Yesterday’s post I mentioned there are multiple types of amplifier topologies used for subwoofers, the majority of modern ones Class D amps. I also mentioned another popular topology called BASH. Several of you asked me to expand upon the BASH amp and explain.
First let me refresh your understanding of Class D vs. traditional Class A/B. Class D amps are also known today as ‘digital amps’. But we’ve learned in these posts that’s a misnomer. Class D is all analog, not digital. However, it is more commonly known as a ‘Switching Amplifier’ because it uses a type of output known as Pulse Width Modulation (PWM) that indeed switches between only two states (which is how it acquired its erroneous ‘digital’ moniker). The design is extremely efficient, getting in the neighborhood of 95% efficient (making little to no heat).
I won’t get too involved with how it works because there’s not enough room in this post. Suffice it to say the amp works by putting more or less energy to its output through a series of long or short pulses at very high speed. A ‘pulse’ is nothing more than a way of describing something that is turned on, vs. turned off. Because these long and short pulses come at a very high rate of speed, much higher than we can hear, it is necessary to remove all that high frequency stuff before it reaches your loudspeaker. To do this, Class D amplifiers require an output filter and there, my friends, lies the issue with these amps.
Placing an output filter between your power amplifier and your speaker has not good sonic results, generally speaking. While the Class D amp itself is quite simple, it is the output filter that’s really tricky to design such that it doesn’t adversely affect the sound quality. In a subwoofer, we worry less about this filter’s impact on the sound because, for the most part, the problems we hear in this topology are in the upper frequencies of music. However, that doesn’t mean they aren’t still problematic.
Now let’s move on to BASH. A Canadian design, this amplifier topology uses a Class D amplifier to feed a class A/B amplifier (replacing the traditional power supply), thus removing the need for the output filter, yet keeping the efficiency high. A class A/B amp is maybe 50% efficient. That is because the power supply feeding this type of amplifier has fixed levels, typically very high levels like 150 volts, and the amp moves between these levels as it produces sound. This is not a very efficient way of doing business. If, however, the power supply of the Class A/B amplifier had very small fixed levels, perhaps 10 volts, then it too would be very efficient. The only way to accomplish this would be to have power supply levels to the A/B amp move up and down with the music. That requires a power amp, thus, adding a Class D to take the place of the fixed power supply solves that problem.
So, in the end, BASH does not have the problem of the output filter between the speaker and the power amplifier and that is what differentiates it from a classic Class D approach. Both are efficient, both work just fine for a subwoofer.