Figure 3. Simplified Schematic of the Heathkit AD-1305. The amplifiers in the Heathkit EQ are made with discrete transistors rather than being integrated circuits. This diagram shows them simply as operational amplifiers instead of giving the internal details. The input goes to the noninverting input of an op amp. The output of the op amp connects directly to the inverting input making it a unity gain buffer. The output of the op amp goes through a 1.2 k ohm resistor to a point we will call pot bottoms. That's because the bottom end of all 5 pots go to this point. Pot bottoms also goes to the noninverting input of a second op amp. The top ends of all 5 pots connect together and go to the inverting input of the second op amp. There is a 1.2 k ohm resistor connected from the output to the inverting input of the second op amp. The output of the second amp is the equalizer output. Now back to the pots. Each pot is 50 k ohms total resistance. There is a fixed center tap on each one that is connected to ground. LTspice simulations indicate that this grounded center tap is unnecessary.
The first pot on the left controls the lowest frequency band. Its wiper goes through a 68 ohm resistor, then a 4 micro farad capacitor, then a 1.75 henry inductor to ground.
The wiper of the next pot goes through a 100 ohm resistor, then a 1 micro farad capacitor, then a 405 milli henry inductor to ground.
The wiper of the next pot goes through a 270 ohm resistor, then a 2.7 micro farad capacitor, then a 94 milli henry inductor to ground.
The wiper of the 4th pot goes through a 200 ohm resistor, then a 0.056 microfarad capacitor, then a 28 millihenry inductor to ground.
The wiper of the last pot goes through a 270 ohm resistor, then a 0.015 microfarad capacitor, then a 6.6 millihenry inductor to ground.
The equalizer has switching for tape recording and tape monitor. All of which is irrelevant to this discussion.
End of verbal description.
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