How to upgrade OEM E30 head unit with modern amp & bluetooth internals

[red328i]

Here is how you can have a working OEM head unit with a volume control knob that works and modern Bluetooth connectivity.

I always hated the 14-year old Pioneer head unit in my E30 Coupe, so I was glad when it died. I've got the original premium audio wiring and original OEM speakers in the back, plus a bunch of soundproofing, so my car has always had the possibility of sounding good, but never has. Hence, I decided to upgrade to a modern amp and a good bluetooth unit.

On eBay, I purchased a not-working OEM E30 head unit with a very nice look (and only one unnecessary knob missing). On Amazon, I purchased a DS18 BT-Two bluetooth receiver, a Kenwood KAC-M1814 amp, and a Rockville SS65P 6.5" under-seat subwoofer.

I then eviscerated the OEM head unit and discarded most of the internals, except for the front grille, the volume control potentiometer (aka pot), and the screen/bass/treble unit. After this, I took a Dremel to all intruding bits inside the now-empty case, to make it as open and bottom-flat as possible.

With a soldering iron and some spare time, I made a custom RCA cable for the Kenwood, wired it up to the OEM volume-control pot, and wired the ON/OFF functionality of the pot to turn the DS18 Bluetooth unit on and off. This worked, and I could play music and control the volume. Also, all the wiring bits fit inside the OEM head unit case, so I was "almost done." However, everything was super messy with flaky soldiering all over the place. There was no way this would give good audio quality.

So, I made a PCB on EasyEDA and had it fabricated in China. This turned all of the messy wiring into what looks like a professional audio part for less $$ than a fancy sandwich, delivered to my door. (This blows my mind.) I'm happy to share the Gerber files or EasyEDA JSON if anybody wants. (If this forum allows me to post URLs, the JSON is at tinyurl.com/4s85nuam) Also, the minimal order is 5 pieces, so I have 4 left over.

I then soldered in some thin wires resistors and LEDs so that the "on / off" light on the head unit still works, and the light on the cassette receptacle lights when music is actually playing.

Pictures attached.

[iXBeater]

This is pretty awesome - thanks for sharing!

[Panici]

Pretty neat!
I like how you kept the OEM looking front radio faceplate.

Do you have any provisions for balance/fade and possibly a basic EQ?
These are things I've found myself wanting with the previous "headless" setup that was in my E30.

[red328i]

Yes, there are a few options on how to do the balance/fade:

1) The Kenwood amp I am using has independent gain control for the two channels, so that is what I used to set the fade (front vs back speaker volume). I can't change it without taking the head unit out, but that's fine with me, since I don't plan to change it often, if at all.
2) I can't control balance at the moment, but if I wanted to, and was willing to do a bunch more soldering, the Bass and Treble controls from the OEM head unit are actually linear potentiometers. So, the left and right channels could be sent through those for independent volume control of each. At the moment I think this is too much trouble for what it is worth.
3) I wanted to keep everything in the OEM head unit case, but it is also possible to put Bluetooth unit under the dash, in the glovebox, under the seat, etc, and couple it with a simple equalizer there (e.g., something like Skar Audio SKA7EQ from Amazon).
4) However, for the equalizer, even better would be if there was a better Bluetooth receiver (that also supported BT 5.0) that directly offered eq controls from your smartphone. These must exist, even though I haven't found them.
5) I've been tempted to build a BT receiver using ESP32, which would also allow me to drive a display on the head unit. But realistically I probably don't have time for this.