Sunday, February 12, 2017

Frequency Over Amplitude

One of the things I really want to do is to listen to my tunes while I'm driving the Midget.

Long ago, I had installed a CD player with 200 watts of amplifiers and dual 8" subs.  It sounded great, but I didn't like the look very well (and it sucked all the juice out of my little dynamo).  I went back to the original AM radio.  It looks cool but even the news is on FM now... and come on, I'm a nerd!  I have to do something cool.

About a week ago, I ran across an article on Spridget Guru about how to plumb a line-in jack into an AM radio.  Bingo!  I'm no electrical engineer, but I can follow instructions and I solder pretty well.  So I picked up some supplies:
  • 1 3.5mm stereo phono jack
  • 2 1k ohm resistors 1/4 watt
  • 1 stereo audio cable 3ft
First things first; I took the case off to see what I had to work with.


The key element here is the volume pot (top left).  That accepts a line level input from the tuner and sends it to the amp.  It's accessible and I identified the positive/negative (red and black, go figure).  This means I can do this.

I cut the plugs off the audio cable and cut it in half, then stripped the ends.  Since this is a monaural radio, I combined the left and right channels on each end.  Then, I soldered the resistors to each leg of the phono jack, trimmed to length and tied the ends together.  Finally, I soldered the mono lead on the cable to the resistors and the ground to the ground leg on the jack.  The result: a stereo-to-mono input jack.  The resistors drop the input level a bit (I probably didn't need them, but was recommended).  I encased it in shrink tubing to keep the smoke inside.

Then I ran the other end of the cable into the radio through a convenient hole in the side where the speaker wire comes out.  I grabbed my iPod Shuffle and another 3.5mm audio cable and plugged it into the jack, and turned it on.  I powered up the radio (I have a portable charger that outputs 12v) and turned it on, too.  Then I touched the ends of the cable to the positive and negative leads on the pot, and incredibly I heard Paul McCartney singing "Wonderful Christmas Time".  (I use the Shuffle for Christmas music in my cabinet radio).

Wow!  It works!  So I soldered the cable ends to the pot, and voila!  I plugged in my phone and I had Van Halen singing "Beautiful Girls".  It sounds really good--good volume and tone without clipping.

I picked the iPod Shuffle because it also has an FM tuner.  So I switched to FM and it came in crystal clear.  Now, the title of this post makes sense, right?

I have volume control of the line input and still have AM reception.  All I have to do is tune to an empty band to listen to music through my new aux jack.  If I don't I get both the aux input and the tuner input, which is pretty funny.  I may have to put a switch on the antenna input, because I might not be able to find a dead band when the radio's installed.

Here's a demo:

Now that it was all working, I repainted the case with wrinkle finish paint.  It's pretty cool to watch this stuff dry.


Once dry, I'll reassemble and it's all done. I'll also add a USB charge port, in a little box that will hold both the aux jack and the USB port (and not ground out the radio--remember, it's positive ground).  The USB port needs a 12v-to-5v converter, which are only about $10 on Amazon.

Update: Here's the finished product, mostly installed.  I used this adjustable voltage converter for the charge port, and put the USB and aux ports in a plastic housing so they're ground-isolated.  It works great with my iPhone (with the Lightning-to-headphone converter you need for an iPhone 8 or newer).  There is a brown 12v lead available on the harness, though it's unswitched... which I prefer, since I can charge my phone and play the radio without running the fuel pump.


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