Over the years I have been asked to write an article on Sound Card Interfacing, I have always resisted.
On the air I admit that I taunt those who have spent hundreds of Pounds/Dollars on commercial Interfaces instead of using “3 bits of wire”.
I have been astounded to see the inconveniences endured by those who have paid out the small fortunes too. Unplug the Microphone from the Rig and plug in the Interface when you want to change from Phone to Digi Modes, wow that is well worth paying good money for.
Recently I ended up on some pages on the web dedicated to an interface design, I think there may even have been a Yahoo group. Was there really a Lecture Circuit too at one time? I began to wonder just how many Billions of words have been written on the subject, how many thousands of videos, how many Millions of pounds profit made by companies. Yet I cannot remember one mention of Safety.
We and our neighbours go out and purchase a Hi Fi Amplifier a Recording Device, Computer and a Radio Tuner and think nothing of connecting them together Output to Input etc. Why? well they are designed to be interconnected. Well so are our Radio Transceivers, particularly the modern ones which have a data socket.
Why do Radio Amateurs panic when they want to connect their rig to a computer. Well you can ‘Blow up the Sound Card and the Radio’ yes that is true but that is due to lack of Good Practice. That lack could kill the Amateur.
Also you could get RF Feedback into the computer Audio, yes true, but will 600 Ohm transformers prevent it, not necessarily, whereas a common mode choke, (winding the screened lead on a Ferrite Core) probably will.
What the transformers will do is drastically limit the Audio Response, may be a cause of some problems in the past, certainly a disaster in modern radio SDR systems.
Anyone familiar with the Icom IC706 (the rig that receives on the frequency on the dial and many others at the same time and sucks more current on RX than 3 other radios put together; but that is another story), will be familiar with what I call the Shaving Brush. A Din Plug for the Data Socket, supplied ready wired with a couple of inches of multi core screened cable, outer cover removed for an inch or so which looks just like a brush with multi colour bristles.
Some years ago there were many IC706’s locally, owned by new Licensees. I was asked to provide details of utilising this for Digi Modes.
I photographed everything provided drawings, parts list and write up. I handed the draft to one keen user who was technically knowledgeable. One criticism, came the verdict, you spend more time, a whole A4 page, writing about Safety before you get down to the Interface, we wanted you to write about Interfacing.
There is my point.
Good Practice will prevent nasty voltages which will destroy the Computer Sound Card or Radio, as a small bonus it may save the Operators Life.
Good Practice will eliminate Ground loops and the need for Transformers.
Good Practice will prevent RF feedback into the audio circuit.
The big difference between the Domestic set up mentioned above and the Computer in the Radio Amateurs Shack is the fact that the Amateur is most likely going to be bringing Real Earth into the Shack and the Radio. He then is going to connect this Earth to a Computer which in the UK is most likely to have its ‘Earth’ connected to the Neutral of the Mains AC Supply. This Neutral is the centre point of a 415 Volt 3 Phase Supply. Under fault conditions, which I have experienced at this QTH when the underground cable Neutral failed, this can be lethal.
For Safety do not have both Mains ‘Earth’ and True Earth anywhere in the Shack, I leave you to implement this, or get in a professional to help. I want neither wrecked Equipment or dead Hams on my hands.
Pay 100, 200, 300 Pounds for your commercial interface and it may or may not protect your Sound Card and Radio. It will not protect your or others lives. You still have a potentially Lethal Shack.
For this reason I have not and will not mention Interfacing without dire warnings.
OK I simplify just a little with my 3 bits of wire statement, only the other day someone thought for a minute and said, not 3 bits of wire you mean 4. No I don’t, the wires are screened lead. Think about it though, you do not want to produce earth loops.
Also there is a need to level change the Computer Serial port for the Rig PTT so we must add a Transistor 2 Diodes and 2 Resistors. This circuit is already published in dozens of places on the web. I just build it into the 9 pin Serial Port plug and forget it. For portable use with my Lap Top computer I required a low profile arrangement. I therefore potted the circuitry onto a skeleton plug. The top of the Transistor can just be seen through the potting.
Off you go then down to the Ham store, pay half the price of your Transceiver for a commercial interface, most modern Hams do.
Whilst you are out be sure to get some good Rubber Gloves and thick Rubber Sole Shoes, always wear them in the shack, have extra pairs ready for any visitors and invest in a good Fire Extinguisher.
Yes some of my postings are meant to promote thought and discussion.