Also, if you are using different ac power outlets for the sound input device, scanner and laptop, you maybe causing a condition called a GROUND LOOP, hence why it was suggested to get a ground loop isolator, you can try to minimize the ground loop my using all 3 devices on a single 6 outlet Power Strip
Also the need for a Ground Loop Isolator when trying to use the rear external speaker jack is the fact the 536HP has what's called a BTL driven output.
That means both the sleeve and tip of the jack or plug are NOT grounded to the radio chassis or power ground.
When you plug that jack into most computers, it grounds the sleeve of the output jack and essentially shorts out half of the audio amps output.
Using a ground loop isolator will eliminate that short if using the rear jack. A ground loop isolator can also prevent an unamplified speaker from working though as most are not really intended for amplified sources.
They will pass plenty of audio to drive a sound cards sensitive inputs though.
Ground Loop Isolators are made that work on amplified circuits as well that still let the amp see an 8 ohm load. Those kind are not usually needed on a scanner that only puts out a couple watts of power though.
Most isolators are intended for the pre-amp stage where the impedance is high and the power is very low. Throw some cheap isolators on an amplified output of 10 watts or more and they will fry.
Most ground isolators will also not supply the 4 to 16 ohm load the amplifier is looking for but the amps used in todays scanners are very forgiving and don't seem to mind a high impedance load on the output.
I built my own isolator so I could use an amplified speaker on my 536.
I used a 1:1 audio transformer that could handle maybe 10 watts of RMS audio power. It provided about 32 ohms at its input so the 536 was not running into a high impedance load. The amplified speaker is a Motorola made to accept speaker level input signals. I still needed to insert a resistor on the speaker side of my transformer to tame the audio level down. I ended up putting in a series resistor in one line as well as a resistor across the two lines to bring the impedance back down to what the speaker liked to see across its input. It was designed to amplify an audio amps output that drove an 8 ohm load but the speaker could actually handle a much greater impedance range across its input. I think the specs said up to 1,000 ohms down to 4 ohms. My impedance after my resistors came out about 50 ohms.
Before I added the resistors, I could only turn the 536 up to 2 before it blew me away! After I added the resistors, I can set the 536 to about 5 for perfect volume in my noisy environment. If I set the 536 at 10, you can hear it down the street a long way. Go even higher and you can hear it over a jackhammer if needed.
It's actually overkill for my needs but I could not find an unamplified speaker that gave me what I needed so going amplified with a home brewed ground isolator did the trick.
Somewhere in one of the Uniden forums, a user posted several home brew schematics to properly get the audio out of a 536 external speaker port but I don't recall who that was. He had drawings for using external amplified speakers as well as for those that wanted to feed the audio into a computer or other external amplifiers that only have sensitive high impedance inputs.
Many do not know that neither side of the external speaker jack on the 536 are not grounded and grounding the sleeve or shield side of the jack will actually short out half of the amplifier.
The headphone jacks sleeve is grounded as far as I know. So using it for a signal into a computer is the better choice if you don't want to mess with ground loop isolator's. Like you mentioned though, different ground potentials between the scanner and computer could create hum or other noise so an isolator may still be needed.
Here's a link to a good post about using the EXT speaker jack:
http://forums.radioreference.com/un...er-connection-considerations-precautions.html
