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Tap speaker wire for 12V Trigger

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Bpet2

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Looking to tap my speaker wire and use a device to activate a 12V trigger when the music is on.

I Found a PAC TR-4
https://pac-audio.com/catalog/specialty-items/tr-4

So, the volts to trip the trigger come from taping the speaker + wire. The speaker wire originates from my Whole Home Amplifier that is in the family room and the speakers are on the back patio and so that is where I will tap the wire. I'm planning to put a second amplifier outside and so the 12V trigger output on the TR-4 will turn on that amp. The TR-4 also has a lead for 12V+ input (which will come from a DC Power Adapter), 12V+ output(which will go to the Outside Amp trigger input), and a Ground lead.

The TR-4 is usually used in a car and the Ground would go to the car chassis - since I'm not using it in a car, where would the Ground Wire connect? I would think the Ground would be connected to the Negative wire on the DC Power Source. But how would the TR-4 Measure the Volts on the speaker Tap if it only has the + side of the circuit? Or should the Ground of the TR-4 be connected to the Negative side of the speaker wire?

Thanks in advance for any advice! :)
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
My response would need to know and claify a few things:

1. Is the whole house amp bridged?
2. Your planning to use a car amp for outside?
3. Would you want to use a 120 V (whatever power is in your region) lamp for outside?
4. The trigger signal can come from a different power supply (wall wart) than the amplifier (high current 12 V supply)

This https://dlidirect.com/collections/frontpage/products/iot-power-relay would be device worth considering.

Bridged: means that the amplifier drives the + speaker 180 degress out of phase with the (-) speaker lead. There is no direct connection to ground to the negative speaker outputs (measured with an ohmmeter).

A whole house system MIGHT be what's called a 70V line system. It's much different than an amplifier per speaker.
 

Bpet2

New Member
Thanks for the response! :)

The Whole House amp is not bridged.
A regular , non car, amp for outside - but it has a 12V trigger 3.mm plug on the back to wake it from Standby mode.
Lamp? I'm not sure what you mean. I have 120V power, both in the house and outlets outside.
The inside unit is not a 70V system, just a standard multi-room amplifier for home use.

I already ordered the TR-4 unit already, just need to know how to connect ground. The PAC TR-4 is simply a device to 'trigger' with 12V.
 
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KeepItSimpleStupid

Well-Known Member
Most Helpful Member
lamp is "amp" mispelled. Sorry.

The ground on the home amp which should be the negative speaker terminal needs to share with the negative of the 12 V adapter and the negative of the trigger. READ BELOW.

The 12 V trigger may be optoisolated which will be an advantage.

You might find that the trigger ground is connected to earth which eliminates wiring them together.

So, confirm a low resistance from the - speaker of house system to a good ground.

Plug in the outside amp and measure from the (-) trigger [not connected to anything except meter] to earth ground.
any AC voltage?
any DC voltage?
Then check resistance if the voltages are near zero.

You could also check the - trigger to earth ground as above.

You want one ground point and that would be the amplifier.
the 12 V should have no earth ground to negative.

If there is a low resistance connection to earth at the 12 V trigger, then you don't want to add another. It may be already connected through the house ground.

I suspect the 12 V trigger is optoisolated, but you can use two 12 V adapters, an optoisolator and resistor to guarantee there are no issues.

One adapter will do the detection and light a LED in an optocoupler. The optocoupler would complete a separate circuit with another 12 V adapter at the outside amp. But, I suspect the 12 V trigger is isolated already.
 

Bpet2

New Member
Thanks Again

“You might find that the trigger ground is connected to earth which eliminates wiring them together.”
Trigger Ground - is this the Ground wire on the TR-4? There are only4 wires that connect to the TR4, 12V+ input, 12V+ output, + Speaker wire Tap and the Ground(which I need to figure out where to connect) So i don’t see where this could get earth ground from...or do you mean the Ground connection on the 3.5mm mono plug that connects to the trigger input of the outside Amp?

“Plug in the outside amp and measure from the (-) trigger [not connected to anything except meter] to earth ground.
any AC voltage?
any DC voltage?
Then check resistance if the voltages are near zero.”
Ok, so the amp, the trigger input - I use a mono 3.5mm plug which has positive wire and negative wire - so i test the negative and earth Ground...where would i get Earth Ground? Should i use the negative side of the wall wart 12V DC adapter?
 
