Audio Isolation Transformer. Why you need one.

[RFI-EMI-GUY]

If you are connecting the speaker output or headphone output of your scanner or any other receiver to a sound card or mixer or any other unbalanced device (with Phono plugs and Stereo jacks), beware that your radio may have a Bridged Tied Load (BTL) amplifier. In fact, unless it is a very old style radio, it probably has such circuitry. You should never connect anything except a speaker, headphone or an audio isolation transformer to the radio. You will likely damage the amplifier because it provides reference directly to A+ and A- rails in the radio. At best you will have noise and distortion on your Broadcastify feed. Many sound pretty awful in my opinion.

This is what a BTL amplifier looks like:


A description of how BTL works:

Bridged and paralleled amplifiers - Wikipedia

en.wikipedia.org

Video below of Big Clive dissecting a stereo isolation transformer. Similar ones were sold by Radio Shack and likely still available. One of these can serve two receivers and two sound car inputs. Note the beefy transfromers inside. There are cheap tiny versions on Amazon that are probably junk and may not actually have transformers inside, just capacitors to lift the voltage from the audio. The BTL may not be happy without the DC continuity of the transformer winding, so don't buy unless it actually has transformers. Big Clive is describing its function as normally used, to safely reduce power line hum and noise from a stereo system.

 

[merlin]

Grounding one side of the speaker can blow the output IC, or if lucky, just cut the output in half.
It works like a pus/pull across the speaker.

 

[buddrousa]

This is why I have several of those in use and spares on hand. Good Post.

 

[bunangst]

Great post! I agree, many scanner feeds sound awful. Unfortunately people are so eager to serve and use feeds for convenience, without any technical considerations of why they sound terrible.

 

[nd5y]

Some microphone inputs designed to work with electret condenser type microphones put out a DC voltage to the mic. A transformer (direct short at DC) could cause problems there.

 

[RFI-EMI-GUY]

Some microphone inputs designed to work with electret condenser type microphones put out a DC voltage to the mic. A transformer (direct short at DC) could cause problems there.

There are always the line inputs.... And hooking a BTL audio amp to a mike input is going to have DC vs DC and overdrive.

 

[bunangst]

Some microphone inputs designed to work with electret condenser type microphones put out a DC voltage to the mic. A transformer (direct short at DC) could cause problems there.

DC Biased audio is common on some commercial radios, that's what a blocking capacitor is for.

 

[chris451rr]

Another issue that may occur is RF noise from the computer. Also AC ground loops.
There is a lot of added noise on low band vjf. hf and AM band. As soon as a connection is made to the radio,
in this case its an AR-3000, RF noise from the computer sound card or USB sound input/output. That is RF noise. I notice when plugging the PC into a receiver the noise jumped way up on AM signals from good to buried. I put together a special audio cord, with ferrite beads on each wire at each end, and a toroid 1/2" od permeability high. This solved the noise problem. But there was still bad audio which was tracked down to be 60hz AC. I grounded the antenna coax to the computer chassis and got rid of that. The feed cleaned up quite a bit. This noise is less noticed on VHF but is present on low band AM and HF.