Audio conversion device

I'm looking to buy or design/build a device to interface one or two speaker annunciators to a mono line level signal.  And provide a 12VDC trigger output to indicate the presence of audio (some form of level threshold and monostable).  I strongly desire that the input(s) be transformer coupled.  I'm NOT going to bridge all of these devices and cause ground loop issues..
 
Looking to couple my Elk M1G indoor speaker output to a WHA matrix controller that has a 'paging' input.  Might want a second speaker input for an automation controller separate from the security Elk panel.
 
The inputs WILL have speaker loads, so my device should have something like a 32ohm input impedance.  I need to do some digikey shopping.  I might be able to locate a dual input coil audio ransformer that will do the mixing too.  With a 32ohm input it shouldn't couple much signal to the other speaker channel, and probably wouldn't care if it did.
 
Not finding anything I'm going to design/prototype/build something (Time to re-tune my EE skills....)
 
Thoughts?
 
I fully understand all of the issues with the WHA system not being battery backed-up (actually it might be...).  So I can't rely on the WHA for alarms.  But I'm going to have WHA speakers in rooms that I don't have Elk speakers in currently.  And this would get me audio alerts everywhere in the house.
 
I can find single speaker to line level converters.  Stereo line level to mono line level.  Nothing with stated transformer coupling.  I may have seen one with the audio detect trigger, but I can't find it again.....
 
Anyway...   Don't want to spend a lot of money...  
 
Thanks.

RDL Radio Design Labs probably has your solution. Maybe ebay, for a used unit?

Thanks.
 
Took a look at RDL...  They have a device which does part of what I need.  Didn't look up prices, but probably expensive.  Need to look into the DIY version a bit when I have more time.  The good thing for the DIY approach is that fidelity (bandwidth, noise, etc.) isn't a huge issue.  The audio output of devices like the M1G isn't exactly HiFi..  My device needs to focus on functionality and not fidelity.  Transformer input stage(s).  Passive mixing after the transformers.  Level detector and one-shot for audio envelope detection.  Variable level output.

Elk has provided some tech notes in the past on how to get line level audio off the board.  In fact, they've also indicated which rules can be written to detect when the Elk is about to use the amplifier.  That can be coupled with rules that will activate a relay to switch between your WHA and the Elk's "backup" internal speakers.  
 
I've thought about this in the past - if it were me I'd set it up so that as long as the system is fully powered, the audio goes out the whole house audio system - but as soon as it loses power, the relay would kick back to the default speakers that are battery backed.
 
Furthering my thought process, as long as there's power I'd run audio through a mixer to balance the inputs or possibly use a PA type amp that will mute audio and give preference to the M1 when it chooses to "speak".

Work2Play said:

Elk has provided some tech notes in the past on how to get line level audio off the board.  In fact, they've also indicated which rules can be written to detect when the Elk is about to use the amplifier.  That can be coupled with rules that will activate a relay to switch between your WHA and the Elk's "backup" internal speakers.  
 
I've thought about this in the past - if it were me I'd set it up so that as long as the system is fully powered, the audio goes out the whole house audio system - but as soon as it loses power, the relay would kick back to the default speakers that are battery backed.
 
Furthering my thought process, as long as there's power I'd run audio through a mixer to balance the inputs or possibly use a PA type amp that will mute audio and give preference to the M1 when it chooses to "speak".

Click to expand...

I've seen the note on speaker -> line conversion.  It's a simple voltage divider + DC-block.  It still ties the grounds together.  I'd prefer galvanic isolation of a transformer....
 
Good idea about a relay gating the audio to either WHA or simple Elk speaker network.  I need to think about that a bit more, but I like the concept.  Use the best speakers/amps available at the time....
 
I couldn't find anything from Elk regarding how to use rules to determine when it wants to use the amp/speaker.  I'll do some more digging into this.  This would make my 'converter' simply passive.