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AnalogKid

Well-Known Member
Most Helpful Member
Since the trigger circuit does not care about audio fidelity, consider a small audio transformer at its input to completely isolate the trigger circuit ground from the primary amplifier output. In this way you don't have to consider bridged outputs, long wire voltage drops, etc.

ak
 

Bpet2

New Member
Analogkid, I looked up audio transformers and I am not clear on ho i would use it? Are you saying i should run the speaker wires through the transformer? The units i found indicate 25 or 70v? Can you except explain how I would connect it for my application? Thx
 

Bpet2

New Member
Does this sound like it makes any sense?
TR4 Speaker Tap to Speaker +
TR4 Ground to Speaker-
That completes the circuit and allows TR4 to detect voltage when music is turned on.

12V DC Wall Adapter + to TR4 12V Input
TR4 12V Trigger Output to + side 3.5mm mono plug that connects to Amp Trigger input.
- Side of 3.5mm plug back to - side of DC Wall Adapter.
This completes the trigger circuit.

Am I looking at this too simplistically?
 

chemelec

Well-Known Member
There are Many types of Audio Transformers.
For Your application, A Small Transformer like a "42TL026" would be Suitable to give some Voltage Gain and Isolation.

This Transformer has a 16 Ohm to 500 Ohm Impedance.
This has a 1 : 5.59 Turns Ratio.
Only about a 3/4 inch size Cube.
 
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AnalogKid

Well-Known Member
Most Helpful Member
I'm suggesting a very small audio coupling transformer, as found in old transistor radios or old modem cards. With one side of the transformer connected directly across the speaker, the other side will have the audio signal but be completely "floating" from the amplifier output. This technique is called ground isolation. Just about any transformer will work as long as the turns ratio is relatively low, like 1:1 to 4:1. An old modem card uses a 1:1 transformer of limited bandwidth, but it would be fine in this application. Because the secondary is connected to a high impedance monitor circuit, there will be almost zero energy through the transformer, so it will work just fine even with the audio cranked up.

ak
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
Does this sound like it makes any sense?
TR4 Speaker Tap to Speaker +
TR4 Ground to Speaker-
That completes the circuit and allows TR4 to detect voltage when music is turned on.

12V DC Wall Adapter + to TR4 12V Input
TR4 12V Trigger Output to + side 3.5mm mono plug that connects to Amp Trigger input.

- Side of 3.5mm plug back to - side of DC Wall Adapter AND CONNECTED to SPEAKER -.

This completes the trigger circuit.

Am I looking at this too simplistically?
That's the way we would like it to work.

If everything is isolated from ground/Earth, then you can connect away. If there is one connection to Earth, that's OK. If the outside amp has a connection to earth already (3rd prong of a cable), then you don't connect another.

By measuring stuff (the common/earths) before they are connected, you try to make sure it's safe.

If you want it foolproof, you isolate something.
The 1:1 transformer is the simplest. No extra wall wort needed.
A relay is another way with two adapters. It may be power hungry.The TR4 has to be able to drive the relay.
An optoisolator with two adapters. Itt can draw from 1-10 mA, so the TR$ can likely drive it. A few extra components can be added for protection.
 

chemelec

Well-Known Member
If you don't need an Increase in Voltage as is the case of a 1:1 transformer.
Than Even Just Using a 10uF Capacitor on Each Wire, Should give Enough Isolation.
 

Bpet2

New Member
Thank you all very much for the advice! It seems like a good first step would be to test to see if I am already isolated and if so, add a transformer?

  1. Where to test?I understand I can take my voltmeter and check for volts and resistance between the negative and the ground/Earth. On the outside amp, I can test the negative of the trigger, but what would I use for the ground/Earth probe? (fyi The amp has a two prong plug) Should I plug a DC transformer into an outlet and use the negative side of the transformer for the ground/Earth probe?
  2. Is it possible for the TR4 to deduce the voltage by only tapping the positive speaker wire? I called PAC and asked them how to connect the TR4 in my application, and they said because it was just a trigger I could simply tap the positive speaker wire. Does that seem possible given that this is not installed in a car where the TR four’s black wire is connecting to the chassis? I’m assuming that’s what completes their circuit? Just based on my limited knowledge, I’m assuming the circuit has to be complete in order for the TR4 to deduce the voltage on the speaker wire line.
  3. Same question as #2 for the positive 12 V line going from the TR4 to the amplifier’s trigger port. The amplifier’s trigger is a 3.5 MM port, I’m assuming I have to connect both the positive (12V+) wire AND the negative side. Is it possible the amp could actually be triggered if I only provided it the +12 V side?
  4. Will adding a transformer increase the Ohms to my inside amp? I’m tapping the speaker line coming out of my inside amp, Will I be overworking the amp by adding a transformer? When I test the positive and negative speaker wire, I get approximately 0.8 V. That is right at the threshold for allowing the TR4 to trigger. Therefore, I may have to add a transformer to bump up the votes to 2 or 3 V to be sure it triggers.
Thanks again for your help
 
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