Memphis rates this amplifier at 115 watts x 4 into two ohms + 1000 watts x 1 into one ohm. The amplifier has four AB MOSFET channels plus one Class D channel.
FeaturesHybrid class AB / Class DHigh / Low pass internal crossoversVery compact designDual Power suppliesLow profile, only 1.7" in height
CosmeticsThe Memphis Belle is highly compact for its power, measuring only 22.5" x 6.25" x 1.7". While cluttered with small fins, the high gloss, polished aluminum heatsink has an overall clean look with only the Memphis Belle logo adding to its character. Mounting the amplifier is easy, granted you have the space, with six tabs to extending from the sink. The connectors are very nice, with the power accommodating four gauge wire and the speaker accepting eight gauge wire. Allen keyed set screws secure the wires in place.
LayoutThe layout uses through-hole components with five upright PCB boards in addition to the main board. The circuitry looks crowded, but this is required to achieve five channels capable of over 1600 watts in such a small package. Standard 85 C capacitors were used in critical power stages (see sidebar). Transformer primar connections were made directly to the power Mosfets to minimize heat build-up on the PCB. All of the class AB channels use two N-channel Mosfets. The class D amplifier uses twelve Mosfets. The 16-ST1300D uses two separate power supplies, one for the four AB channels and one for the Class D sub channel. Audio input connections are standard RCA gold plated PCB mount connectors. Inductors were included for the 12 volt power input and in the supply output section for added filtering.
Electronic CrossoverThe four channel crossover provides an 18 dB/Octave slope. The class AB channels can be configured for high or low pass. The frequency is variable from 50 - 250Hz. We noticed that the high pass crossover's level varied about 3dB when the frequency control was swept over its range.
Power SupplyThe power supply is unregulated, a big advantage for the SPL guys that jack up the battery voltage. The supply went into over-voltage protection at 16.7 volts.
ProtectionThe amplifier is protected by six internal 30 ampere fuses. The four class AB channels went into short protection with loads of one ohm or lower. The amplifier gracefully shut down and recovered without damage. The Class D section went into protection with loads below 0.66 ohms. The sub channel current limits when shorted, but did not shut off. It also recovered without damage. The amplifier survived our over-temperature and reverse-polarity tests.
PerformanceThe four AB channels:This section of the Memphis Belle easily met the rated power and distortion specs.
We found some problems with the Class AB Mosfet output stages. We measured a rather low slew rate, about 7 volts / S. When a signal that approaches this slew rate was applied to the amp, small waveform anomalies appeared. While it's unlikely that one would actually listen to high amplitude 20 kHz sine waves or 1 kHz square waves, it is still worth noting: In figure #1 you can see a glitch when a 1 kHz square wave is applied. In figure #2, a glitch is also seen when a high amplitude 20 kHz sine wave was applied.