DIY Soundcard Measurement Interface (16.06.2010)




Once upon a time a genius called Pete Millet decided to build a soundcard interface, in order to protect the (expensive)soundcard on the one hand, and to provide buffered signals as well as enable measurements with floating ground.
By using only the best components this application delivers excellent results. Distortion and noise figures are very low and on top he added a small display for showing the actual signal level.

Thanks Pete! Very nice job!

My problem: whenever devices like this are designed in the U.S. ore elsewhere, it is hard to find the exact parts in Europe. Therefore a re-design was necessary using parts easily available in Europe. Also I tried to make the PCB smaller and more versatile in terms of how to use it.

Here is the result:





The two part PCB comes in standard 160x100mm Format and can easily be split into two pieces simply by bending it. By doing that you can decide if you would like to use the analog part only, or both the analog and display part.
By using a flexible ribbon cable the user can decide, where to place the display and rotary switch on the front panel of the case.


Regarding component selection: I sticked close to the original design but where possible, I used the best components available: Burr Brown amplifiers (OPA2134 and INA134) as well as BB symmetrical line drivers (DRV134)do not leave a lot of room for improvements. This is top-class.
Also the power supply was copied from Petes design: using a high quality DC-DC converter, which delivers symmetrical +/- 15V out of a single 5V source, eliminates any chance of ground loop problems.

Below you can see the backside. This is the interface to the Soundcard in and out. We used high quality Neutrik connectors, which both accept male XLR and 6,3mm stereo jacks. Input and output are symmetrical ones giving full flexibility for connecting any kind of signal source.
Right of the two Neutrik connectors is the place for an USB B socket (Not yet mounted on the picture). A powered USB hub should be capable of providing the necessary power. However: the used DC/DC converter is a bit hungry in terms of current consumption. When connected to the power source it draws a bit more than 1 Ampere for a few ms. During operation the current consumption out of a 5V source is approx. 400mA.





The front panel offers all swtches and additional input / output.
From left to right: Main Power Switch, Front Input, Front- / Back Input Selector, Front Input Ground / Float Switch, Symmetrical buffered Soundcard Output, Buffered Output Ground / Float Switch





Here you can download a picture with the complete schema.
Here you can download a picture with the PCB layout

You already might got the message: this design without compromises has a downside... it is not cheap!

Here is the Bill of Materials (BOM). I ended up at 90 Euros for all parts. This does not include the PCB (50 Euros) and a suitable metal case (necessary for shielding against interference / hum).

I ordered a small number of PCBs. In case your are interested purchasing one, please drop me a note! I will complete all info on how to build the interface within the next days.