HK Sub Dissected

You can get to the HK sub by snapping off the lid on top of the rear compartment. The lid is attached in four spots. The forward snaps will give way first. Just make sure to keep the lid flat and pull straight up.

The subwoofer is held in place by four screws. Be careful removing the screws because if you drop one it might be difficult to find. Once the screws are removed, you can unhook the wiring harness that attaches the HK sub (it’s tough to undo the first time).

Once the enclosure is removed, you can see how the process was designed to work. One speaker is facing forward and is ported out the front grill into the cabin. The rear speaker is not ported at all; it is wired in reverse phase and is solely designed to assist the front speaker (not to be heard on its own).

The HK sub has two speakers within the plastic box. The rear speaker (pictured on the right) is exposed. The wire plug (coming out of the left side) contains four wires, two for each speaker. (Click on any of the pictures for a larger view).

Looking inside the port hole you can see the second speaker. It is identical to the one on the back. Inside the cavity there is some stuffing. Playing around with the sound characteristic, I found that the sub sounds better at low volume with the stuffing removed, but it sounds much better at high volume with the stuffing. I say high volume, but truth is these are speakers that just can’t handle very much power. The HK sub is a really good sounding speaker up to a point, then everything starts to buzz and rattle.

Some stickers on the speakers claimed 2 ohms, but using an ohm meter, I tested each individual speaker at 3.4 ohms. It would be very interesting to try and find some really good aftermarket speakers that would fit in the same enclosure. I measured the speaker mounting points – it was 4 1/4 inches from the outside of one hole to another and 3 3/4 inches from the inside to inside (click on the picture to the right for a larger view). If anyone finds a possible replacement speaker that will fit, please tell me about it.

You can see there really isn’t much to this design. It’s a plastic molded speaker housing that has just enough room to contain the two speakers. Because of the compact design, there is not a lot of room for air to move. I think the tight design really leads to high air pressure, and the paper/cloth drivers perform up to a point but then the increased air pressure just restricts their movement and they start to distort. But all in all you have to really be impressed with the capabilities of such a small design. I think they could be improved upon, but not bad for stock speakers.

Beating the Buzz

After taking delivery and listening to the “upgraded” HK stereo I was very let down. The rear subwoofer rattled and buzzed whenever it got loud enough to actually be heard. To me it almost sounded like the speaker was blown. When I finally got to compare my stereo to Alan’s HK I knew something was not right.

Turns out a rubber, snorkel-tube that is designed to port the sound to a lower position had come loose and was vibrating. The vibrating rubber tube sounded just like what a blown speaker sounds like.

The good news is it was really easy to fix. The speaker grill just pops off, start with the lower corners then the upper corners. With the grill removed you can see this silly little rubber tube thing. The tube just sticks into the open hole in the subwoofer, a couple raised rings try and hold it in there. To me it seemed like it would only be a matter of time before it worked its way loose again. I was considering putting a couple drops of glue on it to help hold the tube in place.

On a whim I tried listening to the stereo with out the tube in at all. Honestly I could not tell a difference with and without the tube so I decided just to leave it off. It’s a piece of cake to reinstall it later if someone convinces me to do so, but for now the rubber tube is tossed onto the pile of other BMW parts in the corner of my garage.

UPDATE 5/10: The subwoofer sounded good at low volume with the tube removed, but with the top down and the stereo at high volume the subwoofer started to sound muddy. After a little experimentation (this time with the volume turned up high) I found that the rubber tube does make the subwoofer sound tighter at high volume. So the rubber tube came back off the pile of parts in the corner of my garage and is now back with the sub woofer. I was tempted to put a couple drops of super glue on it so it would stay, but decided to give it a try without glue first. If the tube works its way loose then I’ll glue it back in.

1.9 (Non HK) Stereo Upgrade

Pros: Factory Look, No Visual Changes, Much Better Sound
Cons: Still not much bass sound, Not BMW parts (warranty?)
Cost: $500

Radio: BMW AM/FM Cassette

CD: BMW 6 disk CD Changer

Amp: Precision Power 100iX

Spkrs: Boston Accoustic 6.4

Crossovers: Boston Accoustic

Most Z3 owners will agree that the factory stereo just does not cut it. Unfortunately, BMW has done a real bang-up job in deterring owners from upgrading by mixing in some proprietary technology in the stock stereo system. Any owners not happy with the performance with the stock stereo system (which is most) were left with minimal alternatives.

One alternative surfaced when BMW responded to the numerous complaints by offering a “Stereo Upgrade” that could be added to any roadster built before June 1997. This option added two small 3 inch speakers behind the seats, a sub-woofer between the seats, and all the brackets and mounting hardware needed. The down side(s) to this “upgrade” are

1. You loose the storage space behind the seats because that space will be used for the subwoofer (subwoofer is optional).

