This is the inside of my JBL px300.4. I have narrowed down the noise in my system (HISS) to the amplifier itself by

1) Using muting plugs

2) Removeing the amp from the vehicle and using a "test bench" if you will and a test speaker to find the same noise.

It's a low hiss with the vehicle OFF and ear right up close to speakers.

By "test bench" I mean I connected the amp right by my battery with the ground going straight to the battery and used a test speaker and adjusted every setting on the amp with no change - gains at zero - no rca's connected.

So I took off the cover not the entire sink case and looked around. Here are the two things odd I found. I'm no expert and if this won't cause noise, I'll just have to live with some minor hiss till I get it repaired or get a new amp.

Or maybe someone has some ideas of what to look for inside the amp to help me out.

Or maybe, well it has to be the amp only thing that is the same from the test is the vehicle battery.

I used different wires, speakers, grounded to the battery, different turn on lead, different head unit.

Maybe it's the h/u wire harness? Some wire in there?

I did find out after I reinstalled the amp that the noise (HISS) does increase with the gain, but with the gain at minimum there is still noise <-- no rca's connected.

I should say when I set my gain with my oscope the hiss does not increase, only when gain is moved to 3/4 to full does the noise increase.

But from let's say about 1/2 or 5/8 gain to zero gain the noise is the same level.

The Hiss ONLY goes away when I set the filter(s) to low pass on the amp.

The Hiss is there in high pass and in flat

Is that a cigarette burn on your amp, or did something arc and burn off the green coating? o.O

I have no idea, that's why I posted the pic. Maybe someone has an idea what could have caused that or if it even matters.

Is your amp not turning off with the remote lead?

Is your amp not turning off with the remote lead?

Yes it is turning off. Nothing wrong with that. Why do you ask?

if it still makes noise with the car off, then it's not turning off completely.

Well what I mean is the engine is not running, just the key turned to acc. Sorry for the confusion.

I sent an email to JBL about this noise/hiss issue and here is the response.

I spoke with our engineers regarding this. The hiss that you

hear is actually considered normal for this amplifier model.

WRONG! try this.... hook up the rca's ( i know it does it with them unhooked ) and put a noise filter in-line to see if it stops it. try this first and we will continue our troubleshooting. let us know!

WRONG! try this.... hook up the rca's ( i know it does it with them unhooked ) and put a noise filter in-line to see if it stops it. try this first and we will continue our troubleshooting. let us know!

How will that tell us anything? If the amp makes the noise with the RCA's unplugged, what will putting a filter on the RCA's do for him?

I'd guess NOTHING.

The amp is just noisy. It happens. The trick is to play music. No more hiss.

Loud music.