Some BMW 2000 328i (E46 sedan) Non-HK stereo upgrade tips – lessons learned.

David Bagby

Last updated April 8th, 2000


I completed an upgrade of the stereo in my 328i. This saga has unfolded over a 2 ½ month period. I learned some lessons along the way that I want to offer up, in the hope others can have an easier time. Several people helped me with answering questions as I worked on this project; I’d like to give a little back to the net community.




This is what I did, it worked for me, I can’t guarantee you will find things exactly the same – but hey, it’s free advice, worth every penny you paid for it <grin>.


The Stereo Problem:

My primary complaints with the stock stereo echo much of what has been posted to – harsh, raspy, no bottom end. The stock system reminded me of an old 6 transistor radio from the 60’s – I preferred to drive with it turned off rather than listen to it – and I’m not an audio perfectionist. I may have been 30 years ago, but that obsession has long since passed (I think). My goal was a system that sounded like it should in a car of this price/quality class.


Sound wise, I think I exceeded my upgrade goal by quite a lot. I now have all the low end I want, very clean sound and literally wonder if I could rattle the doors off if I’m not careful – and this did not include any sub-woofer work – all from the stock speaker locations. It’s been a few years (like 20) since I last did a custom car stereo system (in my long gone 1968 BMW 1600) – boy, have things changed!


When I bought the 328i (12/99) I explicitly decided to go with the non-HK system and do after market upgrade work if I felt I needed to. I did this because the car I wanted was available at a local dealer, only it was only missing the HK system from my ideal option list – so I went for it to avoid the order and wait agony. Alas, one road trip and I was shopping for stereo upgrade gear – BMW builds a great car, but I agree with whomever posted it that the stereo must have been OEM’d from Fischer-Price. After the upgrade I  now have a system that I think is significantly better than the HK. I didn’t intend make it that good, it just happened – but the result is some really fine audio. Frankly, once you decide to touch the system, the price of really good sound is not much higher than so-so sound.


In checking out how the stock system was set up, I learned that once you touch the E46 system you are into amp and speakers at a minimum – and this is significantly more $ than the HK upgrade due to the BMW use of sophisticated electronic signaling methods in a proprietary system design.


Now for the install saga…


The Upgrade - Overview:

I personally did all the install work – it took me about 105 hours over many weekends and evenings – as with many of these projects I could do it again much faster, now that I know how, and what the right things are to do. The hours quoted include all the thinking, drawing, designing, fabricating and assembly time including mistakes, false starts and corrections. This total also includes the design and construction of some custom interface electronics (which I now believe can be purchased for a tiny fraction of what I invested in time and money – but hey, it was fun). Without the Custom electronics, the total time comes down to around 60-70 hours.


I installed the following speakers:

·       ADS 335is in the front. (5.25” woofer in the stock door location, tweeter in the factory tweeter location)

·       ADS 336im in the rear. (6.5” coax speakers replaced the stock rears)

·       ADS P640 Amp. (The amp is currently configured as 80W per channel front, 40W per channel rear).

·       I kept the BMW factory business CD unit in the dash.


The factory “mids” in the front doors and the “tweeters” in the rear doors are not used – they are now just factory door hole fillers. I left them in the doors so as not to create any randomly tuned bass ports in the door cavities.


I picked this equipment partially because I’d heard Junie’s car in January when I saw it at a local shop in the SF bay area - his car served as “proof by example” for me that the 335is speakers could be mounted in the front doors. I was concerned about physical fit issues when starting the project.


The ADS crossovers I custom mounted to the bracket that used to hold the factory AMP. They are all hidden behind the left trunk trim panel. The P640 is mounted to the forward wall of the trunk.


I considered putting the amp on the left side and using the trunk CD trim panel to hide it but the fit for my amp would have been really tight and looked to be more trouble that it was worth. I also considered using the BMW parcel shelf CD mount to hang the amp in the trunk – but my amp was 10.5” wide and the CD mount is only 10” wide – but for the width this would have been a nice approach as the CD mount is 3” deep and the amp is 2” tall with a requirement for 1” clearance above for cooling air flow – someone with a narrower amp may want to consider this.