2. The replacement amp is better than the stock amp, but still not up to aftermarket standards.

The upside to this option is that the price is very reasonable. BMW sells the upgrade (without the subwoofer for $150). For many, this was the best solution available.

A second alternative usually surfaced after talking to a car stereo shop: “Scrap Everything and Start Over”. Several roadster owners took this route rather than give BMW even more money, especially after the BMW “Stereo Upgrade” started getting mixed reviews. These owners usually started by trying to replace the factory speakers, only to discover that the factory amplifier has a non-adjustable internal crossover that is matched to the factory speakers. So if the speakers were replaced, the amp should be replaced. Once the stereo salesman got on a roll, the roadster owner was then informed that the stock radio has a proprietary plug that would not connect to after-market amplifier, so a new head unit (radio) would also need to be purchased. To finish the domino effect, those roadster owners that purchased the optional BMW CD-Changer then found out that it has a proprietary connection to the factory head unit (radio) so the CD-Changer would need to be scrapped as well. Roadster owners that took this alternative usually ended up dropping some serious cash but left with an outstanding stereo system (and a box of slightly used, expensive BMW stereo components). This option (although expensive) was usually a good value and was the best alternative for those roadster owners who did not purchase the BMW CD-Changer.

Neither of these alternatives fit my situation, so I starting researching for another alternative. I started by creating a list of rules for myself to stick to. Hereafter referred to “Robert’s Rules of Stereo Replacement”.

1. Had to keep the factory BMW CD Changer. I fought very hard to get this thrown in with my roadster purchase and was not about to throw it away.

2. Had to keep the factory BMW head unit (radio). Turns out the head unit is an Alpine-made radio that is quite good. It also has a few really nice features that I wanted to keep (speed sensitive volume, CD changer controls, built-in code alarm with flashing red light).

3. No loss of trunk space.

4. No visual changes.

It took me a lot of research, but I finally found a solution that I would be happy with that didn’t violate any of “Robert’s Rules”. The breakthrough came when I found a way to connect the stock BMW head unit (radio) to an aftermarket amp. This is accomplished by using a device called a “Line Leveler”. This device enabled me to keep the factory head unit and connect it to an after-market amplifier. Best of all, it only cost $20. The factory head unit puts out a low, 2.5 volt signal, but this device increased the voltage and adapted the connection to industry standard RCA plugs. Now I could keep the BMW CD-Changer and BMW head unit.(Robert’s Rules #1 & #2)

The next step was to pick a replacement for the factory amplifier. Before I tell you about the amplifier I purchased, I have to tell you about the factory amplifier. This amplifier was the definite weak link in the stock stereo system. Documentation claimed that this amplifier produced 20 watts per channel into six channels for a total of 120 watts. In the fine print of that documentation it is revealed that the distortion (THD) at that output level was a whopping 10%. In comparison this is 200 times the distortion level of the amplifier I purchased as a replacement. First of all, it is my opinion that this amplifier could not produce 120 watts, even if lightning struck it. It would probably be rated as a 20-25 watt amplifier at 0.05% distortion (the semi-standard after-market amplifier distortion level). The other questionable move on BMW’s part is the fact that only 4 of the 6 channels are even used, so while it might be true that the amplifier could produce 120 watts, only 80 of those watts would be sent to the speakers. The last comment I’ll make about the factory amplifier is that there was absolutely no name, label or identifying mark on it (I guess no one would ever claim to have made it).

Okay, back to picking out a replacement amplifier. To keep from breaking Robert’s Rules #3 & #4 I decided I needed to find a replacement amplifier that was at or near the same physical size. This would enabled the installer to put the new amplifier in the same location without any loss of storage space. As luck would have it, I found an amplifier that was roughly the same size, of high quality, two channel @ 50 watts per channel (0.05% THD), and best of all it was on sale for $149. It was almost a perfect fit; I could have gone with another lesser-amplifier that would have been a perfect fit but this one was close enough. For those willing to sacrifice Robert’s Rule #4, you can get an even more powerful amplifier that is rated for four channels and through the third and fourth channel add rear speakers or a subwoofer.

