We know MA Audio has good taste in spokeswomen. Do they also have it in amp design? We take a look at the company's M680i (sounds strangely Bavarian...) two-channel offering.
A two-channel amp rated at 200 watts RMS a side (400 watts peak), MA Audio's new M680i features user-selectable high- or low-pass variable crossover, switchable bass boost and speaker level inputs, for those with head units not rigged with RCA outs.
On the BenchWe ran the M680i at 4-ohm stereo in two-channel mode and 4-ohm mono in bridged mode. In two-channel mode, we used the onboard high-pass crossover with our 6.5-in. components. When bridged we used the low-pass linked to various reference subs. We gave it a full one volt of signal input-the maximum it can accept-then added 5dB additional gain overlap to bring the signal level up to 1.74 volts. Following are our measurements playing actual music, not test tones, bass discs, or sine waves (see pg. 81 for a full explanation of our test procedures).
MA Audio does not specify in the manual, nor on the box, the ohm load and supply voltage they used to achieve the 200watts/channel spec. We ran it at four ohms since we feel it's the best indication of an amp's "real-world" performance and its long-term health. In any case, the measured output power versus the minimum measured supply voltage of 13.8v makes this amp 60 percent efficient under full power with maximum rated signal input.
We didn't observe any serious heating problems during testing, but the heatsink does need breathing room below. The top cover employs a clear window that reveals the pretty (really) guts of the machine, but doesn't do much to sweep away heat.
We did test the M680i at 2 ohms, but found the power increase marginal and the heat increase noticeable. To run all day long without trouble, best to keep this amp at four ohms. At idle, with no signal on the input, the M680i drew just 0.62a, about par for a car audio amp of this class.
Crossovers and Set UpThe high-pass crossover is variable between 120-800Hz and the low-pass between 40 and 120Hz. Filter slopes are not mentioned in the manual,but we found the crossover points accurate with the corresponding numbers silk-screened on the amp's side. The slope turned out to be about 12dB/octave. A simple switch selects high-pass, low-pass, or full-range (no crossover) functions.
The high-pass crossover goes down only as low as 120Hz, not a problem in most cases. This can sometimes be a blessing with front speakers unable to handle frequencies below 120. If you've gone great lengths to get good front stage drivers that can handle 80-90Hz or above, another half octave lower on the crossover would have been nice.