A Stereo Upgrade to Learn From

In 1994 I made a very costly mistake, I had some extra money burning a hole in my pocket and decided to buy myself an in-dash CD player and Polk speakers for my Ford Explorer. As I was picking out the CD player the salesman made me an offer I couldn’t refuse on a Polk subwoofer that was about to be discontinued. I left the vehicle with them and it was ready later that afternoon. The system sounded awesome but now I recognize what a costly mistake that stereo system was because it forever changed my appreciation for car audio.

In 1996 I found myself with a hard decision to make, I had just driven a Z3 for about a week (dealer loaner) and I really wanted one. While struggling with the decision if I should stick with the Explorer or go for the Z3, the upgraded stereo system almost kept me in that Explorer. However the allure of the BMW Z3 was just too much for me, so in October of 1996 I took delivery of a 1.9 liter BMW Z3.

The stock stereo lasted until February of 1997, with spring weather approaching I decided it was time to upgrade it. Once I had upgraded/fixed the stereo in that Z3 I was pleased with the sound. It didn’t have the deep bass my Explorer had, but the system was very powerful and clean. It could be heard with the top down at highway speed and always sounded great.

In 1998 I had another decision to make, BMW had just released the new 3.2 liter M roadster and the allure of a new muscle bound version of my Z3 was just too much to pass up. I sold my 1.9 Z3 to a friend and purchased the new 3.2 M roadster. The M roadster came with a much better stock stereo system than the original 1.9 Z3 came with. This new HK (Harmon Karmon) stereo was much cleaner and better sounding, in fact I had actually decided that it was “good enough” and didn’t need to be upgraded. I drove around with the stock HK system for three months and never even added a CD player to it.

However the stock HK system suffered a set back when I got to hear my old 1.9 Z3’s stereo system again (I see the current owner about once a month at a poker game). I had forgotten how much better and cleaner the stereo could sound. Top up or top down it didn’t matter because that system had enough power to be heard. The desire for a better stereo and a CD player in my 3.2 M roadster had infected me.

As fate would have it another M owner that I keep in touch with on a regular basis got a deal with a stereo manufacturer to be a show car for a/d/s/ stereo equipment. On almost a daily basis I got to hear the ongoing saga of what they were doing to his car, what kind of equipment they were using, what speakers sizes were fitting into the various locations. This was just too much for me to bare, I decided to start looking into upgrading/fixing the stock HK stereo system. The first step was to figure out as many details as I could about the stock HK stereo system.

Researching the stock HK stereo system

For the 1996, 1997 and 1998 model years every BMW Z3 came stock with the same head unit, regardless if it was an HK stereo upgrade or the base stereo (starting with the 1999 model year BMW started using a different head unit which will not be covered in this article). This common head unit is an Alpine made AM/FM/WB/Cassette unit. It has some nice features like weather band radio, built in theft deterrent system, speed sensitive volume, and the ability to control a trunk mounted BMW CD changer. The stock radio is pretty plain to look at but it blends in to the dash well and shares the same common orange lighting as the rest of the dash. On the downside, the FM reception on the unit is well below average and the tape player ranks right up there with names like Kraco.

Z3’s that have the HK stereo have 10 separate speakers hidden around the cockpit in various locations.

In each of the side door panels there are two speakers, a 1″ 4 ohm tweeter and a 2″ 4 ohm mid-tweeter. These two speakers are running on the same 25 watt, electronically crossed over, amplifier channel that provides a 1.5Khz to 20Khz signal. Some inline crossovers further split and control that signal so that a 1.5Khz to 3.5KHz signal drives the 2″ speaker and 3.5KHz to 20Khz signal drives the 1″ speaker.

On each side of the Z3, down by the driver’s and passenger’s feet there is a single 5 1/4″ 2 ohm speaker in the kick-panel. Each 5 1/4″ speaker is driven by its own 25 watt, electronically crossed over, 100Hz to 1.5KHz signal.

Directly behind each seat is a 3″ speaker that receives its own electronically crossed over 25 watt, 200Hz to 1.6Khz channel.

