Bluetooth APT-X

I got an LG G5 smartphone recently and wanted to try out the APT-X HD audio transmitter, but found this needs a CSR8675 receiver, which are rarer than 3-legged snakes. The best I could find was a CSR8645 that outputs spdif into a Wolfson WM8805, and this provides an I2S to an ES9023 DAC. It’s made by Alientek and cost $25. At the same time, I got a CSR8670 transmitter for another $25, planning to send the rear speaker signal by bluetooth to see if it was better or worse than a 10 metre RCA cable. It’s worse – the noise level is actually lower but, despite both devices being APT-X, there is still a slight delay which creates an echo compared to the original. There is also a little less definition and concentrated listening to demanding tracks suggests some compression-related THD so the treble is less appealing. It’s not much, to the point that I don’t notice it unless paying close attention.

The transmitter can broadcast from either an optical spdif signal (TV, computer, CD player etc) or an analogue stereo input (ADC), with priority on the latter if both are connected. The receiver can work as a USB DAC or a Bluetooth DAC and it outputs the received signal as spdif via optical and RCA sockets. Both devices operate with APT-X low latency to avoid dropouts, but they work best when they are in line of sight of each other and under ~4 metres apart. It’s possible to get dropouts by simply standing in front of either of them. They are also fussy about power and will be extremely noisy if asked to share power with other devices – they need to be isolated, and because I don’t want smps power, I made small independent 5V linear power supplies for each of them. The receiver is the hungriest but still only consumes around 80mA.

The transmitter has an internal battery (400mAH) good for 15 hours on battery power but will switch off automatically if idle for 30 minutes – so it turns off 30 minutes after the hi-fi is turned off. It switches on automatically with the power and searches for pairing, and automatically pairs to any receiver it finds. So in practice, it is fully automatic if turned on after the receiver is ready for pairing – no button presses or beeps and that’s great. The receiver is a little less automatic because after power on, it’s necessary to press the play button to enter pairing, and it then bleeps 3 times, before bleeping one more time when paired. However, that’s it and they always find each other so it’s no trouble really.

The sound quality is surprisingly good, and pretty much what you would expect from a WM8805 and ES9023 combination with reasonable clocks, playing compressed music. Having played with ES9023 quite a lot, I know the sound signature and felt the only worthwhile upgrade would be the output filter caps, C23 and C24. They look like ceramics so they were replaced with Panasonic pps  film and sure enough, the treble took on a sweeter, more detailed presentation, but ultimately, in hi-fi terms, this is still much below what I’ve achieved with other DACs. Despite this, it would be perfectly acceptable for rear surrounds, except for the small delay, and it’s really the delay that means I will keep using cables.