PWM amplifiers - background and info - 2015


CAD HIFI loudspeakers

Audax speaker units

Other HIFI speaker units

HIFI power amplifiers

Amplifier KITS

PWM Audio Technologi

Connectors & Cable

Crossover components

HIFI Special Offers

Audio Compression

Røjlevej 8
DK - 5935 Bagenkop
(+45) 38 33 40 48
E-mail :

You are welcome to email for further information.

All prices are ex VAT/Tax.

last update june 2007



Class-D history.

The basic theory behind pwm amplifiers (or Class-D amplifiers) has been known for 30 years or so, so its not a new invention as "some" would like you to believe ;) 

Power amplifiers are divided into various classes from the way the output stage works, like Class-A, Class-B etc. Some call class-d for digital amplifiers, we think a digital amplifier shall have a digital input and preferably digital pwm generation to be called a "digital amplifier".

Clive Sinclair had a 10W class-d hifi amplifier kit in the early 70´s and Harris released application notes + evaluation boards for a 200W class-d amplifier in 1995, these are the core background for many class-d amplifiers of today.

Class-D topologies.

Basically you can define a class-d amplifier as a clock based design or a self-oscillating design. The clock based version has a clock (typically 250 - 400 kHz) which is fed as a triangle signal to a comparator and the pwm is amplified with an output stage , the high frequency clock is filtered out by an output filter and the amplified audio signal remains.

A self-oscillating design uses an opamp as an integrator and the clock is made by feedback from the output and the whole amp oscillates , the clock is again filtered out and the amplified audio signal remains.

Both versions can have feedback before the output filter or after the output filter, some seem to think it doesnt matter a lot which topology you choose, we think feedback after the output filter is much better and should be preferred.

Both the clock based and the self oscillating amplifier can be half-bridge or full bridge design, meaning the half bridge will have one pair of output fets and output from these to midpoint of the power supply, the full-bridge will have 2 pairs of output fets (in opposite phase) and output between the midpoints of these 2 pairs.

Half bridge designs are often more sensitive to power supply regulation and shortcomings , the fullbridge design will normally have a higher power supply rejection and is less prone to power supply pumping as well. 

Sound quality and specs.

Several examples of bad designed class-d amplifiers have been around and in the last years new types have been brought to market , some of them designed to give good steady-state distorsion figures (with not-so-good sound quality) and others just barely working , with lots of problems with EMC, radio reception... and again far from HIFI sound quality.

To design and produce a pwm amplifier with HIFI sound quality , high power and very high efficiency isnt easy at all, there are many demanding and conflicting design tasks.

If you focus mainly on thd specs you are likely to end up with an amplifier having good thd specs unfortunately this isnt at all the same as having a good sound quality , basically you can see the old linear amplifiers with very high negative feedback and you can compare the sound quality from these with amplifiers being designed with superior sound quality in mind. Many featurers of linear amplifiers can of course be adapted by class-d/pwm amplifiers too, like balanced inputs , DC-servo , protection schemes, etc.

Multicannel amplifiers and sync.

In a multichannel amplifier you would want the channels to be in sync to avoid beating an other undesired effects, this isnt easy achieved with many of the self-oscillation designs but much easier with a clock based design as you can use a "master clock" and slave all amplifier channels with the same clock, this way they are effectively in sync. 

Power supply. 

For most domestic applications a high quality linear power supply , a toroidal transformer, high speed diodes and low esr electrolytic capacitors + regulated power supply for input and driver circuitry will work just fine. For mobile applications or other low weight applications or small size you can choose a switch-mode power supply to power you class-d amplifier, here are a couple of pitfalls, as mentioned above some class-d amplifiers do not at all like being powered with a smps, some of the selfosc. designs seem to pick up the clock from the smps and some of them counteract with the feedback loop of the smps giving a completely unstable system alltogether.

So please make sure your amplifier works well with a switch mode supply or buy the amplifier + smps as an integrated unit, this should guarantee they work together without problems.

Karsten Madsen - CAD Audio DK - june 2007