I am adding this statement on 9-28-2006 to avoid confusion. Further analysis and testing proved out that it is the PAC module producing this rolloff, not the NAV head! I will leave the info for it's own sake but please see http://forums.corvetteforum.com/show....php?t=1507261 for the updated info.

I installed a PAC OEM GM24 interface to get RCA outputs from the NAV head in my C6 vert. Today we hooked that output up to my buddy's test rig PC to see what we were working with. Had mixed results as there seems to be a built-in "loudness" into the output of the head. I triple-checked that all the auto EQ was off, the tone controls were all set to their center positions, auto-volume was off, etc so as to get the most pure possible output.

Here's what we were hoping to see, the pink noise track from the outputs of a CD player, about as close to flat as you'll see.



OK, so this one is at a low volume level, we were looking good at this point.



This is at a medium volume setting, you're seeing the bass roll-off now.



And it just continues at high volume.



Some notes from my buddy, framed as counterpoints to misconceptions:
Have to think a bit on what I'll do based on these results. Even with a CleanSweep you'd have to leave the gain very low from the HU to keep the output flat and I don't know if that will be enough signal level to produce listenable levels with the top down. May just decide to live with the roll off.

If anybody knows of a buried setting that I just missed I'd love to hear it.. but man we scoured the book and pressed every button we could making sure we covered the bases.

 

Very interesting. I was wondering how the head unit with the PAC unit performed. As you know, I left the HU and added the PAC AOEM GM-24. I'm fairly happy with the sound with a CD player, but it blows with FM and XM. I figured that was b/c of compression, but now I wonder.

I'm currently using the 2 JL amps, the 650s in front and a (temporary) Bazooka tube in the rear. One thing I'd definitely recommend is you get the amp that has the remote bass control ****, as I'm always having to adjust the bass depending on the source and music. I got a 250/1, as it should easily power the 2 8's I'm going to put in, but it doesn't have the bass control ****, and for that reason alone, I've considered upgrading to a 500/1. Meanwhile, I'm twidling my thumbs waiting for my subs.

 

Actually I think you're far better off basing your opinion on CD than either FM or XM. XM's sound is far more like mid quality MP3, that flat, compressed sound and FM is... well FM. If it sounds good on CD then I'd take it from there.

You mentioned having to adjust the bass.. have you noticed (or perhaps now will make note of) whether you need to increase bass relative to increased volume? That would be the real issue based on our findings as bass rolls off during an increase in gain.

 

I seem to get a drastic increase in base as the volume goes up with the same set up as Craig. A loudness compensation would be a great addition. Absolutly flat audio perforce is not necesarily optimal in a moving vehicle. I'm running the GM-24 with two 10in subs in the doors, a JL Stealth, modified rear speaker compartments with DLS 6 1/2 and DLS 4in components in the front. I'm very happy with the set up even if I have to adjust the bass a low volume.

 

I haven't noticed whether it varies with volume. It would also be interesting to see how I have to adjust it as it varies with speed (I mostly drive with the top off).

Currently, I'm using both the bass control on the HU and the fader, as I'm only running a front channel. I'm using the rear channel for the sub; therefore, the fader acts as a bass boost. I like this setup, but it is a pain to get to the fader area and back while driving. I really do think a bass control ****, to infinitely adjust the amp supplying the subs, right near the center console would be ideal.

 

Agreed, absolutely flat response is not desirable, but what's being done is the opposite of what would be best to overcome road noise. What is happening is that bass volume is being LOWERED due to that loudness compensation, not increased. There is a perception of an increase in bass because of where the rolloff begins, but at truly low frequencies the volume is being reduced to compensate for the stock speakers.

This from the JL Cleansweep page describes what I'm talking about:

Here's a link to that page:

http://mobile.jlaudio.com/products_c...php?page_id=90

This is a link to the PDF on that page that has easier to see versions of the plots:

http://mobile.jlaudio.com/pdfs/CleanSweep_RCC.pdf

It's looking like I'll be adding a Cleansweep to my list of purchases.

