July 31st, 1998
By: Robert Leidy

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 (6x25 and 2x40). 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