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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. :)

enter image description here

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  • \$\begingroup\$ I'm a bit curious - why would the impedance mismatch be in any way relevant here, with an 8 ohm output connected to a 47k input? \$\endgroup\$ – pipe Jan 9 '18 at 6:31
  • \$\begingroup\$ A pedant writes: is this not a TL071 in the schematic? (Assuming it's a member of the TL07x family). \$\endgroup\$ – DiBosco Jan 9 '18 at 8:27
  • \$\begingroup\$ @pipe, I just assumed that the impedance mismatch might be an issue since it seemed like a similar situation to trying to use a standard low-impedance microphone (e.g., Shure SM57) plugged into a guitar amp with a high-impedance 1/4" input. To get the levels right, you have to use an impedance matching transformer. I thought that might be what's happening here as well, but I don't know. Some users on reddit have also questioned the impedance mismatch here and said that the mismatch the guy described is wrong (or maybe just irrelevant). \$\endgroup\$ – johnnyb1970 Jan 9 '18 at 13:58
  • \$\begingroup\$ @DiBosco, you're absolutely right--I checked the datasheet again and it turns out I posted a schematic of the TL071, not the TL072. My bad. Looking at the pinout of the TL071, it makes much more sense because it appears to be dual channel, so I can use it for the L and R stereo channels. \$\endgroup\$ – johnnyb1970 Jan 9 '18 at 14:02
  • \$\begingroup\$ @johnnyb1970 Yeah, the impedance thing is a red herring, it's not relevant here and I didn't expect a valid answer. Outputs are supposed to have a low impedance, and inputs are supposed to have a high impedance - just as you have it. \$\endgroup\$ – pipe Jan 9 '18 at 14:17
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I have used the TL072 op-amp a lot because it is a quiet amplifier, good enough for at least 16 bit audio. But it is designed for +/- 12 volt to +/- 15 volt power supplies. The trick is to insert a 10 K resistor from the output pin to Vcc.

This causes an offset current that stabilizes the op-amp. Replace your 1 Meg resistor with a 100 K resistor. Delete your 50 ohm resistor. Insert a 10 uF capacitor with the (+) pin connected to your IN- input. Do not use your IN+ input as this is not an instrument amplifier. Now you have a single ended input with a gain of 10, and a stable op-amp.

Because your output has a voltage equal to 1/2 Vcc on it, I would insert a 47uF capacitor at the output with the (+) pin going to the op-amp output.

The pin your pointing at with a red arrow is the Vee or V- pin, which normally goes to a -12 volt to -15 volt rail. You can just ground it if you use the trick I mentioned in the first paragraph. Without that 10K pull-up resistor the op-amp may produce severe distortion. Adjust the 100K resistor if you need more gain. For stability and ripple filtering there should be a 47 uF capacitor from Vcc to the ground (Vee/V-) pin.

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Op Amps designed for bipolar supplies are referred to as Vcc and Vee or V+ and V- and ground is your choice of external 0V or midpoint connection.

Since you are using a single supply your input and output DC levels are both at Vcc/2.

The offset adjust pot is not needed, but you may or may not need a series Dc blocking cap for the output.

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Turns out I posted the wrong schematic--the IC in my schematic is a TL071 instead of a TL072. A user on Reddit pointed me to the following schematic that uses the TL072 and that should do what I need. This schematic shows only one channel, so I'll need to duplicate all the components (except the IC itself and capacitors C3 through C6) for the second channel.

enter image description here

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  • \$\begingroup\$ I would'nt advice on using R2, since the opamp is operating in series feedback, the addition of R2 may increase common mode distortion. \$\endgroup\$ – S.s. Jan 9 '18 at 14:31

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