Audio Isolation Transformer

[cwb212001]

Hello,

I'm looking at building an audio interface to a service monitor for testing things like SINAD, ect. My monitor doesn't have isolation built in and I've read that it is needed to prevent distortion and damaging the radio's audio circuit. I just want to make sure that I wont have any problems with what I was thinking of using. My plan was to use a 1:1 600 ohm isolation transformer (non center tapped) with 2x 8 ohm non-inductive 20w resistors (in parallel for 4 ohm) in parallel with the radio audio side of the transformer. Anyone see a problem with that? Any one have a better way?

Thanks for taking the time.

 

[WA0CBW]

Are you trying to connect the audio output of the radio from the radio speaker into the service monitor? Is the audio connection on the service monitor the SINAD input?
BB

 

[cwb212001]

Yeah, connecting the speaker output of the radio to the audio input of the service monitor... no dedicated SINAD input on the monitor. Just a grounded, unbalanced audio input on the monitor... trying to connect a balanced, unground-able radio audio output.

 

[zz0468]

...My plan was to use a 1:1 600 ohm isolation transformer (non center tapped) with 2x 8 ohm non-inductive 20w resistors (in parallel for 4 ohm) in parallel with the radio audio side of the transformer. Anyone see a problem with that?

That'll work. The only other thing I'd suggest is to be able to reduce the audio level going into the instrument. If you're using several watts of audio power, certainly not all will couple to the secondary side of the transformer, but it could still wind up too high for the service monitor. But otherwise,
I've done the very same sort of thing, and it works just fine.

Any one have a better way?

Not without unnecessarily increasing complexity and cost.

 

[cwb212001]

Thanks! I'll keep an eye on the audio levels. I'm sure it doesn't take much for testing.