Step 3: HOW TO RESOLVE THE IMPEDANCE MATCHING PROBLEM
2 Audio Isolation Transformers - Radio Shack Radio Shack - 273-1380 $2.99
Scrap of perfboard
Okay, here's the biggest hint of the entire mod. In STEP 1, I found a clean line. However, after I wired up the docking connector to test it, the Audio Line Out (Pins 2, 3, 4) Pinout signal was waaaaaay overdriven. I tested it with the headphone output and it sounded fine at about 1/2 iPod volume. However, the volume doesn't work when you use the Audio Line Out from the docking connector. The sound was absolutely gross. Back to the internet for some research I went.
On the internet I ran across some black magic called "impedance matching." Here's a link for some information. http://hyperphysics.phy-astr.gsu.edu/hbase/audio/imped.html Turns out, the Audio Line Out from the iPod is high impedance while the headphone output is low impedance. I figured the cassette tape circuit is designed for low impedance since it sounded fine with the headphone output connected. I needed to figure out how to lower the impedance of the iPod's Audio Line Out. I first tried to insert a simple headphone volume control in between the Audio Line Out and "clean line." All that did was made an overdriven signal less loud but still gross.
Then, I came across this device at DAK Industries. Impedance thingy It takes a low impedance circuit and matches it to a high impedance circuit via an isolation transformer. I wondered if I could reverse that principle.
Back to Radio Shack. I found these audio transformers and bought two of them (one for each channel). I used these "backwards."
I wired and heat shrunk the 8 ohm windings of the transformers to the "clean line" found in STEP 1. The bigger 1K ohm windings I connected to the docking connector (pins 2, 3, 4) Pinout. The grounds can be wired per the diagram. I mounted the transformers on an old piece of old perfboard. Then I modded one of the cassette mechanism brackets to mount the transformers. Lo and behold, the transformers worked! Booya!
Remember I was talking about do-overs? I finished up all the soldering, wiring and reassembled everything. I thought I was finished with the entire project. Not so! I powered up the boombox and eeeeeeeeeeeeeeeeeeeee - audio feedback! When I plugged in the iPod, the feedback disappeared. That meant I had a no load audio feedback loop. Doh! I wanted to drop kick the darn thing. Did I have the courage to tear it all apart to try to fix it? Not courageous, just stupid.
After taking it all apart again, I stripped off the shrink tubing, desoldered the connections and basically started all over. I reconnected it in different configurations. Nothing was working. Finally, I figured it out. Remember those two jumper wires I found on the main board, where I found the "clean lines?" I decided to cut them. Lo and behold, it worked! What I had done is cut out the preamp circuit. The circuit was causing a no load feedback loop.
In fact the sound was better than before. No hiss or humming, very clean. Booya!