (PT2308). Both datasheets attached. The big silvery block on the PCB is some sort of antenna combined with tuner to bring the frequency down I think, but I'm not sure as there are no markings anywhere on the block. The two rotary dials are an FM tuner (3 connections) and volume knob (5 connections).

The entire thing is powered by 2 x AAA batteries of 1.2 V for 2.4 V of available voltage in total.

I've been looking around at off-the-shelve BT receiver circuits that I could easily plug in there with some minor soldering and severing some connections to the RF chip and possibly removing the antenna/tuner-thing, but they almost all come with the following;

1. A voltage requirement of at least 3 V, most often 5 V.
2. An included op amp

This is a problem because:

1. I only have 2.4 V at my disposal.
2. I'd like to keep the audio driver on the current circuit as it would be nice to keep the volume knob working without too much messing around.

Another option would of course be to just re-do the entire circuit. It has however been a looong time since I meddled with PCB's and I'm afraid to waste a lot of time and money on a crappy design that I need to rebuild 3 times to get it working.

Do you guys have any tips as to which off-the-shelve BT receiver circuits could be easily modded in here? Or do you perhaps have some other idea's about how I could achieve my goal? Any advice is welcome!
 

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Papabravo

Joined Feb 24, 2006
15,811
AFAIK bluetooth uses the 2.4 GHz. ISM band. What makes you think such a mod is even possible with the board you have?
It is likely you will need all new electronics inside the headphones. If you do succeed of course you will be fighting with mobile phones and WiFi routers. You might as well try pushing a boulder up a hill.
 

Thread Starter

AliPaca

Joined Nov 18, 2020
2
Hi Papa,

I was initially looking at circuits like this one for example https://banggood.app.link/OicJXTjtwbb
And thinking of modding this board as some sort of a "piggy back" on top of the one thats already in there, after making the necessary adjustments to it.

To state my idea very simply;
  1. Hook up the batteries to the BT receiver circuit
  2. Take the BT input signal from the BT receiver circuit and pass the audio out signal to the headphone speakers via the stereo driver

I was sort of expecting to just be able to pair this BT receiver circuit to, for example, a phone or a computer just like any other BT receiver device. Or am I dreaming too much here?
 

Papabravo

Joined Feb 24, 2006
15,811
Hi Papa,

I was initially looking at circuits like this one for example https://banggood.app.link/OicJXTjtwbb
And thinking of modding this board as some sort of a "piggy back" on top of the one thats already in there, after making the necessary adjustments to it.

To state my idea very simply;
  1. Hook up the batteries to the BT receiver circuit
  2. Take the BT input signal from the BT receiver circuit and pass the audio out signal to the headphone speakers via the stereo driver

I was sort of expecting to just be able to pair this BT receiver circuit to, for example, a phone or a computer just like any other BT receiver device. Or am I dreaming too much here?
This kind of thing has a couple of aspects. The mechanical part is fairly easy to deal with and is kind of a binary thing. It either fits or it doesn't. The other part is more complicated and usually requires some kind of schematic diagram or at least specifications you can depend on. For example, headphones may or may not work well depending on the driving source. If you use existing components on the board you have to know where to break the existing connections and where and how to make new ones. As this is a battery operation it would be good if you can eliminate power to parts of the circuit you will no longer be using. RF parts are heavy users of battery power, especially when they are trying to drive an unmatched load. You know what level of aggravation you are willing to put up with and given the cost of these things it seems hardly worth the effort.
 

fcint

Joined Dec 24, 2020
1
I've been thinking about doing the same thing for my HDR 120. I found this post by putting the RF chip name in a search engine. From what I can tell a range of Sennheiser HDR 11x / 12x are variants of the same design.

I've already modded mine to run on 3.7v lithium batteries. I'm using a lithium cell with TP4056 charger module. I used two 1N4148 diodes to drop the voltage to the 2.0-3.0V range. A lot of these little bluetooth boards appear to be designed to run on 3.7v lithium cells.

One of my first ideas was to wire a 3.5mm stereo jack to the speakers. Then attach small off the shelf bluetooth receiver (with 3.5mm output) to the outside of one of the ear cups. There's an old post in the following link where a dude has added a 3.5mm jack.
https://hatoum.com/blog/2012/01/adding-stereo-plug-to-sennheiser-rs-120.html

He uses a switch to toggle the speakers between wired and wireless. Supposedly there's a way to us a 4 conductor stereo jack as a switch itself. I'm uncertain how this would be wired though. And it was never explained in their post.

I've considered a lower level mod at the circuit board level but I've decided for now it's beyond my expertise. I've noticed a big fat trace on the board that I suspect is the power to the RF part of the circuit. I've theorized about maybe cutting that and putting a switch to control power. I can't tell if the audio channels into the amplifier have separate negative pins or if they're a common ground. Anyways this is getting to complicated for me already because I'm not going to experiment and risk breaking my headphones.
 
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Audioguru again

Joined Oct 21, 2019
3,017
The Banggood module has absolutely no audio spec's (are they hiding poor performance?) but the AliExpress module has very good detailed spec's.
The AliExpress module with Bluetooth 5 is much newer and better than the old Bluetooth 4.2 from Banggbad.
Look at the prices. The new better one costs less than the old bad one.

Hobby stores sell small 3.7V Li-PO batteries and chargers for them.
 

bassbindevil

Joined Jan 23, 2014
157
How about salvaging the guts from cheap Bluetooth headphones from a thrift store or dollar store? I've seen a bunch of Budweiser branded headphones locally for some reason. They probably have terrible transducers, and maybe dead batteries, but the electronics could be adequate for Bluetooth.
The MH-M28 doesn't look terrible, but it may not have a proper charging circuit for the battery.
 
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