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I have a Nutone IMA 4406 intercom system with several intercoms throughout the house.

I am interested in wiring a Bluetooth receiver to its Auxillary line in connection so that I can stream music from my phone to the speakers throughout the house.

The Nutone is powered by 2 801t transformers (18VAC, 72VA) hooked to the intercom in parallel. I also hooked up a Ring doorbell to one of the transformers in parallel without a problem.

Nutone installer manual: http://www.nutone.ca/PDF/InstallGuides/62377.pdf

My project included using an AC/DC converter to convert 18V AC to 12V DC using this: https://www.amazon.com/gp/product/B00R5GO4YU/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

I then connected the 12V DC to the Bluetooth receiver (that can accept up to 24V DC as an input) https://www.amazon.com/gp/product/B07VPG2ZZY/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

This circuit was attached to one of the 801t transformers and run in parallel to the intercom wiring.

With this setup, the Bluetooth receiver worked fine, connected to my phone and output music to headphones connected to the 3.5mm output jack.

However, when I connected the 3.5mm out from the receiver to the auxiliary input jack on the intercom, the transformer fuse blew.

Both the AC/DC converter, and the bluetooth receiver are still functioning without issue.

I am trying to figure out why this happened and how I could prevent it. Would a 1:1 audio transformer help prevent this? What else should I be evaluating to troubleshoot this? I suspect if I use a separate power supply, I would not have this problem, however, that would defeat the ability to have a packaged solution sitting behind my intercom station.

Thanks

UPDATE:

I tried a "ground loop isolator" and put that in the circuit. Based on the description it looks like a 1:1 isolation transformer:

https://www.amazon.com/gp/product/B019393MV2/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&psc=1

This allowed the circuit to work and for the bluetooth receiver to send audio to the intercom, but then it did short out after a few minutes so not sure if it is the component quality or something more.

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