by amb » May 10th, 2014, 4:49 am

Design goal

Digital Inputs

Audio Widget USB-I2S

Digital Outputs

SRC4392

WM8741 DACs

Differential analog output stage and analog outputs

may

Master Clock

Microcontroller and user interface

PCB modularization and casing considerations

Power supply

First, a few words about the goal of the γ3. It's not meant to be a replacement of the γ1/γ2. The γ1/γ2 are excellent DACs that were designed to fit in a particular case and is small and portable while offering features uncommon in DACs their size. By going to a larger desktop form factor (and being only AC-powered), γ3 is no longer portable, but is free from the confines of the small casing to allow more features and capabilities. I hope the result is a DIY high-resolution DAC that would compare favorably against high-end commercial offerings, yet with a build cost that would be quite palatable.The following sections provide an overview of the block diagram posted above.Starting from the left side of the block diagram, you'll find four "Digital Inputs". Each of these can be build-time configurable (by populating different connectors and parts on the PCB) to be coaxial S/PDIF (BNC or RCA), optical (Toslink), or AES3 (XLR). The coaxial and AES3 configurations have galvanic isolation via pulse transformers. Optical is of course inherently isolated. All these inputs support sample rates up to 192KHz.This scheme allows the builder to choose any combination of these three types for the four inputs, based on their preference or need. These four inputs are connected to the Texas Instruments SRC4392 chip, which is a combination of DIR (digital interface receiver), DIT (digital interface transmitter), and ASRC (asynchronous sample rate converter).The γ3 also has a USB input, which is integrated on the "Audio Widget" USB-I2S module. This module features an Atmel AT32UC3A3 32-bit microcontroller, running the Audio Widget firmware, and supports asynchronous mode (rate feedback) transfers in either USB Audio Class 1 (UAC1) or USB Audio Class 2 (UAC2). The USB class is user-selectable. The UAC1 mode supports sample rates of 44.1K and 48K, and does not require any special software device driver on Linux, Mac or Windows. The UAC2 mode supports all "standard" sample rates from 44.1K to 192K, it requires a software device driver for Windows, but is plug-and-play on Linux and Mac. I will provide more detailed information about the Audio Widget in a separate posting.The output of the Audio Widget is an I2S bus. When playing DSD files, the same wires on the I2S bus will carry the DSD clock and data lines, and the Audio Widget will provide indication to the MCU (LCDuino-1) that DSD mode is active.NOTE: DSD support is not yet implemented in the Audio Widget firmware. I hope to drive the effort to get this done.All interfaces from to/from the Audio Widget are via digital isolators for galvanic isolation. This includes the I2S bus, I2C bus and other control/status logic. The digital isolators also perform logic level shifting between LCDuino-1's 5V and the 3.3V logic elsewhere.UPDATE: AMB has released an implementation of the Audio Widget, the ζ1 The SRC4392 provides two digital outputs. We use one of these to provide an optical (Toslink) output, and the other one for either a coaxial S/PDIF (BNC or RCA) or AES3 (XLR) output. The latter is build-time configurable by populating different connectors. The user will be able to select whether this output is taken directly from the selected input (i.e., loop-out), or taken after processing by the ASRC.We utilize essentially all of the functionality provided by the SRC4392. This chip contains a number of internal multiplexers (switches) to allow user control of signal routing to and from its inputs, outputs, and the ASRC. Of particular interest is the chip's two bidirectional "audio serial ports", each one could be set up as a I2S master or slave. We use one of these as input from the Audio Widget, and the other as output to the DACs.Texas Intruments describes the SRC4392's ASRC implementaion as "based upon the successful SRC4192 core" (which is what's used in the γ2 DAC). It goes on to say that it's "further enhanced to provide exceptional jitter attenuation characteristics helping to improve overall application performance". As in the γ2, we will upsample everything to 96KHz before sending the data to the DACs. In the γ2, a 176.4KHz or 192KHz input stream will be downsampled to 96KHz. In the γ3, the user can choose the same behavior, or select to change to upsample to 192KHz so that no downsampling would occur.In DSD over USB mode, the SRC4392 is bypassed, and the data/clock are routed directly to the dual WM8741s.The SRC4392 must be software-controlled, and the LCDuino-1 will provide such control via the I2C bus. I will write more details about this chip and how it will be used in the γ3 in another posting.Two Wolfson WM8741 DAC chips will be used in "differential