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CLASSiC-DAC: a highperformance stereo digital-to-analog converter Pt.1: By NICHOLAS VINEN This high-performance stereo digital-to-analog converter (DAC) is based on the Cirrus Logic CS4398 as used in Marantz and other high-fidelity equipment. It has three TOSLINK inputs, three S/PDIF inputs, a USB audio input and offers playback from an SD card. As well, it has a built-in headphone amplifier, multiple status LEDs and fits in a compact low-profile case. T HE STEREO DAC project publish­ ed in September, October and November 2009 has been a very popular project. In fact, several SILICON CHIP staff members subsequently built one and now use them on a regular basis. Our new CLASSiC DAC supersedes that design and employs a better DAC chip, the Cirrus Logic CS4398, as used in the Crystal DAC upgrade (from February 2012). In addition, we 22  Silicon Chip have added many new features and obtained performance improvements. First, the new unit (the CLASSiC DAC) is more compact than the original DAC at 225 x 165 x 40mm. It requires no mains wiring, being powered from an AC plugpack, drawing about 1.5W in standby mode and about 2.5W while running, so it’s considerably more efficient than the earlier design. The S/PDIF and TOSLINK inputs have been increased to three of each. And the addition of a USB type-B socket means that you can plug it straight into a computer or other USB audio device and play back audio at up to 48kHz/16-bit without the need for any additional hardware or drivers. The addition of a high-quality headphone amplifier means that you can listen to the audio output. This, in combination with the ability to play siliconchip.com.au back WAV files from SD, SDHC or SDXC cards (up to 96kHz/24bit!), means that the CLASSiC DAC can be used as a self-contained music player. In response to reader feedback, we’ve added sampling rate LEDs which indicate either 44.1kHz, 48kHz, 96kHz or 192kHz. There are also status LEDs for each input channel. These show the active channel and whether data is present on any of the other inputs. Like our original DAC, this one also works with an infrared remote control but in addition to switching channels and changing volume, it can also be used to change tracks/folders when playing back from an SD card, put the unit into and out of standby (sleep) mode and perform other functions such as panning. When playing back WAV files from an SD card, multiple directory levels are supported. There is also the option of using digital tone controls and a digital crossfeed circuit for when headphones are being used; these extra features work with sampling rates up to 48kHz. A redesigned output filter offers slightly lower distortion than either of our previous DAC projects. Other features include click and pop suppression at power-up and powerdown for the headphone amplifier and configurable gain to suit different headphone impedances. Automatic input scanning is also improved from the earlier design as this unit can sense the state of its inputs without having to switch to them. Operation Fig.1 shows a block diagram of the siliconchip.com.au Features & Specifications • • • • • • • • • • • • • • • • • • • • Three TOSLINK inputs, supporting 32-192kHz/16-24 bit (with appropriate receiver units) Three S/PDIF coax inputs, supporting 32-192kHz/16-24 bit USB audio input, supporting up to 48kHz/16 bit (no drivers required) SD card playback, supporting up to 96kHz/16-24 bit (MMC/SD/SDHC/SDXC) Programmable automatic input selection 2V RMS stereo line outputs Stereo headphone amplifier with volume control and click/pop suppression Supports 8-600Ω headphones Digital tone control and DSP headphone cross-feed (with SD card only, up to 48kHz) Infrared remote control Powered by 6-9VAC plugpack ~3W operating power, ~1.5W standby THD+N ~0.001% <at> 1kHz (20Hz-20kHz bandwidth) Signal-to-noise ratio ~110dB Frequency response ±0.1dB 20Hz-20kHz Sampling rate indicator LEDs Input selection/status LEDs Standby/input cycle pushbutton & power indicator LED Fits in slim instrument case (225 x 165 x 40mm) with custom front & rear panels Most parts mount on a single PCB CLASSiC DAC. We are using a different digital audio receiver IC compared to our earlier DAC, a Crystal/Cirrus Logic CS8416. This is more expensive than the previously used DIR9001 but has more features which make the extra cost worthwhile. It has two internal 8-channel multiplexers and eight input amplifiers, so we don’t need external amplifiers for S/PDIF inputs nor do we need an external multiplexer to select the active input. Inputs 1-3 are TOSLINK optical in- puts which use integrated fibre optic receiver units and