The forward trunk wall took the least amount of volume from the trunk and was the preferred vertical position for cooling of the amp.


Speaker Mountings:

First you have to get into the door – here are some great instructions someone else posted (anonymously) that I used to get into the doors:

Posted by Alan on March 02, 1999 at 19:44:43: (to E46 forum)

Thank you to anonymous for the pictures and instructions:

Unclip wooden finisher (1) with special tool 00 9 321. If necessary, replace faulty clips (1) in wooden finisher (2).
Driver's side: Using two screwdrivers, lever out mirror switch (2) towards top of control knob (1). Disconnect plug connection (1) and remove mirror switch (2).
Passenger side: Using a small screwdriver, carefully lever out trim (1) at points (2). Remove screws (1 and 2). Remove inserts (3) and release underlying screws. When reinstalling, screw (1) is longer than the others.
Unclip clips (1) in door trim panel with special tool 00 9 317 (trim panel wedge). Carefully unclip door trim panel (1) at top with special tool 00 9 317 (trim panel wedge). Feed door trim panel (1) out of unlocking knob (2).
Unhook cable for unlocking lever: Tighten unlocking lever (1); lift out lock (2); remove cable (3) towards rear/top. To reinstall, press unlocking lever (1) fully against door trim panel. Version with additional speaker: if necessary, disconnect plug connection (1) on additional speaker (2).
Feed out wires (1) from opening (2). Disconnect plug connections (3) and (4) and remove door trim panel. Installation: Clips (1) are longer than the remaining clips. If necessary, replace damaged clips.

After assembling the door trim panel proceed as follows:
Open door window
Lock with vehicle key
Check for ease-of-movement on retaining button linkage If necessary, align linkages

What I installed:
Front woofer mounts (ADS 335is):
The fronts are 5.25" and I created an adaptor to fit them inside the factory hole and recess then into the hole to get door clearance for the magnets. I can't really describe the mounts very well with words only - I'd need a picture and I didn't take any (no digital camera) - but I'll try anyhow...

Think of a plate, sort of cam/triangular in shape, to cover the factory screw hole positions. Add a donut ring to the center of the plate on the door panel side of the plate. This ring fits into the factory mount hole. The speaker is mounted from the panel side of the adaptor and the magnet sticks up thru the donut ring and plate. This recesses the speaker as far as I could go w/o having the speaker cone hit the factory grill mounts at maximum excursion. The speaker seals to the plate and the plate seals to the factory plastic mounting ring. (Which is the only flat reference surface available). Finally, the "ears/corners” of the plate got built up with additional material to bring the plate ear thickness down to the factory screw hole top surface. This required finding longer metric screws to fit the speaker mount threads (I ended up using cap screws and a washer).

Others have taken a grinder to the factory plastic mounts in the door and altered them to fit whatever they were installing - I didn't want to loose the ability to remount the factory speakers – and I have an aversion to cutting up new cars.

By the way - the door grills do not come off the door panel from the outside of the door (inside of the car) - one of the plastic mounting snap tabs is melted over on the inside at the factory.

I choose the ADS 335is as I had heard them already in another 328i - so I knew they could be mounted. In hindsight, I wish I had been able to use the 336is (6.5" separate 2 way version) instead as I like the rear speakers better than the front speakers - but I don't think there is the depth required w/o cutting into the door metal.

The stock speakers are metric so they do not come out to be a nice English size like 6" or 6.5" - if you have the rear speakers out, I believe one of the numbers on the label is the size: 160mm if I remember correctly (the exact conversion is 25.4 mm / inch).

Rear Deck Speaker mounts (ADS 336im):
The front woofer speakers are identical to the rear deck speakers down to the part numbers.

