With all my work on audio amplifiers I have been able to control the signal path from the line level input of the amplifier right up to the speaker terminals. However I have not had control over the source of the signal – be it CD player, DVD player or home theatre, but instead had to settle with whatever quality signal I received from the audio source. I have opened my own CD player and replaced the cheaper op-amps and capacitors in the analogue chain with better devices and benefited from the modifications, however I still had to settle with the DAC and board layout of the CD player. The CD player does however have an SPDIF TOSLINK output and since SPDIF has become very common in recent years it was the logical next step to want to make a SPDIF receiver and DAC to my own specification.
1 Optical SPDIF TOSLINK input
1 Coaxial SPDIF input
1 stereo single ended RCA analogue input
Digital volume control
Stereo balanced XLR analogue outputs.
Microprocessor control interface for relays, volume and device configuration.
I obtained samples of an SPDIF receiver device and DACs from Texas Instruments. I wanted to experiment with two different DACs, one a 16-bit device and the other a 24-bit device to see if one could really hear any difference between them. Connecting the signals between the SPDIF receiver and the DACs is relatively straight forward. I made the design as generic as possible so I could select settings such as the data format and rate by means of populating resistors or adding and removing jumpers.
I obtained samples of a Toshiba TOSLINK receiver for the optical input and designed a simple interface for the coax input. The SPDIF specification requires the input to be terminated in 75 ohms and the signal level is around a few hundred millivolts peak to peak. This needs to be amplified up to 5V TTL levels to be fed into the SPDIF receiver. For the coax input I constructed a simple pulse transformer using a ferrite to provide galvanic isolation for that input.
For the analogue RCA inputs I made use of TI's OPA134 op-amps for buffering and inverting the signal to generate the balanced output signal. I used some Maxim digital potentiometers for the volume control. I used relays to select between the DAC output and the analogue inputs and also to mute the output when in standby mode. The other part of the circuit that required special attention is the power supply. The circuit is powered from a small toroidal transformer. I made liberal use of good quality low ESR capacitors for smoothing and decoupling on the supply.
I provided a number of headers for connecting the various control signals onto the board for controlling from a separate microprocessor board. I laid out the PC board and had it manufactured commerically as a prototype board and then assembled it myself.
SPDIF DAC board assembled in a prototype plastic box
When it came to testing the board performed as expected. I did try both DAC devices but due to some oversight in the design wasn't able to use the 16-bit device, so I left it off the board and just used the 24-bit device. The board can be configured to operate the device in 16 bit mode or 24 bit mode by changing the data format between the SPDIF receiver and DAC, however I am unable to detect any audible difference between the two modes. If the design was tested in specially designed acoustic laboratory it might be possible to detect a difference, but certainly not on my living room.
I am controlling the board from a display card using a Freescale HC08GT16 device, the display card and remote control were some of the left over items from my work with GGAcoustics. I wrote a simple user interface to control the SPDIF board, namely the relays for selecting between analogue and digital inputs and mute, the analogue path volume control and the DAC internal volume control and settings. The display card also receives status signals back from the SPDIF receiver showing lock status, digital format and data rate.
Display card on the outside of the box controls the SPDIF DAC board inside.
The unit is functional and sound quality is acceptable. As a test I connected up the analogue output of my CD player to the analogue inputs of the SPDIF board and optical output of the CD player into the optical input of the SPDIF board. I have set the volume levels of the analogue path and digital path to track each other identically. Thus I can put a CD on to play and switch the source from RCA to SPDIF and back whilst listening to the CD and I can honestly say that I cannot detect a difference in quality between the two.
I use this unit as the source when listening to CDs and watching DVDs on my home theatre system. Although it only has two channels, it has been adequate for watching movies where the audio is down mixed to stereo. It is my intention at some stage in the future to embark on a project to receive and decode a surround sound AC3 encoded stream and then output it in multichannel mode. Thus I may consider designing a comprehensive Digital pre-amplifier which includes surround sound decoding and multichannel outputs.
February 2006 - 2007
Some additional changes to software done later