these produce 3.3V or 5V square-wave outputs which are fed to three inputs on the CS8416 (IC1). The USB input is channel 4 and for this we use a PCM2902 IC (IC2) to do all the hard work of communicating with the host computer and implementing the USB audio protocol. This chip similarly has a 3.3V full-scale S/PDIF output which is fed to another of IC1’s input channels. The three coaxial S/PDIF inputs February 2013  23 TOSLINK RX INPUT 1 LINE OUTPUTS TOSLINK RX L OUT INPUT 2 DIGITAL AUDIO RECEIVER (IC1, CS8416) TOSLINK RX INPUT 3 SPI REF CLK USB TYPE B SAMPLING CLOCK GENERATOR (IC7, PLL1708 & X2, 27MHz) S/PDIF (RCA) INPUT 5 CLOCK DIVIDE BY 2 (IC8, 74LV74) S/PDIF (RCA) SPI DCI (Q15, Q16) HEADPHONE VOLUME (VR1)  TOSLINK Rx POWER (3.3V OR 5V) SD/MMC SKT +15V SW INPUT 8 MUTING CON8 IR Rx S/PDIF (RCA) R OUT HEADPHONES STEREO HEADPHONE AMPLIFIER MICROCONTROLLER (64-pin dsPIC33, IC5) INPUT 6 INPUT 7 MUTING (Q1, Q2) SPI DCI USB-TO-TOSLINK CONVERTER (IC2, PCM2902) INPUT 4 STEREO DAC (IC3, CS4398) DIFFERENTIAL TO SINGLE-ENDED & LOW-PASS FILTER (IC4, LM833) +3.3V ELECTRONIC SWITCHING (DUAL MOSFETS Q12, Q13) –15V SW SPI INDICATOR LEDS 6–9V AC INPUT +5V POWER SUPPLY +15V CON9 –15V AUDIO SIGNALS: POWER: CONTROL SIGNALS: CLOCKS: Fig.1: block diagram of the CLASSiC DAC. Its main components are the CS8416 digital audio receiver (IC1), a CS4398 stereo DAC (IC3), a PLL1708 clock generator (IC7) and a dsPIC33 digital signal controller/microcontroller (IC5). The eight inputs are shown at left, while the line and headphone outputs are at right. use RCA sockets which are connected straight into three more of IC1’s input channels. Coaxial S/PDIF signals can have quite low amplitude so in this case, IC1’s internal amplifiers are required to boost the signal level high enough to allow its decoding circuitry to handle them. IC1’s eighth and last input is used to feed it audio when playing back WAV files from an SD card. The actual SD card reading is handled by microcontroller IC5 (a dsPIC33) and this then converts the WAV audio data to an S/PDIF stream which it feeds to IC1. This means that IC5 does not need to interface with the DAC directly and avoids the need for additional signal multiplexing. Once IC1 has selected and amplified the selected input signal, it then decodes the S/PDIF data and analyses it in a number of ways. Its most important task though is to extract the audio content and output it serially via an I2S (inter-IC sound) bus which 24  Silicon Chip is connected to the CS4398 DAC (IC3). IC3 converts the digital audio stream to a pair of analog signals which are fed to a low-pass filter. This converts the pairs of differential outputs from IC3 into the more typical single-ended analog outputs used by most amplifiers and other pieces of audio gear. New DAC filter We have made some changes to the filtering network in order to improve its performance. The filter used in the Crystal DAC design (February 2012), also based on the CS4398 DAC IC, used the suggested filter from the Cirrus Logic data sheet but we had rounded some of the odd component values (eg, 698Ω) to the nearest E24 series value, such as 680Ω. However we have subsequently discovered, via SPICE simulations, that even this slight shift in component values seriously degrades the ability of the filter to reject common mode signals. Obviously, the actual components won’t have the exact value printed on them so to some extent any actual filter is going to deviate from the ideal but we figure that if the filter can be shown to work effectively with exact values, it should at least perform reasonably well with real components, especially considering that 1% resistors are often much closer than 1%. Capacitors are another matter but 5% MKT types are not difficult to get and 2.5% or better are available. So we again fired up SPICE (“Simulation Program with Integrated Circuit Emphasis”). Using this software, we came up with a filter arrangement that uses only E24-series values for resistors and E12-series values for capacitors and provides (in theory at least) even better common mode signal rejection than the suggested filter in the data sheet and with a flatter frequency response, when the whole circuit is taken into account. This new filter arrangement is shown in Fig.2. Fig.3 compares its siliconchip.com.au frequency response and CMRR (common mode rejection ratio) to that of the filter as specified in the Cirrus Logic data sheet (with the unreal component values) and the filter with rounded values, as used in the Crystal DAC. Note that we have gone back to using an LM833 dual low noise op amp rather than discrete component circuitry for the audio output filters, as used in the Crystal