Ground loop isolator causes loss of audio signal

Captain Jack Sparrow

Captain Jack Sparrow

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I'm pushing the AV boundries here, but please bear with me.

Are there any audio/electric experts here? It's a very peculiar and strange problem that I can't get my head around.
In my car, I fitted a subwoofer and an associated high power subwoofer amplifier. This worked great, until I realized there were several problems:
  • Deep alternator hum (similar to mains 60Hz hum, changes with engine RPM)
  • An ultra loud THUMP when the amp turns on/off.
After all, I guess this is to be expected, cars are electrically noisy by nature due to the common chassis ground.

So I installed an RCA ground loop isolator which fixed both of these problems. But it caused a new problem!
The amplifier now intermittantly drops the audio signal. I checked that the amp is always powered, not overheating or going into protect mode
Fiddling with the amplifier's RCA terminals temporarily fixes it, at least until the next pothole or speedbump.
When I remove the ground loop isolator and connect the RCAs directly, this issue does not occur, even when subjected to the above environmental vibrations.

I have an identical ground loop isolator, which is part of our Home Cinema system, so I swapped them over. This didn't solve the problem.
Taking drastic measures, I opened the amplifier and tried reflowing the solder joints for the RCA terminals, but this also didn't solve the problem.
Is this likely to be a bad grounding issue, or is the amplifier failing internally?

The amplifier's 12V positive feed was confirmed by Halfords to be good. They did not seem to bother checking the ground point, which is using one of the rear seatbelt bolts. Of course, when they examined it, this problem didn't occur and they said I had "done a good job". But I'm not confident with my own work. I'm leaning towards me installing the amp wrong, possibly with insufficient grounding.

Any ideas?
 
Thanks Ian.

It’s a very bizarre one, I’m leaning towards a fault with the amplifier itself.

The only other thing I can think of is the RCAs are intermittently touching part of the grounded amplifier chassis internally. Perhaps this messes with the ground reference, cancelling out the RCA signal.

I’m not sure if the amplifier’s chassis is supposed to be grounded, but it is where it’s screwed into the metal seat frame. Maybe I should have screwed plywood to the seat frame, then fixed the amplifier to that.

Forgot to mention it’s an old class AB amp. Not very efficient, it generates a lot of heat when driving a 2 ohm load. In future I plan to replace this with a class D amp. These are much more efficient, so I can annoy the bad neighbour more effectively. Don’t worry, they definitely deserve this.

Next week, I’ll see if there are any car audio consultants that still exist in my area, but in the meantime, I appreciate any advice or suggestions.
 
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I would not imagine that a rear seat belt bolt is a dedicated earthing point in a modern car. Just because a car has common earthed bodywork does not mean to say that you can connect to any piece of metal. Modern cars have multiple earthing points around the body which are not solely dependent upon the body as a conductor. These earthing points are interconnected by copper conductors in the wiring loom to ensure continuous low loss continuity via copper conductors which is necessary for modern car electronics. I am not an audio expert but as with car electronics I would suggest that optimal noise free performance is best obtained by using a dedicated earthing point. These locations will be indentified in vehicle workshop manual.

I had a problem with an electronic trailer coupling module because I used the nearest bodywork bolt for earthing instead of the more remote and difficult dedicated earthing point as recommended. As soon as I used the proper earthing point - no problem worked perfectly.

Grounding by the way is an old American word for earthing particularly in the radio and TV world. The correct terminology in UK and European electrical networks and vehicle construction standards is "Earthing".

The current standard for vehicles is - Negative earth only. This resulted from the excessive bodywork rusting and corrosion of pre -1970 vehicles - particularly Vauxhalls which were positive earth and corroded quickly and excessively due to the sacrificial nature of electrons flowing from the steelwork in the circulating currents in the vehicle.
 
Captain Black said:
I would not imagine that a rear seat belt bolt is a dedicated earthing point in a modern car. Just because a car has common earthed bodywork does not mean to say that you can connect to any piece of metal. Modern cars have multiple earthing points around the body which are not solely dependent upon the body as a conductor. These earthing points are interconnected by copper conductors in the wiring loom to ensure continuous low loss continuity via copper conductors which is necessary for modern car electronics.
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I agree, this is not ideal. At the time when I originally installed this, I did not want to drill into the bodywork, so I looked for any bolt which had minimal resistance to a known ground/earth connection. It works, and I do not feel that this is the cause of this particular problem. If it was, then I'd expect it to work all the time, or not at all.

I have since located a dedicated ground/earth point, where some of the vehicle's rear electronics are connected to, but the cable is not long enough to reach it. When I contact the car audio consultants, I'll ask about getting a longer ground/earth cable installed properly.
Captain Black said:
Grounding by the way is an old American word for earthing particularly in the radio and TV world. The correct terminology in UK and European electrical networks and vehicle construction standards is "Earthing".
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That's an interesting one, because most equipment I've seen in the vehicle states "negative ground only". This includes the amplifier which is the subject of this topic. The terminology of "ground" seems to be more common, especially in the car audio industry.

