Need help troubleshooting Infinity 7541a

[jaimegmr]

Ive had this problem now for a long time but I just got fed up with it.

When my 4 channel amplifier (Infinity Reference 7541a) is on, it always emits constant static/crackling noise that seems to change in volume slightly in my rear left speaker. Once I turn up the volume it is not as noticeable, but when the volume is turned down, you can hear it crackling and hissing. Also, the rear left speaker pops loudly. At first I thought it was random, but yesterday I increased the motor RPMs while it was popping and the pops also seemed to speed up.

Why would it be doing this? At first I thought the rear left speaker channel of my amplifier was failing, but when I noticed the pops speed up when I increased the engine RPM, it made me think again.

Edit: Now that I think about it, the popping sound speed may have gone up when i accelerated because the alternator output voltage may have also gone up and the amplifier received a higher voltage.

[Perry Babin]

If you believe that the voltage has an effect, you need to confirm that the voltage is indeed changing. You should also try to affect the noise with the engine off. Turn various accessories on/off to make the voltage drop.

This type of problem (not necessarily voltage related) happens when capacitors leak electrolyte onto the circuit board. It also happens in older equipment when transistors begin leaking (electrically) but this amp isn't old enough for that to happen.

[1moreamp]

Sounds like engine noise pickup more then amp related failure...IMO. Perry's correct that your amp is not old enough to have suffered cap failure yet in normal circumstances.

[jaimegmr]

I think the problem got worse. I had not noticed what I heard today before. When I turned on the amp and head unit, at very low volume, everything seemed OK, but as soon as I raised up the volume, the sound would start to garble/jumble/mix/distort. Eventually I turned up the volume even higher and then it stopped having the problem. I went for a drive and even shut off the car for about 20 minutes and the problem didnt occur again. I didnt notice the popping noises today, but the hissing/crackling was still there.

I disconnected both front and rear RCA cables from the amp and the hissing/crackling was still there on the left rear speaker. This leads me to believe the problem is in the actual amplifier or theres a problem with the speaker (and/or wiring), but I doubt the symptoms I've had are from a bad speaker.

Here is a video I recorded with my cell phone. Hopefully the audio is good enough so that you can hear what I described above:

http://www.youtube.com/watch?v=EGkObHI7-bc (I turned up the volume higher at around the 52 sec. mark and then everything was OK)

[I Am An Idiot]

just to make sure it is not a speaker problem, why not move the right speaker to the left channel of the amp. And move the left speaker to the right channel. If the noise moves to the other speaker, the problem is in the amp. If the left speaker still has the problem, there is a speaker problem.

[jaimegmr]

I plugged in the Rear Right speaker into the Rear Left output of the amp and now the Rear Right speaker is popping, so it is definitely the the Rear Left channel that has the problem.

I have the schematics for the amp, so I am willing to open it up and try to track down the problem. What should I look at first?

I forgot to mention that I bought this amplifier from the Crutchfield Outlet store so it was a previous customer return. Maybe it has been repaired once already.

[Perry Babin]

That sounds like a head unit problem. What happens if you swap the RCAs (left to right)? Does the problem follow the RCA input?

[jaimegmr]

Originally posted by Perry Babin
That sounds like a head unit problem. What happens if you swap the RCAs (left to right)? Does the problem follow the RCA input?

I unplugged the front and rear RCAs coming from the Head unit and the static/crackling was still there.

[Perry Babin]

You may have 2 problems.

I don't know how good the recording was but much of what I heard didn't sound like an amp problem. It could have been from overloading of the mic in the phone. It probably wasn't the best choice of music to hear the noise.

I heard something that sounded garbled/jumbled (as you described). That's not something that an amp can do.

[jaimegmr]

Hmm, I do know that the ground connection of my head unit is bad so on strong road bumps, it will turn off for a second and then back on. Could this bad connection be the cause of the garbled sound I heard? Now that I think about it, could the following have happened?:

The ground wasn't making a good contact, so it was quickly opening and closing the circuit. Once I turned up the volume, this drew more current which was able to jump the very small gap and complete the circuit. This "soldered" the wires together enough so that even if I turned down the volume afterwards, it was still a closed circuit.

Definitely need to fix that ASAP. I need to get a harness to connect to the stock head unit cables for power. I just stuck the ground wire from my head unit into the plastic connector on the stock cable and held it together with tape.