A lamp including a body for containing a light engine that includes at least a first and second string of said light emitters. The lamp includes driver electronics including an AC-DC switching power supply section of a circuit including a first potentiometer for adjusting total current to the light engine; and a linear topology LED string current control section of the circuit including an operational amplifier controlled by a second potentiometer to operate a field effect transistor (FET) in the linear portion of the FETs operation range to adjust a percentage of the total current to the first and second string of the light emitters. Adjusting the total current adjusts the photosynthetic photon flux (PPF) of light emitted by the lamp. Adjusting the percentage of the total current to the first and second string of the light emitters adjusts the spectral power distribution of light being emitted by the lamp.