When it comes to car audio amplifier choices these days, it seems like there is an almost endless supply of products. Prices and performance are all over the map, from bargain basement junk, to high end high dollar gear that provides unique features and exceptional performance. If you’re like me and most other gearheads, the problem usually is wanting the features and performance of the high end esoteric gear, but I’m more realistically going to be able to afford the stuff closer to the opposite end of the price spectrum.
Well, it would appear that the folks at Boston Acoustics recognized our problem and did something about it, by creating the GTA series of amplifiers. Using several of the same technologies as their high-end GT series of amps, the GTA amplifiers provide excellent performance and sonics at a modest price. I recently received a GTA-400M for evaluation, and because I have always held the GT series in high regard, I was looking forward to taking a closer look at the less expensive model.
FEATURES AND CONSTRUCTION
The GTA-400M is a Class D mono amplifier, rated at 250 watts into 4 ohms, and 400 watts into 2 ohms. The amplifier is relatively small, measuring 10.25” x 8.375” x 2.375”. The compact chassis still allows all the controls and connections to be located along one side, and Boston’s IMS (Integrated Mounting System) uses mounting points to be located within the footprint of the chassis, without the need for external mounting tabs or feet. The heatsink is finished in silver brushed aluminum, with a gloss black strip that also provides model identification. The heatsink wraps around 2 sides of the amplifier, and the increase in both mass and area combine to provide efficient heat dissipation.In terms of controls and connectors, the GTA-400M uses traditional set screw type gold plated terminals, and installers will appreciate that all the connections can be made using the same 3mm hex key. A 40A ATC fuse is mounted onboard, and there are control pots for gain, crossover frequency, subsonic filter, and a feature Boston calls Q-Tune, which provides bass boost with a variable Q factor.
An optional remote level control (GTA-RSL) allows control of the bass volume from the driver seat. The GTA-400M also includes high level inputs, which make it simpler to integrate with an OEM system. A dual color blue/red LED provides power-on and protection indications, so you always know the status of the amp at a glance.
Inside the Boston amp, a military spec PCB uses a combination of surface mount and through hole parts. The layout and design are well done, and particular attention has been paid to the filtering of the output stage, where all the Class D switching carriers are removed from the output. The amp is ruggedly built, with all the higher mass parts soldered and glued in place, and silicone damping material is used to prevent transformer and toroid ringing. The amp uses a total of 6600µF of capacitance in the power supply, and a pair of 2200µF caps handle the energy storage for the outputs. Power supply and output devices are TO-220 case MOSFETs.
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As you’d expect from a Class D subwoofer amplifier, the internal crossover is always on, and is adjustable from 50Hz to 150Hz. The slope of the filter is -12dB/Oct. The subsonic filter, (a must for vented subwoofer applications) is also a -12dB affair, and can be adjusted between 10 and 50Hz. The Q-Tune feature is one of the technologies carried over from the high end GT series, and is very handy in getting your bass dialed in to perfectly blend with your front speakers. The control is adjustable from a Q of .7, all the way to a Q of 1.6. The higher you turn it, the more bass boost you get at whatever frequency you have the subsonic filter set to, but unlike a common bass EQ knob, the Q-Tune increases the slope of the curve as you increase the boost, to help protect speakers and maintain amplifier efficiency. The effect is more useful than a standard boost control, and can really help bring your bass forward in the car. To better illustrate this clever circuit, have a look at the graph I generated and you’ll quickly understand what it does. In the graph, the subsonic filter is set to 25Hz, and I made 4 curves with the control set to 0.7, 1.0. 1.3. and 1.6.
Read on for Full Results
After reading the manual and understanding the controls and functions, I connected the GTA-400M into my reference system, driving a 2 ohm 12” woofer in a sealed enclosure. I began with the crossover set at 100Hz, and the subsonic filter set at the lowest frequency possible. Because of its relatively compact size, I was a bit surprised to find as much clean power on tap as the small monoblock dished out. While certainly not a power monster, the GTA-400M provided all the power I needed to listen at enthusiastic levels. I also noted that the amplifier drew significantly less current from my power supply than most of the other bass amps I’d tested in recent memory.
From a sonic perspective the Boston GTA-400M was excellent. The bass was clean and low, and I noticed a distinct lack of what I refer to as “Class D syndrome”. I also appreciated that someone took the time to ensure there were no unwanted turn-on or turn-off pops, when the amps powers up and down.
One of the first tracks I played is an amazing solo by the Australian female bass playing sensation Tal Wilkenfeld. Her mastery of the 5 string electric bass is jaw dropping, when you consider she’s barely old enough to vote. Paying close attention to her string plucking and use of harmonics, I thought the Boston amp sounded more like a decent Class AB amp than the Class D that it is. With excellent tonality and definition, the bass was tight and clean, but what I appreciated the most was the way all of the intricacies of Tal’s musicianship were also reproduced.
On a related note, if you have been under the impression that an average Class D amp on a good subwoofer system sounds indistinguishable from a Class AB amp or a very good Class D amp, let me disabuse you of that idea right now. If you listen carefully to well recorded music you’ll notice subtle ringing and overtones which are naturally produced by instruments such as drums, timpani, bass guitars, cello, and even some synthesized sounds. The average economical Class D amp simply won’t produce these subtleties the way a very good Class D amp or a Class AB amp will. Don’t get me wrong, Class D amplification has its place, but there are really mediocre Class D amps out there, as well as some very good ones, and this Boston is one of the good ones.
ON THE BENCH
After listening to the GTA-400M well into the evening, the next morning I moved the amp to the electronics test lab to see if my measurements would correlate with my listening impressions. The measured specifications did show excellence in several areas. The amplifier exceeded its rated power at both 4 and 2 ohms, and it also measured a quiet -82.8dBA at 1 watt of output. The good signal to noise numbers are part of the attention to the output filters that I mentioned previously, but even with extensive filtering the amp still managed a respectable low output impedance of 0.031 ohms at 50Hz. The low current draw I noted during listening was real, the Boston GTA-400M is very efficient, at over 86% at full 4 ohm power. But we don’t drive around with the thing wide open all the time, and most good Class D amps have a 10 watt efficiency of between 20-35%. The GTA-400M excelled here as well, the efficiency was still almost 50%. This means that the amp will draw less current under your typical listening conditions, which means better battery and alternator life, as well as generating less heat. And thermally, the Boston amp is very good as well, because of the thoughtful design that works with the laws of physics to get rid of heat as it’s developed, rather than fighting physics trying to cool an amp that’s designed to look cooler instead of operate cooler.