For the rear deck I built a flat plate, shaped to fit into the recess that the speaker grills cover in the rear deck. I mounted the plate with the screws that used to mount the factory speakers. In the center I cut the plates to mount the ADS speakers. The ADS was mounted to the bottom the plate to recess the speakers (necessary as I put coax speakers in and the center tweeters stick up above the plane of the woofer). I used 6.5" speakers - the paper mache "boxes" on the factory speakers went to the useless pile along with the factory speakers. If you keep everything centered, there is sufficient depth to get this all to fit. The factory grills were then put back over the new speakers and adaptor plate. You cannot tell visually that they have been changed.

Front Tweeter mounts (ADS 335is):
This was a pain to do!

WARNING: Do not attempt to pry the grills out of the pods - this will ruin the grills and still not give you access to the tweeter mounts (I started to but stopped before any damage was done).

Mounting the tweeters was easy, getting into the pods was not! I spent two weeks asking every place I could find how to get into them.

Here is the secret: What you want is the black plastic trim piece that starts at the bottom forward edge of the door and goes all the way up and around the top of the door and down the rear edge. To get into the pods, start at the bottom of the front edge of the doors - get a bright (make that very bright as you will be in a dark corner looking at black on black) light and you will see two black "rivet heads" in the forward edge of the doors - one above the hinge, one below - these pry out.

Once they are out, the black trim piece splits and pulls back from around the hinge. Work your way up and the trim will be fastened an inch or so below the bottom edge of the pod - this is a spring clip that is inserted into a rectangular hole in the door. The hole is taller vertically than it is wide and the clip compresses from the sides. I took a long screwdriver and worked it in (to not scratch the paint I used a paint mixing stick I had around between the screwdriver and the door) and pushed and twisted and pried the clip out.

There is a very loud noise when the clip comes free, your heart pauses as you wonder what this is gunna cost you... and then you decide to open your eyes and look - the clip will stay attached to the trim piece. Now you can work your way up - there are more clips as you go - once you see the first one you'll understand what mechanical fasteners you're dealing with. I only took the trim far enough off to get at the tweeters (the plastic bent some but sprung back and it did not hurt anything - but be careful - it is unclear to me how far one can bend the trim and get away with this w/o leaving deformation marks...).

The factory tweeters are mounted to the door with two Torx bolts - I removed the factory tweeters. This is the one place I had to cut the factory wires - there is no connector on the tweeter and the wires disappear into goop and the water-proof door sealing that I did not want to open.

To mount the tweeters, I got real lucky. The inside face of the factory grill is covered by a light layer of foam. Thick, dense foam also stays in the pods - it used to fit around the factory tweeters. My new tweeters were larger in diameter and the foam is not totally rigid, so I simply pushed the new tweeters into the foam in the pod, against the grill face. Then I inserted a piece of soft foam I had around behind the tweeter and pushed it all back in place - the new foam compresses and the tweeter was sandwiched in padding in the pod. So far so good, no rattles and I did not have to make compound angle mount adaptors (you'll understand when you see the factory tweeter - it has a molded mount to get the face of the tweeter parallel with the pod grill).

The speaker mountings were made from ABS (a non-brittle plastic) and the design goal was to not physically alter the factory mounts in any way. I could drop the factory speakers and amp back in and you would not be able to tell any mod had been made. I also kept the factory grills in all cases so you can’t tell the stereo system has been upgraded by looking.


Wiring the P640 AMP to the factory wiring harness:

During the project I tracked down who made the connector on the factory harness for the stereo. I could not get either Amphenol (AMP) or Siemens to sell me a connector that mates with the BMW harness that plugs into the factory amp. AMP told me they have one but their connectors are restricted by contract to BMW only. Siemans also has one as it is inside the amp. I have seen posts and pictures of the Siemans connector that someone managed to buy a few of – in Germany. Siemans US never returned my calls. It seems an adaptor is not likely to show up on the after market anytime soon given the difficulty of getting the connectors. So I made a custom connector… This was the first project folly that took a lot of time.


I felt I had to do this because I was NOT going to cut off the factory connector – I had nightmares about “Sure Mr. Customer, we can replace that harness connector for you, of course, we only sell full harnesses, so the labor will involve removing the body and replacing the entire harness, shouldn’t take more than a month or so …. No thanks!


