The Mute Audio pupDAC
pupDAC History - 1
|Humble Beginnings -|
While researching the actual history for this webpage, I was shocked to find that the pupDAC can trace itself back to late 2008! As of this writing, it is late 2012, so the pupDAC has been in gestation for 4 solid years. Even before that, the pupDAC can trace itself back to cobaltmute's "cubDAC" on Headwize. Unfortunately, Headwize's forum is now defunct and appears inaccessible. Nevertheless, the actual pupDAC began for the public in late 2008. This was when Head-Fi user cobaltmute (of Mute Audio and the pupDAC designer) made a public post about the software error that was in his PCB design package, Ultiboard. He had ordered several PCBs from a manufacturer, but the footprint used from the software library for the PCM2707 chip was several sizes too small. The boards turned out to be worthless.
(the very first pupDAC PCB - Nov, 2008)
|The First DIY Post About "my DAC Design - pup1 DAC -"
The next time (still Nov. 2008), cobaltmute began a post in the DIY section of Head-Fi and asked for input/review of his design: my DAC Design - pup1 DAC. In the thread, cobaltmute put forth his design principles, along with some sample layouts for review:
- USB only - for power and digital source,
- No capacitors at the output
- High-level design train:
- USB -> PCM2707 -> I2S -> PCM1794/1798 -> IV stage
(U2) accepts the USB signal and converts it to I2S. The chip is running in self-power mode and is configured in a simliar manner to what is done in the gamma1 DAC (see AMB Laboratories). The self-powered mode is a simpler configuration taking up less board space.
(pup1 DAC PCB design - top)
(pup1 DAC PCB design - bottom)
(U4) is setup as to accept the I2S. The implementation is setup as per the PCM1798 datasheet. This is slightly simpler than the 1794 implementation (saves a bit of board space).
IV Stage -
The IV stage is the same as the one used in the ezDAC. All kudos to Evan for his design. The only change is that some resistor values keep the ouput within available voltages (2V p-p). Choice of op-amps at this time for this stage is either the LMH6643 or the THS4222. (The OPA2835 was developed later on.)
USB power is filtered with a ferrite right off the cable. From there is a 3.3V TPS regulator (U1) to provide the 3.3v sections to the PCM2707 and PCM1794 and a 4.75V TPS regulator (U5) to provide the power for the analog section of the PCM1794. In both cases the power is run through another ferrite post regulation.
Power to the I/V stage is provided by a charge pump to create a negative rail. The charge pump is the TPS60403 (U6). This provides an unregulated -5V rail from the USB 5v source. Two regulators (U7, U8) are then used (one from the +5V and one from the -5V) to create +-2.5V. It is important to regulate the charge pumps output to prevent the power rails from drooping under load. Post regulation, there is again another ferrite as well as two rail capacitors to ensure that the output opamp is properly fed.
Given all that effort to create clean power, several test points were added to the board. In the default configuration they can be used to check the voltages of the various stages. They can also be used to run the output at an unregulated + or - 5V or run the entire board on a power supply other than USB.
Originally conceived as a through-hole design, several of the components were simply too large. Many of the components were changed to SMD 1206 series.
|Version 2 of the pup1 DAC -
Several new PCB's were ordered and cobaltmute proceeded to build the DAC. After some troubleshooting, it was determined that were still a couple of errors on the PCB: the pads for R1, R2, and R3 were jumbled and the pins were different for U8. Some creative re-work and soldering yielded the working DAC at right -
Built with the LMH6643 opamp, cobaltmute expressed satisfaction with the results and sound. Another version was built with the THS4222, but the opamp proved unstable in the layout. So, a version r3 was started while cobaltmute waited for interest to develop in the design and to confirm his listening opinions on the DAC. Unfortunately, the Head-Fi thread sat dormant for another two years.
Interestingly, cobaltmute cased up several of his "pup1 USB DACs" using the Hammond 1455C801 case. These are shown in the photo at right, along with a couple of his PIMETA amplifiers.
(version r2 of the "pup1 USB DAC")
|Mute Audio and Beezar -
during the intervening period of two years, cobaltmute designed the grubDAC and entered into a relationship with Beezar.com to sell PCB's and kits for the grubDAC. The grubDAC is a great success, so much so that Beezar was interested in cobaltmute's other designs, namely, the pupDAC.
Cobaltmute shared a couple of the remaining pupDAC PCB's with me and in February of 2011, I built one and posted about it in the Head-Fi pupDAC thread. The results are shown at right -
Having built a number of less-expensive DACs and auditioned many higher-level DACs, I was immediately impressed with the sound. The straightforward linear PCB design, uncomplicated USB-only connection, and small size (designed specifically for the Hammond 1455C801 case) seemed a perfect niche as a higher-level DAC step-up from the grubDAC.
(TomB's r2 pupDAC build)