Audio/Navigation :: Front/Rear Channel Swap
 

Executive summary:

Swapping the front and rear channel connections at the X9331 connector makes a huge improvement in bass response -- in non-HiFi systems (08/24). Doesn't seem to help HiFi systems. Do it! Details follow.

Discussion:

A number of threads have been discussing the performance of R56 sound systems, the standard (Boost) and HiFi.

https://www.northamericanmotoring.co...d.php?t=150567

Turns out that the rear channels in non-HiFi systems are rolled off significantly at 100Hz in the head unit -- for engineering types, down 3dB at 100Hz, 12dB/octave, so the response on the rear channels is down 30dB at 20Hz (from a 2KHz reference).

https://www.northamericanmotoring.co...1/response.png

In a word, sucks!

So, what are we to do?

The good news along the path, is we (a bunch of good folks on Nam, not the "Imperial" we) discovered that the signals from the head unit run to a connector accessible just ahead of the driver's door -- X9331 is its name, and here is where it lives:

http://www.mcaw.info/wds/mini/zi_ima...56_61_0019.png

We don't have to take the centre console apart anymore! Whee!

I got the idea that if the rear channel rolls off at 100Hz, and the front speakers were small anyway, why not match channels that can't do really low notes with speakers that can't do really low notes? That puts the bass on the rear 6x9 speakers, and the 100Hz high-passed signal on the fronts.

We did it. It works. Do it!

More details:

You can expose the X9331 connector with nothing more than fingernails, popping loose access panels. If you want to remove the lower access panel, that involves removing the lower seatbelt attachment, which requires a Torx T50 bit. Optional.

X9331 is the white connector, which has a black connector piggy-backed on to it. Rotate the white connector to free it from the car, then pop it apart.

Here's schatzy62's picture of X9331 in situ:

https://www.northamericanmotoring.co...m/100_0663.JPG

Here's the female end:

https://www.northamericanmotoring.co...m/100_0665.JPG

You did disconnect the negative lead of the battery, didn't you? That takes a 10mm wrench.

Popping the pins out: making a tool

We want to pop out eight of the 12 wires in this connector and move them around.

Note: the part numbers for the real BMW tool:
611 132 for the tool
610 304 for the 2.5mm bit

(thanks to schatzy62 and D unit)

If you want to roll your own, though...

I mentioned trying a shim cut from a beverage can -- didn't work, not stiff enough.

Helpful folks on the list correctly pointed out that a 0.093 inch Molex pin removal tool, commonly available at computer geek stores, doesn't work.

Well, not right out of the box, it doesn't!

Take that Molex removal tool, and put a very thin slice along it. I used a Dremel tool with a high-speed cutoff disc. Spread the split end with the butt end of a #27 drill bit. Do it carefully, you don't want to crack the tool. An alternate approach is to use an Xacto razor saw (or other very thin blade) to split the pin removal tool, and then resize with a #27.

Big hint: Work on the male pins. Far easier. The male pins pop loose pretty easily with the modified tool. Print out a cheat sheet with the color and pin numbers. You might mark it up beforehand to show what's going to go where, and pop things loose in pairs, swapping as you go and marking up the sheet. A pair of needlenose pliers are useful for re-seating things. Don't take the other wires out -- the brown wires and the white wire. Check your work. Hell, check my work!

Plug things back in, swapping front and rear channels. Maintain proper polarity as it's important for phasing. Plug the connector halves back together.

Reconnect the battery and try it out! You'll have to tell your clock what time it is again, but that's a small price to pay.

First test -- the fader should work backwards! Front and back should have swapped places! If they did, you're in business!

Sit back and enjoy actual bass notes coming from the rear speakers!

I'm still going to drop in the subwoofer assembly and see how it flies.

Interesting side effect:

If you have the bluetooth package, your phone will now come through the rear speakers!

What to swap to where: the details

Here are the details on which wires to move where to do the swap:

Note that I recommend doing the male pins, as they are far easier to remove, and are also on the speaker side of things.

Oh, the usual caveat -- you're on your own, don't blame me, no warranties expressed or implied, for prevention of disease only.