My custom connector is not the only solution to this problem – I’m sure there are others. If I could have gotten the connector I’d have made a PC board and brought all the connector signals out to marine grade screw terminals (OK, that’s overkill, but I’m also a boater and I’ve learned not to skip on connectors as they can be a real source of problems). Building a custom connector is the kind if thing that you can do if you are into electronics, but it’s not for everyone and certainly not for the novice – time will tell if my particular solution stands up to the vibration environment in a car.

The connector sub-saga:

OK this is the basic approach I took – But fair warning – this is a pain to do and there may well be a better way I haven’t thought of yet…

This will make more sense if you are staring at the amp bracket and connector while reading this – pictures would really help but I didn’t take any… so here goes… (If anyone is sufficiently frustrated at the lack of pictures to want to give me a digital camera, I’ll promise to take some pics in exchange for the camera - and see if I can sell you a slightly used bridge from London…).

I mounted 2 ADS crossovers vertically to the inside of the factory amp mounting bracket. I mounted another 2 above the first ones to a horizontal panel I made out of ABS that bolts with right angle brackets to the amp bracket – this made the 2nd pair stick into the fender - but not rise above the top of the amp bracket (which is covered by the trunk carpet side panel). The result, from the side is an upside down L shaped assembly of crossovers between the amp bracket and the fender. This does require drilling holes in the amp bracket – but it is a single easily replaced piece so I was willing to do that.

In the bracket for the factory amp there is a square hole in the front (toward the front of the car) side of the bracket. Over that hole I mounted a piece of 0.1 inch spacing prototype PC board. The small pins on the factory connector are on a 0.1” grid. But the gray and black halves are not an even multiple of 0.1 apart…. So you have to take the shell off the connector (it is made to do this to assemble the connector – look at the connector for the release tabs on the end away from the wires) – this let me spread the gray and black portions apart a bit to match the hole pattern in the proto board. It also let me get enough space between them to mate the connector faces flat to the proto board and secure them down (I used cable ties around the connectors and thru the proto board – they pass over the connector in the middle over the positions that have no wires). To position the connectors just right over the proto board holes, I used some 0.1 pins I had around for alignment.

Then had to make long pins to go from the connector to the inside face of the proto board – I made those from two proto board pins soldered end to end – each pin looks like a small “Y” at one end and is designed to press fit into the proto board with the “y” up - so you place the “y”s together, rotated at right angles and solder them – now I had a long 0.1 square pin. You then insert one end from the inside of the proto board into the harness connector (can’t do it from the outside or the connector halves will not sit flat on the board).

Now you have long male pins sticking out on the inside – which try to wiggle around and short against each other – they need to be kept parallel – I did that by using some Molex PCB headers with 0.1” spacing pins - I took the header short pins out (Pull 'em with needle nose pliers) and slide the resulting spacers over the long pins I had made. Then I made the wiring to pin adaptors from some 0.1” spacing female Molex connectors from a local electronics outfit – That finally got me the small connections wired up. You also will have to figure out a way to secure the Molex connectors on the pins so they don’t vibrate off over time.

This leaves the large pins in the connector – they are wider to carry more current. I took some 0.187 spade terminals and filed them down until they fit the connector slots (Pins 1-5 on one half – forgot the #’s on the other half – but they are obviously different from the small pin holes). Then you have to make holes to match in the proto board to insert the spade terms through – that’s a good job for a Dremel tool.

Once you get all those inserted, you also have to secure them against vibration – that part if my solution I am not yet 100% confident in – only time will tell – but I think I’ll be ok. (note: as of 4-8-2000 all is fine so far after about 1000 miles).

If all goes well you end up with a male plug sticking out the side of the bracket than one can plug the factory harness into.

All this requires lots of assembly, measuring and reassembly.

Be sure to test the set up carefully with a meter before applying power. It is also hard because the wire gages involved are bigger than the Molex parts want to handle – so you do lots of little things like removing strands of wire to mate larger wires with the female moles spring terminals etc.

