The QA401 is now available and it has differential inputs and complete isolation from the host PC. Starting around here http://www.diyaudio.com/forums/equipment-tools/231401-quantasylum-qa400-186.html#post4657822 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.
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.
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