Bad Sound?

Fixing distorted audio • Home PageLow Power Radio Blog
Ever experienced distorted audio in low power transmitters using PCs in the audio chain?
The input for your transmitter can be A-OK and your PC Sound card output can be working great, but you're not happy with the sound quality. Perhaps there's too much treble, not enough bass, scratchy high notes or other "ear sandpaper."

There are several possible causes of bad sound. There might be an impedance mis-match. The peak to peak volume levels from the PC sound card might be too high or low. There can sometimes be RFI from the PC leaking through the sound card and into the transmitter. The lack of audio input buffer amplifiers on the audio input side of most low cost transmitters can be a problem, too.

Impedance - The mis-match here could be high or low, but I'm guessing the sound card output is too low, because of other mis-match issues I've experienced. I don't know for sure. If the sound card is in the 300 Ohm area and the XMTR in looking for 600 there could be some clipping-like sounds.  One of the Talking House transmitters I use has a somewhat peculiar input impedance and I had to use a matching transformer (8-1000 Ohm, CT) to get clear audio.

A matching transformer is a passive device that can help bridge an impedance mis-match in the audio connection, example:

VALCOM VMT-1 INPUT MATCHING TRANSFORMER

If the sound card output level is too high then an easy fix can be an attenuating dubbing cord. Some adapters and two cords are needed for stereo. The attenuating dubbing cord simply reduces the level of the audio signal.

There also might be some PC crud (a PC is a nightmare of RFI) sneaking up the audio cable. Perhaps try an isolation transformer in the audio chain. Here's one made expressly for our purpose: "The Pureformer Isolation Transformer isolates the electrical grounds of two pieces of audio equipment. This is especially important in the case of computer audio cards connected to high quality/low noise audio equipment."

If you'd like to add an external buffer amplifier, one easy fix to try is an add-on buffer amplifier using a common, inexpensive IC chip called an LM-386 amplifier chip. My Gizmo AM unit sounded *much* better with a home made LM 386-based amp in front of it.

Sometimes it helps to run audio through an inexpensive mixer for a better impedance match and some pre-amplification. You could try a Behringer XENYX 502 Stereo Mixer or check ebay.