70 Volt Station Paging/Intercom System Noise Question

[chevymano]

Hello all. I recently installed a new station alerting system in our firehouse and I have a bit of a noise/interference issue. But first, the background. We have a VOC system from fire alarm coming through a dedicated phone line to the VOC system controller which trips when activated from fire alarm. It then goes to a new 100w Bogen amplifier/mixer through the telephone/Vox tap so it will override anything being done on the amplifier/mixer. It goes out to 3 wall mounted speakers in the station and 2 loud speakers in the apparatus bays. The speakers and all are coming from the 70 volt taps. The system works great. Clear as a bell when an alert comes over. Now, we have 2 base radios. I cant remember what type off the top of my head, but they are for high band and low band radio for ours/surrounding towns frequencies. I hooked up the radios separate, but the same way: audio out from the radio, to a switch (2 pole/2 position) to select either a small speaker located in the radio room or select audio to the amplifier/mixer to be played through out the station and apparatus bays. When selected to the local speaker, it sounds good. When I select the station speakers through the amplifier/mixer I get a hum or something. Something like interference or noise through the speakers. How can i get rid of the noise/hum? Should I install a noise filter in the audio cables just prior to the amplifier/mixer? The power for the radios come from a ac/dc converter. The power cables are run somewhat close to the audio cables. They are separated as much as possible, but they go into the radio in close proximity. Let me know what you think and how I can solve this problem. Thanks for your input!

 

[RKG]

Hello all. I recently installed a new station alerting system in our firehouse and I have a bit of a noise/interference issue. But first, the background. We have a VOC system from fire alarm coming through a dedicated phone line to the VOC system controller which trips when activated from fire alarm. It then goes to a new 100w Bogen amplifier/mixer through the telephone/Vox tap so it will override anything being done on the amplifier/mixer. It goes out to 3 wall mounted speakers in the station and 2 loud speakers in the apparatus bays. The speakers and all are coming from the 70 volt taps. The system works great. Clear as a bell when an alert comes over. Now, we have 2 base radios. I cant remember what type off the top of my head, but they are for high band and low band radio for ours/surrounding towns frequencies. I hooked up the radios separate, but the same way: audio out from the radio, to a switch (2 pole/2 position) to select either a small speaker located in the radio room or select audio to the amplifier/mixer to be played through out the station and apparatus bays. When selected to the local speaker, it sounds good. When I select the station speakers through the amplifier/mixer I get a hum or something. Something like interference or noise through the speakers. How can i get rid of the noise/hum? Should I install a noise filter in the audio cables just prior to the amplifier/mixer? The power for the radios come from a ac/dc converter. The power cables are run somewhat close to the audio cables. They are separated as much as possible, but they go into the radio in close proximity. Let me know what you think and how I can solve this problem. Thanks for your input!

My guess is that the impedance from the radio output (not sure where you tap radio receiver audio) is way too low.

I seem to recall that we had similar issue and solved it with an impedance bridge between the radio channel card and the Audio Amplifier input module. But I'm not home and cannot search for my notes.

 

[doctordialtone]

RKG is probably correct. You are feeding a speaker output to a line-level input, most likely. That's why it sounds OK hooked to a speaker. The telephone input is 600 ohms, while the speaker out is typically 8 ohms. 3 things I would try;

1) Try the radio on a battery to eliminate the Power Supply as a contributing factor.
2) Tweak the volume level on the radio and the Bogen to get the best results - THEN address the volume level of the speaker.
3) Shielded cable between radio and Bogen and ground the radio to the Bogen.

Being in the phone business here in CT, I run into this a lot - people trying to connect boom boxes or MP3 players to amplifiers in gyms and auditoriums.

Personally, I use a "butt set" to listen to the audio in, because as we know, garbage in - garbage out, and the amplifier will amplify the garbage.

If the radio has a line out, that may solve the noise problem and you'll need to use a powered loudspeaker for the radio room, like PC Speakers.

Hope that helps.

 

[gunmasternd]

If you had acouple of amps around you could try splitting things up instead of that switch idea. fluorescent lights also cause noise issues if the speaker cable is close enuff. You could run new cable? Cable tends to act as a antenna so the shorter the run the better.

ac/dc converter might be a issue another noise generator. grounding things tends to help. If you can fine a way to plug the radios in the wall outlets instead of the coverter that might help.

try and keep things simple seems like people add things inline to fix other issues that just makes more issues somewhere else.

 

[n5ims]

I had a similar problem that was resolved using an isolation transformer (I think we used 8 ohm primary and 600 ohm secondary). This did two things, fix the impedence mismatch as well as removed the ground loop hum.

Also you should check what kind of input your amp connection has. If it's a line input what I suggested should take care of things, if it's a mic input you'll need to reduce the audio levels using a resistor or volume control pot and probably need a higher impedence secondary on the transformer.

 

[scottyhetzel]

Make sure everything is sharing a ground...or try isolating the ground plug. Is it a. 60 cycle hum ?

 

[chevymano]

The "mic 1/2" input from the instruction sheet says "These inputs are for two low-impedance balanced dynamic microphones." and "Screw terminal strip containing the MIC1 and MIC2 balanced, low-impedance inputs". The Aux 1/2 input from the instruction sheet says "RCA unbalanced, AUX1 and AUX2 high-impedance inputs." and "The AUX inputs provide unbalanced RCA inputs for connection to
external signal-level equipment." The phone line from fire alarm is going to the telephone input. I appreciate all your input thus far and I will try the remedies you all suggested one at a time to see what happens. Thanks and stay safe.

 

[k8zgw]

John, N5IMS has the answer.
I do ( or did) a lot of PA work, and you learn quickly that an impedance mismatch
Can cause all sorts of problems.
I always carry a small "line to voice coil" transformer.
If you are "stealing" audio from a radio, the ext speaker plug is likely 8 ohms,
connect this to the "voice coil", and the line side to your "hi imp" input.


Don