PocketADC + D10s + REW for Analog Audio Measurements

[lukewarm58]

I have been "chasing zeroes" lately trying to improve my setup for measuring analog audio circuits. I'm not quite ready to commit to a QuantAsylum QA403, let alone anything from Audio Precision, so I have been trying to cobble together a system that outperforms the USB audio interfaces I have on hand. I recently acquired a MiniDSP PocketADC and a Topping D10s DAC after becoming aware of both through ASR.

Rather than bore you with a flowery backstory, here are three measurements I made to compare my two USB audio interfaces (a Roland Rubix22 and Solid State Logic SSL2) to the PocketADC. I used a 1kHz low-distortion oscillator (details here, commonly known as "Viktor's Oscillator") and set the amplitude feeding the analog frontends to 0dBu (measured using an oscilloscope). For the Rubix22 and the SSL2 I adjusted their respective input gains such that Room EQ Wizard reported an input signal of -10dBFS.
- The PocketADC doesn't have a way to adjust the input gain (REW reported an input level of -8.6dBFS for the 0dBu input signal)
- The oscillator signal is unbalanced and was fed into unbalanced line inputs
- I used the Toslink output on the PocketADC and fed it into a HiFiMe UR27 to convert to USB audio
- The PocketADC was running at 96kHz (192kHz was unstable over Toslink with the UR27)

Measurement 1/3 - Roland Rubix22

Measurement 2/3 - Solid State Logic SSL2

Measurement 3/3 - MiniDSP PocketADC

Here are a couple of overlays that (I think) show just how impressive the PocketADC is:

I will do a follow-up post to show my results when I replaced the low-distortion 1kHz oscillator with a Topping D10s DAC.

Before ending this post (my first), I just want to express a huge thank you to all the members of this forum. I have thoroughly enjoyed digging through so many threads here.

 

[lukewarm58]

Once I confirmed, using the analog 1kHz oscillator, that the PocketADC outperforms my existing USB audio interfaces it was time to test the PocketADC with a Topping D10s DAC. I have had noise issues in the past when using USB devices to test analog circuits so I decided to test the D10s with and without a USB isolator. I used a relatively inexpensive USB isolator from AliExpress, though it is rated for High-Speed USB (many are only rated for Low and Full Speeds, buyer beware). If anyone is interested in the exact isolator, just let me know.

In an effort to test apples to apples, I tried to adjust the output of the D10s DAC to match the 0dBu level of the analog oscillator. I clearly failed given that 0 dBu corresponded to -8.6 dBFS in the measurement from the previous post whereas the measurements below record an input of --8.9dBFS. I hope you can forgive me :)

Measurement #1 - Topping D10s going into PocketADC (No USB Isolation)

Measurement #2 - Topping D10s going into PocketADC (With USB Isolation)

Clearly the USB isolator helps a little bit in reducing 60Hz noise and harmonics thereof.
As a reminded, with the analog oscillator I measured a SNR of 104.7dB and the 2nd harmonic was at 0.000027%.

For good measure, here is the plot of Distortion vs Frequency:

If I have time later this week I will try to do some measurements where I feed the DAC outputs of the Rubix22 and the SSL2 into the PocketADC.

Overall, I am super pleased with the performance of my cobbled together "audio interface" consisting of the Topping D10s and the MiniDSP PocketADC (plus a USB isolator and a HiFiMe UR27 optical to USB converter). What's also nice about the PocketADC, in addition to its impressive SNR and THD, is that is has an input impedance of 100kΩ which is very useable for the circuits I typically measure.

 

[morillon]

PocketADC

The miniDSP Pocket ADC is an extremely compact high-resolution A/D (analog-to-digital) convertor that accepts a stereo analog input on single-ended (RCA) connectors and converts it to SPDIF coaxial and TOSLINK optical digital outputs. This pocket size converter supports high sample rate and...

www.minidsp.com

(Mesureaments with AP555, and here the most important...his generator etc)
;-)
(Ps...VeryStandart level inputs 2vrms asym ;-) )

 

[john61ct]

Yes I wish there were 6-12 channel versions of both PocketADC and HiFiMe UR27

that cost not much more than buying pairs individually.