DAC. This was to reduce the size and complexity of the overall circuit as it would otherwise be over the top. Muting & output The CS4398 supports using transistors to mute the outputs when they are idle or muted, to suppress clicks and pops. We used this feature in the Crystal DAC but we decided to use back-to-back Mosfets rather than special bipolar junction transistors as they are easier to get. We are again using this arrangement, also shown in Fig.2. However, because we have designed the DAC to use the CS4398 from the start, this unit has superior control over the muting FETs and so suppresses clicks and pops more effectively. The line outputs of the unit are taken from the outputs of the DAC filters, at the point where the muting FETs connect. These signals are also fed to the dual-gang volume control pot for the headphone amplifier. The headphone amplifier is a currentboosted op amp arrangement, reminiscent of Peter Smith’s November 2005 design although we actually redesigned it from scratch. It’s based on AmuteC 100k B AoutA+ AoutA– 24 SC 3 220 µF 1.5k 2 1 4 750Ω 1.5k 8 IC4a 1.5nF CLASSIC DAC A 47 µF ZD1 18V D2 G2 –15V 4.7nF 22nF A ZD2 18V 100nF 470Ω 6 .8 nF 2013 100Ω 25 23 470Ω K 100pF 100k 240Ω AmuteC Q3 BC559 C 10k +5V +15V E G1 100Ω Q1b S2 S1 D1 K Q1: Si4804 Q1a CON6 10nF –15V DAC OUTPUT FILTER & MUTING Fig.2: revised DAC differential low-pass output filter arrangement for the CS4398 IC. The components are arranged in the same manner as for the Crystal DAC (February 2012) but the values have been changed to provide better common mode signal rejection and to boost the output voltage level slightly while retaining a flat frequency response. another LM833 and while the performance is not quite as good as the discrete HiFi Stereo Headphone Amplifier circuit published in the September and October 2011 issues, it is still of a very high standard and well above that available from the headphone sockets of most CD players. Again, this was done to keep the overall complexity and size of the unit within reason. The gain is normally set at unity (ie, 0dB) using a pair of jumpers and this suits 8-60Ω headphones, giving an undistorted power output of around 100mW across this impedance range. For higher impedance headphones, 12dB of gain can be selected, giving around 100mW into 600Ω. Lower impedance headphones can also be used in combination with this gain but the result will be slightly more noise, possibly audible as hiss when no signal is present, depending upon the sensitivity of the headphones used. The output is via a 6.35mm stereo jack socket and the headphone amplifier is designed to endure a continuous short circuit, although typically the output is only shorted briefly when the headphone jack is inserted or removed. A second set of muting FETs are connected across the headphone outputs, as with the line outputs. This is designed to prevent the headphone amplifier output from causing a loud Why Build This High-Performance DAC? I F YOU ALREADY own a DVD player of average quality or better, you can hook it up to this DAC and immediately upgrade the sound quality. Most DVD players have mediocre audio quality from their audio outputs, especially in terms of distortion (see “DVD Players: How Good Are They For HiFi Audio?” – SILICON CHIP, October 2007). Some CD players can also have their performance improved with the addition of this DAC, provided of course that the CD player in question has a TOSLINK or S/PDIF digital output. So why are typical DVD players so poor in audio performance? Partly it is siliconchip.com.au L OUT because they are designed down to a very low price and while their on-board DAC might be quite a reasonable component, the supporting circuitry has been cut to the bone in order to keep the overall price as low as possible. It is also true that many cheap (and not so cheap) DVD players are plagued with quite strong extraneous RF in the audio outputs, mainly related to the video output signals that they continuously produce, regardless of whether they are playing DVDs or CDs. In addition, virtually all DVD players, except the most expensive models, use switchmode power supplies. These have the advantage of being very efficient and especially with respect to recent models, have very low standby power consumption. The drawback of switchmode power supplies is that they produce lots of switching harmonics which can get into the audio outputs. Finally, because all DVD players these days are double-insulated and come with 2-core power cords, they inevitably cause hum and buzz when connected to the audio inputs of high-fidelity amplifiers which are usually earthed via a 3-core mains cord. There is no simple way