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Ultra Simple Sound Card Interface Box

PAGE UPDATED: 10/11/2009 , 10/21/2017
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Homebrew PSK box


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N1NKM's PSK31 / SSTV interface
Outside of box

This page contains a schematic & photos to help you build a nice computer sound card interface. It has two pots, allowing complete and independent control of the receive and transmit audio levels between the computer and your rig. The TX audio goes through a transformer to provide step-down AND very low impedence to the mic input. This helps to reduce the risk of RFI, ground-loops, or hum pickup. Even with a LOT of RF in the shack, this design has proven itself to be very effective! :)

This little box provides OUTSTANDING audio performance between the rigs & computer. It also lets the computer key the rig. It does NOT permit easy switch-over from digital to voice modes, though, because it has no provision for the microphone. I never did get to add the mic feature... but you'll see why, below. :)

Inside view of the Ultra Simple Sound Card Interface
Inside box, botom view

Inside/side view
Inside box, side view

PC board view
Inside box, disassembled view


There is no "parts list" for ANY of my projects... you see, I have a massive junk box, and many drawers FULL of parts that I just pick and choose as I go. All of the values given in the schematic, above, are common parts and should be fairly easily obtained online from various sources.

Ask around, tho. You may have a few hams in your local area that have a handful of parts they'd be happy to give to you.

(These parts and MANY MORE are available from places like Mouser or Digi-Key.) Coax (or multi-conductor, sheilded cable) is used for all connections in/out of the Box. Be careful to avoid ground loops! Plugging everything into the same outlet strip and connecting all chassis together is the best way to do this. (This is good practice, no matter what your station configuration.)

You also want to make sure everything is well-sheilded, to prevent RFI getting into the rig or the sound card. (You get some NASTY results, if it does!)

The sound card is wired with both the L & R inputs tied together. The OUTPUTS should not be tied directly together... tie the L & R together through 470 ohm resistors, as shown.

If you are getting some RF feedback into the rig, add .01 uf caps as shown by the dotted caps in the schematic, and re-check your GROUNDING.

If you don't want to go thru the process of building a device like this, you can buy a NICE ready-made unit that comes with all the cables you'll need, even a USB-to-Serial adaptor! The RigBlaster™ also lets you connect your microphone, providing seamless switching between computer and mic use! Check out the RIGBLASTER!™ Please tell the folks at West Mountain Radio that N1NKM sent ya! :)

For several years, I had heard nothing but glowing reports from users of their "RigBlaster"™ line of products, and I have worked many people who are using them. They all said that are VERY good units. In 2004, I received a RigBlaster Plus for CHRISTmas, and yes, it *IS* a very good unit! :) I wired it to the computer upstairs, so that I no longer had to sit down in the basement to work PSK31 or SSTV! :) Now, over 3 years later, this little RigBlaster™ still going STRONG! 100% trouble-free, despite my unusual station configuration!

NOTE: I modified my RigBlaster™ Plus to bring the receiver's audio out to a jack on the back of the unit. (WARNING: Modifying it VOIDS the warrany! Do so at your own risk!) My RigBlaster™ modification brought the Rx audio to the computer via the "CW" jack. I simply re-wired it inside the RB for AUDIO OUT instead of CW. (I never run CW, anyway.) Details & pics of the RigBlaster Plus mods are HERE! :)

If you want a GREAT RigBlaster™ box that you WON'T need to modify, check out the "Pro"! It's worth the money. Trust me! I just got one, (Jan 2008) and have played with it a little.

Unfortunately, I have had some problems getting the "Pro" to work on my setup because of my very unsual station configuration. I have already confirmed that there is NOTHING WRONG with the "Pro"... it simply was never intended to be used in a configuration where the rig is about 40 feet away from the computer, and down in the basement! I'm just getting too much RF and ground-loop problems with these excessively long wires. I will most likely end up using the Pro on my basement computer, instead. In any case, it is definitely a KEEPER! :)

Please E-mail me with any comments/suggestions.

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(Keep the lawyers happy.)

*ALL* Information presented here is done so without warranty or guarantee of any kind. Author assumes no responsibility for the use or inability to use this information. Author also assumes no responsibility for the ability or inability to complete the projects, above. MODIFICATION OF THE RIGBLASTER™ WILL VOID IT'S WARRANTY! If you are not sure of what you're doing, ask an experienced friend to help! Far better to WAIT and have it done RIGHT, than to make a mistake and ruin your expensive RigBlaster™!!

This information is presented as educational information only. No guarantee is made as to its fitness for any purpose. All risk is assumed by the person who choses to use this information. While the author's experience indicates that this proceedure was effective, any attempt to build/modify these devices IS AT YOUR OWN RISK. Extreme care must always be excercised, this is at the builder's SOLE RISK.