Quad DAC – 4-Channel DA-Converter

Published 24.11.2014 (built late-2014)

This design is to be connected to possibly lethal mains voltage. These are not detailed instructions, so do not try to build similar device without extensive understanding of electronics and electrical safety!

4-Channel audio digital-to-analog converter for 4-channel DIY audio system

  • 4-Channel 192 kHz / 24-bit DAC
  • Two transformer-isolated 4-channel digital inputs: coaxial BNCs and RCAs
  • Balanced and unbalanced analog outputs
  • Wired remote control interface with Quad Preamp
  • PCM1794A DACs, OPA2134/LME49724 op-amps, CS8416 SPDIF-receiver

Operation and Design

Operation of this DAC is extremely simple since it mostly works on its own without any interaction as long as signal is coming in. There are only two settings: switching between two inputs and changing the slope of the digital filter in DAC-chips to slightly alter the sonic characteristics. Block diagram is shown to illustrate the design in block level.


Transformer-isolated digital inputs are connected to SPDIF-receiver IC which also has a multiplexer (MUX) for input selector. No microcontroller is necessary for control purposes, IC is set up by pull-up/down resistors (Hardware setup). However, microcontroller (MCU) is used here for wired remote control functionality with Quad Preamp. Differential current-output DAC is used, followed by voltage conversion (I/V) and low-pass filters (LPF). Buffers with single-ended conversion and low-pass filters are used for unbalanced (RCA) outputs and fully differential buffer with low-pass filters for balanced output (XLR). Device is mains-powered with separate transformers and voltage regulators for analog and digital supplies. Only half of the DAC is shown, second HWDAC board without MCU related blocks exists. Power distribution network is not drawn.


Inner design consists of two HWDAC boards I had already earlier designed for HP DAC and two transformer boards, one providing higher-voltage analog dual-supply and one for lower-voltage digital supply. Both HWDACs are equipped with add-on boards also designed for HP DAC. Features of this extra board are balanced outputs and also headphone amplifier which is not used here. However, one of the boards is equipped with microcontroller for wired remote control between the DAC and Quad Preamp. It is also used for the front panel switch to have two functions. Short push changes input and long push filter setting. These are indicated with bi-colour LEDs. Third LED in the front panel signals high sampling frequency (96 kHz or more) signal. I didn’t want to make any modifications to the front panel due to lack of proper tools so I adapted to the LED-holes and one switch I had.


Inside is rather full with two HWDAC-boards with add-on boards and two transformer boards, separate ones for analog and digital supplies. Left HWDAC has microcontroller assembled for input switching, digital filter setting and wired remote control. Voltage regulators are mounted on bottom of the enclosure for good heat transfer.


Schematics

Block-level functionality can be seen in the block diagram above. Electronics schematics are shown for HWDAC-board, add-on-board and transformer-board.

HWDAC is the same board as in HP DAC so description is the same. Circuit is based around TI PCM1794A DAC which is a hardware-controlled version of PCM1792A I have earlier used in DD Preamp, meaning microcontroller is not needed for setting up the IC. It is a high-end 192kHz/24bit DAC with excellent technical performance from Texas Instrument’s wide product range. The chip takes I2S and I2S-synchronized master clock inside and outputs are differential current signals. Therefore, the goal of the design is to provide proper input and output circuitry, clean power supply and good overall system performance to bring out the most of the outstanding specifications of the IC.


Connectors are divided into two sections of two channel pairs. There are two inputs both having two connectors for four channels. One connector pair is BNCs to match with SharcDSP. There is a wired remote control interface with Quad Preamp.


The board supports 4 digital inputs: 2 coaxial and 2 toslink SPDIFs from which coaxial BNC and coaxial RCA are used in Quad DAC. Isolation transformers are used for electrical inputs, and all inputs are routed to CS8416 (IC1) SPDIF-receiver which is also operating in hardware mode, so microcontroller is not needed for setup. High-quality master clock (X1) is needed for PLL of the CS8416 which locks into the selected input signal. When locked, synchronized master clock (RMCK) for DAC is given by the CS8416 along with I2S-audio: data (SDOUT), bit clock (OSCLK) and word clock (OLRCK). These signals are fed to the PCM1794A DAC (IC2) which automatically detects the multiplication factor of the master clock and uses it internally for interpolation filters and modulators. All CS4816 and PCM1794A configuration settings are done by pull-up and pull-down resistors. The only settings to be changed during operation are input signal, using multiplexer of the CS8416, and digital filter slope of the PCM1794A, both by jumper in pin header P7 (here controlled by the microcontroller). There is also a jumper for de-emphasis but it should not be necessary to use. There is a simple mute circuit which should mute DAC if no input is selected or if error occurs in CS8416.

