I recently installed one of Marty's DSP amp replacements and it works superbly however, it got me to thinking. While the door amps are fairly easy to get hold of, the plugs aren't so easy unless you can find a scrap P38 and chop the wiring off. Sooner or later supplies will dry up of both and, while the door amps are still available new, they are ridiculously expensive. So it made me wonder if there is an alternative way of doing it? The amps are just that, amps, but with a built in crossover so have two outputs, one for bass and one for mid/treble. I wasn't able to find a cheap and cheerful amp with crossover but could find amps and crossovers available for not much money at all.
I will stress that I haven't built one of these, and don't have a car with DSP to use for testing, but I can't see any reason why it wouldn't work. If I could lay my hands on a dead DSP amp it would be worth pulling the socket off it to make up a system that could simply be plugged in as a replacement saving an awful lot of time with soldering iron and heat shrink tubing.
I would propose using a couple of TDA7492 based amp units such as https://www.ebay.co.uk/itm/2-x-50W-TDA7492-D-Class-High-Power-Digital-Amplifier-Board-Module/163019456498 feeding into 4 of these https://www.ebay.co.uk/itm/120W-Adjust-Treble-Bass-2-Way-HiFi-Speaker-Frequency-Divider-Crossover-Filter-es/264933727526 or similar. The attenuators may be needed if feeding it with an original head unit but the TDA7492 chip does have variable gain so it might just need the gain turning down. If you are feeding it from an aftermarket head unit with line level outputs, then they won't be needed or if feeding from a speaker output level the amps won't be needed, just the crossovers. If using amps, there is a 12V supply (that originally powered the DSP amp) and a turn on signal from the head unit available at the DSP amp connector so with the addition of a relay the amps would only be powered when the head unit was switched on.
I intended doing a complete write up for someone on RR.net but after the owners removed all the useful information on there I didn't feel inclined to start building it up again. So I've sent the following to the OP by PM and are publishing it here instead. This applies to a situation where the head unit is also being replaced with an aftermarket one so only deals with speakers and crossovers, not the amps, but if it was to be installed as a replacement for the DSP and retaining the OE head unit, then a pair of amps would need to be added before the crossovers.
If you are going to install an aftermarket head unit, you won’t need to replace the DSP amp with separate ones, you just need to link the wiring through at the DSP amp plug. Feeds from the radio all went to the DSP amp and the wiring to the speakers comes from there.
Your aftermarket head unit will have connections for the following:
Permanent power – connect to Purple wire on Pin 4 of C0098 (Grey 8 way connector)
Ignition switched power – connect to White/Pink on Pin 7 of C0098
Illumination – Connect to Red/White on Pin 6 on C0098
Ground – Connect to Black on Pin 8 of C0098
Turn on signal (often marked as for an electric aerial) - Connect to Grey/Black on C0098 (only needed if you intend installing amps)
It will also have the following outputs:
Left Front +ve and –ve – Connect to Yellow and Yellow/Black on pins 5 and 6 on C0092 (Brown 8 way connector)
Right Front +ve and –ve – Connect to Red and Red/Black on pins 3 and 4 on C0092
Left Rear and Right Rear – There will be no wires for the rear feeds at the radio as this was taken care of in the DSP amp. You will need to run two new pairs of wires from here to the DSP amp location.
Subwoofer +ve and –ve – Connect to Orange and Orange/Black on pins 1 and 4 of C0921 (small 6 way connector with only 4 ways used).
Use the diagram above to ensure you get the correct wires on the plug that went onto the DSP amp. With the exception of pin 26 which is the main ground, pins coloured black on the diagram are ones that aren't needed. Although the numbering seems strange, the wires are arranged in pairs just not necessarily on the same row, hence pins 38 and 30 (for example) are next to each other. The back of this plug can be removed so you can cut the wires as close as possible to the plug giving you maximum length to play with and ensure you have the correct wires. I suggest cutting and connecting one pair at a time and not chopping the lot off then getting very confused.
At the DSP amp end you need to connect the wires from the front through to the speakers. Ideally this needs to be done via 2 way crossovers (four, one for each channel) to retain the sound quality.
Left Front – Connect the Yellow and Yellow/Black on pins 38 and 30 to the I/P of the crossover, the low frequency output of the crossover to the Green/Yellow and Yellow/Green wires from pins 4 and 3, connect the mid/high frequency output of the crossover to the White/Yellow and Yellow/White on pins 33 and 34.
Right Front - Connect the Red and Red/Black on pins 9 and 17 to the I/P of the crossover, the low frequency output of the crossover to the Green/Black and Black/Green wires from pins 25 and 24, connect the mid/high frequency output of the crossover to the White/Black and Black/White on pins 11 and 12.
Left Rear – Connect the pair of wires you have run to the back of the car from the Left Rear output of the Head Unit to the I/P of the crossover, the low frequency output of the crossover to the Green/Blue and Blue/Green from pins 2 and 1, connect the mid/high frequency output of the crossover to the Blue/White and White/Blue on pins 31 and 32.
Right Rear – Connect the pair of wires you have run to the back of the car from the Right Rear output of the Head Unit to the I/P of the crossover, the low frequency output of the crossover to the Green/Red and Red/Green from pins 23 and 22, connect the mid/high frequency output of the crossover to the White/Red and Red/White on pins 13 and 21.
If you choose not to use crossovers and are quite happy with a ‘muddy’ sound quality, then simply parallel the Low and mid/high frequency feeds to the speakers.
The sub output from the radio simply loops through the DSP amp so the input and output need connecting together. Connect the Orange and Orange/Black on pins 39 and 40 to a matching pair of Orange and Orange/Black wires on pins 6 and 14 which go to the sub.
You will be left with a few spare wires that aren’t being used. These are for power to the amp (two supplies, one on a Purple wire and the other on a Brown wire), the switch on signal, Navigation mute, Navigation voice input, Road speed signal (for the speed dependant volume adjustment), Ground, DSP data, Phone Mute and Phone audio input (yes, despite numerous people butchering the wiring to add phone kits, there was a simple plug in loom available from LR that integrated a mobile phone with the existing system). All these spare wires should be insulated so they don’t short out against ground or each other.
I’ve gone through this at least 3 times now to make sure I’ve got it correct and haven’t got my +ve and –ve feeds crossed (which, again, will result in poor sound quality), but check through the diagram and legend above just to see if you can see any errors.