Improving Spectra TX audio

[kc7gr]

Fellow techies,

I would be curious to know if anyone has investigated improving the low-frequency response of the Spectra's transmit audio. I'm running into an issue where, according to some measurements, audio frequencies below about 1kHz are getting pretty sharply attenuated.

Advice?

Thanks much.

[Will]

Bruce, Can you give me some details on how you measured this.
Spectra model?

Then I can look into the problem here in the Lab.

[kc7gr]

Hi, Will,

It wasn't me that measured it, actually. I had an odd report on my audio quality, via a UHF T44 series radio, while I was speaking over a linked repeater system in Oregon. The fellow making measurements at the receiving end was comparing my audio response with that of another station, and said that my own audio didn't have a lot of frequencies below about a kiloHertz or so.

Granted, the system in my van is set up to route all the radio audio feeds through an aircraft-type switching panel, so that may be having some effect. I can, however, say that I got the same report no matter if I used a standard Moto hand mic or my normal headset.

This is what led me to ponder the question of Spectra low-frequency response where modulation was concerned. I've not done any actual bench experiments just yet (I probably should, but I'm prepping for next week's run to the Bay Area).

I would be curious to know if you find anything along these lines before I do.

Keep the peace(es).

[kc7gr]

I remembered I had quite a few Spectras here at work, so I decided to do a bit of experimenting.

R413, right next to U400 on the command board, is one of three components that make up the pre-emphasis circuit. You also have C419 as the capacitive component, and they both work in conjunction with an op-amp that's internal to U400.

R413 is normally an 82K chip resistor. I found that by replacing it with a 120K part, the low-end response for the mic input dropped dramatically to around 300Hz, where it started to roll off again. While this may indeed result in improved audio quality, there is a caveat: The 'M' series of PL tones are pretty close to this point, so you may not want to make this change if you're making use of those tones in any repeater system you may be on.

Alternatively, given the results so far, I'm thinking that around 100K may be a better choice. I will experiment further, as time allows, and post the results.

This modification affects only the microphone input. The signaling and PL tones are generated and injected elsewhere in the stream.

Happy tweaking.