Hey, remember those car audio mag ads in the late '90s with the totally average guy wearing a jester's hat and very little else? Remember the name, Macrom? I hated those ads, but I still remember the name, so there you go. (There was also something about a bathtub and some aluminum foil, but it's a little hazy.) The point is, Macrom now makes amps-and video monitors, subs and head units-but I'm only interested in the amps, of course. Macrom sent us one to review, the M1A.2250. Let's find out what the court jester has been up to.
The Macrom M1A.2250 is a Class A/B amp with rated power of 125 watts x 2 at 4 ohms, 250 watts x 2 at 2 ohms and 500 watts bridged to 4 ohms with input power of 14.4 volts DC. The amp features a variable highpass crossover, variable lowpass crossover, buffered line outputs and a remote level control. The crossovers are separate so you can use them asa bandpass filter for more advanced systems.
The shell of the amplifier is an aluminum extrusion, powder coated to resemble sandblasted aluminum. There is a chrome logo strip running the entire length of the extrusion about half the width of the amp, with a cutout for the Macrom "M" logo in the middle. The cutout is filled with a translucent white plastic material that lights up when the amplifier is on. There are two good-sized terminal blocks and a set of three 20-amp fuses at one end. In the other chrome-plated end panel are two sets of RCAs, gain and crossover controls, and the jack for the remote level control.
Inside the amplifier, the layout is orderly and symmetrical, with plenty of room to fit everything in without the power supply's injecting noise into the signal path. The power supply transformer and switchers are at the end with the big terminal blocks; output devices are virtually mirrored across the center section of the heatsink. Even the low-level circuitry looks very even across the centerline of the amp. The PC board is a double-sided fiberglass board with all through-hole components, no surface mount at all. You thought surface-mount was better? Not necessarily, especially when you're using all 5 percent tolerance gold-banded resistors and mostly film capacitors. Even the emitter resistors on the huge output devices have gold bands. Looks like pretty good parts in this one.B+ enters through the 4-gauge terminal block, goes through the three 20-amp fuses (in parallel) and then enters a coil/cap filter consisting of two heavy-gauge iron core inductors and four 2,200f capacitors before reaching the primary transformer windings. The transformer is toroidal and uses multiple heavy-gauge strands per wind to move more current. The 494pwm (pulse width modulator) controller drives the six 94-watt IRFZ44 switchers (564 watts total) at 28kHz. After rectification, a pair of 3,300f 50-volt capacitors (per rail) stabilize the positive and negative 43-volt rails. There are two pairs of output devices per channel, huge Sanken MT-200 devices, each capable of 200 watts. At a total possible 1,600 watts, the output devices are definitely not the weak link in this chain.
Hopping over to the front end of the amplifier, signal comes in through the input RCAs and goes directly into the gain stage. The signal also travels to the line-out RCAs, but passes through an op amp on the way. This is called a "buffered" output and it's better than just having a hard-wired connection to the inputs. There is no feedback to the inputs this way and you can do whatever you want with the output signal without affecting the inputs. The main signal goes through a 12dB/octave lowpass crossover and then through a 6dB/octave highpass crossover before moving on to the amplifier section.
Performance
The frequency response is a very respectable 7Hz to 36kHz at +1dB, a little better than Macrom states in the specifications page. As a whole, this amplifier performed very well on the test bench, though there were some interesting findings.
First, the distortion and damping factor were very impressive, right there with some of the most expensive amps I've tested. SNR and slew rate were excellent. In fact, this is the first amplifier I can recall that actually met its SNR spec, and it's not because the spec is low-it's >100dB! (The actual number was 100.2dB, but it's still greater than 100.) During the listening test, though, I was a little disappointed by some harshness in the upper frequencies. My guess is that they've overdone the feedback loop in the amplifier, which will enhance the THD and SNR readings, but sometimes to the detriment of sound quality. "What's feedback?" you might ask. Well, in this case it's not that loud squealing sound from the electric guitars. Deep inside the amplifier, after the gain and crossovers, but before the output devices, the signal goes through a "correction" stage. While it's usually not a standalone section, the signal will be corrected as it goes through a voltage amplifier stage. The correction is determined by a sample taken from a subsequent gain stage and feeding that sample back into the differential amplifier. In essence, we're taking a look at what's happening downstream and making a correction upstream to make the signal "better." Don't worry about the time factor, this stuff happens fast. In fact, it's not even something that's analyzed by a really smart chip or anything-it's a passive operation. The only adjustment to make is how much feedback to put in. Not enough, and there will be too much distortion, and generally audible as well. Too much, and the distortion figures are great, but the sound quality will suffer. Anyway, that's my guess as to what's going on. The Macrom engineers could probably tweak it a tiny bit.
Second, the lowpass crossover exhibits a strange anomaly. Looking at the frequency response graph here, you can see the lowpass curves are very well behaved, on a precise 12dB/octave slope. The graph here stops at -35dB, but if you follow down to -55dB they actually begin to come back up. In essence, it's kind of like a notch filter. And yes, I did turn off the highpass crossover when I measured the lowpass. They only rise back up to about -45dB, way out at 40kHz so you will never hear anything. Nonetheless, I can't help but think that it affects something.