And lastly there are a pair of 5 1/4″ speakers in a subwoofer enclosure in the center of the console behind the driver. Each speaker in that subwoofer enclosure is receiving an 40 watt, electronically crossed over, 80Hz to 100Hz signal.

The HK amplifier is an eight channel amp that BMW claims is (6×25 and 2×40). However to reach those power ratings BMW had to overdrive the amp to an outrageous 10% Total Harmonic Distortion (THD). To benchmark this amp using aftermarket amp rating standards would probably reveal less than half the power that BMW is claiming. The amplifier is hidden in the trunk behind the lump on the passenger side towards the back.

HK Amp Wiring

M Roadster, 3/98 build date

J1 12-pin AMP connector

Pin Function Wire Color

1 12V red/white

2 on/off white

3 ground brown

4 ground brown

5 RBLO+ blue/purple

6 RBLO- blue/gray

7 LFLO+ yellow/red

8 LFLO- yellow/brown

9 RFLO+ blue/red

10 RFLO- blue/brown

11 LBLO+ yellow/gray

12 LBLO- yellow/blue

J2 26-pin Siemens connector

Pin Function Wire color

1 RB- IN blue/gray

2 RB+ IN blue/purple

3 LB+ IN blue/black

4 LB- IN yellow/brown

5-7 n/c n/c

8 RFHI+ yellow/red (LTwtr)

9 RFHI- brown/orange (LTwtr)

10 LBHI+ yellow

11 LBHI- brown

12 RBHI+ blue

13 RBHI- brown

14 RF- IN blue/brown

15 RF+ IN brown/black

16 LF+ IN yellow/red

17 LF- IN yellow/brown

18-20 n/c n/c

21 LFHI+ yellow/green (STwtr)

22 LFHI- yellow/brown (STwtr)

23 RFHI+ blue/brown (STwtr)

24 RFHI- blue/green (STwtr)

25 LFHI+ yellow/blue (LTwtr)

26 LFHI- yellow/gray (LTwtr)

J3 6-pin AMP connector

Pin Function Wire color

1 GAL IN black/white

2 SPATIAL n/c

3 SUB LO n/c

4 GAL OUT n/c

5 SUB OW n/c

6 SUB HI n/c

Notes:

J1 connector has the thick gauge wires

pin numbers are labeled on the connectors

Function is as labeled on the HK Amp circuit board;

RBLO+: Right Back Low-freq speaker positive

n/c: not connected;

STwtr: small tweeter;

red/white: red wire with white stripe

Knowing these details about the HK stereo system helped me in planning a better stereo system. Figuring out what to keep and what to replace is like putting a jigsaw puzzle together. You have to identify each piece of the puzzle and make sure they fit with the other pieces. Putting this puzzle together is more difficult than most cars, because BMW uses some non-standard equipment. The trouble starts with the stock head unit. It would appear that BMW knew what they were doing when they had Alpine make both the head unit and CD changer. Both units are only compatible with the other, so if you want to use the BMW CD Changer, you have to use the BMW head unit. Or if you want to keep the BMW head unit and play CDs, your only option is the BMW CD Changer. The other “non-standard” part of the radio is that it sends a 5 volt signal from the Alpine head unit to the amplifier. Not very many aftermarket amplifiers can accept inputs in this range forcing upgraders to either limit their choices on a replacement amplifier or to use a line leveling device (also referred to step-down converters).

So when you are planning your upgrade, consider the BMW head unit and BMW CD changer one unit. You either use both or neither. On top of that, if you’re going to stick with the stock head unit then you have to limit your aftermarket amplifier to ones that can accept 5 volt input, or budget for a couple line levelers as part of your upgrade. See why I say this is like putting a jigsaw puzzle together?

Putting the Puzzle Pieces together

Certain parts of my new stereo system puzzle fell into place quickly. In my previous Z3 I had used Boston Pro series 6.4 component speakers. They worked so well with the odd acoustics inside the Z3 that they were the first piece of the puzzle to be locked into place. At the time I was shopping for stereo equipment Boston Acoustics was starting to place advertisements for their new 6.5 component speakers. They weren’t available yet but plenty of stereo places were putting the 6.4 component speakers on sale to clear the shelves and get ready for the new 6.5 models. I have no idea what the new 6.5 speakers will sell for but the 6.4 speakers were normally in the $400 range, on sale you can find them in the sub $300 range.

