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      09-01-2008, 03:58 AM   #1
jonywalker
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Help, annoying "Hiss" after amp upgrade

I posted this problem before, and its been getting more and more annoying day after day since I replaced the factory amp with alpine PDX-4.100. I tried turn the input sensitivity of PDX to minimum, and conntect the ground of headunit and amp together, but it doesn't seem to work. This HISS didn't exist before at least not as strong as it is now. However the level of this noise is constant in relation to the headunit volume, therefore the lower the volume I turn the more demonstrative it is. However, if I turn the volume of headunit to zero there is no noise output from amp anymore. But as soon as I turn the volume larger than zero the noice appears and keeps constant.
1. If the noise came from amp why there is no noise if the headunit doesn't output?
2. If it were from headunit why it was not present with the factory amp?

The alpine PDX has a S/N only 76 dB, maybe this is the reason. Did anyone who also has PDX amp experience with the same problem?
I could not imagine that the alpine has so bad signal to noise rate, it is a downgrading!

Attached is my installation diagram:
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File Type: pdf my_installation.pdf (175.9 KB, 445 views)

Last edited by jonywalker; 09-07-2008 at 04:30 PM.
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      09-01-2008, 08:48 AM   #2
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The hiss origin does not seem to be related to your PDX amp. If you can't figure out what's causing the problem, you may be better off re-installing the factory amp and then feeding the balanced speaker-level outputs of the OEM amp into the PDX. While not an optimal set-up, I'm pretty sure your noise problems would be eliminated, assuming the hiss was not audible before you swapped amps.

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      09-01-2008, 09:17 AM   #3
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Hiss is largely affected by the gain setting on the amplifier. When the gains are at minimum, the S/N ratio is best minimizing hiss. Can you reduce the gains on you amplifier and see if that helps?
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      09-01-2008, 09:35 AM   #4
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Quote:
Originally Posted by kaigoss69 View Post
The hiss origin does not seem to be related to your PDX amp. If you can't figure out what's causing the problem, you may be better off re-installing the factory amp and then feeding the balanced speaker-level outputs of the OEM amp into the PDX. While not an optimal set-up, I'm pretty sure your noise problems would be eliminated, assuming the hiss was not audible before you swapped amps.

Tschuess
you mean factory amp + factory headunit together is a noise eliminating circuit? If I put the factory amp back again between them the sound quality would be rolled back in an old level.

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Originally Posted by scooby9
Hiss is largely affected by the gain setting on the amplifier. When the gains are at minimum, the S/N ratio is best minimizing hiss. Can you reduce the gains on you amplifier and see if that helps?
I already turned the amp gain to minimum, but the hiss is still more than acceptable.
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      09-01-2008, 10:21 AM   #5
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The strange thing is that it wasn't a problem before but it is now. It could be that the amp is defective now...

Do you know anyone with a spare aftermarket amp that you can swap in to test if the hiss is still there? If it is, then it is a wiring or source unit problem. If not, it's probably the amp. The PDX series are still D-Class amps which are generally noisier.
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      09-01-2008, 10:44 AM   #6
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Quote:
Originally Posted by scooby9 View Post
The strange thing is that it wasn't a problem before but it is now. It could be that the amp is defective now...

Do you know anyone with a spare aftermarket amp that you can swap in to test if the hiss is still there? If it is, then it is a wiring or source unit problem. If not, it's probably the amp. The PDX series are still D-Class amps which are generally noisier.
Thanks, its a good idea. I can change my JL slash amp to drive highs and mids to see whether the noise is there. But the JL has S/N rate above 100dB, and PDX only 76dB, the dealer may refuse my claim. I don't know how should a 76dB S/N look like, so am still looking forward for some PDX user experience.
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      09-01-2008, 11:12 AM   #7
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Quote:
Originally Posted by jonywalker View Post
Thanks, its a good idea. I can change my JL slash amp to drive highs and mids to see whether the noise is there. But the JL has S/N rate above 100dB, and PDX only 76dB, the dealer may refuse my claim. I don't know how should a 76dB S/N look like, so am still looking forward for some PDX user experience.
I have no hiss issues with my PDX 4.150, but I'm using a line driver and a processor before the amp, though...

