Ally (PPA DAC Daughterboard)
PPA (Phil’s Portable Amplifier) has become a popular project both in DIY (Do It Yourself) and DIYFSE (DIY For SalE) communities. It can be had as a standalone semi-portable amplifier with built-in battery board and recharger, or as normal AC powered amplifier. That still doesn't address the source, especially if a portable system is desired. That is where the Ally DAC daughterboard comes in. It is a high quality D/A convertor board that can simply slide in Hammond enclosure used with most of PPA amplifiers, but with the addition of a regulated 12V power supply it can also be used as a very capable standalone DAC.
Design of this device is very similar to Piccolo. However, while Piccolo was designed to be very small and is mostly assembled from SMD parts, every effort was given to use through-hole parts with larger PPA daughterboard. Unfortunately, today's electronic industry is almost exclusively commited to machine assembly, therefore shifting away from through-hole components wherever possible, leaving us no choice when it comes to some of them, particularly digital chips and regulators.
Ally uses the brand new PCM1793 which is a relatively low power yet high performance DAC from TI / Burr-Brown. It also uses innovative differential configuration of passive 3rd order Bessel analog filter, therefore eliminating one of the main causes of inferior sound (according to some sources). Operational amplifier is used here for summation of differential signal and to buffer the line out cable from the DAC chip. Thanks to fully differential configuration it is possible to use DC coupling and the concern about offset voltage is minimized, and so is distortion. There are no capacitors in signal path. Not only that, but the opamp is not driving a capacitive load either (except whatever is connected to the output, which is isolated by a 100 Ohm resistor). Capacitors used in analog section are all polypropylene (not even metallized) while resistors are classic Vishay-Dale military grade. Dual operational amplifier is socketed so it can be exchanged for another if desired. Standard issue is LM6172 which works great in this application due to its high performance when used as differential amp. Its sound is natural (i.e. neutral across the spectrum), detailed, with extended and smooth highs, especially with class A biasing, which is done here as well, using SMD cascode FETs. You can also use AD8620 or a pair of OPA627 with a Brown Dog adapter, as some people believe that the sound improves considerably (soundstage gets wider for example and there is more detail, but you may lose some neutrality in my opinion). Using FETs or casocded FETs to bias operational amplifiers into class A was brought into our DIY community' attention by Philip LaRocco, and it can improve sound of an opamp, ranging from minor to considerable, depending on the type of opamp. Operational amplifier is decoupled by the highest performance electrolytic capacitors available - Black Gate N series with red sleeve. Nichicon Fine Gold audiophile grade capacitors are also used as well as PPS film capacitors in other places.
Some of the features of this daughterboard are:
- LM6172 operational amplifier for summing (socketed, exchangeable)
- LM6171 virtual ground driver
- Two voltage regulators, for digital section
- FET rail isolators PPA style for summing opamp
- Black Gate and Nichicon Fine Gold capacitors for the analog section bypass, Panasonic SMD polymer capacitors and Panasonic FC capacitors for the rest
- JFET cascode current sources on the summing opamp
- Full size optical and coaxial digital input, with switch
- On-board switch for analog/digital input selection
- AK4117 ultralow power digital receiver
- PCM1793 Burr-Brown/TI DAC chip (multisegment sigma-delta)
- 3rd order 50kHz passive analog filter
- No capacitors in signal path!
- No capacitors in opamp feedback path!
Schematics is available below (please note: personal, non-commercial use only).
PCBs are available as fully assembled, bare or with SSOP-28 chips already soldered. However, please don't be fooled by the fact that this device was optimized for DIY - as far as DIY goes, this is an advanced project! Engineering still took precedence over the ease of build so there are still several SMD parts to solder (not counting the 2 SSOP-28 chips that require very steady hands and top soldering skills), many parts are tightly grouped together and the total number of parts is high enough that it takes me several hours to assemble this unit (weigh that with the fact that I designed the unit and have 10 years of soldering experience). Also, debugging a DAC in the case it doesn't work REQUIRES an oscilloscope and intimate familiarity with digital electronics.
Parts list is available here.
Component layout for the top layer:here.
Component layout for the bottom layer:here.