The Boston Pro Series 6.4 speakers almost fit without modification. However the speakers did come in contact with the plastic kick panel that installs over the speaker. The solution proved to be simple, a protruding plastic ring was sanded down on the back side of the kickpanel. This provided the extra room necessary to properly install the 6.5 inch speakers.

The next piece of the puzzle involved a new device that I had no experience with. Alpine had a product they were calling Bass Shakers. Alpine was saying that these Bass Shakers could be installed under the seat and they would trick you into thinking you were hearing low bass sounds when what it was really doing was just creating the vibrations associated with low frequency bass sounds. It sounded pretty far fetched to me, but the local stereo shop set up a demo with one of these shaker things mounted to a wood box. The demonstration let you listen to a stereo and step on and off the wooden box to “feel” the difference. It was very strange to be tricked like that but sure enough, when I stepped on the box it really did seem like someone turned on a subwoofer somewhere. I still wasn’t fully convinced of this products capabilities inside a Z3, so I posted my Z3 Bass Shaker question on the BMW roadster message board. It turned out several had not only felt the Bass Shakers before, a few had even felt them inside a Z3. Once I got the virtual thumbs up from a few individuals I started shopping prices on the internet.

After a little research I found out there are a few different Bass Shaker models available. A company by the name of Aura actually makes these devices and offers two different models, the Bass Shaker, and the Bass Shaker Pro. The pro model is physically bigger, heavier, and handles more power which results in more “shake”. I believe that Aura also makes Bass Shaker Pros under the Alpine brand name. Down deep I think the Aura Bass Shaker Pro and the Alpine Bass Shaker are actually the same product, but the Alpine salesman was telling me his version was slightly larger when I was trying to get him to price match on the Aura Bass Shaker Pro. However I think I basically just paid extra for the Alpine name. On the internet I found the Aura Bass Shaker Pros for $180, but I ended up paying just over $200 for the Alpine brand.

To install the bass shakers, the seats needed to be removed. Initially the installer said I would need to take the Z3 back to BMW after the installation because removing the seats would set off the SRS light on the dash. However Larry Nissen (BMW Certified Tech) told me how to remove the seat without setting off the SRS light on the dash. The secret is to never put the key in the ignition. Every time the key gets put in the ignition it triggers a self test that will notice the seat is missing and turn on the light.

My attention then turned to the stock BMW subwoofer. Under the stock conditions the subwoofer seemed to do well up to a point but then it started buzzing and thumping. I removed and dissected the HK subwoofer to see how the thing worked and what could be done to improve it. It was too hot to be working out in the garage so once it was removed I took the subwoofer inside and using some old crossovers I had hooked it up to my home stereo system. (Note: one of the speakers gets wired backwards, reversing the positive and negative wires so the two speakers will work together by pushing in the same direction at the same time even though one is facing the opposite direction).

I was actually quite shocked at what happened, suddenly the bass got much cleaner at high volume, the muffled-buzzy sound was gone. What I learned is that the HK sub still has its limits, but if you give it a good clean power source it actually does pretty well. I could tell that this upgrade was going to get expensive so I decided to stick with the stock subwoofer. As long as I can get a good, clean, power source for the sub it was going to be fine for now (but maybe the subject of a future upgrade). Vibration was really my only concern with the stock BMW sub, especially if I was going to start sending it even more power. I had already found a fault with the rubber porting tube, so I knew I would have to secure it better. But I also suspected that I should invest $20 into some vibration dampening material in an attempt to cut down on the amount of buzzing and rattling the plastic components make as a result of the subwoofer.

