Astell&Kern 120 (RWAK B)

The goal of the RWAK120-S (and B - read lower in the post)  modification is to transform the stock AK120 into the ultimate digital audio player (DAP) used exclusively to send a line level output signal to your portable headphone amplifier.

We achieve this by:

• Replacing the two stock Wolfson WM8740 dac chips with the premium WM8741 chips


• Configuring the WM8741s to run in “hardware mode” – eliminating the digital volume control processing


• Configuring the WM8741s to use their “minimal phase digital filter” option


• Hardwiring the stereo line-out signal to the 3.5mm (1/8”) stereo output jack (formerly the headphone output jack), hence completely bypassing the headphone output stage


• Removing the unused digital volume control wheel and installing a higher quality 3.5mm (1/8") stereo line-out jack that is hard-wired to the line-out signal.


• Installing 1000uF super-low ESR capacitor across the battery rail for increased dynamics and noise filtering

Digital Volume Control and Minimal Phase Digital Filter with the WM8741

The stock AK120 uses two Wolfson WM8740 dacs configured in “software mode.” This means that the AK120 software configures the dac sand allows for a digital volume control of the unit. We discovered that even with the volume set to MAX (setting 75), the output voltage of the WM8740 signal is still attenuated in the digital domain.

The premium WM8741 is not a drop-in replacement for the WM8740 in the AK120. This is because the software control of the WM8740 does not control the WM8741 in the same way. We therefore have to hardwire the WM8741 chips (pulling up certain pins to logic HIGH, and pulling down certain pins to logic LOW) to configure them to run in “hardware mode.” This means they are NOT controlled by software. The digital volume control is eliminated and does not alter the digital bit stream entering the dac, and we manually configure the WM8741 chip to use their optional minimal phase digital filter. The result is a more natural, organic sound that is lively, finely-nuanced, open and transparent, and extended – what you should expect from a true reference quality source component!

More impressive than its superior technical specifications (e.g. 8db greater SNR and Dynamic Range) compared to the WM8740, the WM8741 can be configured to use a “minimal phase digital filter” for the data prior to d/a conversion. Unlike most d/a converter chips that use a standard linear phase digital filter, the WM8741’s selectable minimal phase digital filter has no “pre-ringing” of its impulse response. Pre-ringing is an unnatural side effect of using a linear phase digital filter. Real instruments do not ring before they are excited – a guitar doesn’t produce sound before a string is plucked, a piano doesn’t not produce a sound before a key is pressed, and the impulse response of a minimal phase filter does not have prea-ringing and therefore produces a more natural sound.

Line out

The RWAK120-S modification also hardwires the analog line-output stage following the WM8741 directly to the headphone output jack.  Therefore, the headphone output stage is completely bypassed and the headphone output jack is converted to a true, 2Vrms line-output.

Hardware volume control removal

Since the digital volume control functionality is completely disabled, we also remove the volume wheel that exits from the left side of the AK100, and we install a new, higher quality 3.5mm (1/8") stereo jack in its place.  

RWAK120-B

The RWAK120-B modification is identical to the RWAK120-S, however we install a 3.5mm TRRS jack (4-pin) where the unused volume control was located, and hard-wire this jack directly to the balanced analog output sections of the WM8741 dac chips.  This provides a true balanced output signal to feed the balanced input of external headphone amplifiers that offer this feature (e.g. ALO RX3, RSA SR71B).  

The headphone output jack is still converted to a single-ended line out, just like the RWAK120-S modification.  So you obtain the modded SE ane BAL outputs with the RWAK120-B modification, making it the most versatile AK120 modification!