I've always used the term "earth" for mains wiring, where it refers to the green/yellow cable connected the metal chassis of a device, supposedly to prevent it from going live if I've screwed up badly enough (1/10, don't recommend, it was literally a shocking experience).
 
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I had my setup inspected by a car audio professional.

Their findings were not good. As mentioned, I used the rear seatbelt bolt as a ground for the amplifier. Not only is this a poor ground point, but if it fails for any reason, the amplifier would try to ground itself through its chassis via the rear seat frame it’s attached to.

That could result in potential electric shock, destruction of the equipment, or fire. They basically condemned this and told me not to use it until I address these issues.

I have decided to replace the amplifier. As part of this, I will install a new ground point using one of the factory ground points. To avoid the risk of the amplifier chassis acting as a ground, I’ll also fix plywood to the rear seats, then fix the amplifier to this.

If this doesn’t sort it, I may as well give up!
 
Aw Jeez - don't give up - vehicle earthing has always been a problem when using chassis metalwork. I know everybody does not buy a workshop manual, but the Haynes ones normally have a vehicle earthing point diagram showing the location of formal earthing points which are directly connected to the main harness with copper wire. Using one of these will surely solve the problem - and yes as I said before they will be identified as earthing points - not grounding points. Good luck
 
Captain Black said:
Aw Jeez - don't give up - vehicle earthing has always been a problem when using chassis metalwork. I know everybody does not buy a workshop manual, but the Haynes ones normally have a vehicle earthing point diagram showing the location of formal earthing points which are directly connected to the main harness with copper wire. Using one of these will surely solve the problem - and yes as I said before they will be identified as earthing points - not grounding points. Good luck
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Well I guess it's better late than never! But yes, you were absolutely right.
Last week, I finally removed the old amp, along with its botched grounding point.

IMG_3253.webp

Found these two bolts, these are factory grounding points in the boot. After talking to the car audio consultant, I moved whatever was grounded to the left bolt to the right bolt. Then I ran a new dedicated amp ground cable from the left bolt as shown. I checked resistances, and it was well within tolerances.

To install the new amp, I cut a new piece of plywood, screwed it to the rear seats, and screwed the amp to this plywood piece.
Connected everything and powered it up. Everything seemed to work fine, but then I discovered a very amateur oversight.

The old amp was rated 50A at 12V. Therefore, the engine bay fuse for the circuit was also rated 50A.
The new amp is rated 60A at 12V. Oh no! :mad: Although I couldn't get the fuse to fail, I was not comfortable with this, as it would theoretically fail if the amp pulled its maximum power. Engine bay fuses in my vehicle are non-standard, so I had to get a 60A fuse strip from VW, which was an absolute rip-off.

vw-60a-fuse.webp

That was £10 just for a weirdly shaped piece of metal! :confused:
After I swapped the fuse, I was happy with the install. Everything works perfectly, and the new amp doesn't cut out like the old one did. I'm still trying to properly tune the subwoofer levels and frequencies. It's difficult to get a balance that can compensate for road noise, without drowning out the rest of the music.

IMG_3256.webp

Annoyingly, I have recently got into 70s and 80s music. Music from this era was mastered with sub bass frequencies cut, I guess this was to prevent the turntable stylus from jumping across record grooves. Some remastered music does address this, but it also crushes the dynamic range, cause the mastering engineer thought that louder is 'better'. I hate it when they do this, it just sounds bad. A bit frustrating, I've got all this subwoofer power, but the music I'm currently listening to can't utilize it! :D
 
Good - well done. Car electric have always been a nightmare before they even started fitting radios - in my day suppressing the noise from the dynamo - yes dynamo not alternator - was the biggest headache.

Happy listening
 
Captain Black said:
Good - well done. Car electric have always been a nightmare before they even started fitting radios - in my day suppressing the noise from the dynamo - yes dynamo not alternator - was the biggest headache.

Happy listening
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I can imagine a dynamo would be pretty noisy, as it’s pulsed DC.
But with alternators, it’s rectifying AC to DC, then regulating it. So alternator noise shouldn’t really be a thing unless there’s variance in the ground resistance, or the regulator is failing.

I never had problems with alternator noise/ground loop hum. The only reason the ground loop isolator is there was to stop an ultra loud thump sound when the amp turns on and off.

This was an issue with the old amp, not sure why, I thought all amps had suppression circuits designed to prevent this.

I am not even sure if this happens with the new amp, it was simply carried over from the old installation. It may not be necessary anymore.
 
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