HP DAC – Headphone Amplifier DAC

Updated 8.3.2015 with facelifted version of HP DAC. Originally published 1.12.2014 (built mid-2014, facelifted version Febryary-March 2015)

  • Compact audio digital-to-analog converter and headphone amplifier
  • 2 digital SPDIF stereo inputs: toslink and coaxial (isolated)
  • Analog unbalanced RCA outputs
  • PCM1794A DAC, OPA2134 and LME49724 op-amps
  • TPA6120 headphone amplifier with PGA4311 volume control
  • Powered by wall-wart type AC-adapter or dedicated DC-supply (to be designed later)

HP DAC is a small 192 kHz / 24-bit capable audio DA-converter with digital inputs, analog outputs and built-in headphone amplifier. The main use is as a headphone amplifier but with analog outputs it can also be used as a stand-alone DAC. Volume control only affects the headphone amplifier so it is not suitable for preamplifier. Unlike my preamplifiers, the DAC operation do not require microcontroller but mechanical switches are used. However, headphone amplifier part uses microcontroller for volume control. See below for details. The device is not mains powered but uses 12 VAC wall adapter. There is also an input for an external dedicated power supply which I will design later to be used with various devices.

Information regarding the updated facelifted version: Original HP DAC was built in quite ugly enclosure and the headphone amplifier part was actually never finished although it was published here. When the time came to finish it, I decided to put it in a proper enclosure with the finishing it deserves. The new enclosure is smaller so I needed to remove some parts. Balanced outputs and one optical and one coaxial input were removed to save some space. To keep the front panel simple and clear, input selector (now only two inputs) and DAC filter switch were moved to the rear panel. Otherwise the design is the same.

Design

The initial idea was to design a DAC board with SPDIF inputs and high-quality DAC and keep it hardware-controlled, meaning no need for a microcontroller. The board would have unbalanced analog outputs and a possibility for an add-on -board for balanced outputs. Another add-on -board would also add a headphone amplifier. These addon-boards would have been so similar so only one was designed with the possibility for both functionalities.

HP DAC is currently powered by an AC power supply for dual supply voltages. Despite being a small device, it has some power-hungry ICs and can generate plenty of heat in its linear regulators if fed by high input voltages. Therefore, pre-regulators were installed on the bottom of the enclosure to dissipate the excess power instead of the actual regulators without heatsinks (due to lack of space). After the pre-regulators is the HWDAC board with SPDIF input circuits, DAC, analog stage and some control logic. Add-on -board is stacked on the HWDAC providing accurate volume controller and headphone amplifier. As a headphone amplifier the operation is fully differential but analog outputs are unbalanced RCAs due to lack of space.

HWDAC board was originally designed for this use but it is also used in Quad DAC.

Below is a block-level diagram of the device, except some connections are removed from the board for the updated version of HP DAC. HWDAC itself uses ICs in hardware-mode and does not need a microcontroller. In the add-on board I use PGA4311 for accurate volume control as in many of my designs, and it is controlled by SPI bus so a simple microcontroller software is needed for the headphone amplifier.


Digital inputs (electrical signals transformer-isolated) are connected to SPDIF-receiver IC which also includes a multiplexer (MUX) for input selector. No microcontroller is needed for control purposes; input is selected by mechanical switch and IC-setup is done by pull-up/down resistors. 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 headphone amplifier. Microcontroller is used to control stepped attenuator IC before the amplifier IC. Device is powered from AC-wall-wart with dual half-wave rectifier to generate dual supply. Power distribution network is not drawn. Chassis-mounted pre-regulators are used to dissipate most of the power of the linear power supply scheme.


Two SPDIF inputs are used in the HP DAC and the selection is done with a switch, as well as the DAC filter slope selection. Line out is fixed-level as stated – volume control only affects the headphone amplifier. Power switch has dual-functionality. It works as an on-off switch but the direction depends on the used power supply (only one at a time). Now the device is used with an AC-supply but I have plans to design a dedicated power supply to be used with various designs.


Schematics and Layout

Schematics for HWDAC DA-converter board are here. The 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-quality 192kHz/24bit DAC with excellent technical performance from Texas Instrument’s wide product range. The chip takes I2S serial audio data 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 the most out of the outstanding specifications of the IC.


HWDAC board. In HP DAC only one toslink/coaxial connector is used.


The board has 4 digital inputs: 2 coaxial and 2 toslink SPDIFs, from which one of each is used in HP 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. Clean master clock (X1) is needed for the 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 (and here rear panel switches are wired to the pin header). There is also a jumper for de-emphasis which should (almost) always be off. There is a simple mute circuit which should mute DAC if no input is selected or if error occurs in the CS8416.


Unfortunately interior is not as neat as exterior. I made a compromise by selecting a small enclosure so the inner appearance became slightly messy. Panel connectors do not make it easier. However, all wires are DC power wires and all audio except headphone connector go on the PCBs. Pre-regulators are chassis-mounted behind the front panel. Inside the ugly duct-tape are rectifier diodes and capacitors for the AC supply.


Analog part consists of operational amplifier circuits performing current-to-voltage conversion (I/V), buffers and low-pass filters. OPA2134s as U1 and U2 operate as I/V-stage with low-pass filtering for high-frequency components of the DAC outputs. This balanced voltage signal is fed to another OPA2134 (U3) for further low-pass filtering and single-ended driver, followed by RCA connectors. Low-pass filter cut-off frequency is approximately 200 kHz and output level 1.5 V RMS.

After U1 and U2 signal is routed to the add-on board via pin header P12. Schematics can be seen here. Balanced voltage signal is low-pass filtered and buffered with LME49724 (U1) fully differential op-amp and connected to a PGA4311 SPI-controlled stepped attenuator IC for volume control, followed by a high-performance current-feedback TPA6120A2 single-chip headphone amplifier. Again filter frequency is approximately 200 kHz maximum level being 1.5 V RMS. ATmega168A microcontroller with simple software is used to control the PGA4311. I chose to use normal potentiometer for volume control because remote control is not used. Microcontroller uses AD-converter to read the potentiometer. This may sound nonsense but PGA4311 is very accurate compared to (cheap) potentiometers and also all 4 channels are used for differential operation. Some hysteresis and software filtering are provided for neat user experience as well as oscillation and fault prevention. Two jumpers are also used to set 15 dB or 30 dB software limitation for the maximum volume level.

The circuit contains numerous power supplies. For analog circuits, ±12 V are used for operational amplifiers and headphone amplifier and ±5 V for the volume control IC. Separate 5 V is for the DAC analog side and separate 3.3 V for the PLL of the CS8416. For digital circuits, 5 V is only used by the toslink receiver and the rest use 3.3 V.


Add-on board with differential buffers, volume control and headphone amplifier.


PCB is a 2-layer one although for mixed-signal 4 layers would be a great improvement. The design is not that old when writing this but I have learned a lot since that so I am not completely happy with it. But it is decent. When designing HWDAC I had an idea of modularity and included pin headers for add-on boards. The plan was to make two add-on boards: one for balanced outputs and one for headphone amplifier. Soon I realized the design would be so similar that I included both in the same board. Anyway, I won’t go into the details of the layouts here.


Add-on board stacked on HWDAC board.


Enclosure

Full aluminium enclosure is from eBay. It is very small, measuring approximately 115 (W) x 135 (L) x 50 (H) mm. I knew the size would cause some problems but also that I can fit everything inside. Pre-regulators are mounted on the bottom for efficient cooling. I drilled the front panel holes myself (in my kitchen) but the back panel is custom-made from Schaeffer AG to give the final professional touch.


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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