The γ1 modular miniature DAC

Schematic diagram

The following is the schematic diagram of the γ1 DAC. Parts values are not shown. Please see the Parts list section for more details. The first diagram is of the DAC board, and the second is the USB board. All the parts on the DAC board have part IDs with a "D" suffix while those on the USB board have a "U" suffix.

Here is the schematic diagram in PDF format (117KB).

Circuit Description

USB board

J1U is the Mini-USB jack. Its VBUS line provides 5V bus power when plugged into the host. The VBUS and ground lines are filtered by ferrites and shunted by a capacitor to reduce noise. VBUS is connected to the PCM2707's HOST pin for USB attach/detach detection. The VBUS line also goes to pin 1 of J2U, which is one of the pin headers that mate to the DAC board. In bus-powered mode, the VBUS power is essentially "looped back" through pin 2 of J2U. If an external power supply is connected, the power that comes back on pin 2 would then be from the power supply (See the DAC board descriptions below for more details). In either case, the returned power on J2U pin 2 feeds a 3.3V voltage regulator which supplies all the circuitry on this board. The voltage regulator could be either a LP2985AIM5-3.3 or a TPS79333DBV. If the USB board is used standalone as a USB-to-S/PDIF or USB-to-I²S converter (i.e., no DAC board attached), then the USB board can only be bus-powered, and a jumper must be installed across pins 1 and 2 of J2U.

The USB jack's serial data lines feed the PCM2707 chip's D+ and D- pins. The PCM2707 is used here as a USB-to-S/PDIF or USB-to-I²S converter. Its DAC section is unused. The choice of output format (S/PDIF or I²S) is set with a jumper at JP1U, which changes the state of the FSEL pin. The default mode is S/PDIF, and the output data stream is at the DOUT pin. When set to I²S mode, which has 4 wires (CLK, MCLK, DATA and SYNC), the data and clock comes from the FUNC1, FUNC2, DOUT and FUNC0 pins. The S/PDIF and I²S signals go to pin header J3U, which mate to the DAC board. The jumpers JP2U-[1-3] must be removed in S/PDIF mode, and installed for I²S mode.

If the USB board is used standalone as a USB-to-S/PDIF converter (i.e., no DAC board attached), then the parts in the "optional" box of the schematic may be populated. U4U is an SN74HCU04D hex inverter chip, three of its gates are paralleled to serve as a buffer with lowered output impedance and increased output current capability. It drives both U5U (TOTX147PL), the optical transmitter module as well a resistor voltage-divider to attenuate the amplitude to standard coax S/PDIF levels, which is then transformer-coupled (T1U) to the coax output RCA jack. Note that these "optional" parts will preclude the USB board from mating with the DAC board because the connectors are located at the front panel, and occupy the same space as the DAC board's output jack and selector switch.

If the USB board is used as a standalone USB-to-I²S converter, then the I²S signal can be wired from the J3U pin header to the appropriate destination or connectors, depending on your application.

The PCM2707 datasheets says that the USB D+ line must be pulled up to VDD with a resistor under normal circumstances, but in self-powered mode, if the USB cable is detached (i.e., loss of VBUS), then D+ should not be pulled up. This is accomplished by a positive AND gate chip SN74AHC1G08. The D+ line pullup resistor R2U will get power only if there is 3.3V power and VBUS is also powered.

The rest of PCM2707's connections are essentially identical to what's recommended in the datasheet. The only thing worthy of note is that the PSEL pin, which normally selects self-powered or bus-powered mode, is hardwired to self-powered mode in this circuit. This is because of the fact that we allow on-the-fly switching between the two modes, and that we don't use the PCM2707's built-in 5V-to-3.3V regulator.

LEDU and R7U provide power-on indication. These may be omitted if the USB and DAC board will be set up in the default configuration, because the DAC board also provides power and status indication (see below).

DAC board

U1D (TPS2115A) is a power multiplexer chip that allows selection and transition between two input power sources. The external 5V power from the DC input jack (J1D) and the VBUS 5V power from the USB board (through the docking pin socket J4D pin 1) are connected to the INPUT1 and INPUT2 pins of the TPS2115A, respectively. This chip is configured such that normally the power comes from VBUS, but if external power is plugged in, then it will switch to external power. The output goes to the USB board (via J4D pin 2) as well as to two local voltage regulators U5D and U6D, which regulates down to 3.3V and 4.5V (or 4.75V if the TPS793475 is used) for the digital and analog power, respectively. The analog power for the CS8416 and WM8501 chips are further filtered by separate ferrite beads and decoupling capacitors.

