[sdiy] Low voltage synthesis?

Mike Bryant mbryant at futurehorizons.com
Fri Jan 14 23:48:43 CET 2022

Well Rohm, TI and ST still call this sort of amplifier Class-D so I have as well.  Maybe it should have a new class name to distinguish it from ones with an analogue input and analogue feedback but that’s not something I can secure.

And as the last paragraph shows, Wikipedia is hardly the font of all knowledge, as what they say is ‘unattractive’ is how the best systems have been doing it for the past 25 years.  The maths is complex – you need a decent DSP and good modelling – but it gives a better result.

From: Mattias Rickardsson [mailto:mr at analogue.org]
Sent: 14 January 2022 22:37
To: Mike Bryant
Cc: Synth DIY
Subject: Re: [sdiy] Low voltage synthesis?

Aha, I didn't think it would be called Class-D if it was just shoving out digital pulses, but maybe it can be either or. :-)

The Wikipedia article on Class-D amps has some interesting passages:

The term "class D" is sometimes misunderstood as meaning a "digital" amplifier. While some class-D amplifiers may indeed be controlled by digital circuits or include digital signal processing devices, the power stage deals with voltage and current as a function of non-quantized time. The smallest amount of noise, timing uncertainty, voltage ripple or any other non-ideality immediately results in an irreversible change of the output signal. The same errors in a digital system will only lead to incorrect results when they become so large that a signal representing a digit is distorted beyond recognition. Up to that point, non-idealities have no impact on the transmitted signal. Generally, digital signals are quantized in both amplitude and wavelength, while analog signals are quantized in one (e.g. PWM) or (usually) neither quantity.


Error control
The actual output of the amplifier is not just dependent on the content of the modulated PWM signal. The power supply voltage directly amplitude-modulates the output voltage, dead time errors make the output impedance non-linear and the output filter has a strongly load-dependent frequency response. An effective way to combat errors, regardless of their source, is negative feedback. A feedback loop including the output stage can be made using a simple integrator. To include the output filter, a PID controller is used, sometimes with additional integrating terms. The need to feed the actual output signal back into the modulator makes the direct generation of PWM from a SPDIF source unattractive. Mitigating the same issues in an amplifier without feedback requires addressing each separately at the source. Power supply modulation can be partially canceled by measuring the supply voltage to adjust signal gain before calculating the PWM and distortion can be reduced by switching faster. The output impedance cannot be controlled other than through feedback."


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