JBL Premium Audio System uncovered

This is a long read, but believe me, worth for those with the JBL Premium Audio System and interested in upgrading.

Tired of the poor quality the JBL Premium Audio system delivers I decided to make some changes in order to improve it to a decent level. In doing so, I captured some data that I though would be worth sharing with you.
I have attached four zipped AVI files that show the Voltage-Frequency response of each independent output of the JBL Amplifier component. I created a CD with multiple samples that I used as input source including a logarithmic 0-20KHz frequency-sweep, which was used to produce the attached results.
The Amplifier has independent drivers for the following components:
- Front Doors Woofers (digital crossover involved)
- Read Doors Woofers and Tweeters
- Front Doors Tweeters (digital crossover involved)
- Front Central Speaker (digital crossover involved)
One interesting point to note is the low frequency response (right side spectrum in the first two attachments). There is a peak response at about 70Hz, which turns very unpleasant when applied to the front and rear doors woofers, after reaching certain power level different components in the doors start to rattle. In my opinion, most pleasant bass sounds are in the range of 25Hz to 50Hz, and yes, there is a big difference between 50Hz and 70Hz. Unfortunately, as you can see in the attachments the JBL amplifier does not respond very well on that range, however the signal that comes from the head unit and feeds the amplifier shows a pretty nice response even at 10Hz!! The only reason that is forcing me to keep the JBL amp component in the car is because it contains the logic that understands all commands sent through AVC-LAN protocol from the touch screen and steering wheel to adjust volume, equalizer, fader and balance. Why is the logic in the amp and not in the head unit? I really wonder.

So, the first stage on improving the system was to replace the speakers. I selected the following brands:
- Front and Rear Woofers: Blaupunkt TS660
- Front and Rear Tweeters: Bose (same used in the Jewel Cube speakers)
- Central Speaker: UNPLUGGED!!
- ADDED: SubWoofer: Bose, Acoustimass Series.
- ADDED: Four 3.5Khz Low-Pass & High-Pass passive crossovers.

I also added a four channel Amplifier.
- Amplifier: Pioneer.

After all speakers were replaced I was astound to find that the front tweeters were delivering an incredibly awful sound compared to the ones in the rear doors, even though they were same brand, same model. To my surprise, when I rewired the front tweeters to be fed from the same signal that feeds the rear ones the sound quality was of superb quality. This simple exercise showed a definite problem in the JBL Amp component where the digital filtering (digital crossover maybe) causes major deterioration of the signal that is delivered to the front tweeters, I presume that since the signal delivered to the rear doors goes with full bandwidth, no digital filtering is involved and therefore the quality is preserved. Note that this problem does not appear to be related to output bandwidth range as shown in the attachments, but to bogus signal processing algorithms.

I actually removed the JBL amp component and opened it in an effort to find a point where I could take the audio signal before it gets fed to the power IC’s. Unfortunately, I didn’t have much luck in doing so, this is a highly integrated circuit, with multi-layer PCB and two large scale integrated chips, most likely dedicated to decoding AVC-LAN and Signal Processing (equalizer and digital crossover). As a consequence I had to use the JBL Amp speaker outputs to feed the second amplifier, not the ideal solution of course, but after all it worked pretty well.

Conclusion (to put it in brief):
- The poor sound quality associated to this system comes mainly from the power amplifier itself, not the head unit (which is somehow good news).
- For better quality the JBL tweeters need to be replaced. Ideally, woofers as well.
- There’s a major quality flaw in the amplifiers section that powers the front tweeters. Use the rear doors output instead.
- The central speaker becomes useless if the tweeters are replaced with better ones.

--Jorge

 

Yes, that is the one. Other packages have the amplifier built in with the head unit.

Well, unfortunately there is no way to pick selectively what you want, so in my case, the other features included in the package such us the NAV and the SmartKey were good enough to still consider taking the upgraded package and extra cost. But from the begining I knew not to expect too much from JBL, there are far better brands out there.
What still amazes me is the high level of technology put into this amplifier and yet the results it delivers leave a lot to be desired.

 

The signal fed to the amp from the head unit has a fixed level, the amplitude is changed internally in the amp via AVC-LAN commands. This is somehow good because it will allow you to feed an auxiliary audio input source (MP3 player, etc) directly into the amp and still be able to use the controls in the steering wheel and touch screen.

 

I have not confirmed if the beeps are generated in the amp, I'll check that real quick later today. But my first assumption is that they do get generated since you can always hear them with the same amplitude regardless the volume setting. I'll let you all know a definite answer later today.

With regards to the center speaker, as you can see in the attachment its frequency range is limited starting from about 1Khz to about 14Khz.

 

I thought about that too at the beginning, but dropped the idea as soon as I saw the results.

 

I have to rerun the test for the central speaker so that you can see the full spectrum. When I first ran that test (the attachment that you see) I forgot to disable the auto-trigger feature in the software, so it started capturing at about 1KHz. I clearly remember that below 100Hz, which is the range that you'd use for a subwoofer, there wasn't any output.

 

I tested this. The input level is constant regardless you take the signal between (+) and (-) terminals or one of them and ground.

 

I just ran a test and confirmed the beeps are actually generated from within the amplifier. The signal that feeds the amplifier does not get affected by the beeps. This makes sense so that the beeps don't get louder as you increase the volume.
This makes very complicated to figure out a way to remove them altogother.

 

When you say with the amp disconnected from AVC-LAN, do you mean by simply unplugging the TX+ and TX- terminals? My first thought would be that this will not work.