I used to have hiss issues with my old E46 M3 OEM HU and aftermarket amps/EQ so I started using line drivers before the amps/EQ/processors since then to simply eliminate the issue by raising the S/N ratio not so much of the aftermarket amps but of the OEM HU.
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      09-01-2008, 11:56 AM   #8
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The PDX amps have won IASCA SQ finals so it's not the S/N ratio. Perhaps you have a bad amp, but highly unlikely. To make sure the noise is coming from your HU rather than from inside the amp, you can disconnect the RCA's while the HU is on and then listen for noise or hiss. If there is nothing, then your amp is fine.

Like Technic pointed out, a line driver may be a good choice to raise the S/N ratio of the head unit. Or you can re-connect the factory amp which is virtually the same as a line driver. The factory amp however is "optimized" for your HU and may give you the best results.

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      09-01-2008, 02:12 PM   #9
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I just swaped my JL amp into high pass to drive the highs and mids, it is the same, hiss. It is now clear to me that the noise does come from the HU, however the input gain of JL amp can be turned much lower so that the noise can be kept lower, but the alpine not. I am frustrated absolutely. Since my JL amp has just two channels it can only be used to drive subs. The BWM HU is also made by alpine why the alpine amp doesn't work together with alpine HU! I think it is true that BMW amp has noise deduction but aftermarket amps don't.
What should I do? Restall the BMW amp?
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      09-01-2008, 02:30 PM   #10
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I have an AudioControl LC6i that I use post-amp to sum the sub and front channels together and also boost the line voltage going to my amp. I then use the amp crossovers to split the outputs back to the front speakers and the underseat subs. The sound is amazing, crystal-clear and no-hiss.
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      09-01-2008, 02:50 PM   #11
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Quote:
Originally Posted by jonywalker View Post
I just swaped my JL amp into high pass to drive the highs and mids, it is the same, hiss. It is now clear to me that the noise does come from the HU, however the input gain of JL amp can be turned much lower so that the noise can be kept lower, but the alpine not. I am frustrated absolutely. Since my JL amp has just two channels it can only be used to drive subs. The BWM HU is also made by alpine why the alpine amp doesn't work together with alpine HU! I think it is true that BMW amp has noise deduction but aftermarket amps don't.
What should I do? Restall the BMW amp?
Re-solder the RCA plugs or try a LC6i without RCAs/a Matrix with RCAs... no need to reinstall the OEM amp.
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      09-01-2008, 03:00 PM   #12
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Quote:
Originally Posted by scooby9 View Post
I have an AudioControl LC6i that I use post-amp to sum the sub and front channels together and also boost the line voltage going to my amp. I then use the amp crossovers to split the outputs back to the front speakers and the underseat subs. The sound is amazing, crystal-clear and no-hiss.
Fine, it seems to me I have to use a LC6 too. Maybe the LC6 could be configured to set very low input sensitivity, as I noticed the noise output of HU is constant, nearly independent to the volume adjustment of HU. Or, sell the PDX and buy JL HD600/4 since it has the same high level input as my JL slash.
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      09-01-2008, 03:20 PM   #13
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Quote:
Originally Posted by Technic View Post
Re-solder the RCA plugs or try a LC6i without RCAs/a Matrix with RCAs... no need to reinstall the OEM amp.
My soldering skill has been improved by lot since I started working on it :-). I need a little time to encourage myself to perform the further steps. If I take the LC6i, then I also have to add the DQXS.
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      09-01-2008, 04:25 PM   #14
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The reviews on the HD600/4 are very mixed over the web. Similar to PDX in that it is another D-class amplifier that can be used for duty other than just subs. I've been on a few audio forums and the consensus seems to be that the Slash V2 series is much better in terms of SQ. Maybe a 300/4v2 could be considered? These amps also take balanced inputs so you could use the same connectors as your PDX amp.

