M1225 Remote Volume Control

[kb1nbn]

Hello All,


At our fire station, we currently have a M1225 setup for selective call. We have a Cat5 cable running from the rear radio to the "front room" in the front room is a microphone and a radioshack amplified speaker. The audio connection is actualy coming from the internal speaker connections... The problem is that the audio coming from the front speaker is so loud that nobody wants to use it... They just turn the speaker off... is there a way I would install a variable resistor in line with the amplified speaker? Would this damage the audio PA in the radio if the imedance isnt 8 ohm?



Any help would be great


Thanks, Chris

[Bat2way]

If the speaker up front is being turned off anyway because it's too loud, remove your hook-up to the radio speaker and use the options plug low level disc audio output (pin 11), or just put a local remote up there. This allows the remote speaker volume to be adjustable and independent of the radio volume control.

[Will]

Better yet since you are using an amplified speaker, get the receive audio from the Handset pin in the Mic jack. This way they can turn the volume anywhere and the Handset audio pin stays at the same level.

[kb1nbn]

is there any way to just add a variable resistor to the speaker circuit? is it that easy or would that kill the audio PA in the radio

[Al]

Generally, the only thing that will destroy the audio PA in the M1225 is applying a ground to either of the external speaker lines or connecting a load impedance that's too low for the ratings on the audio PA. Yes, you can use a wirewound variable resistance rated for 5 or 7 watts and 10-15 ohms in series with the remote speaker - maybe mounted in the same enclosure that the speaker's in. You may have more luck finding an 8 ohm L pad in your local parts store however. This can be used as well if none of the resistance elements are connected to the housing of the pad(they usually aren't).

[kb1nbn]

So I could just use a 50 Watt 8 Ohm L pad and wire that to the output of the Radio...? The wires for the audio come out of the front of the radio which makes me believe that its internal speaker audio which is being fed to the "front" speaker... Which would mean its 16 ohm...?

[Al]

Well, not exactly....If I remember, the internal speaker on the M1225s is 32 ohm. Any external Motorola speaker that you would normally connect is probably 8 ohm or 6 ohm(the 13 watt accessory). Since the audio PA is only capable of 13 watts or so in the BTL configuration that it's used in, a 10 watt L pad should be more than sufficient, and it's a lot smaller than a 50(!) watt pad would be.
The internal output impedance of the TDA1519 chip used in the audio PA is very low, with builtin current limiting to limit output current should the speaker or wiring develop a direct short. So the audio output power that the IC develops- maximum - is what the chip manufacturer specs. If I remember, that's with a 4 ohm load. So, disregarding the idea that there's a fixed output impedance that the chip presents to any connected load(s), the power that the chip delivers to any connected load(s) is just E^2/Z. Z can be as low as the chip design allows before it goes into current limiting and starts distorting your audio on peaks.