Ground loop isolator

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bigRoN18

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I am wishing to connect two scanners to the input of one of my computers to serve live scanner audio and have seen there may be an issue with having a ground loop. I've looked at Radio Shack and see that their ground loop isolators are on the expensive side and don't have the connectors I was wanting to use anyway. I was thinking that I may want to make my own, but wish to get input from others before actually purchasing parts. To my understanding, each source audio needs to run through a 1:1 transformer... Is that it? Is Radio Shack part number 273-1380 the proper transformer for the job? I may also go to Fry's Electronics to see if they have a better or cheaper part.

Provided this is correct, here is my plan. I'll get a small project box. Two mono 1/8th" jacks and 1 stereo 1/8" jack. Two transformers and miscellaneous connector wires. One side of each transformer will connect to each mono jack. Is this good so far? Now... on the stereo side... can I combine a wire from each of the transformers to ground and the other to either left or right on the stereo plug? Is there anything else that is needed in the circuit? I feel like I'm missing something important here.
 

n5ims

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That should do the trick, although Radio Shack doesn't list on their web site much about the transformer in question. Since you're building it yourself and buying components you may want to get a different transformer than a 1:1. Most speakers are in the 8 ohm range and line input jacks are around 600 ohms so what I would recommend (assuming prices are similar) is a 600 ohm to 8 ohm transformer. Wire the 8 ohm side to the radio end (mono jacks) and the 600 ohm end to the sound card (stereo jack).

You may also want to put in a 0.1 mf disk capacitor on the radio side's between the jack and transformer. This should isolate any DC that may be on the radio's speaker leads. Adding it to the sound card side shouldn't be an issue if you connect to the line input connection. If you use the mic connection, adding it to that side may be good as well. Please note that 0.1 mf isn't a magic number, but since this could affect audio quality some it's best to stay around this value. If the audio is affected too much, simply experiment with going a bit higher or lower in value until the sound is what you like.
 
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N_Jay

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I would not use a 8ohm to 600 ohm transformer.
You do NOT want to couple all the energy across.

A 1:1 will work fine. You may want o load the speaker connection with 20 to 100 ohms (About what a headphone is)

If DC is a problem (it should not be) then the cap can be used.
(A sign of a DC problem is usually a click when the speaker comes on.)
 

rasputen246

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This is a great one for $5.99 with m/f 1/8" stereo plug/jack.

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Here is a link for a nice 1:1 audio xformer.
It is intended for a balun but it will decouple the ground fine for a scanner.

https://taweber.powweb.com/store/balun.jpg

Order this from the very bottom of the next link,$2

WBALUNClick For Specs $2.00 - This is a transformer you can use to create a balanced output sample of your amps output signal. You connect the output of this transformer to an XLR or TRS phone jack and you have a balanced signal to take to the board. Eliminates ground loops .

https://taweber.powweb.com/store/magnetic.htm
 

rbecker1963

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I just purchased one of those isolators from Ebay. I'll let you all know how it works out. Appreciate the link and the info on that.

Ray
 

gmclam

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Scanner audio to the PC

I am wishing to connect two scanners to the input of one of my computers to serve live scanner audio and have seen there may be an issue with having a ground loop.
I just went through this issue, and the BIGGEST factor is related to the exact scanner and sound card(s) you are using. The headphone outputs of the GRE scanners do not have a ground signal. They are wired with resistors so that either a mono or stereo headphone can be connected. I found the PSR-400 has both an EXT SP jack and a headphone jack. The EXT SP jack does provide a ground. Otherwise I was able to significantly reduce the hum by grounding my audio with either the antenna connector ground or ext power ground when using a hand-held model.

To my understanding, each source audio needs to run through a 1:1 transformer... Is that it? Is Radio Shack part number 273-1380 the proper transformer for the job? I may also go to Fry's Electronics to see if they have a better or cheaper part.
But I like the idea of isolation anyway. In my situation I am using some of those $1 sound cards from eBay. But they only have MIC inputs (no LINE input). I decided to use coupling transformers to do both the isolation and dropping down the signal. I did NOT use 1:1 transformers. I went to a local surplus store and found some audio transformers that worked great; they were about 10:1 and cost me 50 cents each.

I'll get a small project box. Two mono 1/8th" jacks and 1 stereo 1/8" jack. Two transformers and miscellaneous connector wires. One side of each transformer will connect to each mono jack. Is this good so far?
The signal from the scanner is mono, but the plug you need depends on the exact scanner and the exact jack you are connecting to. If connecting to a headphone jack, you might want to consider where your ground will come from.

Now... on the stereo side... can I combine a wire from each of the transformers to ground and the other to either left or right on the stereo plug? Is there anything else that is needed in the circuit? I feel like I'm missing something important here.
The signal to the computer depends on whether you are using MIC in or LINE in. The MIC inputs I have dealt with are MONO; the LINE inputs are STEREO. You can use a stereo plug, and wire both signal wires together. Again, I did everything mono with mic inputs.

I'd be careful about putting two transformers next to each other in the same box. Just don't cram them next to each other, leave some space. Otherwise you might get some crosstalk.

I did not put my transformers in a box at all. I used heat shrink (fairly large heat shrink on the outside) and wired the transformers inline. The transformers I used were fairly small, and only created a small bulge in the wires. This way I was able to keep them away from each other and did not have to find a place for another box. Plus I have 6+ of them on each of 3 computers, and if I want to move one, or add one, I won't need a different box.
 
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rbecker1963

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That isolator has not taken out the hum. For $6 plus shipping it was worth a try. Looks like I will need to run a regulated power supply and eliminate the wall wart. Always used Radio Shack in years gone by, so unless there's a better flavor, I'll pick up a new one.

Any suggestions on power supplies?
 

gmclam

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That isolator has not taken out the hum.
Are you getting your audio from the headphone jack of a GRE-made scanner? If so, that's why the isolator does not work.

Looks like I will need to run a regulated power supply and eliminate the wall wart.
Do you hear a hum out of the scanner's speaker? Or perhaps with headphones? If so, then a good/regulated supply should fix it.

As I wrote before, I just went through this scenario and the hum was being caused because of the lack of a direct ground for the audio output. I was able to eliminate the hum by using the power ground and/or coax ground. I ended up using the EXT SP jack rather than headphone jack.
 

rbecker1963

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It's a Uniden BCT8 and it's the ext speaker jack. I'm pretty sure these radios were not made to run off wall warts.
 

doctortube

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You do not need the capacitor as transformers do not pass dc . BCT8's do run off wall warts . If it does not hum on it's own you need more shielding . .You can put more filtering caps right inside the scanner . I had to do this to my 2096 and now it's dead quiet .

Just my 2 cents
 
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