The final step was to pick out replacement speakers. Stock, the roadster has six speakers (2 woofers, 2 midrange, 2 tweeters). I first started looking for an after-market three-way component speaker set but quickly found out that these are rare and I would have better selection, performance and value if I looked for an after-market two-way component speaker set. Three brands quickly jumped to the forefront. (Let me qualify that statement: hundreds of speakers are available, but I had found a car stereo shop that had worked on roadsters before. I really wanted them to do the work so I was limited to the speakers that they offered). The brand I wanted to purchase was Polk, since all my home stereo speakers were Polk. Unfortunately, the only place in town that sold Polk I wasn’t comfortable with in the installation area. I reluctantly ruled the Polk brand out. The two that were left were a/d/s and Boston Pro. Both sold for the same price so it came down to the sound show-down. Both speakers sounded good but the Boston Pros were just a little too shrilly (the salesman nicknamed them diamond cutters). The a/d/s sounded much deeper (ie lower bass), so I chose them. However, once the install was complete, these speakers sounded horrible in the roadster. All the sound seemed to come from the floor and it was horribly muddy. These were excellent speakers; they just didn’t do well in the unique locations in the roadster. So out came the a/d/s and in went the Boston Pros. The overly high ended Boston Pros suddenly became perfect once installed in the roadster. The extra high end is especially appreciated with the top down. The other bonus with the Boston Pros was that a 6.5″ driver fit into the factory position. The new speakers had external crossovers, but these we were able to hide under the amp.

Let’s review: new amplifier, new speakers, old head unit, old CD-Changer, no visible signs of a stereo upgrade, tons of audible signs of a stereo upgrade, and most important, Robert is happy.

This information isn’t really important, but some readers might find it interesting. The install process was done by Earmark Audio in Dallas. Very nice guys, but I did have to yell at them a little (okay, for awhile it turned ugly). Initially, I was sold 6.5″ a/d/s speakers and when they didn’t fit, Earmark swapped down to the 5″ driver without telling me. After the install I caught it and the finger pointing started. I never really resolved who did what, and if it was on purpose or not because I ended up with the Boston Pro 6.5″ speakers. The second trouble I had was leaving with a rear tail light not hooked up. After a friend almost rear ended me he informed me of the non-functioning tail light so I returned a little pissed to Earmark. They apologized and quickly hooked up the brake light, perhaps a little too quickly because when I left Earmark (turning right out of their parking lot) I discovered my right rear turn signal didn’t work (they busted the bulb). I stormed in once again and they quickly found out that they had busted the bulb re-installing the tail light assembly. I drove down to the local auto parts store, bought new bulb, and replaced the busted one in the parking lot.

Hold on… I’m not done yet (not even close). Over the next couple of days, I noticed a high pitched sound; this is commonly referred to as alternator whine. I returned once again, and they blamed the sound on the gain control (adjustment on the amp). They turned the knob down and it went away. However, the next morning I heard the whine again but rather than drive across town so someone else could turn a knob, I crawled into the trunk, peeled back the carpet and turned the knob down myself. That is when I saw it. A screw hole through the wheel well. Needless to say I hit the roof. It was a bright sunny Saturday morning and Earmark was packed with customers when I entered took one step inside the store and loudly asked “Who’s the Ass-Hole in Charge!” (which produced a manager quite quickly). He initially took a defensive stance and escorted me outside to discuss the situation. The conversation pretty much ended when he saw the hole himself. He promised a serious ass-chewing and monetary loss for the individual involved. He also was pissed when he heard the alternator whine (which ended up being that the speaker crossovers were too close to the amplifier). The final nail was when I told him of the brake light, turn light, and speaker swap .

The entire install process spanned two and a half weeks which is totally unacceptable, but the end result is exactly what I was wanting. The stereo is clearly audible top up or down, at any speed, and there is no visual indication of any upgrade. The final install is very clean, very professional looking. I think I just had a run of bad luck at Earmark. I almost can’t believe I’m saying this, but if the situation arises I might even give them my repeat business.

Alan’s 1st Stereo Upgrade

Pros: Great Sound, Great Bass
Cons: Lost trunk space
Cost: $2000

The top picture is of my trunk with half of its space taken by the Kicker 10″ SoloBaric subwoofer and Xtant 4180c amplifier. I find this to be adequate for weekend trips–a couple of soft-side overnight bags will still fit in the remaining space.

The second picture shows the trunk with the subwoofer removed. The two small boxes at the front of the trunk are the Alpine crossovers that came with the DDDrive 6.5″ drivers and tweeters. Note that the amp is forward just enough to still be able to raise the panel for access to the battery and toolkit.

The last picture is a close-up of one of the Alpine crossovers. Note that the Xtant amp has built-in crossovers. Mine are set for a 90 Hz low-pass crossover for the rear channels, and a 70 Hz high-pass crossover for the front channels. The rear channels are bridged to power the subwoofer. The front channels are fed to the Alpine crossovers. These 2nd crossovers then further split the signal to send highs to the tweeters and mids and lows down to 70 Hz to the 6.5″ drivers up front.