hissing in my setup

Hi all! I know this is a bit long, but please bear with me :) I recently upgraded the speakers and amplifier in my BOSE system on my Mazda3. The amplifier is a Alpine MRV-F540. I decided not use a LOC, instead creating a harness from a metra adapter which converts the high level speaker inputs to RCAs. The head unit was kept stock and I want it to stay stock, if possible.

Everything sounds great, but I have one nagging issue. I have this persistent "hissing" sound coming through my speakers. There is no crackling and there is no whine - it is pure hiss. The noise is mid to high frequency in nature and becomes louder as the gain and volume controls are turned up. It is not related to incorrect gain settings as the noise is still present and noticeable when the gain is all the way down. The noise is not affected when the car is turned on.

I have tried many things including rechecking all the grounds, adding a ground wire to my head unit, checking and going over my harness, using a PAC-AOEM Harness with built in gain controls to bypass the input wires running in the car, and using different RCA cables. None of these things had an effect the hissing noise in the slightest.

The one thing that DID eliminate the noise was when I hooked up my IPOD directly to the amp's RCA inputs. This is the only thing I changed; The speaker outputs were still running through that adapter I created. During the "IPOD test", I paused the music, turned the gain all the way up and maxed the volume on the HU. There was not one hint of noise when doing this.

This makes me believe that the noise is coming upstream through the amp and being induced through the RCA cables somehow. Is it possible that my HU's power supply is leaking noise into the RCAs, or that my amp is having compatibility issues with the HU? How can I test for these things? What "type" of noise is hiss (IE - what is it usually indicative of?) I have no idea what else to check to pinpoint the problem, and any help would be appreciated. Thank you!

Hiss is usually referred to as System Noise.  The device you used to convert speaker level to RCA level is a Line Out Converter. 

Exactly what did you use?

i am an idiot wrote:

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Hiss is usually referred to as System Noise.  The device you used to convert speaker level to RCA level is a Line Out Converter. 

Exactly what did you use?

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I used a metra adapter that plugs into my BOSE actory harness (under my drivers seat where my BOSE amp was). It converts my BOSE speaker level inputs to RCAS for my amp/

What is the effect if you disconnect the RCA inputs? If it goes away, then your source is to blame. If I have read your post correctly, then I understand that a source OTHER than the OE amplifier (your iPod, but connected to the same inputs of the Alpine) is quiet. Did I get this right?

Almost a guarantee, though, is that the Bose amplifier (seeing as you are converting speaker level - notoriously noisy - to RCA) is the noise source.

Is there a Metra harness to bypass the OE amplifier? If so, this is a FAR preferred method of connection.

It's not power. It's not ground. It's source. LOCs do nothing to reduce background noise, they are simply voltage dividers or step down transformers.

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It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

haemphyst wrote:

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What is the effect if you disconnect the RCA inputs? If it goes away, then your source is to blame. If I have read your post correctly, then I understand that a source OTHER than the OE amplifier (your iPod, but connected to the same inputs of the Alpine) is quiet. Did I get this right?

Almost a guarantee, though, is that the Bose amplifier (seeing as you are converting speaker level - notoriously noisy - to RCA) is the noise source.

Is there a Metra harness to bypass the OE amplifier? If so, this is a FAR preferred method of connection.

It's not power. It's not ground. It's source. LOCs do nothing to reduce background noise, they are simply voltage dividers or step down transformers.

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When I disconnect the RCA inputs, the noise completely goes away. When I plug in an empty set of RCAs, there is no noise. When I plug an RCA cable (With a 3.5mm jack on one end, and RCA on the other)to my IPOD and max the volume on the IPOD and maz the gain on the amp, there is no sign of noise. I tried the same thing with my cell phone as I did with my IPOD - no noise.

The BOSE amplifier is completely gone from the system, unless you are talking about something in the head unit. I have removed it and replaced it with my Alpine. The speaker outputs and high level inputs are both being converted through the adapter I made by following this tutorial - BOSE blows no mo - and then running to my Alpine. I tried to use a PAC-OEM adapter behind the head unit, then run the signal through the RCA out's on there in an attempt to bypass the adapter; the noise did not go away.

Initially, because I thought my head unit was the culprit, I purchased a touchscreen caska unit which had RCA out's built into it. The noise was still present even when running the signal through the RCA outs on it.

This whole thing has confused me to no end, lol. I cant even figure out what else to check, or what type of noise filters to try out.

I would connect the input plug back into the Bose amp and use a Line Out Converter on the output of the amp.