Here is a simple building block for making just about any repeater
or RF link sound as it should.
This circuit originally appeared in the May 1990 issue of QST Magazine, "An Audio Equalizer for Communications Use" by Robin Rumbolt, WA4TEM. I've made no changes to the original design, but merely laid out a printed circuit for it.
This unit is not unlike a 5-band "hi-fi" graphic equalizer, and in fact uses the same audio chip as hi-fi units. The difference is that instead of covering 20Hz to 20kHz, this equalizer concentrates on the "communications" audio passband of 100 to 3000 Hz. Five boost/cut bands within this passband is more than enough to compensate most link and repeater audio paths. This is a very simple circuit, and in lots of 10 will cost about $10 each to build. I say lots of 10 because most of the parts come in packages of 10 from Digi-Key, though there's nothing stopping you from building just one. I needed a number of them to integrate into my various remote bases and repeaters, so I made several at once.
After the circuit is built, it is as simple as inserting in in series with the audio path to be equalized and the pots are adjusted for the desired response. A quick way that I use to restore "flat" (input=output) response is to generate tones with a signal generator at each of the 5 passband frequencies to a test receiver, and note the peak-to-peak voltage of each tone at the receivers' speaker. Then, without touching the receiver's volume control, change the signal generator and receivers' frequency to go through the link or repeater with the equalizer. Adjust the gain pots for each passband to match the peak-to-peak response noted during the direct path, and that's it. It might take a few passes through the pots to be able to get the range you might need depending on how much equalization is required, but it's almost as fast to adjust it as it is to explain it. I have several of these equalizers in service and they really make a difference in repeated fidelity, especially when there's more than one link or repeater linked up.