Sneak Peek: The QA402

Post by Matt

In late October, a massive 82 hour fire destroyed AKM's ability to deliver their flagship ADC and DAC products to customers, and this included the AK4490 DAC and AK5397 ADC used in the QA401

This has put enormous pressure on countless companies (including QuantAsylum) that relied on AKM parts. And frankly, given AKM's size, it's surprising they didn't have an alternate fab that was geographically diverse to help insulate them from a severe earthquake (or fire). But alas, here we are.

A follow-on to the QA401--the QA402--has been in the works for a long time and is mostly complete, but the AKM fire has prompted a pivot to new converters. The ADC in the QA402 has been replaced with an PCM4220 from TI and the DAC has been replaced with the PCM1794A

The purpose of this post is to share some of the details and early measurements on the QA402. And if you've been wanting to buy a QA401, this post will hopefully give you some indication that what is coming is worth waiting for. 

The Features

The QA402 offers a lot of improvements over the QA401 (all are subject to change, of course). Let's look at a few below.

Lots of input relays. The QA401 had a single stage attenuator of 20 dB, with a max input of +26 dBV (the two input ranges were +6 and +26 dBV max). The QA402 offers two additional relays, ensuring the ADC can be fed at its optimal point over a much larger dynamic range. This manifests on the UI as with the following gain range options as shown below: 7 steps 6 dB apart with the max input being + 36 dBV (the max input on the QA401 was +26 dBV). 

All CMOS opamps. The QA401 used a very low-noise bipolar front-end (OPA1612). When your source impedance was below 1K, the performance could not be beat. But at higher source impedances, the noise increased quickly--that is the nature of bipolar. But since the QA401's introduction in 2015, the OPA2156 and its audio cousin the OPA1656 arrived, and they have reset everyone's opinion of CMOS and low noise. This new breed of CMOS opamp is simply phenomenal from a noise and distortion standpoint (though still not as good as bipolar on noise when source impedance is below 1k). And it means there's no longer a painful noise penalty for those times when your source impedance rises above 1k. 

Lots of output relays. The QA401 didn't have any output relays. The QA402 has two, offering 0, 10, 20 or 30 dB of output attenuation. The max output of the QA402 is 8Vrms = +18 dBV (single ended) and 16Vrms = +24 dBV (differential). With some higher power amps, the +6 dBV drive of the QA401 was insufficient to let the amp hit max power. The QA402 won't have any problem driving those amps.

USB Powered. The QA401 and QA402 are both USB powered. The current demands on the QA402 are steep--about 5V at 800 mA. But it's no problem for a USB hub to provide. But this means you will most certainly need a powered hub to run the QA402 (or a USB Y-cable if you really want to use your laptop without a hub). Luckily, hubs are cheap and since all our instruments are USB powered (except the QA460), you'll probably want a hub anyway. 

Wider Internal Rails. The QA401 uses +/-6V internally for the audio input and output voltage rails. On the QA402, that increases to +/-13V. 

Advanced Power Reporting. Because the power demands increased, you will need some advanced diagnostics to ensure your QA402 is running right. The QA402 has the ability to report the USB voltage inside the QA402 as well as the total current AND the current spent on the split rails generation. In the picture below, you can see the QA402 is reporting the USB voltage is at 4.8V, the USB current is 801 mA total, and the current feeding the rail generation DCDC converter is 566 mA (meaning the non-rails current is 801 - 506). These will turn red when a problem is present. For example, a skinny USB cable will have too much voltage drop and you'll see the USB voltage reported as low. Or, if you are driving into a low-impedance load with a high output level, the ISO current will increase (and can hopefully detect and protect the QA402's output stage as accidental shorts at +18 dBV output will be a problem). In short, the pieces are in place to ensure you have the tools to manage the power on the QA402 and verify the QA402 is functioning as designed.

Improved Noise Floor: The QA401 delivered a noise-floor around -115 dBV (20 kHz, no weighting). The QA402 should achieve slightly better. And it's always easy to use an external low-noise amp for those times you need to measure even lower. For example, the QA471's low-noise 30 dB amp let the QA401 measure noise floors down to about -125 dBV (20 kHz, no weighting).

Connectors. The front panel of the QA402 has L/R input XLR (female) and L/R output XLR (male), and also a pair of BNC for single-ended outputs. There's also a digital connector with I2S signals present, so that you can drive some of the newer digital class D power amp ICs directly. That isn't working yet but the path to make it work seems clear.

A picture of the current hardware (REVH PCB) with TI silicon is shown below:


And the performance? Well, the board above with the TI converters has been in-house for 24 hours, and the ADC (driven by the QA480 at +6 dBV) appears as follows. 

In single-ended loopback, at -25 dBV out, you can see the DAC + ADC together. The -25 dBV level shows the benefit of the DAC attenuator (0, 10, 20 or 30 dB atten). 

In the plot below, you can see ADC sweeps (made with the QA480) showing two different input ranges: +6 dBV max, and +0 dBV max. This gives you an idea of how all the different gain ranges combine. When your input level is around -20 dBV, you'd want to be in the 0 dBV input range. And when your input level is around -14 dBV, you'd want to be in the +6 dBV range. At each point, you keep the distortion optimized, allowing you to deliver very solid distortion numbers across a huge dynamic range. Something a sound card can never do, even when using a state of the art ADC and DAC. And yes, a simple autoranging button will be provided that will allow you optimize for THD or THD+N.

The 2.0 Software

The measurements above don't have all the normal QA401 markers because the software is being re-written. The REST engine from the QA401H was pulled in, which offered a good excuse for a lot of cleanup. This will give a super-clean SW interface for external measurements via REST, making it simple to measure THD from a Python or even Matlab, for example. And if you want headless, then you can "disappear" the GUI and run headless, just as you do with the QA401H today. 

And the other reason for the re-write? Here's some very early testing of the QA402 hardware on Ubuntu. Yes, cross platform is coming.

Risks and Schedule

As mentioned in the beginning, this is a very early look at the QA402. The product isn't without risk, but this is all normal at this stage of development. For example, the DAC output levels might be a concern (too large) and have to be dialed back. The I2S might have be abandoned and saved for a second product. The power consumption might end up being too steep. The drive levels might be weak at higher output levels (that is, only high-Z loads can be driven at high output levels). The TI ADC has  noise bump at 192Ksps that might be more objectionable than anticipated. THD is where we want, but THD+N isn't quite yet there yet.

In short, right now the expectation is shipping in March 2021 on the new 2.0 cross-platform software (the QA401 will move to that software too and will pick up the cross platform benefits). And the price of the QA402 will be very close to the QA401.

In a perfect world, I think the QA402 with AKM converters would have slightly better performance, but the number of relays on the QA402 means the QA402 performance will be quite a bit better across a larger range than the QA401. Could some more esoteric converters have been used? For sure. But longevity and supply reliability are very important. And TI are very good in those areas.

And finally....Remember, the goal with the QA401 (and QA402) isn't the best performance in the world. That space is well covered if you have the funds. Instead, the goal is to create something that is as trusty as your DVM for audio (meaning no calibration needed) and that has enough performance and margin to reliably measure 99% of the audio products out there today. I think the QA402 will continue to serve that segment very well.

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