A class-D amplifier lacks a transformer, which means it’s much lighter than a traditional amp and it generates very little heat. But if the power module fails, it typically can’t be repaired. Replacing it is so expensive that most players opt
to buy a new amp instead.

Know Your Needs
Despite what I’ve just said, I want to make it clear that I’m not saying class-D amps can’t sound good and perform well for bass players. But whether they’re right for you does depend on what you play, how you play, and what other gear you use. For example, if you’re a slap player who uses the bass as more of a percussive instrument (like Mark King of Level 42, for example), the waveform going from your bass to your amp will be very similar to general music waveforms. It’ll have a series of large peaks (transients) and a lower average power demand. That means serious output power is not required for any length of time. This is more or less perfect territory for the typical class-D amp.

The situation changes somewhat for players who use compressors, because compressors remove the large transients and elevate the decaying note so that the resultant wave more closely resembles a continuous waveform. The more compression, the more continuous the waveform gets (Fig. 4). And the higher average volumes that compressors and limiters promote require more amplification power for a longer period of time. Throw in some pedals, perhaps a sub-harmonic generator or an oscillator, and the problem is compounded even more.

Some manufacturers of class-D amps have started to address this problem by fitting extra heat sinks and cooling fans to the power modules in an attempt to push the continuous power ratings higher, prevent thermal shutdown, and make the modules more suitable for musical-instrument amplification. But there is still some distance to go in this regard.

Another crucial thing to remember when buying a bass amp is that frequency response is intimately connected to power ratings. In a tube amp, the lowest frequency that can be reproduced is determined by the size of the output transformer and its ability to transfer the low frequencies to the speaker. It’s usually around 30 Hz. In solid-state linear amps (class A, B, AB, etc.), the low frequency that can be reproduced is approximately 20 Hz—the lowest frequency humans can hear. But some class-D amplifier manufacturers limit the lowest frequencies to around 60 or 70 Hz to use less of the already-low continuous power and make the amp sound louder. The result is usually an amp that sounds boxy and has no real low-end substance.

Similarly, it’s critical to consider how an amp will react to being overloaded by, say, stepping on an external boost or fuzz pedal, or by cranking the gain and volume to max. Whether this is a big deal to you will depend largely on the genre of music you play. Tube amps tend to growl and snarl in a way that can be very pleasing. For rock bassists, nothing comes close to the sound of a good tube amp at full bore. Traditional linear class-A and class-B transistor bass amplifiers also tolerate quite a bit of overloading, although most players don’t find the distortion to be as pleasing as tube distortion.

Because of this greater efficiency, class-D amps generate far less heat than class-A and class-B amps. But they are not
without their problems.

Class-D amps, on the other hand, do not tend to react well to overloading due to built-in protection mechanisms. Under slight overload the amp will limit or cut the output momentarily, and under heavier overload the output gets switched off completely for a few seconds. The leading manufacturer of OEM class-D modules has fitted the latest models with soft clipping in an attempt to solve this problem, but there is some distance to go before the problem is solved. However, if the style of music you play is less likely to result in overload situations (for instance jazz or fusion), this may be less of a concern.

Worst-Case Scenarios
Bass amps from reputable makers should be reliable if you treat them properly. However power sections do die sometimes, and here is where another big difference between amp types emerges. Tube amps usually blow tubes, which is no big deal: You buy some more, plug them in, maybe have a tech re-bias them, and enjoy months or years of great tone. It’s easy, simple, and not too expensive. This is why hundreds, maybe thousands, of old tube amps are still being used onstage and in the studio decades after they were built.

Traditional class-A or class-B transistor power amps are also usually very easy to repair. Parts are available from most electronic suppliers, and a good tech can diagnose and fix the problem at reasonable cost by replacing only the parts that have blown up.

The power modules in class-D amps, however, usually cannot be repaired. If the power section fails, paying a tech to replace the power module will cost about half as much as the complete amplifier. In these cases, most players will opt for a new amp instead. This represents a total loss to the customer, and is also bad for the environment because the surface-mount circuit boards in most of these amps are hard to recycle into anything useful. Then there is the second-hand value to consider: A lot of musicians primarily raise funds for new purchases by selling old gear. Tube amps seem to go up in value as they get older. Traditional heavyweight linear amplifiers also do well, largely because their low repair costs mean if you buy a duffer it can be fixed without breaking the bank. Sadly, many modern lightweight class-D amps with surface-mount modules that cannot be repaired have almost no residual second-hand value.

Armed and Dangerous
We’ve covered some pretty heady territory here, but now that you have a better understanding of power-amp types and their power ratings—one of the most mysterious and significant areas of amp design—you’ll be able to make your next amp purchase much more confidently.

It’s worth noting that almost all the big bands and pro musicians use old-school, heavyweight tube or solid-state amps, and the particulars we’ve discussed here are a huge reason why. But like I said, class-D amps can be solid performers. Just be sure to pay attention to the fine print. If you’re interested in a lightweight class-D unit, be sure to select the right output power. If a 200-watt tube or traditional solid-state amp would meet your power needs, double that and get a 400-watt class-D amp to be safe—it won’t weigh much more.

There are a lot of very good amplifiers out there—tried-and-true designs that are still in production after many years, as well as modern heavy and lightweight designs from outfits new and old. So try as many as you can, don’t rush into anything, and buy the best that you can afford. Happy hunting!