Output oscillation in car but not on bench.

[ben12345]

I recently fixed a Dragster 5 channel amp, with the help of Perry here on the forum.

It tests fine on the bench, but when I installed it into a car, the outputs oscillate like crazy. Its not engine noise, as it happens with or without the engine running.

If I directly connect an MP3 player to the amp via a short RCA cable, it plays fine through every channel.

I thought maybe the head unit is faulty (i've never used the RCA outputs on this one yet), so I swapped that for another and the problem is still there. I swapped the RCA cable from the head unit to the amp, and the problem still occurs - even with the cable running outside of the car to the trunk away from any other wires.

I even tried a different 6 channel amp, and thats working fine with the original head unit and wiring.

Any ideas?

[Perry Babin]

You need to confirm that the RCA shield ground is intact.
RCA Shield Repair

Also measure the DC voltage on the shields of the amplifier's RCA jacks.

[chrismh9999]

Ive had this same problem, but with an amp made from scratch im certain it has something to do with a ground loop

[ppia600]

Does it only oscillate with the car running? If so, Perry and chris have it right. It could be the deck or the amp causing the issue. Even though you tried another amp, some amps will be less susceptible to ground shield issues.

[ben12345]

Yeah, I thought first it could be a ground loop. I grounded the head unit directly to the body, and made sure the amp had a real nice short ground, but still no change.

The oscillation occurs with or without the engine running, and it seems to happen only when a quiet signal is inputted.

I did have a good brainstorming last night, and I thought the same as Perry... maybe the RCA shield is no longer intact on the amp. I'll take a look at this in the next day or so and report my findings.

[ben12345]

Ok,

So I checked out the guide on Perry's website regarding the ground shield on headunits.... are they tested in the same way on amplifiers?

I have infinate resistance between the rca shields and the amps ground terminal, so it looks like that's my problem.

[Perry Babin]

Amplifiers are different. For budget amplifiers, the RCA shield is generally connected to the non-bridging speaker terminals.

For amplifiers with balanced inputs or amps using some form of noise canceling circuit, it's more difficult to test the input circuit.

[chrismh9999]

I remember measuring the rca out on the head unit it was open, but at the time didnt know if that was right. i know now thanks perry