Recently, I had a request from my brother to take a look at the 3.5mm auxiliary input line into his car stereo. Over the years, pulling at the cable has seemingly mangled the plug and he was looking to get a fix for it.

Instead of simply repairing the cable by putting a new plug (or better, socket) onto the lead, I decided to investigate the use of a Bluetooth audio receiver, so that he can wirelessly connect his phone to the system and not wear out the 3.5mm plug on the phone as well.

I’m wary of putting a self-powered receiver in a car where it could be inadvertently left on a hot day – those lithium ion batteries would get toasted, but could also (in the worst case) catch fire. Instead, as there is a source of USB power within the cabin, I decided to purchase some simple USB-powered Bluetooth audio receivers just to see if they would do the job. On eBay, I found one for under AU$2.80 delivered – so I bought three just to try out.

The Item

For under AU$3 delivered, we certainly don’t get much. Inside a padded envelope were the cellophane-wrapped devices.

This comprises of the USB receiver itself, with a cap, and a short 3.5mm stereo male-to-male lead. No manual is supplied – but no manual is really required. It should really be a case of “plug and play”.

The first one I took out of the bag seemed to work just fine when connected to my set of computer speakers and a USB charger. In fact, I thought it sounded quite good – so I decided that further investigation was necessary – to understand what’s inside, what the power consumption looks like, what the codec support is like and how well it actually performs.

Testing Methodology

To test the units, I used my Windows 10 PC with a Bluetooth radio to connect to the unit and transmit audio. Generated wave files with 1kHz tone or frequency sweeps are played for measurements. The output from the receivers is sent through to a load PCB (see below) with the Rohde & Schwarz RTM3004 used to assess the quality of the output, using high resolution mode and 20MHz bandwidth limit.

The load is designed to be switched between 32 ohms (made of 10 + 22 ohms) and 11kohms (made of two 22kohm resistors in parallel). The lower impedance emulates the load of a headphone attached to the output, whereas the higher impedance is closer to what you would expect when connected to an amplified speaker input (normally 10kohm, but I didn’t have any of them handy). The measured resistances were 32.1/32.2 ohms and 11.01/11.04 kohms which is fairly well matched, but some mismatch is expected.

Power consumption was measured using the Rohde & Schwarz NGM202 supplying power via a USB socket using the energy statistics, averaging over time. Audio prompts were recorded from a Zoom H2n handy recorder.

During further testing – it was discovered that in the three units I purchased, all were slightly different in some way despite sharing the same shell and form factor, meaning it would be good to examine their insides separately.

Variant #1 – PiX LV-B02L

The first variant I will call the PiX LV-B02L based on the printing on the circuit board. One variant identifies on Bluetooth as BTMR-6313, while the other identifies as YET-M1. Using the HCI logging capabilities on my smartphone, both come up with the same capabilities – SBC support (seemingly all modes) with SCMS-T copy protection support.

The shell design is based around a PCB-based USB-contact, thus the metal shell is part of holding the case together. The rest of the case is held together by pegs on both sides.

The first variant of the board seems to have used the cheapest-possible single-sided paper-type PCB to construct the unit.

In fact, this is the simplest Bluetooth device I have come across, with an SMD chip that could possibly be hand soldered. The chip doesn’t seem to have any codes which lead to a datasheet, which is quite interesting. There are very few components necessary to make the system work – even the antenna is a printed trace with no matching components. The only other major component is the 26MHz clock crystal.

Being a single sided board, it has nothing on the rear. But there is another variant of this.

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