The 3-pin jumper block JP2D is normally unused. However, if the builder wants to omit the power mux chip U1D and hardwire the board to either external or USB power, then a jumper can be used to select the power source. The power mux chip provides the benefit of controlled-slew voltage ramp-up, therefore it is not recommended that you omit it unless you have a very good reason to do so.

The external 5V DC power supply must be regulated, and have an output voltage tolerance of between 4.85V to 5.4V. This is to ensure that the voltage is high enough so that the regulators do not drop out of regulation, yet not so high as to damage the TPS2115A and other chips.

S/PDIF coax input from the RCA jack J2D is transformer-coupled (T1D) while optical input is handled by U7D (TORX147PL). The S/PDIF or I²S data from the USB board comes through J5D. U2D (CS8416) is a digital audio interface receiver chip, configured to operate in "hardware mode" (standalone, not requiring a micro-controller), and allows up to four S/PDIF input sources. In this circuit, we have three sources, and the source selection is by toggle switch SW1D. The CS8416 also performs low-jitter clock recovery and is configured to convert the data to I²S format. This data is then sent to U3D, the Wolfson WM8501 DAC chip.

If the USB board is not attached, then the DAC board has only the coax and optical S/PDIF inputs to select from, and must be externally-powered because there is no bus power from USB.

If the builder only needs USB input (no S/PDIF coax or optical), then there are two build choices. The first one is to fully populate the DAC board (except the coax input RCA jack, T1D transformer, TORX147PL optical receiver and SW1D toggle switch), and set the jumpers on the USB board for S/PDIF output. This method converts the USB audio into S/PDIF on the USB board, goes through the CS8416 for clock recovery and conversion to I²S, then goes to the DAC chip. In this method, the JP1D jumper should be left open.

The second choice is to build the DAC board without populating the coax input RCA jack, T1D transformer, TORX147PL optical receiver, SW1D toggle switch, CS8416 receiver chip and all its support components. Then, set the jumpers on the USB board for I²S output. This method converts the USB audio to I²S and feeds the DAC chip directly. The JP1D jumper should be shorted.

The second method, known as the "lite" configuration, is the simplest and cheapest way to build a USB-only DAC based on the γ1, but it skips the clock recovery step that the CS8416 chip provides, and requires populating more parts if at a later time it is desired to upgrade the DAC board to support S/PDIF inputs.

The selector switch SW1D is a bi-color illuminated miniature toggle. It serves as the power-on indicator as well as CS8416 status. Normally, it glows green. If the datastream is not audio, or if there are errors, then it changes to red. The status is taken from the NV/RERR and -AUDIO pins of the CS8416, and the NOR gate chip U8D plus two 2N3904 transistors (Q1D, Q2D) drive the two LEDs in the switch. If SW1D is not populated (such as in the "lite configuration" mentioned above), then the USB board has an alternate provision for a power indicator LED.

On this board, the WM8501 DAC chip is set up to accept I²S format input only. It is a voltage-out DAC with high output current capability (can handle a low impedance load), and swings up to 1.5Vrms (0dBFS) into a 10Kohm load on a 4.5V analog supply (1.6Vrms on a 4.75V supply). The analog line outputs are capacitively-coupled to the 3.5mm stereo mini output jack.

The Wolfson WM8759 DAC chip is pin-compatible with the WM8501 and can be used instead. All functions and specs are identical, except that the WM8759 has somewhat less output swing of about 1Vrms (0dBFS) into 10Kohm load on a 4.5V analog supply.

A MCP100 reset chip (U4D) holds the CS8416 in reset for a short time after power-up to allow the chip to stabilize. It also feeds an additional R-C network to the DAC chip's ENABLE pin, to ensure proper power-up sequencing. Another reset chip, a MCP101 (U9D), holds the Wolfson DAC's DEMPH pin high while the chip is enabled. This enables "high-performance" mode, which increases the internal analog output stage bias currents and (according to a Wolfson engineer) decreases THD and increases S/N ratio. The DEMPH pin state is then set to low after the chip is enabled so that we don't turn on the de-emphasis function.

Both the CS8416 and PCM2707 are set up to produce a I²S master clock of 256 times the sampling frequency in order to optimize the WM8501 DAC's digital filter response. Note that while the S/PDIF coax and optical inputs support resolutions and sampling rates up to 24-bit 192KHz, and the CS8416 also supports these modes, running the master clock at 256*fs limits the DAC's maximum supported sampling rate to 96KHz (see Table 1 in the WM8501 datasheet). The USB input resolution and sampling rate is limited by the PCM2707 to 16-bit 48KHz.

The DAC's analog outputs are capacitively coupled to the 3.5mm stereo mini output jack. The board has provisions for electrolytic and film-type box coupling capacitors. The default recommendation is a Elna RFS (Silmic II) 22uF cap in parallel with a Wima MKS2 1uF cap per channel.

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