ABSTRACT
This chapter focuses on techniques for driving loads heavier than can be handled by a single 5532 or LM4562 opamp–basically any impedance less than 500 O. This is typically a headphone load, the impedance of headphones varying widely from 600 O to 16 O or less, with sensitivity inversely related so the higher impedance models need more drive voltage. The wide range of load impedances is accompanied by a wide range of sensitivity. Headphones of 600 O impedance require ten times or more voltage to achieve the same sound pressure level than do those of 30 O impedance, though there is much variation. The traditional solution to the wide impedance/sensitivity range is a resistor of the order of 50 O to 100 O in series with the amplifier output, which reduces the maximum output voltage available as the load impedance is reduced. It is believed by a small minority that a balanced drive to headphones can be of advantage.