mono" mode, one per stereo channel. This improves performance (increases dynamic range by 3dB). These DACs will also operate in software controlled mode (in contrast to the γ2), provided by the LCDuino-1 via the I2C bus. Running the DACs in software controlled mode makes available many features not accessible in hardware controlled mode.There will be a total of five digital filter responses to choose from (compared to three in the γ2), including linear-phase or minimum-phase apodising filters that are not available on the γ2. We are also taking advantage of the WM8741's internal volume control attenuator. The WM8741 provides a 10-bit control range which gives 1024 steps of 0.125dB per step. However, we will implement 256 steps of 0.5dB steps. The WM8741 natively supports PCM as well as DSD. For DSD we will use the "DSD Plus" mode which is only available via software control.As on the γ2, an "Anti-clipping" mode will be selectable.The γ3 will feature an analog output stage module implemented as a differential amplifier with buffered inputs. Premium opamps will be used for this section. Both unbalanced (RCA) and balanced (XLR) outputs will be available. This stage will also provide a 3-pole analog low-pass filter (LPF) to remove digital artifacts from the DACs. It will be powered by a dual-rail power supply, and there will be no output coupling capacitors.UPDATE: This analog stage has been released as the α24 fully-differential line amplifier An alternate module based on discrete componentsbe developed in the future.The master clock is a low-jitter 24.576MHz oscillator. It will be located close to both the WM8741s and the SRC4392 for best performance. In addition, there are additional 24.576MHz and 22.5792MHz oscillators which are used by the Audio Widget.The LCDuino-1, its LCD display, and the handheld IR remote control provides the user interface to all functions. Instead of Volu-master, the LCDuino-1 will run different firmware specifically tailored for the γ3. It will allow the user to select any of the four digital inputs or USB as the playback source, change the volume, mute the audio, select a digital filter, select digital output mode, etc. A front panel pushbutton switch offers "Learn IR" and power on/off and mute functionalities, and an optional front panel motorized potentiometer gives you a knob to change the volume with. As in the LCDuino-1/δ1 combination, the audio signal does not go through the pot. The volume change and mute control are implemented in the WM8741.The LCD display will show the current input sample rate (PCM mode), volume/mute setting, selected filter, anti-clipping mode, etc. Big font modes will also be available that shows only the volume.The Menu setup will offer adjustable fine/coarse volume increments, volume adjustment range limits, user-programmable input names, adjustable backlight intensities, and several other user-selectable DAC settings.In the γ3 application, the LCDuino-1's real-time clock option will not be supported. All parts related to that function should not be populated on the PCB.All interfaces from to/from the LCDuino-1 are via digital isolators for galvanic isolation. This includes the I2C bus and other control/status logic. These digital isolators also perform logic level shifting between LCDuino-1's 5V and the 3.3V logic elsewhere.The Audio Widget USB-I2S converter (ζ1), the SRC4392+DAC core (γ24), and the differential analog output stage (α24) will each be on its own PCB module, to be plugged-in to the main γ3 backplane PCB. The γ3 is not designed to fit any particular case, so you can choose what you want as long as it fits.The front panel will contain the on/off/mute/config pushbutton switch, the LCDuino-1 with its LCD display, the IR sensor, and the optional motorized pot. Connections from the LCDuino-1 to the main γ3 board will be similar to how it is connected to the α10 preamp backplane, via Molex KK connectors and wires.Not shown in the block diagram is the power supply. There will be three power supplies, each one with its own transformer (or a special transformer with multiple secondary windings), as follows:+5V - always-on supply for the LCDuino-1 and Audio Widget (Audio Widget power is enabled only when the unit is "powered up:).+5V - digital supply for the SRC4392, WM8741 and oscillator (regulated down to 3.3V and 1.8V with multiple onboard LDO regulators).±10V - analog supply for the differential analog output stage and for the WM8741 (regulated down to 5V by onboard LDO regulators).These three power supplies are not implemented on the main γ3 PCB. You may use two σ11s (or σ25s with additional heatsinking) and a σ22 to get all of these, or other power supplies of your choosing. None of these require a high-VA rating transformer. You could mount them in the main chassis if there is enough distance and clearance from the DACs and analog output stage, or you can use a separate chassis.More to come...