to fix any of these problems but this new DAC project fixes them all and provides first-class audio performance. February 2013  25 +1 80 0 70 -1 60 -2 50 -3 40 -4 -6 -7 10 30 FR (Crystal DAC) CMRR (Crystal DAC) FR (Data Sheet) CMRR (Data Sheet) FR (CLASSiC DAC) CMRR (CLASSiC DAC) -5 20 50 100 200 500 1k 2k 5k Common Mode Rejection Ratio (dB) Frequency Response (dBr) CS4398 DAC Output Filter Comparison 20 10 10k 20k 0 50k 100k Frequency (Hz) Fig.3: comparison of the frequency response and common mode rejection ratio (CMRR) of the various filter arrangements. This includes that of the Crystal DAC (using standard component values), that from the CS4398 data sheet (using non-realistic component values) and the revised standard values used in the CLASSiC DAC. The CMRR has been improved by about 18dB <at> 1kHz. thump in the headphones when power is applied or removed, due to various capacitors charging up. This works in combination with the electronic power supply switching arrangement, shown at the bottom of the block diagram. When power is first applied, the ±15V rails are switched off and it is these rails that power the op amps in both the DAC filter and headphone amplifier, as well as the current-boost transistors in the latter. This gives the muting FETs time to switch on first, after which the ±15V rails are brought up. Once the amplifier has stabilised, the FETs are switched off and the DAC can then be enabled to drive the headphones. What Are S/PDIF And Toslink? The acronym S/PDIF (or SPDIF) stands for Sony/Philips Digital Interface. Basically, it is a standardised serial interface for transferring digital audio data between consumer-level equipment such as DVD and CD players, DAT and DVD recorders, surround-sound decoders and home-theatre amplifiers. S/PDIF is very similar to the AES3 serial digital interface used in professional recording and broadcasting environments. In operation, each digital audio sample (16-24 bits) is packaged along with status, control and error-checking information into a 32-bit binary word. This is then modulated or encoded into a serial bitstream using the Biphase Mark Code (BMC). BMC involves combining the data bits with a clock signal of twice the data bit rate, in such a way that a binary “1” results in two polarity reversals in one bit period, while a binary “0” results in a single polarity reversal. This double bit-rate signal is selfclocking at the receiving end and has no DC component. The BMC encoded serial bitstream is then transmitted as a 400mV peak-to-peak signal along a single 75-ohm coaxial cable. In most cases, the cable connectors used are standard RCA or “Cinch” connectors, as also used for analog audio and composite video. Although originally developed for conveying linear PCM (LPCM) digital audio signals as used in CD and DAT audio, 26  Silicon Chip A similar procedure is used during switch-off but in reverse, ie, the muting FETs are switched on and the ±15V rails are switched off before the supply collapses entirely. WAV playback While we mentioned this capability earlier, here are some more details on WAV file playback. Note the clock generation section shown in Fig.1, to the left of microcontroller IC5. When reading a WAV file from the SD card, the micro determines the sampling rate used from the file header and must generate a matching clock, both to time the data transfer to the DAC IC and also for the DAC IC to use to reproduce the analog audio. A PLL1708 audio clock generator (IC7) is used for this purpose. It uses a 27MHz crystal and an internal PLL (phase locked loop) to generate all the common audio sampling rates, from 16kHz to 96kHz, with nine different options. It outputs up to four different clocks which are multiples of these rates, at 256, 384, 512 or 768 times. For WAV playback, to generate the required S/PDIF serial stream to send to the digital audio receiver, we need a clock that’s 128 times the sampling rate. So we take the 256x sampling rate output from IC7 and divide it by two using a low-voltage, high-speed Digital Audio Bitstream Formats SOURCE & CODING SAMPLING RATE MAX DATA BIT RATE CD-Audio (LPCM) 44.1kS/s DVD-Video & DAT (LPCM) DOLBY DIGITAL (AC-3 COMPRESSED) 96kHz 192kHz 96kS/s 192kS/s 48kS/s 6144kb/s 12.288Mb/s 448kb/s 6144kb/s 12.288Mb/s 24.576Mb/s 896kb/s 48kS/s 2822kb/s 3072kb/s SPDIF (TOSlink) 5644kb/s BMC BIT RATE DVD-Audio (LPCM) S/PDIF has also been adapted for conveying compressed digital audio, including Dolby Digital (AC-3), DTS and MPEG-2 audio. TOSLINK is essentially just the S/PDIF signal format converted into the optical domain, for transfer along optical-fibre cables. The accompanying table (see above) shows the most common domestic