Flute 2 Portable DAC/Amplifier
Figure 1 - Flute 2 Portable DAC/Amplifier
Flute 2 is a successor to Flute. It better addresses the same problem as our first product Sostenuto-1 - the one that many audiophiles on the go are facing. Namely, a low power DAC and headphone amplifier combination, that can operate on batteries and is small enough to be portable.
What's extra compared to Flute 1? First, DAC has been upgraded to Crystal's top of the line CS4398 - the same one used in a lot of high end equipment (e.g. E-MU 1212m). Second, optional daughterboard can provide USB input (galvanically isolated and still using CS4398 for conversion)! Third, redesigned panels with deeper holes now hide the shaft of the knob making the unit better looking. Fourth, metal headphone jack is used which provides grounding for the chassis and reduces sensitivity to static electricity discharge that plagued the older unit. Fifth, upgraded firmware allows input switching, low battery indication, low power shut-off, inactivity shut-off, timer shut-off, choice between low power or high sampling rates, configuring the unit as mono (left or right) or mono balanced out (left or right) to allow dual mono and dual mono balanced operation. Sixth, two indicator LEDs are now bi-colour, allowing more indication possibilities.
Actually, firmware has not been fully written yet so none of the features mentioned in 5 are working yet. Also, USB daughterboard hasn't been tested yet (but it is fully designed and the same schematics was successfully used before so it should work fine).
Flute 2 has the same dimensions as Flute 1. The unit's size is therefore 4.72" X 2.125" X 0.91" (plus the knob's length). This extra space is where the new amplifier and control circuitry resides. Its panels have been redesigned as the rear/front distinction is now obvious, unlike with Piccolo. That is, the rear panel has optical/coaxial combo input, USB input and power jacks, while the front panel contains volume control, two bi-colour indicator LEDs and headphone jack.
Figure 2 - Flute 1 Portable DAC/Amplifier Insides
Unlike Sostenuto-1, this unit cannot be used for amplification of analog signals, only for digital signals. However, it still allows coaxial (through RCA to mono 3.5mm jack) and optical digital audio input, as well as USB input. The size is still small, and that is thanks to brand new TI chip - the first proper audiophile grade headphone amp in a chip, the TPA6120A. This chip is capable of driving almost any dynamic headphone, providing massive amount of current as necessary.
Flute can be used as a DAC using the same jack that is used for headphones. Unlike a typical portable, with TPA6120A being a current feedback amplifier - which are often used to drive cables - and thanks to its high performance, there is no loss in performance; it's effectively a real line out too. First set of measurements I did was made on a Flute 2 which did not have one of the DAC capacitors installed due to my oversight - after I soldered it down - almost a year after I first made it! - a massive improvement in measured intermodulation has occured, so if you've seen the old graph, you've seen nothing. Anyhow, these are Flute 2.1 measurements driving nothing but cable and E-MU 1212m ADC. You can see the improvement compared to Flute 1.
And of course Flute does a good job driving headphones. Here is a comparison (Flute 1 though!) of various loads (at maximum Flute 1 volume). As always, take all RMAA measurements with a grain of salt. Flute has single ended output while E-MU 1212m used for measurements has balanced inputs. Also, these measurements do not reflect real listening volumes, especially for low impedance headphones. If we use attenuation on Flute to get closer to real listening volume range (still much higher than it should be for low impedance phones!) then we see that IM distortion decreases. I will post measurements with different soundcard at a later time.
Figure 3 - Flute Front Panel
Quality parts are used to produce Flute. Specifically, the finest available surface-mount PPS film capacitors (2%) and tantalum film IRC resistors (0.1%) are present in the analog section. A mix of Panasonic FC, Nichicon Muse, Panasonic polymer capacitors and multilayer ceramic capacitors is used for power bypassing, as appropriate. Flute also contains SMD FET cascode class A biasing yielding massive improvement in measured performance - such as 20dB reduction in second harmonic distortion.
Furthermore, the sound quality is kept intact by using digital attenuation right on the DAC chip! That's right, 120 step by 0.5dB attenuator, built onto the CS3298 Crystal chip achieves the best possible volume control, better than even the best stepped attenuator! Attenuator works by simply multiplying each sample with a number equal to desired attenuation, before doing D to A conversion. It's a linear operation and as a result we don't have to add anything to signal path, and there is no distortion of any kind (at least as long as the operations are done in higher precision than the original signal itself). To be fair, there is one drawback, and that is the decrease in the dynamic range commensurate with the amount of attenuation. But this isn't much of a problem when using 16-bit CD quality signal on a 24-bit DAC with a high dynamic range. If 24-bit usage was the primary purpose of the unit, then this loss should be taken into account as it might not be acceptable. To eliminate this loss, if the unit is used as a DAC, the volume should be set to maximum, at which point there is no adjustments to sound and no loss of performance. The gain is set so that for for hi impedance phones you're likely to listen at between 5 and 20dB of attenuation, and for low impedance phones between 10 and 25dB, depending on the type of music and your preferences. For high impedance headphones such as Sennheiser HD580/600/650, at normal listening volume of -10dB with uncompressed 16-bit music you'll get a 3dB measured loss of dynamic range which is minimal. Here is a comparison of measurements taken with different attenuation settings - first two are with 10dB of attenuation and second two with no attenuation. The other variations reflect the volume set in the RMAA (it allows -0.5 to -4dB range) as well as different input configuration and gain of the E-MU 1212m and they can lead to up to 1dB difference in measurements. Again, this shouldn't be used as a guide because of many variables that aren't obvious, such as influence of the gain on the soundcard itself as well as different configuration setting (-10 or +4 dBu) which do influence the noise floor and consequently the dynamic range). Also, change in performance when driving headphones will overshadow smaller changes in dynamic range anyhow.
Figure 4 - Flute 2 Rear Panel
The "volume control" is actually not a potentiometer, but a digital device which is read and interpreted by the microcontroller. It also has a momentary switch (activated by holding the knob pressed down) which is used to power the unit on and off. The advantages of a microcontroller are many, mostly in added functionality and flexibility. At this time only volume control and on/off operation are operational.
Remarkably, only 67.6mA is used by this whole unit when powered on. When powered off but plugged in - which is achieved by pressing and holding the knob for 2-3 seconds until LED turns off (or on, if you're turning the unit on) - the unit uses 1.6mA (this might be improved later on with firmware changes). This should lead to about 30 hours of battery life if using modern AA batteries of 2100mAh (10 AA battery holder with appropriate jack is included). Flute has both optical (mini Toslink) and coaxial (via RCA to 3.5mm adapter) digital inputs, using the same jack (these types of jacks, used on devices such as minidisc players, are capable of accepting either optical or electrical jacks). Flute can operate either on batteries (using an external 10AA battery pack 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 (150mA).