Strange engine noise problem?

[The Lord Flashheart]

Hi all,

I have developed a strange engine noise problem since putting in a JBL 1400.1 amplifier.

The problem manifests as the alternator noise being heard through the front stage amp only when the RCAs from the sub pre out on the head unit AR connected to the JBL.

The JBL May be plumbed into the power circuit but the alternator whine doesn’t become apparent until the RCAs are plugged in.

I’ve tried swapping the RCAs to the front stage amp and the noise is independent of which set of RCAs are used; it only occurs when the JBL RCAs are connected to the head unit.

Any guesses as to what is going on chaps?

 

[TBTL]

Based on your description, I think it is a ground loop. Insert a ground loop isolator (= small transformer) in the rca wire between head unit and the JBL as a quick fix.

 

[The Lord Flashheart]

Thank you TBL, I shall try that.

I should have That the level of the the whirring noise is independent of head unit volume position.

 

[Perry Babin]

With no RCAS plugged into the JBL, what's the resistance between the input RCA shields and the B+ and ground terminals of the amp?

 

[The Lord Flashheart]

With no RCAS plugged into the JBL, what's the resistance between the input RCA shields and the B+ and ground terminals of the amp?

Hi Perry, with power applied and RCAs connected it reads a dead short.

With no power and no RCAs connected it reads several megaohms and climbing.

With power and no RCAs it reads a variable amount between 200kohm and 20megaohm differing on separate measurements, but I saw the higher figure most often.

 

[Perry Babin]

Hi Perry, with power applied and RCAs connected it reads a dead short.

Dead short? It can't read shorted to both B+ and ground.

Confirm that you read 0 ohms between all RCA shields on the head unit and the case of the head unit (no RCAs plugged in).

With the RCAs disconnected on both ends, confirm that you get infinite resistance between the RCA cable shields and the vehicle ground.

 

[The Lord Flashheart]

You’re right of course, I think it’s this shitty cheap multimeter I have running out of batteries and reading all over the place.

I’m buying a decent one tomorrow and will check again.

 

[Perry Babin]

See if you can borrow one. You may be looking at the charging of isolation or filter capacitors. They will make your meter read oddly.

 

[The Lord Flashheart]

I think there is a certain amount of that but the jumping about by two orders of magnitude is a lesson on buying once and crying once...

I’m buying a known good second hand fluke 11 tomorrow from a friend but I will try and get reading tomorrow with a fresh battery just to see.

I Want to build up a decent lab anyway, I’m getting more into playing around with Electronics more and more anyway.

 

[Perry Babin]

For repair work, it's hard to beat the Fluke 10, 11 or 12.

 

[The Lord Flashheart]

It matches my 105b scopemeter, it’s almost like it’s miniature offspring side by side...

 

[phase]

I had an annoying alternator noise that was finally cured by placing a choke on the front amp power cable near the battery. This is just 10” of wire wrapped around a 1.5” dia. ferrite core multiple times.
Also had tried installing a 5uf polypropylene cap on the alternator, grounded to the case. While that didn’t do much to help the obvious noise, it improved the sound quality.

Before this, I had messed around with different power/ground schemes and nothing made a difference.

 

[The Lord Flashheart]

With no RCAS plugged into the JBL, what's the resistance between the input RCA shields and the B+ and ground terminals of the amp?

With power applied but no RCAs connected, the ground reads infinite resistance to the shield and B+ A fluctuating value that settles around 100kohms.

 

[Perry Babin]

You can't check resistance with power on the circuit.

What's the exact model number of the amp (or a link to the service manual)?

 

[The Lord Flashheart]

You can't check resistance with power on the circuit.

What's the exact model number of the amp (or a link to the service manual)?

Apologies, I had thought the amp needed to be connected to see if the ground leak was elsewhere in the system.

Measuring with the Fluke and with the amp disconnected from the car completely (both in RCAs and power connections):

B+ to rca shield:~5 megaohms and slowly climbing.

Ground to rca shield: infinite resistance.

Exact model of amplifier is a JBL GTO 14001.

 

[Perry Babin]

For that amp, all of those seem right.

Did you check the RCA cable shields to ground?

 

[The Lord Flashheart]

Head unit chassis to HU RCA shields: 0.2 ohms.

RCA cable shields to chassis: infinite ( cheap meter) ~15 megaohms and slowly dropping ( Fluke)

 

[The Lord Flashheart]

I tried a transformer based rca isolator Between the JBL and the head unit and it removed the noise.

I now notice that when I put my foot on the brake or use an indicator I get a very faint buzz through the front stage that seems to be in time with them.

Could it be that there is a fundamental problem elsewhere that causing noise to get in the system, google has thrown up something about alternator regulators possibly going on the blink and having this effect?

 

[Perry Babin]

What you posted seems OK. Could it be the other amp has a problem that only causes an issue when you have both amps connected?

Did you try running the RCAs through the interior over the seats or even outside of the vehicle to see if the problem was associated with the proximity to something in the locations where they're run?

 

[The Lord Flashheart]

Quite possibly, I shall have to investigate. I tried the running the RCA through the cabin but only the sub channel RCA as I thought it had the problem.

I shall try it with the front stage RCA tomorrow and measure the RCA shield to B+ and ground on the other amp. I'll also try the noise suppressor on the front stage cable too. It's a Sony XM 3040 for reference.

Thank you for your help.