It works for me, and I think it will stand up – but I’d happily have paid for a commercial solution! I also considered making a PCB to mate with the male connector from the factory AMP – but I didn’t want to cannibalize the factory amp for the connector.

Factory harness Wiring information:

Now that you have the connectors made, you need to figure out how to wire things up… 

Some of the wiring harness info available on in the archives and the info faxed to me by ADS for the E46 is WRONG.


In the course of making the connector I started with info from the archives and a faxed page of info from ADS re E46 wiring – both had (different) errors in them – be advised that it pays to check all wiring for yourself in a project like this. Both sources had incorrect pin numbers and colors that did not match the wires physically in the harness. I have provided (to the best of my knowledge) corrected information below.


The notation I used below is:

Pin # xx / Signal

(main wire color) / (wire stripe color)

no connection literally means that there is not a wire in the factory harness at that pin.

Signal name followed by an * are signals that I believe to be correct, the color code is verified with the harness etc, but I did not personally use these signals in the upgrade so I can not claim to have proven the connections out personally. In particular I did not verify the +/- polarity of the factory mid range speakers. The non-* signals lines I am using to run the upgraded system so I am sure they are correct (at least for my car).


The harness connector pin numbers are embossed on the plastic. There are two “halves” to the connector, looking at the female harness connector its pin lay out looks like this:


          Grey ½ of connector                 

Pin # 13 / Tweeter Rear Right door + *          Yellow                                                                        


Pin # 21 / Tweeter Rear Right door - *

Blue / yellow

Pin # 12 / Tweeter Front Right -                     

Blue / Brown


Pin #20 / no connection

Pin # 11 / Tweeter Front Right +                    

Blue / black


Pin #19 / no connection

Pin # 10 / Switched +12v remote turn on      



Pin #18 / no connection


Pin # 9 / Head unit output Front Right +              

Blue / red


Pin #17 / Head unit output Front Right -

Brown / orange

Pin # 8 / Head unit output Rear Right +               

Blue / black


Pin #16 / Head unit output Rear Right -

Brown / orange

Pin # 7 / no connection


Pin #15 / no connection

Pin # 6 / no connection


Pin #14 / no connection

Pin # 5 / +12v factory amp +12v supply *

Red / green



Pin # 4 / Woofer Front Left +

Blue / white



Pin # 3 / Woofer Front Left -

Blue / brown



Pin # 2 / Woofer Rear Left +




Pin # 1 / Woofer Rear Left -





          Black ½ of connector                

Pin # 34 / Tweeter Front Left -                      

Yellow / brown                                              


Pin # 42 / Mid Front Right door + *



Pin # 33 / Tweeter Front Left +                     

Yellow / red                                                   


Pin #41 / Mid Front Right door - *

Blue / green


Pin # 32 / Tweeter Rear left door - *             

Yellow / brown                                            


Pin #40 / Mid Front left door + *



Pin # 31 / Tweeter Rear Left door + *           

Yellow / black                                                           


Pin #39 / Mid Front left door - *

Blue / white


Pin # 30 / Head output Front left -                 

Brown / orange                                            


Pin #38 / Head output Front Left +

Yellow / red


Pin # 29 / Head output Rear Left -                

Brown / orange                                              


Pin #37 / Head output Rear Left +

Yellow / black


Pin # 28 / no connection                                


Pin #36 / no connection


Pin # 27 / no connection                               


Pin #35 / no connection


Pin # 26 / DC ground factory amp




Pin # 25 / Woofer Rear Right +




Pin # 24 / Woofer Rear Right -




Pin # 23 / Woofer Front Right +

Blue / Red



Pin # 22 / Woofer Front Right -

Blue / Brown




Pins 1-5 and 22-26 are heavier connections that then other pins, these are the ones that feed the woofers and provide power to the factory amp. I did not use the power connections for the factory amp for the ADS amp – the ADS can draw 30A and the factory connector had wire much to small for that type of current draw. Instead I powered the ADS amp directly (with safety fuses etc) from the battery (easy since BMW puts the battery in the trunk).


