The Class D Amplifier Revolution

The Class D Amplifier Revolution

Making a bass amplifier with enough clean headroom to be heard over drums, keys and guitar requires a lot of power. This is partly because the human ear is tuned to hear midrange frequencies better than bass frequencies, and partly because bass players tend to want to avoid the distortion that comes with pushing an amplifier to its limits. While a guitar player might want power tube distortion, most bass players prefer the clarity that comes with lots of wattage.

When Andy Hefley of Great American Sound built a set of four 2500 watt amps (2 for stage and 2 for backup) for Phil Lesh under the name Precision Innovative Electronics in the early 1980’s, the amps weighed 150 pounds each and were 12” high. Fortunately for those of us who are not in the Grateful Dead, there is a new(ish) technology that is allowing bassists to bring lots of power to the gig without breaking our backs. The name of the game these days is Class D power amplification.

Class D amplifiers use MOSFET transistors in pairs acting as switches rather than as linear gain devices. They are much more efficient than class A or AB and as a result can put out more power and generate very little heat. Because they can be made with compact circuitry and do not need large and heavy heat sinks, these amps can be very small and light. Class D is sometimes mistakenly referred to as digital amplification. This is still an analog application.

While class D has been used in power amps for years, it is only recently that the technology has taken off in the bass world. It seems like there has been a tipping point and now there are a slew of small but powerful bass heads. Since most of us don’t have roadies and giant tour buses, small and light is a serious selling point, as long as the tone is there.

Ashdown has a number of Class D amps including the ABM1000LT, weighing only 21 pounds. They also have the Pi Bass 240, weighing in at 3.1 pounds and putting out 240 watts. This is an amp that can fit in a gig bag yet packs plenty of power for a club gig!

Truly amazing is the DNA 1350, a 7 pound amplifier designed for professional touring musicians that delivers 1350 watts and has all of the features you would expect in a world class bass amp including compression, DI, and plenty of tone shaping.

There are entries in the Class D field from Warwick, Phil Jones (who is combining his super light amps with super light speakers to make some very cool portable gear), Mark Bass, G-K, and pretty much every other bass amp manufacturer. Because the technology that makes these amps so light is in the power amp section, many of these companies are combining their already great sounding preamps with the class D power amp. As a result there are a variety of features available including some with tube preamps.

It’s no surprise that these amps are more popular with bassists than guitarists. Guitar players love pure tube sound even if they are heavy and prone to failure. But there are some offerings for the six string including the hybrid (and amazing sounding) Mighty Minnie from Demeter Amplification and virtually everything made by Quilter, as well as the 44 Magnum from Electro Harmonix, which is the smallest 44 watt amp ever as far as I can tell. Also, the short lived but really useful Crate Powerblock was the first class D amp I ever bought and I still love it and use it as a PA head regularly.

The world of bass is changing. Between the new batch of small, powerful amps and the recent advances in lightweight speakers, it is no longer necessary to break your back for wonderful tone. Smaller, lighter rigs are becoming the norm. Because most of the newer amps have a line out, it’s no problem to go through the PA for really big shows. Now you can rock hard without getting a hernia!