I then started looking at those tiny speakers behind the seats, I just wasn’t sure what to do here. It was obvious the current speakers were not worthy of being part of this new stereo system, but what could replace them. I had seen what a/d/s/ did in their show car in regard to rear speakers and liked that solution. But one quick check of the rapidly depleting budget made this decision a little easier. I decided to physically leave the stock speakers in place, but they would not be hooked up to anything. The budget played a big part in this decision, I decided to give up rear speaker sound fill in order to save some money right now. If the desire for rear speaker sound hits me in the future I will deal with it then. However if possible, I should attempt to plan my amplifier selection so I could add rear speakers at a later date if I choose to do so.

There were two pieces left to the puzzle, the CD player and the amp. I went back and forth on the CD player. On one hand I could spend $500 on the BMW CD changer, mount it in the trunk and stick with the stock head unit. This option has many advantages such as retaining the speed sensitive volume, and weather band features on the stock head unit. It also would maintain a totally stock look to the stereo system which is practically a stealth mode shield against car stereo thieves. On the other hand spending the same $500 could get you a much better in dash CD player installed which would probably sound better and offer tremendously better FM reception. The hangup on the in dash option that eventually led me to choose the overpriced changer route was purely cosmetic. It seems the trend on aftermarket stereos now is to add lots of flashy multi-colored light displays. But what I really liked is how that constant simple orange display lights up the dash at night. I didn’t want to break up that look so I choose to purchase the BMW trunk mounted CD Changer. I’m not sure if I made the right decision here, but I figured I could sell the changer at a later date and install an in-dash CD without loosing too much money.

About a week after taking delivery of the CD Changer one additional fact came to light that might have led me to choose an in-dash player if I would have know about it ahead of time. The BMW CD Changer now has two different mounting location possibilities. I had only seen the side mounting location in the Z3 and assumed that is where the CD Changer would be mounted in the M roadster. However now there are some rumors that BMW is mailing me an M Mobility system that will fit in roughly the same area as the side mounting location making me either relocate the CD to a less desirable location (pictured) or remove the nice gray flannel cover around the CD changer. If the M Mobility kit ever shows up I’m probably going to wish I would have gone the in-dash route.

So now the only part of the puzzle left to put in place was the amplifier. A quick count of speakers tells me I need to power 6 separate channels

Boston Pro series 6.4 Component Speaker Set (Left – Front)

Boston Pro series 6.4 Component Speaker Set (Right – Front)

Alpine Bass Shaker (Left – Front)

Alpine Bass Shaker (Right – Front)

BMW Subwoofer (Left – Rear)

BMW Subwoofer (Right – Rear)

Picking the Amplifier

Choosing the right amplifier proved to be the most difficult part of this upgrade. Initially I choose a pair of amplifiers from Phoenix Gold new line of amps that were all chrome. The salesman showed me how Phoenix Gold’s QX4150 4 channel amp and the QX2100 2 channel amp could be mounted together and joined using this cosmetic piece to make them look like one big amp. We took the amps out and laid them in the trunk to see if they would fit. Sure enough they fit just perfectly, almost the entire width of the trunk. The band of all chrome across the back of the trunk looked really good too so I decided these amps were going to be the ones.

The Phoenix Gold amplifiers sounded really good, however they started overheating and shutting down after only 30 minutes of hard use. The stereo shop that installed the amplifiers diagnosed the fault to be the amplifiers and suggested trying a different brand.

The salesman suggested a more expensive Rockford Fosgate 360.6 six channel amp with the promise that it would not only run cooler but it would also sound better. This single unit six channel amplifier delivered 30 watts per channel so the overall output was lower however the salesman promised it would run cooler.

After the new amplifier was installed I found that the bass sound had improved (probably due to the rockford bass EQ feature) but the Boston Pros were not as clear and couldn’t get nearly as loud before distorting. It still sounded pretty good, but not as good as with the previous pair of Phoenix Gold amplifiers. The lower power of this amp didn’t sit well with me especially considering the additional cost and poorer sound quality, but I had resigned myself to accept it as long as this one didn’t overheat.

However after putting the Rockford Fosgate amplifier through a road test this amplifier overheated as well. After talking with Rockford Fosgate’s customer service they suggested that something was wrong with the installation. I returned to stereo shop to question the installer but they didn’t find anything wrong with the installation, and suggested I choose another amplifier. Since I wasn’t happy with the sound quality with the Rockford Fosgate anyway I started looking for another replacement.

