I have an early 70's Magnavox turntable/radio console that I want to be able to plug my smartphone into so that I can use it as an external speaker. I'm plugging into the Tape In input, which is an RCA cable. If I go straight in, the volume is too low--I have to crank the phone and the console to get a decent level. I suspected an impedance mismatch and asked for advice on the VintageAudio subreddit. I got this response:
"The output of the phone is probably a modern "line out" level of 150mV. The output of the tuner circuit is more likely around 1-2v. The function switch in equipment of this era is almost ubiquitously a literal switch with no additional amplification adaptations between the various input selections. Also, the output of the phone is likely 8 ohm for headsets, vs. 47k ohm the input expects. You're correct that this would be an impedance mismatch. What I've done in these cases is build a small preamp module using a TL072 dual-channel opamp. They're less than a buck each and I just use the simple preamp circuit provided by the manufacturer in the data sheets for the IC. Hang a half dozen capacitors and resistors off that 8-pin chip, feed it 12V and it can take care of all the level and impedance mismatches like magic."
So I bought a TL072 and figured I'd build the "AC amplifier" circuit that is listed on the datasheet (schematic shown below). I have all the necessary parts but I have a few questions as I am a total noob when it comes to building circuits:
1.) Is this in fact the correct circuit (or a correct circuit) that will do what I want? If not, is there a better or simpler circuit that will accomplish the same goal? 2.) Notice the line that the red arrow is pointing to. What does that connect to? All of the other pins on the IC are labeled (VCC+, N1, OUT, etc.) but I don't understand what that line, which is connected to ground, is supposed to connect to on the IC. 3.) I'm assuming that I would connect IN+ and IN- to the mini jack that I'm going to plug my phone into. And on the output, I would connect the OUT to one of the L/R leads of the RCA cable, and the ground to the other RCA lead. Are those assumptions correct? I guess I'm trying to figure out how a single output connects to a stereo Tape In. Please enlighten me. :)