Station Alerting System (Humming Issue)

[treyboothe]

So we have completed a station alerting system (Audio only). Which is really just a 70v audio system attached to a mobile radio. Below are the photos of the system.
(Ignore the mess with the rack, we're still in the process of finishing it up)
Amplifier with rack mounted volume controls. This is mounted in our server room where access is restricted to a few admin and IT personnel. Hopefully in the future we will replace the m1225.


Below is diagram for the audio system. Everything is wired with 16 awg twisted pair.


RFS BA1012-1 Antenna with 1/2" Hardline, polyphaser is inside the server room mounted to a grounded bus bar.


After I grounded the antenna, we have a low level hum which is annoying for some of our personnel. When I run test audio (music from my phone) the audio does not have this hum. Should placing a ground isolator after the audio out of the radio fix this issue? Any other thoughts or ideas will be appreciated.

 

[doctordialtone]

Should placing a ground isolator after the audio out of the radio fix this issue?

It should, yes.

Here's the one I use:

https://www.amazon.com/gp/product/B019393MV2/ref=ppx_yo_dt_b_asin_title_o05_s01?ie=UTF8&psc=1

Let us know how it works out. Good luck!

 

[RFI-EMI-GUY]

Indeed an isolation transformer is needed.

But I would be careful with that Mpow device. It is a stereo device, like most and it has a stereo jack. The radio speaker jack may be different. Or it may plug and play just fine. You are using only one of the two channels.

I assume it has transformers inside . Some cheapies don't. You will have to make sure that you connect the radio speaker output to exactly one input transformer and the PA input to the secondary of that same transformer and that there is no DC continuity between the two. It also has a tiny transformer which means it may saturate on any DC differential from the radio speaker output and on loud audio. There are probably commercial PA audio isolation transformers that are suitable.

 

[12dbsinad]

And while you're at it, run a ground wire from your antenna clamps to a good suitable ground or follow the coax back to the buss bar. By code it needs to be grounded.

 

[treyboothe]

And while you're at it, run a ground wire from your antenna clamps to a good suitable ground or follow the coax back to the buss bar. By code it needs to be grounded.

Didn’t realize the antenna clamps also needed to be grounded. Thanks for the info. This was installed through our local dealer to avoid any code violations or grounding issues like that. Starting to loose faith in them especially after this installation. I guess if you want it done right you’ll have to do it yourself.

 

[12dbsinad]

Didn’t realize the antenna clamps also needed to be grounded. Thanks for the info. This was installed through our local dealer to avoid any code violations or grounding issues like that. Starting to loose faith in them especially after this installation. I guess if you want it done right you’ll have to do it yourself.

Don't feel bad, 75 percent of antenna installations I see are not properly grounded. Most antenna's are direct DC grounded thru the support pipe/clamps. This isn't limited to LMR antenna's. TV antenna's, satellite dishes, etc. all need to be directly grounded per the NEC. A lot of TV dish mounts for example provide a specific grounding location, though this is commonly overlooked.

I concur about the done right, do it yourself statement. That's why I have a endless amount of projects to do.

 

[jeepsandradios]

You didn't specify what audio you took from the 1225. Is it speaker audio or discriminator ? I have seen hum using the speaker level audio.

 

[treyboothe]

You didn't specify what audio you took from the 1225. Is it speaker audio or discriminator ? I have seen hum using the speaker level audio.

Sorry I forgot to mention that, it is ran by the discriminator/flat audio.

 

[cmdrwill]

You need the Filtered Squelched audio and that is on the radio's 16 pin connector Pin 11. That 'audio' will be low level and should go to the AUX input on the audio amp.

 

[treyboothe]

Update, the hum has now diminished (barely audible) once I tried a different ground isolator. Happy with the results considering it's analog. I'll have higher expectations when we switch to p25.

 

[kb4mdz]

Sorry I forgot to mention that, it is ran by the discriminator/flat audio.

Yes, if you're running discriminator audio, you will still have the PL tone on that signal. That's why cmdrwill is right; take audio after that PL is filtered out.

 

[cmdrwill]

I'll have higher expectations when we switch to p25.

You are going to have to get a different radio.

 

[12dbsinad]

You are going to have to get a different radio.

And a different radio company