Differential Balanced Signal Electronics:


IMPORTANT NOTE: All inputs (i.e. the outputs of the head unit) to factory amp and the outputs (the speaker wires) from the factory amp are isolated from the vehicle ground! This can be very important to the electronics you install. Make the electronics mad and you’ll let out the magic smoke…


Here is a real good example of why this is important:

The ADS folks say that you do not need level converters to connect the factory head to an ADS amp (at least the P640 model), this influenced my decision to use an ADS amp. ADS does NOT (or at least did not) know what they were talking about re the E46.


During the install I wired the outputs from the head to the ADS amp inputs. Positive input signals to the center of the RCA jacks, negative to the shields. Ah grasshopper, life is not that simple…


When I got all the speakers installed and wired up the ADS amp, I double checked all the wiring with my meter, took a deep breath, reinstalled the fuses, checked the power leads to the amp for 12v, and all looked good so far... then I turned on the CD player and the amp power light did not even light up! Sigh.


I checked the remote turn on signal - the head unit was correctly switching it. Clearly Mr. Murphy had come to visit my garage – whatever could go wrong was happening. Eventually, I disconnected all inputs and outputs from the ADS amp except for power and the remote turn on lead – now it powered up.


By process of elimination, I figured out that the amp would not power up if any of the input RCAs were connected to the amp – weird. After some head scratching and a call to the local shop where I bought some of the speakers, I decided to try hooking the positive input leads ONLY to the ADS amp, leaving the ground side of the circuit disconnected.


Success! Sorta…. Now the amp turned on as expected, and I heard sound, but there was a horrible amount of background noise and a high pitched whine in all the speakers (like the sound of DC-DC inverter interference for those into such things). Yech. Totally unacceptable.


The next morning I called ADS at the suggestion of their dealer. ADS had never heard of this problem before and said that the only time they had heard high pitched noise was when an input or output ground signal had been shorted to chassis DC ground in error, which had been known to cause instability internal to the 640 amp. I double-checked the grounds. Alas, all input and output ground lines were isolated from vehicle DC ground as required. ADS was out of ideas, and so was I – a definite bummer re the Bimmer was beginning to set in…


I got to thinking about what the signals looked like coming from the BMW/Alpine head unit to the factory amp – so I took the factory amp apart to see what I might learn from the circuitry…. Ah, every signal in and out of that amp is internally bypassed via a 0.22uF cap to ground; or what looked like ground, it wasn’t really ground – you technically inclined readers will understand in a bit. Then I checked the leads from the head unit – the ground leads are not tied to chassis ground – they are floating! So I reasoned that I had a DC vs. AC ground potential issue. I was wrong! I should have kept looking at the BMW amp circuits…. But then I was ignoring a prime rule: Never, ever work on a project when you are tired! Make this triply true if power tools, sharp objects, or electricity is involved! At best you’ll waste time, at worst you’ll get hurt.


If you search for some of my posts on you will find the initial approach I took to getting the AMP to work – I have purposely not retained that information here. It is out of date and wrong! Do not look for those posts and do not follow the first path I took – it won’t help you in the long run.


After several days of phone calls, frustration, etc., I decided to get serious and trace out the input circuits of the factory amp to figure out what it was doing… I’ll skip the rest of the trails and tribulations and just report what you should do instead…


(Note: Some folk’s posts indicated that they may be trying to apply E36 information to E46 stereo setups – this can be problematic. The heads in the E46 and the E36 are not the same and so not generate the same types of signals).

E46 Amp electronics and noise issues:
The E46 head does not put out ground referenced outputs - it supplies differential signal pairs that are terminated in an impedance matching network that feed an Op Amp (in the factory amp input circuitry) that is set up to remove noise induced into the lines on the way from the head to the amp in the rear. This is very good engineering - it's easier to remove the noise than to keep it out - but it does not utilize "standard" unbalanced line levels and it is not unbalanced speaker level signals. There have been many posts claiming either of these are what the input signals to the factory amp are – they are wrong! I have the schematics and oscilloscope traces to prove it.

