Axiom Home Page Forums General Discussion Stereo Amplifier Question?

There are no digital amps. The closest any come are the Zetex, Direct Digital Feedback Amplifiers, which can take PCM and convert it to PWM to directly control a MOSFET pair. But the amplification is still happening in an analog way.

No, they run cooler at higher volumes, as more of the energy is going into the speaker rather than being turned to heat behind the gate.

Hi kcarlile,

OK, get ready: these things are complicated. You are correct that class D does not stand for "Digital", although through frequent usage, it has taken root.

Class D amplifiers, although there are a number of different design variations, are essentially switching amplifiers or Pulse Width Modulator (PWM) designs. B&O's "ICE" modules are also switching designs.

The incoming analog audio signal is used to modulate a very high frequency PWM carrier that works the output stage either fully on or off. This ultra-high frequency carrier must be removed from the audio output with a reconstruction filter so that no ultra-high frequency switching components remain to corrupt the audio signals. Class D designs are extremely efficient, typically in the range of 85% to 90% or more. Axiom's A1400-8 reaches 95% efficiency at full output. Conventional class A/B transistor amps' efficiency is about 50% to 60%. The unused energy is dissipated as heat.

Regards,
Alan

Hi Sir Quack,

As Club Neon has pointed out, that's true. Further to my explanation to kcarlile, I'd add that an analog amplifier works in analogous fashion, regulating the output stage devices (transistors) to release power from the amplifier’s power supply to the loudspeakers in a manner that exactly mimics the tiny incoming audio waveform. So-called "Digital" amplifiers use high-frequency switching circuitry to modulate the output devices.

Here is an explanation of how the B&O "ICE" amplifiers work and how Axiom's A1400 differs in operation:

Denmark’s Bang & Olufsen (B&O) holds patents on its “ICE” amplifier, which is basically a Class D switching design (Pulse Width Modulator) with variants that B&O claims reduce distortion to levels associated with Class A amps, while retaining the high efficiency of Class D switching designs. ICE amps use a very high switching frequency of 384 kHz, which B&O says is 20 times as high as the highest frequency the ear can detect. The ICE amps also use feedback control to minimize the effects of the PWM design.

Axiom’s engineering division took a different approach in the A1400-8’s amplifier design. Axiom worked with International Rectifier to develop new silicon output devices that drive the MOSFETs in the output stage in such a way as to produce a perfect Pulse-Width Modulated square wave at the output before the reconstruction filter. This approach also simplifies the A1400-8's feedback network, which makes the amplifier more robust in its operation without being subject to oscillations or instability. The A1400-8 also uses a very high clock frequency to allow for excellent transient response and non-aliasing in the audio band. The massive power supply is able to accurately output very high current and voltage to the loudspeaker over extended time periods.

Alan

I'll give you my shipping address.

Ken, as the replies have indicated, although the term "digital" is sometimes thrown around rather loosely, the class D amplifiers, such as the Axiom unit, in fact operate on analog principles. The analog waveform at no time is converted by digital sampling. It's used to modulate a sawtooth triangular waveform which results in what is sometimes said to look "digital-like", but the analog waveform remains as such, but concealed by the modulation. When the several hundred KHz sawtooth waveform is removed at the end by low-pass filtering, the original(but now amplified)analog waveform is again revealed and is used to power the speakers.

Excellent information here!

Thanks, guys! I think I understand it a little better now.

Although the modulation waveform is a triangle wave /\ not a sawtooth |\.

Your welcome, glad I could start a thread too bad I can't contribute LOL