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Factory Subwoofer Amplifier and Bluetooth Adapter Frequency Response Curves

Introduction
New Subwoofer and Enclosure
Bluetooth Adapter Frequency Response
Factory Subwoofer Amplifier Frequency Response
Factory Subwoofer Amplifier Power and Efficiency
Soundstream Stealth Subwoofer Amplifier

Introduction

My 2000 Land Rover Range Rover Rhino/HSE (P38), with the "High Line" radio in the electrical troubleshooting manual, has been through quite a few subwoofer iterations since I bought it in early 2007. The original factory subwoofer system consists of a pair of 6.5" 8 ohm subwoofers (in parallel) in a ported composite box fed by a Harmon Kardon amplifier. The original subwoofers had cracked foam surrounds and the previous owner was running the factory HK amp output to a speaker-to-line-level converter. He could have just tapped into the line-level input and added some RCA connectors, but I guess that isn't obvious unless you have the wiring diagram. He ran the line level output to a Sony amplifier and a pair of Rockford Fosgate 12" subwoofers in a huge sealed box. It sounded great, but it occupied far too much of the cargo area and it was way too heavy to be moved easily. I sold the aftermarket stuff on Craigslist, replaced the blown factory 6.5" subwoofers with JL Audio replacements, and reconnected the factory amp to the factory subwoofers. It added some decent reinforcement, but it didn't hit all that low. I added an auxiliary input adapter in place of the CD changer. Some time later, I ditched the factory subwoofer box and mounted the factory amp to an aluminum plate. I used that to drive an Audiobahn (remember them?) ALUM12Q (awesome subwoofers in the right box) 12" subwoofer in a random several cubic foot slot-ported box. That deck-of-cards sized factory amplifier did pretty well with that setup, but eventually I got tired of having another big heavy box consuming my cargo area and I was not thrilled with the extreme low-end frequency response of that setup. I ditched the Audiobahn and random box, then lived subwoofer-free for about a year.

New Subwoofer and Enclosure

I finally got tired of the anemic sound and decided to attempt a balance of of size, weight, and frequency response. I came across a bunch of sonotube (concrete cylinder form) subwoofer projects in my search for lightweight subwoofers. The theory makes a lot of sense. The cylindrical shape can be very thin since pressure and stress are evenly distributed, which results in minimal flex. A 1/4" thick 14" cardboard tube with MDF end caps will be much lighter than an equivalent volume rectangular MDF box. I decided on a large sealed box for a single 12" subwoofer. That should manage a good balance of low frequency response, reasonable size, and sound quality for my purposes. I selected a Dayton Audio Ultimax UM12-22 and a 3 ft^3 sealed enclosure that should hit at least 30 Hz at -3 dB before factoring in cabin gain, according to
WinISD. The end caps are 2x0.75" MDF and I used 0.5 lb/ft^3 polyfill. After gluing the end caps in, I coated the inside and outside with polyurethane, specifically Minwax Indoor/Outdoor Helmsman Spar Urethane, which makes the sonotube stiffer, more durable, and water resistant. Here's the unpainted/uncovered enclosure with two coats of polyurethane. It's ugly, but highly functional. Theft deterrent? ;)


Bluetooth Adapter Frequency Response

I hooked up the new subwoofer to the factory amplifier and played some test tracks. It sounded good parked in my garage, decent while driving with the windows up, but absent while driving with the windows open. That led me to break out my oscilloscope and do some
measurements. The measurements didn't make a lot of sense at first and I eventually discovered that the frequency response of my HTC Car A200 Bluetooth adapter is terrible in the subwoofer region. The bass was audible, but much quieter than the rest of the music. The adapter works well overall, but I now do not recommend it for anything capable of reproducing low bass. I used the PA Tone app on my Google Nexus 10 tablet and the RMS voltage calculation on my oscilloscope to record points at different test frequencies. Then I double-checked a few points using my HTC One phone as the source to make sure the problem was the Bluetooth adapter and not my tablet. Here's the plot.


Factory Subwoofer Amplifier Frequency Response

After figuring that out, I used a wired aux-in connection instead of Bluetooth and then measured the input and output of the factory subwoofer amplifier. I figured these measurements would help me set the subsonic and crossover frequencies when using an aftermarket amplifier with my new subwoofer. Here's the plot.



The factory subwoofer amplifier has a subsonic filter (high-pass) frequency around 25 Hz and a crossover (low-pass) frequency around 70 Hz. After using a wired connection instead of the Bluetooth connection, it sounds awesome. Great low frequency extension and surprisingly loud for a tiny factory amplifier. I decided to get an aftermarket amplifier so I will have control over the frequency range; a little extra headroom will be really nice too. I decided on the Soundstream Stealth ST1.700D for its efficiency, features, and small size.

Factory Subwoofer Amplifier Power and Efficiency

Here's the factory Harmon Kardon subwoofer amplifier.


I performed the frequency measurements above at pretty low volume, just to understand the filters built into the amplifier. I thought I read on a spec sheet somewhere that the factory subwoofer amplifier was 150W, but that assumption was way off (as pointed out by a fellow rangerovers.net forum member). I performed output voltage measurements with a 4 ohm non-inductive 100W test resistor. I turned it up to the onset of clipping and recorded a screen capture with Vrms on my oscilloscope. I measured supply current with my clamp-on current meter. Then I replaced the resistor with a 4 ohm subwoofer and took another screen grab at the same volume. The results are humorous and eye-opening. At 50 Hz, I measured 7.67 Vrms @ 4 ohms = 7.67^2/4 = about 15 Wrms. I also measured 2A @ 13.85V = about 28 Wrms at the supply to the amplifier. So, the amplifier is about 15/28*100 = 54% efficient. Even maxed out with tons of clipping, I did not get over 9 Vrms or close to 20 Wrms. With the right subwoofer and enclosure, I'm still amazed by what 15-20 W can do. After disassembly, I found out the amplifier chip inside is a Toshiba TA8221AH.


Soundstream Stealth Subwoofer Amplifier

I mounted the amplifier, connected all of the wiring, and connected a non-inductive 4 ohm test resistor. With the car running and around 13.9-14V from the alternator and battery, the ST1.700D produces around 27 Vrms at the onset of clipping with a 50 Hz test tone. I measured this with an oscilloscope. That works out to around 27^2/4 = 180 Wrms into a 4 ohm load or 27^2/2 = 360 Wrms into a 2 ohm load. I set the subsonic filter frequency all the way down and I adjusted the low pass filter around 90 Hz. I'm a little disappointed that it doesn't make the rated RMS power with reasonably low distortion, but I knew the ratings weren't CEA 2006 compliant before I purchased. However, the price and size are right for my application and it sill produces over 12x the RMS power of the factory subwoofer amplifier it's replacing. Plus, it sounds good, so overall I'm happy and I'll stick with it.

But I'll say it again, this amplifier will distort like crazy at the rated power of 250 Wrms into 4 ohms listed on the Soundstream website. It will produce the roughly 31.5 Vrms required, but only with lots of clipping and distortion. If you like smooth sine waves instead of square waves, consider this a 180 Wrms amplifier when the gain is properly set.