PCM 1794 Bluetooth receiver

I bought this device from eBay to see if I could get a decent sound from my smartphone when used as a source connected wirelessly via Bluetooth to the audio system. This is a more direct way compared to the case where a smartphone is used only to control the playback by means of communicating with a dedicated streamer via the local network.

It comes without a power supply, so the first thing I did was to build a 12-volt one based on an L7812CV voltage regulator, two smoothing capacitors, an old 20-volt transformer, and a simple EMI/RFI network.

After the initial sound check, it was clear that the digital part had some potential, but the output stage, together with the I/V conversion and the low pass filtering, was ruining the sound. And there is no wonder about it, for the analog stage relies on operational amplifiers surrounded by tiny filter networks built with SMD components. It looks like the implementation strictly follows the typical application circuit shown in the original Burr-Brown PCM1794 datasheet. According to it, the balanced current outputs of the DAC are fed into 5532 dual operational amplifiers for I/V conversion, then the outputs of each channel are passed to differential amplifier stages, which sum the signals into a single-ended output and provide the low-pass filter function.

The good thing is that all op-amps are installed in sockets to facilitate op-amp rolling, but in my case this helps to ease their removal and provide a convenient signal take-off point.

Circuit board modification

I decided on the straightforward passive I/V conversion based on a pair of resistors per channel. Since the DAC’s current outputs are balanced, both the positive and negative output pins must be resistively coupled to ground in order to achieve a proper voltage conversion. For that purpose, I used 150 ohm tantalum resistors soldered directly to the respective pins on the op-amp sockets. The positive output of each channel is then wired directly to the respective RCA connector’s hot pin on the back side of the PCB, thus completing the balanced to single-ended conversion.

This kind of modification preserves the device integrity, so that the original aluminium enclosure and connectors can continue to be used. The signal can now be taken directly from the RCA connectors by means of a standard interconnect cable and fed into a dedicated amplification stage. One can hear how the modified board sounds right away, but the signal is way too weak to properly drive a preamp or integrated amplifier and still needs to go through a low-pass filter to clean up the residual high-frequency images.

Low-pass filter

Since the PCM1794 DAC is performing 8x oversampling, the sonic artefacts produced due to the D/A conversion are pushed quite high in the frequency spectrum, thus requiring a gentle roll-off filter with a rather high cut-off frequency. After plenty of experimenting, I arrived at a second-order low-pass LC filter consisting of a 270 ohm resistor and a 0.1 mH inductor in series, followed by a 2.2 nF capacitor in parallel with the load. Built on a separate prototyping board and equipped with RCA connectors, this filter can be easily inserted between the Bluetooth receiver and the output stage, which facilitates the assessment of its effects on the presentation.

As gentle as it is, the low-pass filter endows the sonic images with a certain clarity, differentiation and density, which makes for a more realistic portrayal of the reproduced music and consequently for a more satisfying listening experience.

Vacuum tube output stage

That’s the most time- and cash-consuming part to build. I decided to try the output stage circuit based on a pair of 6922/ECC88 tubes in an SRPP configuration. This circuit derives from the early Audio Note DAC production and has seen various implementations across the do-it-yourself community. The stage uses 220 kOhm termination resistors across the signal input and output, 330 ohm cathode resistors at both upper and lower triodes and .47 uF signal coupling capacitors at the output. I also added .22 uF input coupling capacitors to block the DC offset present at the DAC outputs and a pair of 1 kOhm grid stopper resistors. The stage expects 180 volts of clean high-tension power to perform as intended.

Contrary to the original PSU schematic, which relies on tube rectification, I decided to use what I had at hand and went with a solid-state bridge rectifier fed by an Audio Note TX005 transformer with 290 volt secondary winding. Filtering is choke-input utilising a 30H, 40 mA choke followed by a 22 uF capacitor, then a 2 kOhm resistor, followed by a 220 uF reservoir cap paralleled with a 22 kOhm bleeder resistor. Afterwards a pair of 3.9 kOhm resistors branches the supply off into left and right sections, and each section terminates with a 22 uF capacitor. Since the SRPP circuit requires the tube heaters to be elevated in order not to exceed the maximum heater-to-cathode voltage ratings, the PSU features a 50-volt biasing tap as well.

Listening impressions

I am very pleased with the sound of this Bluetooth receiver; it allows me to fully immerse myself in the listening experience and enjoy a very lifelike presentation. In the smartphone’s settings, I have chosen the available LDAC Bluetooth audio codec developed by Sony, as it has the greatest transmission bandwidth and, to my ears, clearly outperforms the rest. To stream music, I use the YouTube Music app, and although the streaming quality is capped at 256 kbps, utilising lossy audio codecs like AAC and Opus, I don’t miss anything. The music swings with great dynamics and authority, and I am easily able to forget myself. Differences in recording quality of varying program material are easy to spot without being disturbing. Of course, some of my favourite CDs are already transferred and stored on my smartphone in lossless WAV audio format, and I can tell that there is even more nuance when listening to uncompressed music, but whatever the differences are, they are not that essential; the music is still able to transcend the format limitations and bring great joy, provided that it is skilfully recorded.

Happy listening!