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 Post subject: Audio isolation transformer for adding aux port to radio
PostPosted: Jan Mon 01, 2018 11:06 pm 
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Location: Yukon, OK USA
There have been a number of posts on using transformers back to back to make an audio isolation transformer. I found these on eBay and they looked promising. I ordered several and hooked one up between my iPhone and a tube radio. The transformer is a 10k:10k. It handled the signal level just fine. No distortion I could detect. I swapped back to direct connect and could tell no difference in sound level (measured by calibrated ear). Price is good at $3.50 each. I am planning on ordering some more to have on hand.

https://www.ebay.com/itm/Audio-Signal-Transformer-10K-to-10K-1-1-Great-for-Isolation-or-Couping-New/311544540618?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

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 Post subject: Re: Audio isolation transformer for adding aux port to radio
PostPosted: Jan Tue 02, 2018 4:03 am 
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Location: Vancouver Island, Canada
I've found that a 1:10 step up makes the signal stronger and you can then turn the digital device down to about 1/3 volume. Just saying, because it works 1:1 as well, and that is a good price.

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 Post subject: Re: Audio isolation transformer for adding aux port to radio
PostPosted: Jan Tue 02, 2018 8:49 pm 
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Joined: Aug Thu 27, 2009 7:47 am
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Location: Seattle 98125
That one looks like it only provides 100V of isolation...I'm not sure that'd be a good choice, what with line voltage being 120V RMS or so. If you want to use an isolation transformer on a transformer radio, that's probably sufficient, although is also not strictly necessary in that application

I've found these are a good choice for series-string radios: https://www.mouser.com/ProductDetail/Tr ... ZQsw%3d%3d

They're 20 Hz - 20 kHz +/- 1 dB, 1:1 or 2:1 ratio, and have 1500V isolation voltage. $5.39 (plus $8 shipping) for quantity 1. Would also make a good interstage transformer in some applications.


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 Post subject: Re: Audio isolation transformer for adding aux port to radio
PostPosted: Jan Tue 02, 2018 9:11 pm 
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Joined: Jun Sat 18, 2011 3:36 am
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Location: Milton, FL 32570
jkoebel wrote:
That one looks like it only provides 100V of isolation...I'm not sure that'd be a good choice, what with line voltage being 120V RMS or so. If you want to use an isolation transformer on a transformer radio, that's probably sufficient, although is also not strictly necessary in that application

I've found these are a good choice for series-string radios: https://www.mouser.com/ProductDetail/Tr ... ZQsw%3d%3d

They're 20 Hz - 20 kHz +/- 1 dB, 1:1 or 2:1 ratio, and have 1500V isolation voltage. $5.39 (plus $8 shipping) for quantity 1. Would also make a good interstage transformer in some applications.

Do you suggest that because of the possible hot chassis in an AA5? Can line voltage intersect with audio signal? I've just been using phono inputs on my radios.

Thanks,
Jason

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 Post subject: Re: Audio isolation transformer for adding aux port to radio
PostPosted: Jan Tue 02, 2018 9:30 pm 
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Joined: Aug Thu 27, 2009 7:47 am
Posts: 2790
Location: Seattle 98125
Yeah, mostly the possible line voltage on an AA5 if the plug is in wrong or the outlet is wired wrong.


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 Post subject: Re: Audio isolation transformer for adding aux port to radio
PostPosted: Jan Tue 02, 2018 9:33 pm 
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Location: Detroit, MI USA
You need ones that are UL certified and hi-pot tested to at least 1500 or 1600 volts to be safe for use with any device that connects to AC power. Unfortunately that sort of small transformer is extremely difficult to source. It's very unsafe to use anything with a lower rating unless the radio you are connecting to has its own power transformer providing complete isolation from the AC line.

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