Analog part consists of operational amplifier circuits performing current-to-voltage conversion (I/V), buffers, low-pass filters and balanced line-drivers. OPA2134s (U1 and U2) operate as I/V-stage with low-pass filtering for high-frequency components of DAC output. This balanced voltage signal is fed to another OPA2134 (U3) for further low-pass filtering and single-ended driver, followed by RCA connectors. Balanced voltage signal is taken to the add-on board with LME49724 (U1) fully differential op-amp with low-pass filtering, followed by XLR-connectors providing balanced outputs.


Outputs are provided with unbalanced RCA and balanced XLR.


HWDAC has voltage regulators on board so rectified input voltage is needed. Analog ICs operate at +-12 V and digital logic with 5 V and 3.3 V. There are 6 regulators in both boards: LM2940-12 (U5) and LM2990-12 (U8) for operational amplifiers, LM2940-5 (U9) for 5 V digital voltage (only for toslink receivers) followed by TPS77633 (U6) for 3.3 V for all digital ICs, LM2940-5 (U11) for 5 V analog parts of the DAC followed by TPS79133 for 3.3 V for the PLL of SPDIF-receiver. There are plenty of decoupling capacitors and serial ferrite beads in the power distribution network.

I have designed general transformer board which is used in this DAC and also in Quad Preamp, and most probably in some future designs. It has holes for various common PCB-transformer footprints, primary and secondary fuses, rectifier diodes, capacitors and connectors. Dual-secondary transformers can be configured in parallel or serial configuration. Here 2x 7.5 V transformer is used as parallel-secondaries providing voltage for digital parts and 2x 12 V is used secondaries in series providing dual-rail voltage for analog parts.


Two transformers provide power for separate supplies for analog and digital parts. Voltage regulators on HWDAC boards are mounted on the bottom of the enclosure for efficient heat transfer.


Layout

PCB is 2-layer one although for mixed-signal 4 layers would be necessary. The design is not that old when writing this but I have learned a lot since that so I am not completely happy with the layout. But it is decent. When designing HWDAC I had an idea of modularity and included pin headers for add-on cards. The plan was to make two add-on cards: for balanced outputs and headphone amplifier. Soon I realized the design would be so similar that I included both in the same board. Here only differential op-amps are used but in HP DAC the whole board is assembled. In fact, I made a mistake in the board and the headphone amplifier IC has wrong footprint in the version used in this Quad DAC. XLR-connectors are panel mount type due to space limitations in HWDAC HeadAmp. In later designs I have tried to use only PCB-connectors.

Voltage regulators are assembled on the bottom side of the PCB and mounted on the enclosure with insulators. DAC boards don’t consume much power but with rather high voltages and linear operation principle they tend to become quite hot.


HWDAC board only has unbalanced RCA outputs. Add-on card is designed for HP DAC DA-converter/headphone amplifier and is used here to provide balanced outputs. Digital inputs are isolated with dedicated transformers. Ribbon cables come from the second add-on board with microcontroller to switch input and digital filter rolloff.


Software

Microcontroller is not needed for the boards since CS8416 and PCM1794A are operating in hardware mode. However, to include wired remote control function and to change input and filter slope with only one switch, microcontroller with very simple software was added to one of the add-on boards. It replaces the jumpers of input selector and filter and also receives remote commands from Quad Preamp and sends remote controls to it in case of changing settings using the front panel switch.

Enclosure

Full aluminium enclosure is from China via eBay. It is very sturdy, affordable and looks good. It was meant for DAC with some specific connector holes in the rear panel and one switch and three LED-holes in front. Obviously my connectors didn’t fit the ready-made holes so I ordered a custom rear panel from Schaeffer AG. I have written a blog post about getting the plates from there. Otherwise the enclosure is just right size for two HWDAC-boards and two transformer boards.


Sturdy aluminium enclosure is from eBay.


I have not given references list but used links directly to references. However, there is a list of some documents and books I think are worth reading on the main projects page. Feel free to comment and ask questions so I can clarify some things and further develop the description. At the moment it is still quite vague. It will not become detailed building instructions though.

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