Mobile DAC: AKM and ESS
First off, both AKM and ESS offer outstanding performance and Sound Quality.
The Sound Signatures slightly differ from each other, but it's a matter of preference. Some audiophiles say ESS DAC demonstrates a little more similar sound signature close to R2R DAC, but it depends on the system design, DAC filter option, and other design choices.
Here's the excellent video link. It helps you understand what ESS wants to deliver.
Again, both AKM and ESS are the major players in the market and deliver industry-leading Sound Quality. In this posting, we'd like to talk about why ESS is less popular than AKM, especially in mobile and portable applications.
DAC(D/A Converter) produces weak analog signals, and eventually, we need an OP-AMP(Operational Amplifier) to make those signals stronger and powerful enough to drive earphone/headphone load. HPAMP is a kind of OP-AMP designed to drive earphones/headphones, with low noise figure and moderate output power.
It used to be off-chip and discrete OP-AMP IC, but IC makers started to integrate both DAC and OPAMP (HPAMP) into a single IC package. The mobile DAC ICs, designed for portable devices, came to have the built-in HPAMP in a single IC package. It helps set makers save PCB space, power consumption, and cost all at once. However, as a result, one single IC became to define both Sound Quality and Signature.
Other than the Sound Signature of each DAC, the main difference exists in the output power and voltage as below:
Eventually, AKM works with a single 1.8V power supply, while ESS also needs an additional 3.3V supply as well as 1.8V.
Regulation for limiting the sound output level
In short, smartphones and personal listening devices can't exceed a certain level of sound pressure level by law. Any smartphone with the packaged bundle earphone MUST follow the guideline and MUST limit its HPAMP output power, keeping the SPL(Sound Pressure Level) through the bundled headset not to exceed the safety guideline. Consequently, most smartphones need to provide the output voltage smaller than 1VRMS, unless otherwise bundled with high impedance low sensitivity headphones.
*Note that Portable DAP, DAC/AMP, they don't need to follow the guideline and law, because they don't include any earphone/headphone in the package.
Smartphones and Bluetooth earphones/headphones, they don't need the extra 2VRMS output from ESS DAC. Those applications can't utilize the benefit of the higher output power while requiring more battery time and BOM cost.
LG V Series
LG V series is an excellent Hi-Fi device, demonstrating the benefits of ES9218p. However, they need to detect the connected load impedance and switch the output level to 1VRMS or 2VRMS depending on the load impedance. For example, if connected with the bundled earphone, the device checks the impedance and set/limit the maximum output level of ES9218p. The only way to switch LG V ES9218p to the 2VRMS mode is by connecting the higher impedance headphone. They need to follow the safety guideline and limit the output power as long as bundled with high sensitivity low impedance earphones.
For this reason, some users put the additional resistance between the audio jack and IEM to enable the high gain mode. But it's not a good idea, and most of the signal power goes to the resistor, just making the heat and thermal noise. It would sound differ anyway, but it's like using a custom earphone cable with 50~100 ohm impedance.
Design & Implementation
In an application design viewpoint, AKM mobile DAC/AMPs are very easy to handle. They usually perform almost the best, as noted in the specification, even at some poor PCB layout and passive components. And they consume less power.
Meanwhile, ESS mobile DAC/AMPs are more sensitive to HW design and implementation. Unfortunately, there seem to be many application products that don't demonstrate the true nature and performance of ESS DAC/AMPs. This might be the main reason why there are so many reviews with different thoughts and feelings on ESS applications.
Again, we'd like to mention both AKM and ESS are the best option for hi-fi applications. They have their unique and specific specialties for their target. They might sound slightly different, but it doesn't mean which one is better. If some device doesn't perform an expected level of sound quality, it's a matter of system design and implementation, not the DAC.
We hope audiophiles enjoy both AKM and ESS without any prejudice on DAC itself.