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Old 11-15-2011, 01:18 AM   #1
kebas239

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Hissing (I think?) noise

Hi all! I was wondering if any of you guys may be able to help me with a minor (albeit very annoying) problem that I have with a recent car stereo installation.

I installed an amp for my regular speakers - Alpine MRV-F540 and an amp for my two subs - Sony Xplod XM-502Z. I am a n00b installer, but I was able to get it to work in my Mazda3 w/ a BOSE system. I did it by creating an adapter to convert the supplied BOSE plug into RCA outputs and speaker inputs. I kept the rest of the wiring in the car stock, and upgraded my regular speakers to four Alpine SPR-68's and then used two Sony Xplod subs that I had leftover from an old installation for my bass.

I set everything up and it sounds great, except one thing - There is this constant hissing noise in the background - it is not overly annoying, but it is very intrusive and hurts the overall sound quality of my system.

By fiddling with the settings, I have figured out a few things about the source of this noise:

1) It does not appear to be engine related since when I turn the car on and drive it has no effect on the problem. As a result, I suspect it may have something to do with the stock speaker wiring, or some other type of RCA interference.

2) It is directly linked to the gain level on my Alpine amp, as when it is turned all the way down the noise goes away. I can even put it 1/4 up with only a minor, but tolerable version of the noise.

3) When I apply the low pass filter on the Alpine amp to the regular speakers, the noise completely goes away regardless of how high the gain is. Of course I cannot use this setting as it causes the regular speakers to only play lower frequency sound.

Some may say to set the gain low to eliminate the noise but I am unsure that I want to do that. My head unit supplies a line level signal, and when the gain is set to a low level the sound cannot go very loud. Also, my amp puts out 80W RMS per channel to each of my four speakers which handle 100W RMS. It is inefficient and a waste of my equipment's power if I keep the gain really low (Right?)

I guess what I'm asking with all of this is if anyone knows the possible source of the noise, or how to find out (instead of me ripping all of my speaker wire without knowing what it may be). I was also wondering if some type of filter could be installed to eliminate this noise. Thank you!!!
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Old 11-15-2011, 05:09 AM   #2
Mordrid1

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A source of the noise could be the way you created your own adapter for RCA outs.(no way to know without knowing exactly what you did)
You can try a ground loop isolater, but not sure if it is a ground noise issue. Another way to go might be getting a LOC to bump up the input signal to your amp, so that you can lower the gain.

Last edited by Mordrid1; 11-15-2011 at 05:11 AM.
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Old 11-15-2011, 08:08 PM   #3
kebas239

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Quote:
Originally Posted by Mordrid1 View Post
A source of the noise could be the way you created your own adapter for RCA outs.(no way to know without knowing exactly what you did)
You can try a ground loop isolater, but not sure if it is a ground noise issue. Another way to go might be getting a LOC to bump up the input signal to your amp, so that you can lower the gain.
I don't think it's a ground loop, but it may have something to do with the RCA's or the combination of stock speaker wiring, floor noise and the original signal being line level. This is exactly what I followed to create the harness - BOSE blows no more!. Essentially, i bought an adapter that fit over my BOSE plug, and repinned it. After doing that, I soldered and electrical taped the wires together so that I created a harness I could use as "plug and play".

I discovered that after turning the LPF on and adjusting the settings, it would filter the noise out depending where I placed the crossover. Unfortunately, this is not realistic because you can barely hear normal portions of the music. It does show me, however, that the noise is high frequency in nature. I contemplated installing an LOC between my adapter and amp, but I've been reading reviews about how varying portions of the music get filtered out. I may just wait until I replace my head unit. In this case, the original signal (and not the noise) should be amplified a bit and allow me better use of my gain. I'll probably just keep it at 1/4 for now and adjust my sub amp's gain a bit lower to compensate.

Last edited by kebas239; 11-15-2011 at 08:17 PM.
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Old 11-16-2011, 11:43 AM   #4
KaeZoo

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It sounds a lot like some sort of induced noise. You might try relocating the RCA cables. It's possible that your cables are routed close to some source of radiated noise in the vehicle, in which case moving them away from the noise source should help. Another option is a line driver, which increases the signal voltage at the head unit, allowing you to reduce the gain on the amplifier. This would only work if the noise is entering the system between the head unit and amp, though; if the noise is somehow being generated at the source, then a line driver would just boost the noise along with the signal.
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