More car audio tweaks

In the previous post, I described how I'd replaced the front door speakers, added an extra set of tweeters and replaced the rear bass-mid speakers in the new car, a 2014 B8-series A4. Although the improvement in sound quality was nothing short of dramatic, there was still some room for improvement.

Firstly although I'd added extra tweeters in the doors by using the B&O trim panels and making use of the mounts that were behind the original blanking plates, there was an apparent lack of top-end sparkle. The factory sub was swamping everything with lower midband (more on this later). There was also a lot of harsh midrange coming from the factory tweeters in the dash. In order to make the system sound ok I had to turn the treble control up a couple of notches and back off the sub by 3 notches. Although this sounded acceptable, work still needed to be done.

So the first thing is to replace the factory tweeters in the dashboard.

First of all, some preparation work. I used the same Vibe Slick tweeters as I'd used in the doors. Initially I'd planned to use the ones I wasn't originally able to fit in the rear doors, however in the meantime I'd come up with an idea for those. So I ordered an additional set of matching tweeters, which came complete with the inline filter capacitor. The wiring adaptors were left over from the front door install, as the new tweeters had connected directly to the bass-mids via a connecting cable which had its own inline capacitor.

So, to make up the adaptor leads:

Now to fit the tweeters into the car:

After replacing the grilles, the dashboard looked absolutely factory. It wouldn't be possible to tell just by looking that the system had been upgraded, however once turned on the difference is night and day. I was immediately able to drop the treble control back to the 0dB position and the harshness had gone, apart from the factory centre speaker which I'll come to later.

Now onto the next problem. There is an 8 inch subwoofer located on the rear shelf, which normally has a low-pass filter applied to it. This filter is implemented in the amplifier module, and unfortunately when I turned off the Audi sound profile it also disabled the filtering. As a result, there was a significant amount of lower midband coming from the sub. In order to fix this, a hardware crossover needs to be installed.

I located a suitable unit from Amazon for around £12:

After reconnecting the amplifier, I powered up the system. The midrange "honking" from the sub had gone, and turning it back up to 0dB gave a decent amount of low-end kick. As a test, I increased it past 0dB; this worked, there was way too much low end but no midrange and no audible distortion. Setting it back to the 0dB position restored the correct tonal balance.

Now onto the rear door speakers. I'd already changed the bass-mids, but didn't previously have time to work out how to fit the replacement tweeters. The light was fading that day and I needed to get the car back together. So, time for another go:

Again, a quick test demonstrated a dramatic improvement.

There is a further 'tweak' that can be done at this point. The Audi sound system in the A4, whilst being equipped with DSP, lacks the "front" focus mode present in the A6, and the B&O system in the A4. It only has settings for "driver" or "all". This little tweak is very effective, but does involve cutting wires on the factory loom. I wasn't able to get photos of this, but the basic principle is this:

• Locate the rear speaker wires on the brown connector going to the amplifier. These should be the top and bottom pair, looking from the wiring side of the plug. The left speaker connects to the bottom pair of pins, the right to the top pair.
• Cut the two negative (-) speaker wires (top right and bottom right). Make sure you leave enough wire to the brown plug to re-attach!
• Cut the positive (+) wire from the rear right speaker (top left). Leave the left (+) attached to the brown amplifier plug.
• Join the left speaker (-) to the right speaker (+) with a suitable terminal block (eg Scotchlock or screw terminal).
• Join the right speaker (-) to the right amplifier (+) on the brown plug.
• Plug back in and test. If it all works, tape up the connections and re-fit the wiring loom back into place.

Essentially what this does is connects both rear door speakers in series, then connects across the rear L+ and rear R+ outputs of the amplifier. This gives the effect of subtracting the right channel from the left (so-called L-R) and feeding the stereo difference signal (sometimes called the S-channel) to the rear door speakers. Whilst the effect can sound quite weird if you fade the sound to the back of the car, when listened to along with the normal stereo signal coming from the front, it will give a really good ambience and surround effect. It still works for rear seat passengers too, as they can still hear the sound from the front of the car. Of course this modification can easily be reversed by re-attaching the wires in their original position, but once you've heard it you'll want to keep it.

Having sorted all the above issues, one problem remained. The factory centre speaker. There was still some 'squawking' midrange coming from the centre speaker, which vanished when I covered the speaker. Clearly this also needs to be rectified. My options seemed to be to either disable the centre by cutting the wire at the amp or opening up the dashboard and unplugging it, or replacing the speaker. I didn't really want to cut the factory wiring any more than necessary, and figured if I was going to have to remove the speaker anyway I might as well stick a new and better one in there and keep the functionality.

There doesn't really need to be any midrange coming from the centre as with it covered and inaudible the tonal balance was correct. Replacing it with a tweeter will suffice. Also, in this application, having a diffused sound (rather than a point source) would be more effective, as it would just pull the soundstage towards the centre of the dashboard rather than sounding like "there's another speaker there".

A quick check on eBay turned up a single Vibe Slick tweeter (matching the ones I'd used in the dash and the doors), which came complete with the inline capacitor. I ordered this, along with a set of adapter leads (only available in pairs unfortunately; apparently the tweeter had been bought as part of a pair to replace a blown unit and the seller was just offloading the unused item).

Research on various Audi forums suggested that removing the centre trim is an absolute pig. I've found the best way to do it is to use a table knife with a rounded end to lever up the trim, then get a flat-blade screwdriver in to pop the clips. You can get special plastic tools to do this job, but I've found they often flex and won't open the clips. The table knife doesn't flex, and the rounded end and relatively blunt blade on a table knife is unlikely to damage the trim, as long as you're careful. Using this method, the trim popped straight off without breaking any clips or damaging any of the surrounding plastic on the dashboard.

With everything back in place, all that remained was to give the system a test. Cranking the volume up (moderately!) no longer results in a shrieky midrange coming from the middle of the dash; instead a lovely smooth clear treble is consistently reproduced across the front of the car, whether you're sat in the driving seat or as a passenger.

The whole project has cost less than £250. That's a third the cost of a mObridge (which would also need an additional amplifier), or a third the cost of having the B&O system fitted when the car was first ordered. This probably sounds quite a bit better than the B&O system too. There's plenty of bass, you can feel the thump from the kick drum, vocals are clear, and the top end is airy and spacious. It still sounds good when turned up at motorway speeds.

Yes, I could probably have gone further and installed Focal or similar speakers, but the Vibe Slick range are more than good enough, and any further improvements would likely be drowned out by road and wind noise, even in a quiet car like an A4. Overall I'm very happy with the sound now, it hasn't cost a fortune, and I still have all the factory functionality such as Bluetooth, sat-nav etc. Best of all, the car looks absolutely stock so isn't going to be a target for the light-fingered. Hopefully I'll get the best part of a decade out of this car too.