Piccolo Portable DAC
Figure 1 - Piccolo Portable DAC
While Sostenuto-1 portable DAC / headphone amp was created in response to a desire to have a portable DAC, some people expressed interest in an even smaller and cheaper portable DAC at the expense of headphone amplifier section. As I was designing PPA DAC Daughterboard, I realized that I could shrink the size of that circuit to about a third and fit it into smallest Hammond 1455 series enclosure- so I decided to do it. Dubbed Piccolo, it is in most regards the same design as Ally and carries the same sound quality. So if one were looking for a standalone DAC with Ally sound, Piccolo would be the thing to get. It was designed to be as small and cheap as practically possible without compromising the most important feature - sound quality. It has both optical (mini Toslink) and coaxial (via RCA to 3.5mm adapter) digital inputs, using the same jack. Piccolo can operate either on batteries (using an external 10AA battery pack, which is available (sans batteries)) or using a standard 12V wallwart (AC/DC adapter) - or basically whatever else you want to plug into its 2.1mm DC jack as long as it's DC (doesn't have to be clean) and can provide necessary current (100mA),
Figure 2 - Real panel of Piccolo
Piccolo is a small and light portable DAC that is to the point. Size reduction in comparison to Ally has been achieved by extensive use of SMD parts so even though sporting an extra power regulation stage it fits the smallest standard 1455 series Hammond enclosure, the 1455C801, which has the size of 80 by 54 by 23 mm (3.15" X 2.125 X.91") and is only about one third of the size of the Ally.
Piccolo draws only 45mA of current (at 12V supply). It uses the brand new PCM1793 which is a relatively low power yet high performance DAC from TI / Burr-Brown. Digital receiver chip used here is the ultralow power one Asahi-Kasei, the AK4117. 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. Capacitors used in analog section are 2% PPS film SMD capacitors while resistors are 0.5% - all same as before.
This time around the (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 - for example it has very high CMRR that stays flat in most of the audio range. The sound is natural, detailed, with extended and smooth highs. It is additionally improved by using class A biasing on the opamp, with SMD cascode FETs. Using FETs or casocded FETs to bias operational amplifiers into class A was brought into our audiophile community' attention by Philip LaRocco. Operational amplifier is decoupled by Nichicon Fine Gold audiophile grade capacitors as well as precision PPS film capacitors.
Unlike Sostenuto-1 it uses AK4117, which is a very low power digital receiver chip and while it doesn’t quite match the low jitter performance of DIR1703E, it does not suffer from source compatibility problems the latter does and it requires far less power to operate (7mA as opposed to 29mA). Some of this saved current is applied to power the more modern DAC chip that Piccolo uses. This unit also uses new Sharp ultralow power digital optical receiver.
Piccolo has total of 4 power supplies, first one allowing for +/- 5.5V supply for the opamp (with the buffered virtual ground driver featuring brand new very high performance current feedback chip), a 5V regulator for the DAC chip, and 3.0V / 3.3V regulators for the optical receiver and digital section. All regulators are modern high performace ultralow noise integrated types. In fact, regulators in the digital section are very fast types with PSRR that holds up well into the MHz region. Bypasses are PPS film where they might influence the sound quality, and X7R ceramics for the pure digital paths.
Figure 4 - Insides of Piccolo
There are of course other major differences between Sostenuto-1 and Piccolo – on the later, coaxial input is done via an adapter instead of using a separate RCA jack (due to size considerations) and it’s a DAC without any amplification so there is no volume control and no analog input. This DAC does have both digital (on chip) and analog filtering, therefore yielding much better measured performance than the older unit which doesn’t contain a digital filter, although in some audiophile circles digital filtering is considered undesirable (some of the most expensive DACs use non-oversampling non-digital-filtering approach). Deciding the merits of this approach are best left with the listener, but it stands to reason that the digital filter can influence the sound, therefore making its quality an issue. PCM1793 doesn’t have the hottest filter around as it isn’t top of the line chip, but then this isn’t top of the line DAC either and the power consumption is certainly one of the design concerns and so this chip was selected. This unit has also much better channel separation than Sostenuto-1, because it doesn’t have analog potentiometer as volume control.
Figure 4 - Closeup of front panel