Many thanks to:

Robin Casady, rkw, ScottRiqui, basil49, schatzy62, Minidrivr, Doc Cooper, D Unit, and undoubtedly many more. The effort we all put into NAM makes life easier on the next person.

Wheee!

NO SHIZ!!!

Im gonna try this out ASAP since my stereo installer hooked my 10" with a loc to the rear speakers; I cant hear anything below like 50hz; I thought it was just my box or something.

I'll probably have to re-tune the gain controls.

Will post results asap!

Good writeup:thumbsup:

Awesome writeup! :thumbsup:

I finally understand the genius in the madness! Thanks a lot! BTW, does this mean the 40W will go to my 6.5" woofers and 20W to my 6x9s? I really hope so because my new 6.5" could really use the extra uuummmph.

Unclemeat--

A 14 year old, 9 pound yorkshire terrier will produce more bass than what you've got hooked up to those crippled rear speakers!

Switch things around and you'll be amazed, and so will the guy who installed things for you! And try 80-100Hz, not 50...

It's GO time!
 

Congrats ... that was some nice EE-detective work :thumbsup:

cant we eliminate the cut off filter?

Wish I knew -- send me a head unit and I'll dissect it for the offending circuitry. If we had schematics for that stuff, it would be pretty easy to spot. I'm guessing even without schematics it's going to be easy to spot, but it's parts on a board inside the bleeding radio head (sigh, grumble). Not easy to get to, that's the problem.

I'm still perplexed why the engineer's purposefully rolled off the rear speakers so dramatically. If, as speculated here, it was because they're mounted in plastic panels that might have had boominess or rattling issues, I'm interested to see whether you guys making this change will experience those problems.

Very nice discovery/writeup!

Plus, this mod gives you *two* significant benefits. Not only will you be feeding the rear speakers with a full-range signal, but you've also prevented the smaller front speakers from playing low bass notes they're really not designed to reproduce. Normally, you'd have to do this by either installing passive filters on the fronts (aka "Bass Blockers"), or going with an additional amplifier or signal processor that has active filters, but by swapping the front/rear pins around, you're actually putting the stupid factory 100 Hz high-pass filter to good use!

Thanks for the info. I'd probably have to redo my speaker boxes for the 6x9s before doing this (got some bad info from a Crutchfield tech), but it might be a reason to consider the effort.

I look forward to reports as to how it works out. I'm also interested to hear if anyone gets vibration problems on the back panels.

I'm putting together an iPod playlist with lots of thumpa-thumpa material, but balanced thumpa-thumpa. Sheffield Drum record, Sheffield Track record, Joe Sample, lots of other stuff. With the small selection we played this afternoon (after installing the subwoofer), didn't notice any buzzing or rattling from the rear speakers.

Mystery to me why someone would design it this way!

Scott has it right -- not only do you put the bass where it belongs, you keep it away from where it can't do any good! A winner all around!

So far the only interesting side effect (and I edited it into the initial post) is that my bluetooth phone answers through the rear speakers now, which once you think about it, is to be expected.

A very successful day. Tomorrow I work on the ham radio gear.

And thanks again to all the NAM participants who made it possible!

wild guess: perhaps after they had specified the head-unit, they was to be a subwoof option from MINI that never happened because it
(pick one: never got designed / was too pricey / was too large to fit anywhere / your guess)

So, by swapping the front and rear channels, does the fronts get 40 watts now? Do the rears get 20W now?

I thought the non-HiFi system was around 20 watts per channel all the way around. Not sure why, but that's what stuck in my head.

Since you're swapping channels, whatever went to the front now goes to the back, and vers visa (sic).

Another aspect to consider on that is that the rears are single 4-ohm beasties (even in my car with the upgraded Polk db691). The fronts, however, each have two 4-ohm or so speakers in parallel, the original 6.5 inch, and the original 4 inch. Impedance on that parallel combo is going to be under 4 ohms, and could be 2 ohms or less. If the amp has the wheeze, producing the same voltage swing, it will pump more power into a lower impedance load, so the fronts could still be seeing more power.