At this point I was getting very frustrated and confused. Alan’s a/d/s/ show car was running nearly twice the power with his a/d/s/ P840 amplifier that I was with the Rockford Fosgate. And he was telling me his amps were cool enough to rest your hand on. I really wanted to just give up and retreat back to the stock stereo, but even this option had additional costs. In the end I decided to stop listening to advice from the shop that was installing the equipment. I did my own research and decided to try the same a/d/s/ P840 amplifier that Alan was using in the a/d/s/ showcar. This amplifier has 8 separate channels rated at 40 watts per channel. The amplifier also had full crossover control over each channel. With this much power and flexibility the new amplifier required a lot of thought and planning on how everything would be set up. I ended up setting the new amplifier up in the following configuration.

Channel 1&2 bridged: Source should be the front, left channel with the high pass crossover set at the lowest point of 45Hz. This signal would deliver 120 watts from 45Hz to 20kHz to the driver side (left) boston pro series 6.4 component speaker set via the boston crossover.

Channel 3&4 bridged: Source should be the front, right channel with the high pass crossover set at the lowest point of 45Hz. This signal would deliver 120 watts from 45Hz to 20kHz to the passenger side (right) boston pro series 6.4 component speaker set via the boston crossover.

Channel 5: Source should be the rear left channel with the band pass crossover set to drive the range between 80Hz and 100Hz. This signal would deliver 40 watts from 80Hz to 100Hz to the front facing driver in the BMW subwoofer.

Channel 6: Source should be the rear right channel with the band pass crossover set to drive the range between 80Hz and 100Hz. This signal would deliver 40 watts from 80Hz to 100Hz to the rear facing driver in the BMW subwoofer.

Channel 7: Source should be the rear left channel with the low pass crossover set to drive the range below 100Hz. This signal would deliver 40 watts from 20Hz to 100Hz to the driver’s side (left) alpine bass shaker.

Channel 8: Source should be the rear right channel with the low pass crossover set to drive the range below 100Hz. This signal would deliver 40 watts from 20Hz to 100Hz to the passenger’s side (right) alpine bass shaker.

Fighting Amplifier Heat

After installing the a/d/s/ P840 amplifier I was still having some amplifier heat problems. After discussing the trouble I was having with other people and with a/d/s/ I decided to give up on stereo shop that had been working on my Z3 *Earmark Car Audio in Dallas). Several other Z3 owners as well as a/d/s/ recommended I talk to Custom Sounds in Austin Texas. I eventually gave into my frustration and paid Custom Sounds to fix the problem.

What they discovered was that a couple of the speakers were wired out of phase, some line leveling (or signal knockdown) devices that were not appropriate for use with the a/d/s/ amplifier were installed, and in general the wiring was in their opinion “messy”. Custom Sounds cleaned everything up, wired the speakers correctly, removed the signal knockdown devices and also suggested installing some fans just for “overkill” to make sure I wouldn’t have any heat related problems. Taking their advice I had them wire a power source for the fans, however they were out of the particular Radio Shack fans they wanted to use so they took the time to show me how to add the fans myself.

On the way home I stopped by Radio shack. The fans were only $9.99 each and I picked up a plug as well so the fans and cover could be unplugged and removed if the need should ever arise. With four fans there is quite a bit of air flow directly over the amp. The fans are attached under the facade but blow air through the groves in the heat sink. It is a very productive setup, the fans are quiet and do not affect the sound quality.

While at Radio shack I also picked up a fancy digital thermometer that has two separate temperature sensors. I used this device to measure the temperature directly over the amp as well as in the corner of the trunk. After comparing these temperatures to those I took from other Z3s I’m ready to declare the heat problem resolved.

Six Month Update


Over six months have passed since this stereo upgrade initially began, and there are several things that have happened so I felt the need for a “six month update”. I will go into detail one issue at a time below but basically I’m happier with the stereo, but even more upset with Earmark Car Audio, the stereo shop that initially installed everything. At this point there is not much of the upgrade that was initially installed that hasn’t been uninstalled and reinstalled by someone else.