 

Komoman: Please forgive me, I'm a noob. Your results show that the only way to stop the sub frequencies from being "rolled off" by the head unit would be to add a remote bass control **** and manually adjust it as you turn the bass up. Is that a correct assumption?

Thank you for posting the results.

Ryan

 

Ryan,

The way around this is to use something like the JL Cleansweep which takes the signal and "flattens" it through signal processing prior to sending the signal to the amplifier.

I seem to be the only one with these findings and it has been suggested that the PAC OEM GM24 interface I am using may be the culprit. I am going to be bypassing the "noise inducing black box" (as Steve Germany calls it) part of the module and re-evaluating the output. I will again post my results when I have them.

Dan

 

My original set up included the line level adaptor form the PAC GM-24 but I quickly realized that I was not going to be happy with it. I by passed the black box (line level adaptor) by cutting the color matched input wires and hooked RCA jacks directly to the inputs of the PAC black box. I've been in sonic bliss ever since!

Can't wait until we get to see what Komoman (Dan) comes up with on the signals (not that it will change my set up) but I'm curious on what pleases my ears!

 

Steve my understanding is that the PAC module isn't a line level adapter but a breakout box with gain controls. The wiring diagrams in the service manual don't show any speaker level (amplified) signals coming out of the head, they specify "low level" as a matter of fact. OK, now I have to go out to the garage this morning and do some splicing.

 

Sounds correct to me! Now let's see if you notice the difference with that box out of there!

 

Thanks for the response, Komoman.

Steve: If there is no need for the black box, what exactly does it do besides boost the signal? Million dollar question: Do the RCAs really need a boosted signal? From what you're saying, no .

I'm about to finish the install of my sub today (nice timing ), and I'll probably just use the box for the time being. With that in mind, I've never used one with gain control.

Where do you start the settings on the gain control? I was assuming to just leave them like they cam from the factro...?

Ryan

 

BIG difference.. I'll write up what I did later because it's getting too close to kickoff now but I am sure glad the evil little black box is gone!

 

Ryan,

The PAC OEM GM-24 was never designed to be compatable with the Vette NAV unit. In fact if you check the PAC web site it does not list the NAV C6. I'm not sure what the function is with the Base C6 (impedance matching??? stereo front back loudness level adjustment???) all I know is that it is not needed and all adjustments can be achieved through amplifier adjustments.

 

 

Steve,

I gotcha. I am using a PAC converter, but it is for a CD Bose radio. Big detail.

Thanks for the info, though. Nice to know .

Ryan

 

I'll post a quick write-up tonight with pics when I get home about how exactly I bypassed the PAC but I wanted to share the results here.

After splicing in the RCA's I re-calibrated the Cleansweep at the same level I had when using the PAC and then played the 2 CD's I had listened to just before doing this so I had a good A/B. The difference was rather dramatic. The immediate difference was that no matter how much gain *increase* that module said it had the signal output was obviously much higher without that in the signal path. While there previously wasn't distortion evident the sound is kinda like a veil was lifted off the speakers. Everything was tighter, highs clearer, bass punchier.

Thanks for putting the idea in my brain Steve, it was well worth the effort!

 

I really thought at first I was going to miss the HUD thats integrated into the Bose system when I bought my vette new a few weeks ago! At the time I didn't realize it, but I'm glade I went with the base model since having a very high end aftermarket audio system is a must for me and I would have lost the usage of HUD anyways when I upgraded to a better system.....MONEY SAVED! It seems even after spending hundreds on trying to make the factory "junk" preform up to par in an upgraded setup, it'll NEVER be as good as an all out Eclipse or Alpine HU in sound quality.

 

Can't so therefore won't argue the point.

I was lured by the cool factor of the HUD integration but if I were to do it again I would have gone with the non-nav and started completely from scratch. The NAV head isn't awful but you're absolutely right, it ain't Eclipse or Alpine quality.