My AudioFire-12's at $120 are top notch but a PC is required - apparently a Raspberry Pi solution is "coming soon" we'll see

 

[morillon]

Yes I wish there were 6-12 channel versions of both PocketADC and HiFiMe UR27

that cost not much more than buying pairs individually.

My AudioFire-12's at $120 are top notch but a PC is required - apparently a Raspberry Pi solution is "coming soon" we'll see

In a multi-channel interface, we're in pro mode and therefore mostly balanced... so not really the point...
(but we can imagine that in the future, professional ADCs will incorporate these latest ADC chips ess etc, like the one in the Adept, if there are no unforeseen drawbacks)
(On the hardware side, perhaps a future rme ucx "3", which is a few years old, could benefit from it, for example..However, in practice, tried and tested solutions with a track record are often preferred.)

 

[john61ct]

Sorry, but do you mean my AF12's?

all 24 ports are balanced/unbalanced TRS 1/4", both input and output individually switchable +4dBu/-10dBV input sensitivity / output level

If you mean the PocketADC, yes I wish it were the same, or that miniDSP would release a balanced version like Topping does for D10S.

If you meant something else I didn't grok it

 

[morillon]

MiniDSP simply created two devices to add analog inputs to their machines.



As for the rest, it's up to other manufacturers to perhaps take advantage of these new ADC chips, etc...in pro or consumers material...
MiniDSP has demonstrated its potential here, even at a reasonable cost.

 

[lukewarm58]

I took a couple of quick measurements where I used the DACs of my two audio interfaces and fed them into the PocketADC. No surprise, the Topping D10s outperforms both audio interfaces.
- both audio interfaces were isolated from my PC using a USB isolator (power to the isolator was provided by a USB power bank)
- the SSL2's outputs are balanced (with both signal lines actively driven), to convert to an unbalanced signal I used a custom 1/4" cable that goes from TRS to TS (the ring signal is unconnected/floating)

Measurement #1 - Rubix22 feeding a ~0dBu signal into the PocketADC

Measurement #2 - SSL2 feeding a ~0dBu signal into the PocketADC

I couldn't help myself and I ordered an SMSL PS200 Pro yesterday as I am curious to see how it compares to the Topping D10s. And yes, at some point I will stop chasing zeroes :)

In the meantime, does anybody have any suggestions on how I can reliably get 192kHz data from the PocketADC into my PC?

 

[john61ct]

Use a high-quality USB cable

confirm specific ASIO driver installed

disable System Audio

set resolution in Playback/Recording Devices

 

[lukewarm58]

Erratum
I was redoing some measurements today and realized my original plots for the PocketADC weren't scaled properly (in REW), thus giving the impression of a lower noise floor. Please ignore plots 3, 4 and 5 in my original post. Here are the corrected plots:

Analog Oscillator connected to input of PocketADC

Comparison of Rubix22 and PocketADC (with the same analog oscillator input signal)

Comparison of SSL2 and PocketADC (with the same analog oscillator input signal)

And out of curiosity I also took a measurement where I increased the level of the input sine wave such that REW reported an input signal of -8.9 dBFS (to compare it to the erroneous measurement in my opening post):

Fingers-crossed I didn't walk into another unforced error with these measurements :)

 

[morillon]

You need to understand where this level issue comes from (which skews your measurements intended to be compared to 0 dBFS) because the PocketADC does indeed have a very standard 2v rms asymmetric input (just verified for you with mesureaments)
;-)
(The measurements provided by minidsp forum this pocketadc on AP555 (those from asr never appeared) on their websites were done rigorously and without marketing gimmicks, it seems.... You can refer to it...)

It was not produced for measurement purposes...but it will allow it to some extent if there is a sufficient margin on its end relative to the source...otherwise, of course, it will be useless...
;-)