Amplifier Heat

This is no longer a concern for me, the a/d/s/ amp is running relatively cool with the help of the four additional fans. When I compared the temperature in my trunk to the temperature in other Z3 trunks during the homecoming convoy, I discovered that I am well within the norm. Most (stock stereo) trunks were cooler than mine but not by much, and any Z3 (with the exception of Alan’s) that had an upgraded amplifier were actually running hotter. I am happy to report that the a/d/s/ amplifier has never shut itself off even with extended high-volume stereo listening. The radio shack digital thermometer was very handy, during the homecoming convoy I was able to toss it into a different Z3 trunk every time we stopped for fuel. By the end of the trip I had a very accurate representation of what was acceptable and I was within that range.

Boston Pro 6.4 Speakers

I was disturbed to discover that the passenger side tweeter stopped working one day. It took nearly two months but a replacement was delivered at no cost. The speakers still sound great, but a recommendation is worthless at this point since the 6.4 model has been discontinued.

Alpine Bass Shakers

While my M roadster was on the lift at BMW one day I asked the Tech “what are those sheet metal screws”. He smiled at me and said “those are holding down your bass shakers”. I could feel my blood pressure rise as I looked at these inch long sheet metal screws ripped through the bottom of my M roadster. One of the screws had been bent (probably from a speed bump) and the installer hadn’t even put any kind of sealant around them to stop water/rust. The BMW Tech cut the screws off so they didn’t stick down quite so far and added some sealant but the damage had already been done. About a month later that bent screw finally gave up and the drivers side bass shaker shook itself loose and was now a bass rattler.

One weekend I ended up taking out both of the M roadster’s seats and then removed the bass shakers. I reinstalled the bass shakers using nuts and bolts and a generous amount of sealant. Once everything was reinstalled the bass shakers were working as good as they had ever worked, however my opinion of them has gone way down over the last six months. Their shaking affect is quite impressive when the car isn’t moving. However once the car is moving at more that 30mph the slight vibrations from normal driving seem to cancel out the bass shaker affect. Maybe they work better in other cars, but the sports car (stiff) suspension in my Z3 appears to make the bass shakers a poor decision.

BMW subwoofer

Finally some good news, for several months a/d/s/ has been using my BMW HK subwoofer to develop a upgrade kit. I haven’t received the finished product yet but they are telling me they have made some improvements to the stock enclosure and are now using two new drivers. I can’t wait to hear it in my car.

Rear Speakers

The desire for rear speaker fill has hit me, I’m going to hold off until I get the a/d/s/ subwoofer installed but I suspect I will be adding some rear speakers (myself) sometime this summer. I will probably work them into the same channel as the bass shakers or a/d/s/ sub via some external crossovers.

BMW CD Changer

The rumors about the M mobility kit and its interference with the side mounted CD changer were correct. To get them to fit in that location the gray flannel cover must be removed. It might be my imagination but the CD Changer also seems to be skipping more frequently. I’m starting to second guess my decision to keep the BMW equipment rather than get an indash CD player.

a/d/s/ Subwoofer

My HK subwoofer was rebuilt by a/d/s/ as a development project while they were evaluating the possibility of making a speciality kit. The product they created works very well, however it was determined that the stock HK amp was not powerful enough to truly drive the (now) a/d/s/ subwoofer with enough power to audibly tell the difference between the stock sub and the much improved a/d/s/ subwoofer. As a result of this, a/d/s/ canceled the project.

The individual within a/d/s/ that had done the research and development of the prototype replacement subwoofer got permission from a/d/s/ to build the subwoofers on his own for those HK owners that had also replaced the stock amplifier. However even that project was canceled with the news that BMW had a new HK subwoofer with the model year 2000 Z3.

So as it stands I have the one and only a/d/s/ prototype subwoofer. I do not have any details about the drivers, they were specifically unmarked so I can’t tell you anything about them from looking at them. All I can add is that they sound very good and its a shame the way things worked out.