Adventures in in-car audio

Those who know me outside of the blogosphere will know that I've recently changed my car. Previously I had an Audi A4. I now have an Audi A4. There's a surprise.

Actually to be more specific previously I had a 2003 2.0 FSI (petrol) model. The engine on this spluttered and died after nearly 140,000 miles. So I've replaced it with a newer 2007 2.0 TDI Quattro model.

So as per usual, I set to work on getting a decent audio system in the car. Audi actually fit fairly decent speakers, but the electronics leave a little to be desired. Even the RNS-E satnav unit (which was fitted to my car) isn't that great. The onboard amplifier is underpowered. The SD slots won't read SDHC cards. And having tracked down a couple of 4GB SD (non-HC) cards I discovered that the RNS-E will only read the first 500 or so tracks. And the CD changer won't read MP3 format discs. All in all about 20 years out of date.

I'd already replaced the RNS-E with a Zenec ZE-NC3141D, as I was fortunate enough to have an A4 with a double-height slot for the radio. Because it had the RNS-E sat-nav system the heads-up display on the dashboard is also in full colour, rather than the red-on-black of the standard model.

The Zenec has a pretty decent Class D amplifier built in, which powers the front door speakers. The rears are powered from the Audi Active OEM amplifier in the boot, which also runs an 8" subwoofer mounted on the rear deck. Actually it's technically a "woofer" rather than a "subwoofer", but never mind.  The problem is that although the stock speakers are quite decent, the amplification isn't really powerful enough. Plus Audi use a weird arrangement for their OEM sound system. Basically there's preset EQ curves for the front and rear channels, with the HF rolled off to the rear and the LF rolled off to the front. There's a strange bit of EQ applied to the sub as well which gives a nasty peak around 120Hz (what us hi-fi enthusiasts refer to as "Bose bass").

In previous cars I've solved this by fitting a 4 channel JBL GTO-4000 amp behind the trim and using this to power the front door speakers and a pair of 10" subs in the boot. The downside of this is that the 4 channel amp is too big to hide and causes the trim to bulge out, and the pair of subs take up half the boot space. As I didn't need to add separate amplification to the front channels in this car I thought I'd try something different.

So I obtained a newer and smaller 2 channel JBL GTO-752 amp which can be bridged. I've connected this up in monoblock configuration, chopped the wires from the Audi active amp to the sub (leaving enough slack to reconnect them in future) and routed them to the new amp:

The new JBL amplifier (unlike the old monster GTO-4000) is small enough to fit behind the trim panel. This is now fed from a dedicated sub out on the Zenec head unit, crossing over at 80Hz. With some clever positioning it fits in a space behind the trim panels.

The silver cylindrical thing in the well is a 1 farad power capacitor. The black box next to this is an interference suppressor.  I did find that I needed to connect a ground loop isolator between the audio cable from the sub and the new amplifier.

I also packed some bubble wrap behind the amplifier (being careful not to obstruct the cooling fins) to stop it moving around, and more bubble wrap underneath to protect the capacitor.

Finally I replaced the trim panel. The smaller JBL amplifier fits perfectly into the empty space behind the trim panel, and there's even sufficient space for the cubby hole in the trim:

And it looks completely stock standard. No massive great bass bins in the boot. The head unit is styled to look like an original Audi unit. It keeps that pristine "factory" look. But it sounds far far better. There's now proper deep bass from the factory sub rather than a resonant boom at 100Hz. And because the sub is no longer fed from the rear pre-outs I've been able to re-insert my home-made little device that retrieves the S-channel and sends it to the rear speakers, effectively giving a matrix-decoded surround effect. I had this in the old car and it sounded great.

The next project? I have a multi-function steering wheel (with volume and CD changer controls) which I'll be fitting over the next few days.