The crossovers in the E46 amp are NOT passive (as I hear they are in the E36 amp) - they are active op amp filter circuits. (I've spent several days reverse engineering the factory amp circuits). I have learned that to correctly terminate the head unit outputs w/o introducing noise into the system, and drive an after market amp requires some active electronics. This insight lead to Dave’s second folly during this project – I designed, built and debugged some custom electronics to interface my ADS amp with the stock BMW Business CD head unit… About this point my wife began wondering why the new car was always apart in the garage.  J


I’m going to skip describing the custom electronics – it does not pay for anyone to build it. Don’t worry about recreating my electronics - Just before finishing the project folly number 2, I found a commercial unit that I believe will do essentially the same thing – and it only costs $40-$60 depending on where you buy it! That beats the parts cost of the custom route and saves all the labor.


Here is what you want for an interface and why:

Some shops will try to talk you into having the head unit modified to add what they call “line level” outputs then try to run new cables from the head to the trunk – this is a waste of time and money – it they push this, they do not understand the BMW E46 stereo system.

A primer on some signaling theory:

Most line level signals are run using Coax cables. The reason for coax cable on most RCA jacks is that the outside tubular coax is shielding the inside wire with the signal (well, that's a simplification, but it'll do for now).
The actual signals from the E46 head unit are approx 11V peak-peak (I got the scope out) when the head is all the way up - that's a lot. The real reason the signals are driven the way they are is to be able to take noise out of the system. Each wire of the pair has a mirror image signal of the other wire. Any noise that gets into the wires tends to be the same on each wire (since they are run together down the car harness) - the factory amp uses a differential circuit to only amplify the differences between the two wires - thus the noise gets dropped out of the audio information. This is the "we can't keep noise of the wires, so let it get in then take it out at the other end" approach - very common in professional audio & instrumentation electronics. It also allows use of plain wire instead of more expensive (and not always effective) coax cable.

However, differential/balanced signals are not the type of signals that RCA jacks typically are set up to take (ok, this is not really a function of the jack type, but RCA jacks are 98% of the time associated with a particular type of signal setup called unbalanced or common ground signals). Typically, RCA jacks have one side (the negative/outside) tied to DC ground and the info signal on the +/center lead.

Now then, why do some aftermarket amps work and not others? Well, the differential signals can really be thought of as having the center of the signal at 6V DC (bottom at 0v, top at 12v) - some amps tolerate this some don't. The reason some speaker-to-line converters work (for example the P15 I hear) is that most speaker signals are NOT connected to DC ground on one side (that would introduce ground noise directly into the speaker) - hence speaker converters also tend to have isolated inputs - the balanced diff signals are also isolated (floating) from ground - sometimes the combo is close enough to work out – but this is NOT correct from an engineering viewpoint.

Speaker level converters will NOT really be doing what the factory amp is designed to do - they will not cancel out any noise induced in the lines from the head unit. If your particular car has little noise, you may think all is fine, but if you have noise involved, the speaker converters only reduce the amplitude of the noise (along with the desired signal) but do not cancel it out. The noise gets amplified again along with the wanted audio signal as it goes thru the amp. The balanced/differential to unbalanced converters actually cancel the induced noise out.

The last wrinkle is that these days "line level" is not a specific voltage amplitude - in the old days "line level" was between 0 and 1V max. These days (for cars) "line level" can be up to 12V - because of this the converters have gain controls so you can set the output from one box to match the input level needed/wanted by the next box - the mfcts simply can't predict the level needed so they make it adjustable - and for the same reason the amps get input gain controls. The installer gets to figure out how to get all those things tweaked to play together w/o over driving a single stage and causing distortion.

Is that all clear as mud? Never mind if it sounds too complicated – just buy the right interface widget and you can forget all the techno jumble – just learn enough not to get sold the wrong thing from the local (probably ignorant) car stereo shop (I found many in this category close to home while figuring all this out).