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Originally Posted by jonywalker View Post
Fine, it seems to me I have to use a LC6 too. Maybe the LC6 could be configured to set very low input sensitivity, as I noticed the noise output of HU is constant, nearly independent to the volume adjustment of HU. Or, sell the PDX and buy JL HD600/4 since it has the same high level input as my JL slash.
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      09-01-2008, 06:46 PM   #15
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Quote:
Originally Posted by scooby9 View Post
The reviews on the HD600/4 are very mixed over the web. Similar to PDX in that it is another D-class amplifier that can be used for duty other than just subs. I've been on a few audio forums and the consensus seems to be that the Slash V2 series is much better in terms of SQ. Maybe a 300/4v2 could be considered? These amps also take balanced inputs so you could use the same connectors as your PDX amp.
I agree with you, and for that reason I used my slash 300/2v2 to drive subs, and the PDX for front and rear channels and it almost did its job pretty well till the input sensitivity, the sound is very clear and precise. I didn't pick an another slash 300/4v2 because of its large size and high power consumption. The PDX fits wonderful into the place where the oem amp was. If the HD600/4 were not that over priced 800$ I would have already tried it. Its 8V min. (PDX 4V) input sensitivity may have halfed the hiss issue. Anyway, BMW should really improve the HU sound quality.
Now I recall VP Electricity mentioned the noise problem issue, he said the JL amps were just balanced input compatible but not the real balanced, it just took the half the input signal amplitude, so as to the alpine amps.
Maybe only the audiocontrol linedriver has the real balanced input?

Last edited by jonywalker; 09-01-2008 at 07:07 PM.
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      09-01-2008, 11:29 PM   #16
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Quote:
Originally Posted by jonywalker View Post
I agree with you, and for that reason I used my slash 300/2v2 to drive subs, and the PDX for front and rear channels and it almost did its job pretty well till the input sensitivity, the sound is very clear and precise. I didn't pick an another slash 300/4v2 because of its large size and high power consumption. The PDX fits wonderful into the place where the oem amp was. If the HD600/4 were not that over priced 800$ I would have already tried it. Its 8V min. (PDX 4V) input sensitivity may have halfed the hiss issue. Anyway, BMW should really improve the HU sound quality.
Now I recall VP Electricity mentioned the noise problem issue, he said the JL amps were just balanced input compatible but not the real balanced, it just took the half the input signal amplitude, so as to the alpine amps.
Maybe only the audiocontrol linedriver has the real balanced input?
Ah I see. I hope it all works out with the LC6i. I know it really added a lot of flexibility for me and my system and I hope it can do the same for you. Good luck!!!
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      09-04-2008, 10:53 AM   #17
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Quote:
Originally Posted by Technic View Post
I have no hiss issues with my PDX 4.150, but I'm using a line driver and a processor before the amp, though...

I used to have hiss issues with my old E46 M3 OEM HU and aftermarket amps/EQ so I started using line drivers before the amps/EQ/processors since then to simply eliminate the issue by raising the S/N ratio not so much of the aftermarket amps but of the OEM HU.
do you installed the line driver near the HU or near the amp in the trunk? I just ordered a line driver bcuz i also some hiss, hope it will reduce the noise
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      09-04-2008, 11:26 AM   #18
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Originally Posted by ontopofm View Post
do you installed the line driver near the HU or near the amp in the trunk? I just ordered a line driver bcuz i also some hiss, hope it will reduce the noise
In the E9x with HiFi (OEM amp) you will not have any other option than installing the line driver near the amp in the trunk, just because the wiring and the space to put it is in the trunk.

For example, my old configuration (E46 M3) was:

OEM Nav Tuner -> Matrix line driver -> 3SIXTY.2 Processor -> 6-channel amp



The great thing about the Matrix (if you do not want to create RCA plugs out of your OEM HU speaker outputs then you can use the LC6, it works the same) is that it will give you 6 pairs of high voltage, high S/N ratio (110dB) and gain adjustable outputs out of 4 pair of inputs, so you can individually adjust the front from the rear from the underseat subs OEM HU output gain levels (up to 13V peak) to the aftermarket amps.

You can keep the aftermarket amps and processors gain levels at minimum, thus radically reducing or simply eliminating the "hiss" at the very source.
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      09-04-2008, 12:18 PM   #19
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so if i have the 2ch amp to power the sws subs then i only need a 2ch line driver, correct?
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      09-04-2008, 01:20 PM   #20
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so if i have the 2ch amp to power the sws subs then i only need a 2ch line driver, correct?
Are you having issues with hiss with the subs?
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      09-04-2008, 01:27 PM   #21
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after subs and amp installed, there is hiss w/ about 1/4 volume turn up, i really do not know if it's coming from sub or amp
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      09-04-2008, 01:44 PM   #22
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Originally Posted by ontopofm View Post
after subs and amp installed, there is hiss w/ about 1/4 volume turn up, i really do not know if it's coming from sub or amp
How do you installed this amp?
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