A Closer Look at Sony PHA-2 (Part 1)



Here, I am going to test Sony PHA-2. It is a good sounding portable headphone amplifier to let you enjoy your music anywhere. With various inputs, including PC, Sony Walkman, Apple family, and analogue input.

PHA-2 could be simply divided into digital domain and analog domain, their ground planes were separated on the PCB in order to minimize the noise coming out from the digital circuit. The most amazing feature is that, it supports DSD format. In fact, quite a few portable amps can handle DSD file on the market. However, it still not so useful for portable use as DSD format is not so common due to  file size and not easy to extract from a SACD.



Audio Signal Flow


The analog audio signal will flow in this way, it first go to the line-in buffer, and then passing through the volume control. Finally, the "amplified" signal will then come out from the power amp section. Similar way, just swap the line-in buffer for the digital part. The digital audio stream will then be decoded by the DAC and passing to the amplifier stage.


Digital to Analog Converter

PCM1795 is a DAC has 32-Bit resolution and DSD decoding capability. As it is a current output DAC, a pair of IV converters are required to convert the current to voltage then followed by a differential-to-single converter. Instead of using the recommended op-amp NE5534 for the IV converters, Sony select OPA1662 with higher gain bandwidth and faster slew rate for improving the audio dynamic performance of the IV section. For the differential section, The LME49860 was chose for higher fidelity.



Volume Control

Pot volume control is common in the market. It is not a bad thing, proper design could obtain a linear volume gain. In this test, I find the volume control is smooth but the gain is not quite linear. Please refer to the figure “The Relation of Volume Gain and Volume Control Position” for more info. It is very sensitive when the volume pot is running in the first quarter. You may find 1 hour (9-10 o'clock) equal to 32dB gain. A worst thing was found at around 8-9 o'clock, the left and right channel is not balance. Especially, when pair it up with small IEMs.



Power Amplifier

TPA6120 was used this PHA-2, a more than a decade old chip. No matter how old is it, this current feedback amplifier is a good sound amplifier. According to TI, TPA6120 has following pros:
  1. High slew rate that prevents odd order distortion anomalies.
  2. Current-on-demand at the output that enables the amplifier to respond quickly and linearly when necessary without risk of output distortion. When large amounts of output power are suddenly needed, the amplifier can respond extremely quickly without raising the noise floor of the system and degrading the signal-to-noise. 
  3. The gain-independent frequency response that allows the full bandwidth of the amplifier to be used over a wide range of gain settings. The excess loop gain does not deteriorate at a rate of 20 dB/decade.



Power Supplier

As PHA-2 is running with a 3.7V lithium battery. In order to turn the chips on, PHA-2 step up the voltage from 3.7V to +-5V. However, this method will introduce switching noise to the circuit. By properly adding passive filtering component can suppress the noise but never reach the same level as linear power supplier. Theoretically, the voltage output swing of PHA-2 could be 10Vp-p, i.e. 3.5Vrms. So, it is pretty enough for driving a pair of Sennheiser HD650 at 112dB. (You can refer to Headphone Amp Power Calculator to find out how much amplifier power is enough)


To be continue…

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