For those interested, I've seen the insides of the M2 and M4. All the specs below are identical in both the M2 and M4. The ADC in the M4 is the same chip but supports 4 preamps instead of 2.
M2 & M4 chips:
- ADC - AK5552VN (AK5554VN in M4)
- DAC - ES9016S
- Integrated Circuit - XMOS U11680C20
- Headphone Amp - OPA1688 (50mW @ 32Ω, 3.3Vrms)
- Mic Preamp: THAT 6263
Very interesting.
The current top AKM is AK5572EN / AK5574EN / AK5576EN / AK5578EN
That has THD+N -112 db vs -106 db on Motu M2/M4 and 121 S/N vs 115 on Motu M2/M4
https://www.akm.com/eu/en/products/audio/audio-adc/
The headphone amp is from early 2016 / December 2015, quite good - can't find better in other buget audio interfaces, but needs low ohm headphones like AKG K371 because of USB power only.
The OPA1688 and OPA1689 are a family of SoundPlus™ 36-V, single-supply, low-noise operational amplifiers capable of operating on supplies ranging from 4.5 V (±2.25 V) to 36 V (±18 V). This latest addition of high-voltage audio operational amplifiers, in conjunction with the OPA16xx devices provide a family of bandwidth, noise, and power options to meet the needs of a wide variety of applications. The OPA168x are available in micropackages, and offer low offset, drift, and quiescent current. These devices also offer wide bandwidth, fast slew rate, and high output current drive capability. The dual and quad versions all have identical specifications for maximum design flexibility.
Unlike most op amps that are specified at only one supply voltage, the OPA168x family is specified from 4.5 V to 36 V. Input signals beyond the supply rails do not cause phase reversal. The input can operate 100 mV below the negative rail and within 2 V of the top rail during normal operation. Note that these devices can operate with full rail-to-rail input 100 mV beyond the top rail, but with reduced performance within 2 V of the top rail.
The OPA168x series of op amps are specified from –40°C to 85°C.
Mic Preamp: THAT 6263
http://www.thatcorp.com/626x_Dual_Programmable_Preamp_ICs.shtml
Also good.