In our ongoing series on class D amps and listener fatigue I made the point (hopefully) that the input stage is a critical path element in the chain – because if you can’t get this part right then everything else that follows only contributes to making things worse. It’s like putting bad gas in your car; there’s little a conventional engine can do to make up for the problem.
I would guess many of the first class D amps had hastily attached input stages and these contributed greatly to their poor reputation for sound quality.
Once the input stage has amplified the incoming music signal, it’s the job of the Pulse Width Modulator to “convert” the music into a series of long and short pulses – which is how this amplifier works. The lower the musical signal level, the shorter the pulse width (time that it is on) and the louder the musical signal the longer the pulse width is. What determines these pulse width lengths? A very simple, yet critical, waveform called a triangle wave.
A triangle wave looks more like a series of ”V” strung together. Here’s a picture of it.
It’s really a simple wave with a voltage that rises at a perfectly steady rate, then falls back down at an equally steady rate. The music is compared with this rising and falling triangle wave and as long as the size of the triangle wave is smaller than the music, the output of the amplifier is on – and as soon as the triangle wave size exceeds that of the music, the output of the power amp is off.
Critical to the sound quality of the class D amplifier is the purity of this waveform. In older first generation amplifiers this triangle wave was generated with a simple timer circuit that wasn’t all the accurate and steady – the results were that the music on the output of the power amp was not linear – and that’s a disaster waiting to be rejected by one’s ears.
This modulator has to be perfectly linear for the output of the class D amp to also be linear.
So the two elements we’ve discussed so far, the input stage and the modulator, are two pieces of the class D puzzle that absolutely have to be right before we can call this thing a reference quality amplifier. Most of the early generation amps were not anywhere close to reference quality – certainly not in the input stage.
The modulator part is fairly easy to get right, the input stage not so easy – as it takes decades of experience to know how to design such a stage that sounds good.
Trusted high-end audio companies adding their own input stage should be something we can all rely upon in this day and age and the excellent modulators from companies like Hypex, B&O and Tripath (the recognized leaders in class D modulators and output stages) have progressed to a point where we no longer need question their sound quality and linearity. As an example, the Hypex module we will be using in our upcoming PerfectWave Power Amplifier has a linearity at the output of the amp that exceeds any class A or A/B amplifier we have ever measured; exceeding 100dB without the aid of feedback or analog crutches which is really excellent.
So let me magnify that thought because it’s important. Class A and A/B amplifiers generally use feedback to linearize their output performance – and feedback has a significant impact on sound quality – if not properly implemented it can injure the way the amp sounds. This is one of the reasons low feedback amps sound more open that your average high end power amp – but they also suffer from poor control of the loudspeaker. Unfortunately, a low or zero feedback power amplifier’s linearity is not all that great, falling off poorly as the signal gets louder.
A properly designed class D amplifier, on the other hand, uses no feedback or crutches to linearize its output and the 100dB we get on our output stage sounds identical at the lowest or the highest output levels – something critical to achieving effortless music.
So with the input stage and modulator issues a thing of the past, what’s left to worry about on a class D amp? Just the single biggest contributor to bad sound there is on these devices – so big, in fact, that it can overshadow both the input stage and modulator combined.
Tomorrow we tackle this beast.
Paul McGowan
