I am trying to make a voltage adder/summer circuit using a LM358 op-amp. I got the schematic off the internet and it matches the one that I have in one of my electronics book at home. I bread boarded the whole thing using a 5 volt voltage regulator. I made 2 voltage division networks using a 20K ohm variable resistor and placed it from (+) to (-) with the wiper output to my fixed resistor that went to pin 2 of the LM358. The fixed resistors are 1k ohm and the reference resistor (fixed resistor) is also 1K ohm. I measured the voltage drop across the fixed resistors. It came to about 1 volt each. So as a summing circuit it should put out about 2 volts. For some unknown reason though it puts out only .6 volts. When I change the voltage drop across the fixed resistors to 2 volts each I still only get a .7 voltage output between Vo and Gnd. Does anybody know why. I used the circuit diagram found at Summing Amplifier (Voltage Adder) for my circuit. Only I am using a voltage division network to get my different voltages. I have tried using a spare op-amp with the same results. Please help.