|The Radio Design Labs TX-LM2 audio isolation transformer has proven to be a neat little solution to a vexing audio problem in the operation of my home Amateur station. That problem is the isolation of audio lines shared between different audio components and the avoidance of ground loops. This product was suggested to me by a friend and fellow ham who works in the professional audio field, and it has worked very well.|
As an external microphone audio processing system becomes more complex, audio line isolation problems such as ground loops begin to appear and have to be fought. This is not a problem, of course, for a microphone attached directly to a transceiver.
Ground loops can form when two different chasses are directly connected by the shield on an audio cable running between them, and when that line’s audio return also is carried on the shield. The resulting ground loop can impress hum, buzz, possibly RF feedback, and other garbage into the transmitter audio input. The most sure-fire way to break a ground loop? An audio line isolation transformer, which breaks the continuity of the audio line’s outer braided shield.
If the connecting audio line that needs to be isolated is “unbalanced,” i.e. if the circuit consists of an audio coaxial cable with just one hot lead and a surrounding outer braided (or foil) shield, there is a simple product available to provide the isolation: a Radio Shack #270-054 Audio System Ground Loop Isolator. The product actually consists of a stereo pair of transformers in one plastic housing plus attached flying leads, and it does work well.
But, if the external processor was designed for use in professional audio, its I/O ports will typically be set up for “balanced” audio lines, i.e. the circuit consists of an audio coaxial cable containing two hot leads and an outer braided shield. The audio is carried only by the two internal conductors, and not on the shield. The shield will still connect two chasses, but since audio does not flow on the shield the chances for a serious ground loop are lessened.
Still, for the ultimate in protection it is more than worthwhile also to use an isolation transformer in a balanced system and the Radio Shack product will not fit into this type of service. This is where the TX-LM2 comes in, since it is a passive device specifically designed for isolation in balanced line systems. But it also has an ingenious second application as well.
The transformer primary is intended for use on a balanced audio line. The secondary has dual windings. One of the windings will both isolate and convert from a balanced to an unbalanced audio line (one hot conductor plus shield). The other secondary winding is even better: it also connects to a balanced line, but transforms the audio voltage from line level (~1 volt) to microphone level (~ 20 mV)! Thus the line level output of the processor can be fed directly into the microphone input of a transceiver, without running the audio through a pi or T-pad attenuator.
The TX-LM2 is housed in a small sealed plastic shell with removable screw-down audio line connection blocks. It is very light weight and can be tucked out of the way easily. Since these RDL products are intended for professional audio work, their specs are more than adequate for ham radio purposes. And they are relatively inexpensive.
I use a pair of TX-LM2s in my microphone audio processing system, one in the line from the unbalanced output of the mike pre-amp/mixer to the balanced input of the audio processor (application 1), and the other in the balanced line from the processor output to the transmitter’s “quasi-balanced” two-wire microphone input (application 2). They have worked perfectly for the intended purpose. I could not be more pleased.
RDL has a complete line of small audio transformers for various specific applications. Another model, the TX-1A, converts from balanced to unbalanced line and has a variable “audio level adjust” pot. See their Web page for other solutions.