Theoretically, going from a 4 ohm load to a 2 ohm load should double the output power, which would explain the 20 watts to the back and 40 watts to the front. (P=E*E/R -- cut R in half and P doubles)

If that's the case, you're going to see the same overall power in the fronts and backs, with the important change that you'll be seeing low end power going to the 6x9 speakers which have a chance in hell of doing something with it, and you won't be seeing low end power going to the 6.5 and 4 inch fronts, which can only turn that low frequency energy into heat!

This is one of those situations where less can come out being more -- just like in a bi-amped situation, you're putting the amplifier's energy where the speaker can make use of it.

It sounds good to me, anyway!

If you have the HIFI setup, I'm pretty sure the connecter that they're modifying is between the head unit and the amplifier, not between the amplifier and the speakers. So, the rear speakers are still connected to the 40W outputs on the amp and will still receive the same amount of power. The only thing that's changed is that the front-channel outputs from the head unit now feed the rear-channel inputs on the amp, and vice-versa.

On the non-HIFI stereo, all four speakers receive the same amount of power (~15 W), so nothing's changed there.

Is there any kind of passive crossover network for the front speakers, or are both the 6.5" and 4" speakers being fed a full-range signal? If there's any kind of filtering going on at the speakers, it would increase the overall impedance of the combo, so hopefully it would never dip all the way down to 2 Ω.

Yipeeee... Thanks guys! I'm gonna try this on Tuesday or Wednesday.

I bet I am not the only guy out there watching this thread for when the proper tool for the pin extraction is identified.
I gots no dremel and the wife won't let me play with exacto knives.
Besides, modifying a tool would go like this: find tool, buy tool, modify tool, pull panel, RATS!, throw uselessly modified tool, put panel back on, start over.
I did a search for extraction tools, seems there are a bunch out there. One of them is the cat's meow.

Oh. Thank's to all you audio/electro folks that figured this out. Proving Nam is THE MINI WEBSITE!

Scott--

With the OEM setup, you've got a 6.5 inch single coil speaker in parallel with a 4 inch single coil speaker. Don't recall seeing any impedance displayed on either; I could run the 6.5 inch OEM and see. My new 6.5 inch speakers have a cap-coupled tweeter, and that's about it.

Way back on the frequency response thread, comparing the front curves, it's apparent that the front amp is loaded down -- the difference in response between curve 1 and curves 2, 3 particularly in the midrange. Curve 2 is the 6.5 replaced with a 5 ohm purely resistive load, and 3 is just the 4 inch speaker (5 ohm load pulled). Pretty apparent that the 6.5 is presenting a load less than 5 ohms in that midrange dip, and then again at the high end! But the little amp is handling it.

4xAAA--

One of the other East Coast folks was checking with his MA/shop to try and get a part number for the tool Mini uses -- you can bet on two things for said tool: (1) it's got a multi-digit part number, and (2) it's not going to be cheap.

If you find one (or more) that looks good, please post info about it!

I picked up two of the Molex pin removal tools at a local geek emporium (benefit to living in Silicon Valley). I did the dremel cutting disc mod, and my son did the razor saw mod. Both work at popping the male pins loose.

That was our "Plan A." "Plan B" was to cut the cables and install an additional pair of connectors -- I was going to use DB9 connectors, as I happen to have a bunch of those handy. Luckily, we didn't have to go to "Plan B!"

And when I went to hook up the sub, I popped out the male pins again, and soldered my wires onto the back end of the pins, making for a very clean deal.

I'm sure an easier solution will present itself over the next few days.

!

The BMW service manual shows the tool part number as 61 0 303. It is like a screwdriver with an interchangeable sleeve at the end. You should be able to get by with just the separate sleeve in 2.5mm, part# 61 0 304.

Or search around for any 2.5 mm pin removal tool.

Personally, I would first try cutting up a soda can and making a tool.

Thanks for the part numbers! I'll probably check on them Tuesday; I need to replace some plastic clips I've bashed in the process of removing and replacing panels...

Tried the soda can, and the material wasn't strong enough to do the job, so we went for hacking the Molex pin removal tool.

Interesting find! From an audio engineering perspective, MINI really got their signals crossed [pun!]. There's no reason to have a high-pass rear stage...ever. As long as the frequency response differences between the front and rear don't mess up the overall response too much and the 6x9's can handle the low frequencies, I see no reason to discount this "correction".