Cutaway picture: Magnepan Ribbon  The most frequently cited disadvantages of Magnepan's overall design are relatively low efficiency and, given the absence of an enclosure, difficulty in reproducing very low bass frequencies. Because of the low resistance (4 ohms) and low sensitivity (86dB), it requires a surprising amount of current to drive the Magnepans to moderately loud listening levels. Many inexpensive receivers simply do not have the required current capacity so they go into various distortion modes including crossover distortion, clipping and slew limiting, with the result that the Magnepans may not sound very good. Driving the Magnepans with a 100 watt or better high current amplifier can result in much better sound. Many suitable modern and vintage amplifiers can do this, but it is dependent on their design. Originally Magnepan sold the MG and the Tympani series. The Tympani series has been discontinued for several decades, and is replaced by the 20.7 and 30.7 speakers. The MG series has been produced continuously but the "MG" has been dropped from the name; thus the MG-I is now the 1.7; the MG-II is now the (discontinued) 2.7, and the MG-III is now the 3.7. Magnepan also produced a small, relatively inexpensive speaker, the SMG, which became the MMG and is now called the LRS. The Tympanis and MG speakers were all made from wire-on-mylar; the speakers with no "MG" in the name are all made from aluminum foil on mylar ("Quasi-Ribbon"). The MG-III, 3.7, Tympani IV, 20.7 and 30.7 all include Magnepan's ribbon driver and thus produce sound up to 40khz. The other speakers do not include a true ribbon and therefore produce sound only up to about 18khz - 25khz. All Magnepans are 4 ohm speakers, with sensitivity of 86dB / 500Hz / 2.83v