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Discussion Starter · #1 · (Edited)
I tested the output of the factory head unit on a 2017 LT. I used both an oscilloscope and a PC based RTA to measure both signal amplitude and frequency response.



Signal into the amp is 2V p-p balanced, 3V with EQ boosted.


Input to factory amp



Sound Devices USBPre line input


Both left and right channels measured at 5 volume steps (10, 20, 30, 40, 50, & 60)



While the output voltage is sufficient for feeding an amp directly, some EQ compensation is needed.

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Awesome information! Thanks for doing this. Couple questions if you don't mind. I understand car audio but not when it gets this detailed.

So, according to your measurements, the stock head unit puts out between 2 and 3 volts on the input side (before amp)? And if so, wouldn't this indicate that the head unit puts out a pre-amp signal rather than a speaker level signal like others have indicated?

Is there any way to tell from your last graph if (or how much) the stock head unit attenuates bass frequencies at higher volumes?
 

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Discussion Starter · #3 ·
Awesome information! Thanks for doing this. Couple questions if you don't mind. I understand car audio but not when it gets this detailed.

So, according to your measurements, the stock head unit puts out between 2 and 3 volts on the input side (before amp)? And if so, wouldn't this indicate that the head unit puts out a pre-amp signal rather than a speaker level signal like others have indicated?

Is there any way to tell from your last graph if (or how much) the stock head unit attenuates bass frequencies at higher volumes?
Correct, the output I measured from the HU is low level, not speaker level.

From the graphs, I didn't measure any bass attenuation as volume increased.


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Thanks again! You may have dispelled two myths about this system very quickly. Everything I've read says that the pre-amp signal was speaker level going into the amp (or processor) in the trunk. And I assumed bass frequencies would be attenuated like most OEM systems do.

So this means I possibly could skip a line out converter when tapping before the trunk amp to supply my sub. Just go straight into the amp.
 

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Discussion Starter · #5 ·
Yep, this is why I measured it. I suspected the same and was happy with the signal level.

I am disappointed with the factory EQ and will have to correct that with the new system.

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What didn't you like about the factory EQ? We're talking about just the factory head unit here, right? I have a 2016 LT1 without Bose.

And if you don't mind me asking, what do you have planned for your "new system"?

I have been seriously disappointed to find out that it's almost impossible to change the head unit in these new Chevys. Or at least nobody has attempted yet and posted to the web. That's why your post was so intriguing. There's just so little about the new Malibu stereo system upgrades out there.
 

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Discussion Starter · #7 ·
LT1 non-Bose.

I would prefer a flat output so I didn't have to EQ the signal to make it flat.

New system will be the stock HU feeding a DSP (tbd) with an Audison LRx5.1k amplifier feeding Hertz HSK-165 components ran active and a pair of Hertz HX380.

Doors will be treated with deadener and pvc encased fiberglass sound absorption. Openings will be sealed.

A bank of six Maxwell BCAP3000 will support the electrical system.


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Discussion Starter · #8 · (Edited)
HU can't be replaced without losing all vehicle preferences and factory control. You could relocate to the trunk if you wanted a double din in the dash.

I should clarify this isn't my vehicle, I'm doing this for a friend. I compete in IASCA SQ with my 2014 Accord.
http://www.caraudio.com/forums/car-...14-accord-sport-sq-build-keep_hope_alive.html

I looked for info and didn't find it so I figured I'd share what I find during this build.

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Let me say thanks again for the posts and insight. I really hope you'll continue to update the progress on the friend's Malibu. You've already provided valuable information and I'm sure more will come.

Your Accord looks awesome. It was fascinating to read about your build. Sorry to hear about your "mishap" a few weeks ago. Glad you got it back.

I hope you don't mind me picking your brain. You obviously have tons of knowledge about these modern systems and I'd love to ask more questions. I've always installed my own systems for many years, but my last car was an 07 Focus and things have changed immensely since then in the car audio upgrade world with the new factory units. If you'd rather do this via pm then please just say so.

So with the Malibu, you are replacing and amplifying all the speakers with signal from the input side of the factory amp? I would like to do this as well for at least the front speakers. But if you bypass the stock amp in the trunk how do you retain bluetooth audio for phone calls and all vehicle warnings and messages? Wouldn't the signal have to be taken after the factory amp? And if so then we're dealing with the ANC system problems. Or maybe you plan to defeat the ANC system?
 

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Discussion Starter · #10 ·
Good questions. I am looking for more info on the system schematics. I had access for service manuals and grabbed a decent amount of info I can share. My hope is that the head unit does the matrix switching and control for BT. I plan on cutting ANC mic wires (as is typical with modern cars).

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You may have seen this already, but according to this thread (below) there is one wire in the top plug on the amp which triggers the amp to play digital alert information and turn signal sounds. Possibly from the ECM unit? It's the same plug that contains the ANC wires. When this wire is cut all these alerts disappear. This may indicate that this info is NOT passed through the head unit. So amplifying signal before the amp would eliminate these sounds.

But I'm such a rookie at this that I may be confused.

http://www.chevymalibuforum.com/for...oofer-installation-non-bose-system-1lt-5.html
 

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Discussion Starter · #12 ·
I can that that the door chime and safety/demo mode announcement came through the HU output. We observed it through our tests

My experience has been base models have BT routed through the HU while premium models tend to route through the external components.

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Discussion Starter · #13 ·
I read through that thread, a good read. I see where my low level measurement seems to be the first one. My scope doesn't lie. :)

It will be interesting to see what additional tests reveal.

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I see where my low level measurement seems to be the first one. My scope doesn't lie. :)
Exactly! I was going on that thread because there was no other info, and was about to purchase an Audiocontrol LC2i to take the speaker level signal to pre-amp and recover lost bass frequencies. Your scope has saved me (and anyone else) from needing this.

It would be great to find out that everything is routed through the head unit. Then I assume we're free to amplify this signal and route to any or all upgraded speakers while retaining all other system sounds. But I just don't get why the turn signals are eliminated by cutting that wire.
 

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Did you ever have a chance to determine if everything was routed through the stock head unit? Like turn signals, warning sounds, safety chimes, Bluetooth, etc?

I'm ready to start my system and just trying to figure out if I can freely amplify my front speakers by tapping the pre-amp signal to the factory amp without losing any of this needed info.

Thanks!
 

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Did you ever have a chance to determine if everything was routed through the stock head unit? Like turn signals, warning sounds, safety chimes, Bluetooth, etc?

I'm ready to start my system and just trying to figure out if I can freely amplify my front speakers by tapping the pre-amp signal to the factory amp without losing any of this needed info.

Thanks!
Yes it is, all the output is coming from the H/U to the factory amp. I have a 2016 LT with a 4 channel amp and processor connected from the factory amp....suffering because my chimes are LOUD! One installer said he can somehow bypass only the speaker outputs from H/U to my 4 channel amp.
 
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