audio bitstream formats and the S/PDIF/TOSLINK bit rates for each one. Note that LPCM audio is rarely used for DVD-Video, because even a stereo audio track requires a BMC bit rate of 6.1Mb/s. Many current-model DVD players and recorders are provided with either coaxial S/PDIF or TOSLINK digital audio inputs and outputs, or quite often a mixture of both. Similarly, many home-theatre amplifiers are provided with coaxial S/PDIF and/ or TOSLINK inputs. This is also the case with many up-market PC sound cards. siliconchip.com.au The new CLASSiC DAC is built on a single PCB and is much more compact than our previous Stereo DAC which was built into a rack-mount case. It also has more inputs and has better performance. flipflop (IC8). This is then used to clock the dsPIC33’s Data Converter Interface (DCI). The micro selects the clock speed for the required sampling rate by sending commands to the PLL1708 using an SPI (Serial Peripheral Interface) bus. We also send the 256x (undivided) clock to the CS8416 digital audio receiver (IC1). This is used as a reference clock, when one of the other seven inputs is selected. The CS8416 contains circuitry to measure the ratio of the sampling rate of the incoming audio stream to the reference clock. Thus, by setting a known clock rate output from the PLL1708 and reading the control registers from the CS8416, the micro is able to accurately determine the incoming audio sampling rate and then displays it using the four LEDs. Control & user interface Besides the aforementioned infrared remote control and headphone volume control, the only other control input on the CLASSiC DAC is an illuminated pushbutton. This can be used to cycle through the inputs and switch the unit into and out of standby mode. The button is brightly illuminated when the unit is on and dim when the unit is powered but in standby. The rear panel carries the power input socket, seven digital audio inputs and two line outputs. The front panel has the headphone amplifier volume control, headphone socket, power siliconchip.com.au switch/LED, SD card socket and 12 status LEDs. Power supply The power supply for the original SILICON CHIP DAC was purely linear, with an 18V-0-18V toroidal transformer, full-wave rectifier, filter capacitors and linear regulators. This worked well but was quite inefficient, with the unit consuming 6-8W and the case lid above the power supply PCB becoming slightly warm during operation. To make the power supply more efficient and to allow the use of a common type of AC plugpack, the CLASSiC DAC power supply is a bit more complicated. The ±15V rails are derived using linear regulators but their input voltage is boosted using two full-wave voltage doublers. These rails are also switched off in standby mode, as the doubler greatly decreases the supply efficiency for these rails. This arrangement means that the rectified and filtered supply from which the lower voltage rails (5V & 3.3V) are derived is substantially lower and so no dropper resistor is required, with its inherent inefficiency. Also, for the 3.3V rail, from which substantial current is drawn during operation (for the micro and some of the other ICs), a switch-mode buck pre-regulator is used, to drop the 10V or so input to 3.9V with high efficiency (90+%). The 3.9V output of this switchmode regulator is then regulated to 3.3V using a linear regulator as the switchmode regulator has relatively poor ripple rejection and the CS8416 digital audio receiver IC requires a clean 3.3V rail for its PLL to operate properly and provide a low-distortion audio output. There are also several LC (inductor/ capacitor) filters in the power supply to further reduce switchmode noise, so it won’t affect analog audio performance. Since much less current is required on the 5V rail, this is derived using a 7805 linear regulator. The TOSLINK receivers operate from either the 3.3V or 5V rail, depending on which type are used, and these draw power while ever their supply is present. So to save power during standby, the supply for these units is also turned off using a Mosfet. All linear regulators use the PCB as a heatsink. Total dissipation is about 3W, spread across about 10 devices (regulators and ICs), so the copper on the PCB (and to a lesser extent, the fibreglass) is more than adequate to spread and radiate the waste heat. In operation, the case only gets slightly warm, directly above the power supply section (rear left corner). What’s coming Next month, we’ll present the full circuit details for the CLASSiC DAC and publish several graphs showing its audio performance. A final article will then describe the construction, SC testing and set-up. February 2013  27