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Old 9th January 2012, 02:21 PM   #1
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Default Methods of Handling Various Devices and Grounding - Ground Loop Issues

My motorcycle has a Garmin zumo GPS and a Escort Redline radar detector. I've been looking at building a simple headphone amplifier which accepts both the inputs. I know I can buy something to do the job, but I want an educational project. This is an early design consideration that is stumping me.

Each device, including the amp will be powered from the motorcycle's 12v battery. Each device's audio ground reference is seems to be different.

If I want the ability to plug in which ever consumer electronic devices I want, with undersized power leads and various amp designs, how can I break the ground between devices and the amplifier?

Commercial intercom systems such as the Autocom and Starcom unit utilize 1:1 isolation transformers which appear to be for telephone use. The ground for each input is tied together. (i.e. shield of all headphone amps is 0ohm resistance between each other).

I've considered trying to source better isolation transformers, but am running into size and cost issues. I see many 1:1 transformers which don't mention audio applications. Any suggestions? I'm looking for smallest possible package.
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Old 9th January 2012, 05:21 PM   #2
Mooly is offline Mooly  United Kingdom
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Are you sure there is a problem in reality ?

I know nothing about bikes and have never used a GPS but I would have though that if it were 12 volt powered then the "0" or zero power lead would be ground as far as any audio outputs were concerned. Same for the radar detector

Thats as assumption on my part... have you done a quick measurement to see.

Some transformers here of various sorts but I can't really see that one is needed.
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Old 9th January 2012, 05:26 PM   #3
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Are you asking for resistance from shield on 3.5mm jack to power negative?
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Old 9th January 2012, 05:39 PM   #4
AndrewT is offline AndrewT  Scotland
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I would expect the "bike frame" to be connected to battery and alternator -ve (Negative Earth).

Everything else is powered from +12Vdc to +14Vdc + spikes.
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Old 9th January 2012, 06:28 PM   #5
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by mattwalston View Post
Are you asking for resistance from shield on 3.5mm jack to power negative?
Yes, is the audio output "ground" from these two devices connected to the power negative lead.

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Originally Posted by AndrewT View Post
I would expect the "bike frame" to be connected to battery and alternator -ve (Negative Earth).

Everything else is powered from +12Vdc to +14Vdc + spikes.
The "spikes" bit could be interesting...

I would guess anything manufactured for an auto environment should be designed to take into account noisy rails. Anything you build (headphone amp) should probably include a decent line filter although the low currents involved probably allow a simple R/C network to be all that's needed.
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Old 9th January 2012, 07:39 PM   #6
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Quote:
Originally Posted by Mooly View Post
Yes, is the audio output "ground" from these two devices connected to the power negative lead.
One is and one isn't.

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Originally Posted by Mooly View Post

The "spikes" bit could be interesting...

I would guess anything manufactured for an auto environment should be designed to take into account noisy rails. Anything you build (headphone amp) should probably include a decent line filter although the low currents involved probably allow a simple R/C network to be all that's needed.
Understand power supply needs.
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Old 10th January 2012, 10:56 AM   #7
Mooly is offline Mooly  United Kingdom
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If one is then that one shouldn't be a problem.

The other one... lets cover all bases on the one that doesn't. I would measure on DC volts from the power lead ground to both outputs with it powered up and check there is no DC volts present. If there isn't then it's well it's worth just coupling it up "as a normal" output feeding into a headphone amp and seeing if there is a problem. It can't do any harm. I'm assuming the outputs are marked or identifiable as signal and "ground" If not then try either way around.

Not quite sure how these will all be used in practice but if the idea is to have these two units to overide the headphone amp (music) then you could feed them all into an opamp mixer stage. Then any audio from any device will come through. Easy to set the levels for each too.
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Old 10th January 2012, 11:08 AM   #8
AndrewT is offline AndrewT  Scotland
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You could also add a "detect signal" to the two "alarms" and use the result of "detect" to mute the normal source to zero or low.
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