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1audio 5th March 2013 04:58 AM

QuantAsylum QA400 and QA401
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The QA401 is now available and it has differential inputs and complete isolation from the host PC. Starting around here is discussion of the QA401 and some measurements. With the new version my efforts on a balanced out and differential in become less important but they still may help someone.

Several of us have been playing with these for a few months now. They are an inexpensive "sound card" USB capture device that has been optimized for audio measurement. More here: QA400 Audio Analyzer

The performance is quite good. More than adequate for any normal audio measurement need and the software works well, avoiding the usual problems with the Windows sound engines. It provides many good measurements as is and hopefully will get extended by others for specific tasks.

The one problem is that the I/O is single ended and limited to around 1V. This is fine for mobile audio stuff like iPods but not enough to work well with more conventional audio with 2V+ levels and amps good for much more.

In my prodigious free time (as in none) I worked out a design for an interface card to match the QA400 to real world audio devices. Its similar to Pete Millet's interface Soundcard Interface but not as refined or pretty. It also is two channel and doesn't have the metering, since the QA400 is quite accurate.

For the I/O I realized that making interface cables with appropriate connectors would resolve several issues. I have something like 5 pounds of adapters to go from anything to anything, however most end up as part of a link from one connection type to another. Usually for me its a dual BNC to Banana adapters to single BNC to dual banana to clip leads. Pretty dumb. My idea (borrowed from the Praxis interface PRAXIS INFORMATION ) is to make cables that have the appropriate adapter as part of the cable. The I/O is 3.5MM stereo connectors. This would be a stereo 3.5mm connector used as a differential interface terminated into an RCA for line level or two bananas for spoealer level, or XLR for balanced interfaces. Then the cable can have attenuators built into it eliminating the need for overload protection required for a dual banana type input.

I decided to steal power from the USB interface and make it a pass through. I'm using a DC-DC converter to get enough voltage to get 10V RMS out. I won't know until its built if this worked so easily.

The IC's will all be socketed on mine so they are easy to replace when they get fried.

I got my assistant Tim to do most of the heavy lifting on the layout. We have checked it and have sent it out for fab. We should have PCB's and parts in a week to build and test the design.

I am attaching the schematic and layout, all in Expresspcb ExpressPCB - Free PCB layout software - Low cost circuit boards - Top quality PCB manufacturing since I feel its very appropriate to this project. Please feel free to point out the dumb mistakes I made and improve it. The software is free from them.

PLEASE DON'T ORDER ANYTHING UNTIL I KNOW IT WORKS! I will post a revised version after I have made the first ones work.

mkc 5th March 2013 05:42 AM

Hi Demian,

Thank you for opening the thread and making this interface.

I hope this thread can be a source of hints and experience on how to get
the most out of the QA400.

I would be most interested in your interface card.


31697B 5th March 2013 06:15 AM

I too am looking forward to knowing how well it works and what thd and noise it may contribute to testing a DUT. The convenience and usefulness is very much needed to make everything more like P-n-Play operation.

I am still upset with the USB not working and tired of fooling with it. I'll have the company people tell me what to do to make it work right. I have 3 computers and it doesnt work on any of them. I dont have th time nor patience for usb issues. Software should be fixed to work better. The unit itself and the help files etc are very good.


dennismiller55 5th March 2013 10:38 AM

I am looking forward to building this also. Thanks for sharing your work.

Bob Cordell 5th March 2013 10:55 AM


This looks like a very nice piece of work and a good contribution to the diy audio community.



Telstar 5th March 2013 11:42 AM

Looking forward to your results, and yes, I also agree about cables without adapters.

paba 6th March 2013 06:28 PM


between this PCB and the QA400 it would probably be best to have BNC. BNC to BNC makes sense no? Besides the unit comes with two BNC to BNC they would naturally be used for this purpose...Now on the input side of this interface jig, I guess that is more open to choice/application... RCA comes to mind for my case. (so I'll probably make that change on mine).

nice contribution.


1audio 6th March 2013 06:51 PM

I thought so at first. Then I realized that for the link to the QA400, the ground is common, the signals are low impedance and its a lot of connections and the BNC cables pull the box off the table.

For the inputs if you use an RCA and connect to an amp you will fry the chips. An RCA is also unbalanced. My choice was to make a cable with a 3.5mm minijact to RCA with a separate ground connection the chassis to get the best SNR for connection to line level signals. For an amp, making a minijack to clipleads or banana connectors with an internal voltage divider is a pretty clean way to make sure you don't fry the internals.

Also there isn't much real estate on the board for bigger connectors.

However you are free to revise the system to suit. But you should wait until I find out what doesn't work so you won't be repeating those mistakes.

paba 6th March 2013 07:04 PM


Originally Posted by 1audio (
I thought so at first. Then I realized that for the link to the QA400, the ground is common, the signals are low impedance

yes so that is a good point.

Myself, I know better then to connect audio amp straight to anything but speakers... or a proper dummy load....but good to mention for a wider audience.

I would use the QA400 for source components and most have 2V RMS coming out the output RCAs so even that I would need an interface before the QA400.


gk7 6th March 2013 07:19 PM


would you mind to post the schematic as some graphic file,
to have a look at it without installing software ?
Looks (or rather reads currently ;-) ) like a very useful project,
thanks for sharing it.

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