The best commercial unit I have been able to find that does what is needed is the VEN 4 by Peripheral. See

The VEN 4 is just what the doctor ordered. It is a 4 channel, balanced/differential to line level/unbalanced converter so it should do the noise cancellation stuff. Of course if you really want the most effective 100% noise cancellation possible you have to match the design of the line drivers (in the head unit) to the line receivers (in the factory amp) - that's why I choose to build a custom circuit to copy the factory amp electronics (well, that and the fact I couldn't find a commercial unit until I was already 90% done with the project – and I like the fact that my custom stuff works as I intended).

OK, I can’t personally vouch for the VEN4 – I haven’t used it since I did my own thing – but it matches the technical requirements – and I know some others are planning to use it – so check more recent posts for an update on their success.

I have also heard from one person who has used a Sony amp that has balanced inputs built into the amp – his system works fine.

Other Noise issues:

Alas, the head unit interface it is not the only potential source of noise problems. Some amps are very sensitive to power noise (Alternator whine) – my ADS is one of them. At one point in tracking down noise issues I ran the amp from a bench power supply - if you are not connected to the car power system you can't get alternator whine thru the power leads. This did produce a quiet stereo system in the garage (but it’s a bit hard to road test this set up – requires a looong extension cord). So I put more effort into cleaning up the DC power to the amp. In my case (yours may be different) the secret was to keep the ADS amp ground lead as short as possible – literally 4 inches – this licked 99.9% of the DC noise problem for me.

Also keep the interface gain up and the amp gains down when balancing the system setup. This keeps the signal to noise ratio up and keeps the amp from overly amplifying any noise that is on the input signals (the BMW head is not 100 noise free for the signals it generates).

Keep all signal lines away from power lines and if they have to cross, make the cross at right angles (never run them parallel) – this will keep the power lines from inducing noise in the signal lines.

The Cost:

I now have screech/whine/noise free signals (well at least as noise free as the factory head produces – it does produce some white noise – but I heard that before the upgrade so it is not an artifact of the new gear).


All done I have about $1500 in the speakers, amp and misc. material, wiring etc. More than the cost of the BMW Harmon-Kardon upgrade? Yes. Worth the difference in price? Without any doubt whatsoever!


I think you could duplicate this system’s equipment for a bit less than $2000 from an ADS dealer, perhaps less if you bargain well (I got some deals via Ebay on the 335is speakers and a used amp, but spent more on custom stuff). You can do well (price wise) buying ADS equipment over the net.

But a good dealer is worth some $! They helped even though I was doing my own install and some of the equipment was used…  I strongly recommend taking that into consideration before deciding to go shopping for rock bottom price only.


The biggest ticket item in the project is the install labor. My local shops run between $60 and $75/hr. The $75/hr rate is for the shop that actually understood what I meant when I’d say things like “Balanced lines and Differential Op Amps”. The shop labor charge for what I did would have been over $8000! Most shops would not have gone to the trouble I did to avoid cutting the factory harness, they might have been quicker in figuring out speaker mounts (if they have already learned on someone else’s 2000 Bimmer) and if they knew enough to provide the correct type of electronic interface they would save time (but rightly charge you for their expertise).


None of the local shops I talked to knew how to solve the interface problem – I suppose they will, eventually, but as of 3/2000, the E46 is still too new. They just haven’t seen many of them – and the average stereo installer is an installer, not an engineer – so until someone teaches them what to do, you can’t expect them to figure it out for you.


By reading this, you now know some things that will make your shopping much easier as you will not have to spend time on the detective work I had to do.


The sound:

Ah, this makes it all worth the trouble!

The system sounds great, no noise that I can hear (I can see some on the scope but I can’t hear it), plenty of bass (for me, I don't share the desire to give up the trunk to sub-woofers, but to each his own) and the detail I hear is amazing! I really helps that the Bimmer is so quiet when closed up. I’ve driven the car for about a month now since I finished up the system – I love it. I seem to have a double case of SAG (Silly Assed Grin); once for the car and once for the stereo. The work commute is infinitely more pleasant now.

I’m left with just one more novel idea to try out - I think I'll try driving the car and enjoying it instead of taking it apart...