Progess! Episode 6 - Smoke 'Em If You Got 'Em
I wrote out a post on a car audio forum yesterday after looking at my options for attenuation, figuring that this was the only option now unless I went for a full amplifier at great expense. I had been looking at DIY circuits using a pair of resistors to create a voltage divider and while I had asked the question on there my mind was already working on the problem.
I needed to know the voltages I was working with, so I got the head unit back out of the dash. Finally a chance to show you the cable spaghetti that gets vomited out and which needs to be crammed back in. This isn't everything, either. You can see the metal bracket I've mentioned before.
I set it up on the bench, played a test tone through an FM modulator and probed the output. 12V at full whack. I did the same with the factory head unit (I was using an FM modulator on my phone for both tests as the old unit doesn't have an aux input and I wanted to ensure the test signal matched). Around 2V at max volume.
Calculations showed I needed a 2K resistor to get the voltage down to 2V but the closest standards are 1.5K and 2.2K, so I set up a pair of voltage divider test leads with a 10K resistor and 1.5K or 2.2K respectively. Both worked well but the 1.5K gave my more volume sweep so I went with that one. I got out the perfboard and made a simple mock-up circuit with the resistors and cables.
That looked fine so I cut up some board to make a little one for each channel, which were all covered with heat shrink. The little bits of dowel stop the cables from moving and straining the solder joints. I admit that these could've been made smaller but I wanted space around the resistors.
The measurements of the resistances were all correct and on the bench the voltages all looked good when plugged into the head unit so I tried it in the car again. Bingo. There was no distortion or noise, it seemed a little louder than earlier using the same resistor values but that was only using one channel so combined I suppose it would be louder.
It gives me plenty of scope to adjust it in the head unit now, something I've never been able to do before. Strangely the navigation voice and Siri are always quieter by default. I'm happy and assuming the resistors don't explode during future testing this will finally fix my issues!
While the dash is out again there are a couple of things I need to show you. Firstly I remembered that there's a fourth 3D printed part in there, supporting the top edge of the dash and stopping it from flexing. Sorry about the strange angle of the photo, it was upside down originally.
Finally there was one job to do that I hadn't done before, some extra decoration. When I first made the part that sits below the screen I thought it looked very plain, it needed something to break up the block of black. As it happens, when I was shopping on eBay for some putty to smooth out the parts I found a 1:10 scale number plate kit. Now there was an idea.
OK so the letters aren't all completely straight but whatever.
Hopefully I'm now back to the position where I can see light at the end of the tunnel again. Unfortunately, refitting and longer tests may have to wait until next weekend as work and dark nights will get in the way.
. And yeah, like Thomas says, that makes me want a 3D printer!
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