QA401 Audio Analyzer Teardown

The overview of QA401, USB and DSP section are isolated from DA/AD.  TI ISO2401 provides signal isolation, and WE parts provides DC-DC galvanic isolation.

DAC: AK4490

Output: OPA1612 low noise and distortion OP

ADC: AK5397

XO: 24.576MHz

There is a preserved XO soder pad. CCHD-957 could be therefore an upgrade options. Supply of this XO is 3.3VDC.

Input buffer and differential amplifier provides input signal to the ADC. 

OPA1612 dual channel OP and OPA1632 differential output OP are used. 

There is barely no competitor of OPA1632 regarding distortion and noise, OPA1612 is also the best performer for its ultra low THD and noise. AD's ADA4807 has very good and close performance to OPA1612, but it maximun supply voltage is too low, not suitable in this system. 

For OPamps, QA401's supply is +-6VDC, those tiny precision OPamps with max 5V or 10V supply is thus not able to be used.

According to ASR, the performance limitation or bottleneck of QA401 is its ADC, the AK5397.

Looking at the PCB and parts around AK5397, there are some way to improve its performance, namely THD and noise. 

The datasheet of AK5394 indicate the importance of Vref bypass capacitor, which is C1112, C108 or so soldered on the left side of AK5397. The larger the capacitance, the lower the distortion.

It looks like that the manufacturer of QA401 adopted some MLCC as Vref cap, so the capacitance would not be large, maybe just 100uF.

So, one can solder some electrolytic capacitor to parallel those MLCC, maybe 1000~22200uF is fine.

Also, the XO can be upgraded to provide better performance of ADC. The performance of the tiny black XO on the right is unknown. The designer of QA401 preserves a solder pad that is connected to the original XO. 

Hence, a good XO like NDK NZ2520SDA or Cyrstek CCHD-957 can be used to improve AK5397's performance. 

We don't know how these will provide how much improvement though, because AK5397 is a good but not state-of-the-art best ADC we have.

However, given its price, about 429USD, QA401 provides excellent cost-effectiveness of a audio analyzer. You can't find any other analyzer that adopts AK4490+AK5397+software for $429USD.

Right now, QA401 is still the best option for 20~20KHz analyzing.