[RTTY] RadioShack isolation transformer replacement

[Kok Chen]

By the way, there is one more trick that I use to produce a cleaner transmit signal.  It works with many rigs, but not all.

If your rig has an audio attenuator at the line input of your radio (e.g., the "Mic Gain" control in the FT-1000MP series when you use PKT mode), send as large a signal as possible from the sound card to the rig (short of clipping any input amplifier or saturating transformers), and attenuate the signal at the other end for proper operation (no ALC, etc, etc).

If there is any remaining ground loop noise, the attenuator will help reduce that noise.  

(The Elecraft K3 has likewise menu items to provide different line input attenuation.)

This is analogous to what frequent travelers used to do on airlines before the days of noise canceling headphones.   You first plug your ear with one of the foam plugs that pilots use.  Then you place a pair of headphones over the ear, and cranking the volume of the music way up.  The foam plug is the attenuator.  The ear hears music with much better SNR.  Like ALC and saturating transformers, you need to make sure the headphones/DVD player are not driven into distortion.

The corollary is that the worst thing you can do is to inject a tiny audio signal from a sound card into the microphone input of a rig. You maximize ground loops when you do that, not to mention how wimpy most microphone preamps are.

73
Chen, W7AY