Simple device to resolve radio to PC impedance mis-match and grounding issues

This document describes how to construct a simple box to eliminate impedance mis-matches between your Radio and PC sound card line-input. This device will handle both grounding and impedance issues unlike the simple in-line ground isolators.

Here are a couple of WAV samples that you can listen to that demonstrate what this device can do for your audio stream:

<Without Transformer Sample>

<With Transformer Sample>

The following assumes that you have a Mono input on your computer's sound card and a mono output from your radio.

Note: If you are not handy with a soldering iron, you could simply take a long pre-made audio patch cord with the right connectors, cut it somewhere in the middle, and splice the transformer onto the cut ends rather than mounting jacks to the project box.

Here is the finished product:











Radio Shack Parts List (Read entire document before you go shopping)

1) #273-1380 Miniature Audio Output Transformer ($2.99)

2) #274-251 1/8" Mono Phone Jacks (Package of 3, you only need 2, $3.99)

3) #270-408 Enclosed "AA" Battery holder (Optional, see text below, $1.49)

4) #274-286 1/8" Mono Phone Plug (Package of 2, Optional, see text below, $2.79)

Total price $11.26 + Tax.

Step #1 - Connect the Mini transformer to 1/8" jacks

Connect the red and white wires to one jack, connect the green and blue to the other jack. Cut off the exposed wire on the black wire (not used). You could put a piece of tape or a small shrink tube over the end of the black wire so that it does not make contact with the other metallic parts.

Step #2 - Preparing the project box

I chose the smallest and least expensive box I could find at radio shack. You may want to buy a larger one that can more easily accommodate the 1/8 phone jacks. When I purchased this tiny box, I did not realize that there is a battery divider that must be removed to accommodate the transformer. Unless you are handy with a Dremel tool, I would suggest buying one of their larger project boxes.

To use this box, you remove the Phillips screw so that the cover can be removed.

Use a pair of needle nose to remove the battery contacts and wires (you just want to use the box).

Remove the small battery divider that is built in to the case (get out the Dremel!).

Drill two small holes at the end of the box to hold the 1/8" mono jacks. If you use this particular battery box, the spacing is pretty tight so double check before you drill the holes.

Step #3 - Mount the 1/8" Jacks in to the project box

Use a permanent marker to mark the jack that has the red and white wires connected. This jack will be connected to the Radio side.

Step #4 - Replace battery cover

I put in the shopping list the 1/8" Phono Plugs so that you could make a short patch cable that will go between this box and the radio. Plug the patch cable in to the jack you marked earlier.

Step #5 - Attach patch cord to box

Step #6- Putting the box inline:

Plug the line from your computer's line input in to the box, plug the patch cord in to the radio. You are all done!

Technical Details about impedance mis-match

The following was posted by Jim Kelleher in a forum discussing what is causing the hum when you interface your radio to your PC:

In the past, I have found that audio issues between equipment are frequently related to impedance mis-matches and grounding issues. The audio out jack on a radio is usually a low impedance/balanced output (8 ohm/neither side connected to ground). However, the input of a standard amp is a high impedance/unbalanced input (10K ohm/one side connected to ground). The impedance mis-match is not that big of an issue but the grounding one can cause excessive hum, low output or a blown input/output circuit. A line matching transformer between the two pieces of equipment will match the inputs/outputs but, more importantly, isolate the grounds. In the past I have used an output matching transformer from Radio Shack (Catalog #:273-1380 $2.99) that costs a few bucks but you need to package it and add some connectors. Usually I use a commercial version that is fully assembled and ready to go but it costs about $30 (Bogen WMT-1). BTW, the Radio Shack version is used backwards to the way it was originally designed for, but it works fine.

This transformer has been used for many years to match push/pull transistor output circuits to 8 ohm speakers. As such, the designated primary is a high impedance center tapped one while the secondary is an untapped 8 ohm winding. If you turn it around (swap primary and secondary) you connect the 8 ohm side to your radio (mono only or one side of your stereo) and the other side to your audio card (don't use the center tap wire, just insulate it with a piece of tape or whatever). I've used this technique before to eliminate hum caused by ground loop issues. In the old days, ground was ground. Nowadays, everything is floating ground and not floating in the same spot. You can also use it to turn a mono signal into a pseudo-stereo signal. You run the mono into the 8 ohm side, use the center tap as your 'ground', and take your L and R off the other two leads. Bear in mind that these two signals are 180 degrees out of phase. If you set your stereo to mono, you'll get nothing, the two signals cancel each other out. If you listen in stereo, you get a hollow, open sound with no sweet spot and no bass.

Jim jkelleher_gk6513 at

Label on the #273-1380 Miniature Audio Output Transformer