sbartoli

I have
I have a mini headphone amplifier (12vdc 500mA) and aviation portable intercom (12vdc 1A) connected to separate power supplies (12v sealed lead acid batteries 7.5AH) and the intercom and the amp work great … very clear and crisp and amplified.

The problem I have is that when I connect the intercom and amp to the same power supply … sealed lead acid battery 7.5A 12V DC, there is major interference or crackling of the sound coming through the headphones from the amp. The sound is not clear but very broken up and not working at all. Both positive wires are connected together and both negatives together and then to the battery.

When I separate the positives on to different fuses but still using same battery still get the same problem. The only way to get them to work is on separate power supplies.

Question: How can I use the same battery (1 power supply) to power both these devices without risk of interference from each other i.e radios, intercoms, amps, etc ? This is also happening with radios. I think this is called dirty power or interference.

Tarsil

Try puting a diode in series and a .1uF capacitor in paralel with each consumer.

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OutToLunch

you might want to try an LC filter on the inputs of both loads

Tarsil

Hmmm...I'm whoried about the size of the coils.
And BTW...the capacitor in paralel with each consumer on it's own might solve the poblem.The capacity isn't critical.

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If is too complicated.....most of the times is the wrong way.

Hero999

It sounds like you're having problems with and earth/ground loop to me.

http://en.wikipedia.org/wiki/Ground_loop_(electricity)

You need to break the loop, connect the head phone amplifier input to the intercom's output via an audio isolation transformer, either that or power one of them from an isolated DC-DC converter.

What's happening is there are two paths to 0V DC, each having a different impedance and carrying differend currents. The amplifier's input is 'looking' at the intercom's output which is floating at a different potential, so it picks upt the noise.

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sbartoli

Thanks Hero999. I believe it is a problem with and earth/ground loop also.

How do I obtain or make an isolated DC-DC converter for each device so I share the same battery.

What parts will I need to buy. Thanks for the help much appreciated.

RadioRon

Hmmm. You know, I think it is overkill to provide two separate isolated supplies for these two. My gut says that this should work fine on one supply, and the fact that it doesn't has something to do with the audio interface between the two, not with ground loops. Ground loops are typically a problem when you share grounds and/or when you have very sensitive interfaces followed by a lot of gain.

If you connect each device to the battery ground directly with its own wire, and you use the lead-acid battery (which is very low impedance) then there is no shared ground currents on the power supply negative wires. The problem may be in the two connections that join audio from one box to the other. It could be that one is balanced while the other is not. I suspect something like this, or perhaps there is a DC bias on one line that the other cannot tolerate. A simple experiment is to make the audio connection using a large value non-polarized cap on the Audio+ and the same thing on Audio-. You can form such a capacitor by joining two large electrolytics + to + and then use the remaining two leads as one capacitor.

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RadioRon

sbartoli

I agree that the problem is the difference in the ground potentials. I'm sure that the intercom box uses the DC ground as its reference, while the amplifiers use an internally generated ground that is half of the positive DC voltage.

The maximum current that each amplifier can draw is 500mA, but that would only happen in an extreme condition.

Can someone please help as to the exact type of isolated DC-DC converter I would need. Im trying to find a cheap solution as I can simply run each amp of a seperate battery.

audioguru

That is your problem. The grounds are at different voltages. connect the ground (0V) of the intercom to the 0V supply wire (not its ground) of the amplifier. If the amplifier doesn't have an input coupling capacitor then add a 0.33uF capacitor between the intercom and the amplifier.

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Uncle $crooge

hjames

That seems like a lot of work - you'd have to open up the amplifier to find it's reference point, plus it might not quite work if the devices happen to use different internal voltages - 1/2VCC isn't exactly guaranteed.

I think it would be simpler to just take the single signal line coming out of the intercom and plug it into the amplifier. I'd bet that the amplifier already has a coupling cap in place, so the main thing is to get rid of the accidental short between ground and the intercom's speaker return - and leave the speaker return "floating."

i.e. get rid of the wrong wire...

audioguru

You cannot couple an audio signal on one wire. You need two wires.
The amplifier has a 0V power supply connection. Use it as the signal's ground.

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Uncle $crooge

Hero999

I agree also, I had got my terminology wrong, it is a difference in ground potentials that's causing the problemn here.

I don't see how large coupling capacitors can help though. If the grounds on both the headphone amplier and intercom are bobbing up and down then the interference will pass through any coupling capacitors. I think the only real solutions to the problem are balancing using an audio isolation transformer or running one device of a different supply using a DC-DC converter.

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Hero999

I agree also, I had got my terminology wrong, it is a difference in ground potentials that's causing the problemn here.

I don't see how large coupling capacitors can help though. If the grounds on both the headphone amplier and intercom are bobbing up and down then the interference will pass through any coupling capacitors. I think the only real solutions to the problem are balancing using an audio isolation transformer or running one device of a different supply using a DC-DC converter.

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