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Wednesday, August 5, 2020

Doug's Domain

Doug Vetter, ATP/CFI

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Saturday, March 23, 2013

Parts for M3 Mirror Upgrade

Some new to BMWs, particularly an older specimen like the E36, may not realize the side view mirrors on non-M cars equipped with the cold weather package and all M3s are heated. These heaters will nicely shed ice and snow from the mirrors when the temperature drops below freezing.

For years I assumed that the heaters were thermostatically controlled just like the winshield washer jets are, and some Internet lore originally convinced me that the mirror selection switch served a dual function to control the heaters but both points are false. According to my technician the mirror heaters on the E36 are designed to run continuously as required to eliminate condensation that can form on the mirror surface at almost any temperature provided the surface of the mirror is cooler than the dewpoint. So this means the heaters run continuously, in all weather. Unfortunately, while run in this mode the heating elements don't last forever. There's no way to tell exactly when they failed on my car but it's been at least five years.

Of course, that's not the only thing wrong with my mirrors. The face of the mirror glass has been etched with water spots almost since new when I used to wash my car with water supplied from what was ultimately determined to be a malfunctioning softening system. A couple years ago I noticed the reflective mirror surface had started to creep away from the edge of the glass. The effect is small enough to be a non-issue for most people, but enough for me to notice every time I look at them. And most recently I noticed bubbling in the metal base of the mirrors, indicating subsurface corrosion that if left alone will soon cause the paint to flake off. For these reasons I figured it was time to replace the mirrors.

The first decision was to whether to go with stock replacements or a popular upgrade – M3 mirrors. For me this had more to do with price than functionality, particularly given that the M3 mirrors are not foldable and I do occasionally find that feature convenient – like during the recent front and rear suspension overhauls when the garage bay in which I worked was a bit cramped. After browsing realoem.com for the part numbers and getbmwparts.com for current pricing I ultimately discovered that the stock mirror assemblies are only $30 less expensive for the pair -- a wash, really. With price removed from the equation and given how they are more aerodynamically efficient, the decision was easily made in favor of the M3 versions.

On the advice of bimmerforums members and a couple readers I also decided to go with the euro version of the mirror glass. Why the euro versions? The passenger side euro glass is the same convex mirror used on the passenger side of US vehicles except it's refreshingly free of that stupid “Objects in mirror” text. The driver's side is apparently different in that the outer portion of the glass is convex and the boundary between the flat and convex portions is marked with a vertical dashed line.

When the parts came in I noticed the passenger side was indeed free of the dissertation concerning the physical effects of a convex mirror but the driver's side appeared to be completely flat and that's not what I wanted. I'm not sure what happened as I thought the part numbers were correct. I'll have to do some additional research to see if I can find the correct part number.

One thing I should point out for anyone looking at the parts diagrams -- as you can see in the pictures the mirror drive motor is provided with the mirror assembly. There is no need to buy the motor separately -- and that's a good thing since it retails for nearly $100. Total price for the mirror assemblies and the rubber "window guides" I figured I'd replace at the same time came to $630. I'll paint the mirrors when the car goes in for some rust repair tentatively scheduled for the end of April, and then I'll install them myself. No sense paying others to do what I expect to be a simple job.

More Stereo Design Decisions and Parts

(Image: Stereo configuration version 1)After a lot of research I have decided to “go active” and implement a system involving a couple MiniDSP 2x4 balanced boxed units based on the Analog Devices ADUA1701 DSP, driven with the Advanced Filter plugin. At a mere $145 each +$10 for the plugin these devices will allow me to easily implement high order filters in the digital domain without all the hassles associated with passive filters I outlined earlier. Perhaps best of all, they will allow me to accurately tweak the filters in the future with nothing more than a click of the mouse.

The plugin also supports the processing of spectral data obtained using the Room EQ Wizard software in combination with a calibrated measurement microphone, so this will allow me to take live samples of the E36's acoustical environment and plug the resulting data into the MiniDSP's parametric EQ (PEQ) to help flatten the system response. The 16 band equalization feature of the headunit will remain operational, of course, and I'll use that to tweak response for individual tracks.

The plugin provides up to 7.5ms of time correction. The speed of sound in a standard atmosphere at sea level is roughly 1116 feet per second or 1.16 feet per millisecond. That translates to roughly 8.4 feet, which is more than enough for this application. The basic goal will be to delay the left speakers by some amount. That will push them farther to the left, acoustically speaking, and thus center the soundstage. The headunit also provides time correction and it may be easier to use that for basic left/right correction and implement finer-grained control of the front high and low frequency drivers in the MiniDSP.

Although the balanced version of the MiniDSP will directly accept the unbalanced / single-ended output of the headunit, I've decided to go ahead with plans to insert some Jensen line transformers in the circuit to convert the single-ended output of the headunit into a balanced signal acceptable to the MiniDSP and provide ground loop isolation as well. The transformers aren't cheap but they're made in the US, require no separate power connection, and will provide a CMRR near 100 db -- something not achievable by any active balanced line driver. According to Jensen the units are best deployed as "input transformers", which means mounting them close to the MiniDSP and therefore in the trunk. That works out well since there isn't much room to mount them near the headunit.

The balanced outputs of the MiniDSP should integrate with the amplifiers' "high level" inputs. Though this connection will involve differential signalling, it won't be balanced in the strictest sense since the term "balanced" refers to the impedance on either side of the wire and the output impedance of the MiniDSP is 600 ohms while the input impedance of amplifiers varies and is usually much higher. To their credit, Arc Audio is the only manufacturer I've noticed that documents the input impedance of their amplifiers, which varies from 13K to 47K across their model line.

Assuming an impedance mismatch is unavoidable (as it is in this case unless I install yet more, possibly custom-made, line transformers) it is best to pair a low output impedance with a high input impedance. Because the input impedance of the amplifier is higher than the output impedance of the MiniDSP there should be no reduction in the signal amplitude. That will allow me to keep the gains set as low as possible and thus reduce the overall noise in the system, while leveraging the benefits of differential signalling. It's important to note, however, that to fully appreciate the benefits of differential signalling the amplifier must implement a true differential input stage. The problem, I've found, is that basically no amplifier manfacturers provide this level of technical detail. Only Arc Audio appears to make a subtle reference to a "fully balanced input stage". Not exactly what I was looking for, but better than most.

Naturally, I'm still undecided on which amplifiers to buy, but now that the crossovers will be in the MiniDSPs I can once again consider some models I originally eliminated from the search because they failed to provide the necessary crossover specifications. Always trying to balance price and quality I'm now looking at everything from the Soundstream Picasso line to the Arc KS/KAR series and even to vendors I never previously considered like JL and Focal.

Mileage: 233533, Parts: $630