This project was an opportunity to test an upgrade path which some of you already know (I’m looking at you, Ray, Derek, and Chris).

The last time I got to work on one of these trucks was 2014, and I didn’t know what Bose was up to back then. Now, armed with better understanding, I signed up to help my friend upgrade his 2016 GMC Sierra. To be fair, he was really after a CarPlay upgrade (which we will discuss in another article), but I managed to get him onboard with audio as well.

He’s not an audiophile. He’s a cinephile – his day job is video production – and he likes good audio, but he’s not an audiophile.

For those of you who haven’t already committed this vehicle to memory, Bose has a 7-channel amplifier, in this case getting its signal off of the MOST 50  “MOST-over-copper” network. GM seems to be moving away from MOST 50 to a new “standard” called AVB, as of 2019, but the concept here is the same.

These Bose systems lack a center channel, and they lack any unusual settings in the tone-control menu. Using the Educar TestTune app and following the instructions, we get a center image from either front seat. In point of fact, from the driver’s seat, the image was maybe 2 inches left of center. From the passenger seat, the center image was, well, in the center of the windshield.

Now, knowing what I know about how comb filtering works, and how phase equalization works, I am sure that if I ran through the 20-20kHz range with a sine generator, I could find frequencies that weren’t in the center. I don’t care. I tested with the same four segmented pink noise bands used in the Audiofrog UMI-1 test CD, and that was close enough for me from a practical point of view.

So we know – based on the article last month on The Six Stereo Presentations – that this system is using Two-Seat Phase EQ. The plan is to retain it, but during the tuning process, I tested three different approaches.

In this article last week titled Use the OEM Speaker Sizes, I talked about some of the benefits of the 2-and-6×9 arrangement. This truck uses it too, and for more on why we retained that arrangement, please read that article. We used an Audison AP2 in the dash and an Audison AP690 woofer in the door.  Both required us to make adapters – in the door, pretty thick adapters to keep the magnet off the window glass:

The signals from the Bose amp were passed into the new DSP amp.

We left the OEM Bose amp on the OEM Bose speakers in the rear.  We left the subwoofer in place in the console, disconnected, and we ran the sub signal, the door signal, and the dash signal into the Audison AP F8.9bit DSP amplifier. The output of the Bose amp was 8.8V AC at full rail, without clipping.

That amplifier is 85 watts by 8 channels, and we ran it in a “5-channel mode”, where we sent 85 watts to the dash and front doors, and then bridged the back 4 channels down to two, and send 2×260 to a single Audison Prima APS10D dual-voice-coil subwoofer. We loaded the subwoofer into a vehicle-specific enclosure from Ryan at SRQ Customs in Sarasota, Florida.

System Equipment:

Audison AP F8.9bit DSP amplifier in “5-channel” mode

Audison AP2 wideband speakers in the dash

Audison AP690 6×9 woofers in the front doors

Audison APS10D DVC subwoofer in a vehicle-specific enclosure

Now, if you use this amplifier in this application, and you define the inputs as Front High and Front Low (which is technically what they ARE), the amplifier’s DSP section will sum the high and low channels back together and try to get a full-range signal out of them. I didn’t want to do that, because I suspected that those channels – which have a LOT of overlap – were not in phase with each other at all frequencies. I tested, and that theory was correct.

So, I set up the inputs as Front and Rear, and I didn’t use Audison’s de-EQ or summing functions. I just passed them straight through, and used the OEM crossover filters.

Same for the sub signal. This was pretty easy, as it skipped a few steps common to most installations, and let me get right to the tuning step.

During tuning, I wanted to do three tests. The first test was using level ONLY, with L and R tied together, and tried to get a good-sounding response by getting the relative levels of dash-to-doors-to-sub correct. This depends on the OEM equalization being decent even with new speakers. I can report this worked the charm. The stereo location tests were identical – you can upgrade this vehicle with just a basic 5-channel amp and still get much-better-than-average imaging and great tonality.

For the second test, I left L and R levels tied together, but I used equalization. I kept L and R equalization tied together as well, and I put a mic in the driver’s seat and adjusted both sides at the same time. I did NOT use delay at all. The result was a better-sounding system, with the same imaging results as before. This is the result I shipped the truck with, but I wanted to do one more test.

For this third test, I unlinked L and R level, and I unlinked L and R equalization, and I did a “normal” one-seat tune, where left and right levels are matched at the driver’s seat, and left and right response are matched at the driver’s seat. However, I still used no delay. Instead, I relied on Bose’s phase equalization processing to get rid of the worst of the path-length-difference-caused phase cancellations. This approach gave me the best sound, and tightened up the imaging quite a bit.

Why did I do all these tests? For two reasons. The first reason is, I love to see what’s been done in the area of two-seat presentations and how to retain them. Secondly, using an external preamp in this vehicle adds a significant cost, and also deletes the two-seat processing. If a good result can be attained without the external preamp, whether it’s a one-seat tune or a two-seat tune, that is important to know.

Now, remember, this approach depended on my use of OEM speaker sizes. Going off-book and using other speaker sizes would cause problems and break what made this work.

Takeaways:

• This truck can be upgraded with a standard 5-channel amp, either with new OEM-sized speakers or with the OEM speakers, plus a sub.
• This truck’s 2-seat presentation can be retained, with or without DSP.
• DSP can retain and improve the 2-seat presentation’s sound, as long as you don’t sum.
• A one-seat presentation can be achieved in this truck without an external preamp.

Thanks to Musicar Northwest, especially Patrick Rollins and Pierce Barrett, for their assistance.

Thanks to Elettromedia and Elettromedia USA for their assistance with this project.

Thanks to Ryan Pepsin at SRQ Customs in Sarasota, Florida.

Finally, thanks to Andy Wehmeyer of Audiofrog for explaining to me years ago what Bose was doing, and for suggesting this approach in the first place.