I have been watching with some interest a thread on audio circle about a new form of digital amplifier.
http://www.audiocircle.com/circles/viewtopic.php?t=17913&start=0
http://www.audiocircle.com/circles/viewtopic.php?t=17913&start=0
Analog Switching Amplifier Technology
NuForce™ amplifier technology is based upon the principle that a power oscillator can be modulated by an audio signal so that it produces an amplified audio signal obtained with a reconstruction filter, without the bandwidth limitation of a fixed frequency carrier-based conventional PWM control. It uses a high-performance analog modulation technique and a close-loop control system. Therefore NuForce refers to its audio amplifier as Analog Switching Amplifier.
NuForce believes that uncompromising real world music reproduction can only be achieved by having:
High Bandwidth
NuForce™ Analog Switching Amplifier provides a ruler-flat response from 5 to 100,000 Hz. Only a handful of very expensive high-end linear amplifiers have a bandwidth of 20-100,000Hz. Most linear or digital switching amplifiers hardly achieve 20-20,000Hz of bandwidth.
Low Distortion
The majority of today's amplifiers provides a good number, typically less than 0.1% of Total Harmonic Distortion (THD), when measured at 1W using a 1000Hz sine wave. The manufacturers of these amplifiers do not tell you that at full power, the distortion of their amplifiers could be 10 to 100 times worse, and at higher frequencies, their distortion increases further. NuForce's patent pending technologies actively cancel out the distortion at every cycle of operation. As a result, NuForce's analog switching amplifier has the same low distortion characteristic independent of and indifferent to the output power level or audio frequency.
Unique close-loop design
Unlike the Class-D amplifier, NuForce's analog switching amplifier does not require a sawtooth waveform for modulation, but a proprietary naturally occurring modulating signal. In conjunction with taking the signal at the loudspeaker terminals thereby eliminating all distortions, NuForce's natural switching signal does not add noise into the system. NuForce™ amplifier also does not suffer from the 180-degree phase shift caused by the output filter.
The high bandwidth and unique close-loop system provide very high forward gain, well beyond the audio frequency range, at up to 10 MHz. All non-linearities are hence greatly reduced to achieve an operating power bandwidth as high as 150 kHz, while at the same time maintaining a consistently low distortion across that frequency spectrum.
NuForce™ amplifier technology is based upon the principle that a power oscillator can be modulated by an audio signal so that it produces an amplified audio signal obtained with a reconstruction filter, without the bandwidth limitation of a fixed frequency carrier-based conventional PWM control. It uses a high-performance analog modulation technique and a close-loop control system. Therefore NuForce refers to its audio amplifier as Analog Switching Amplifier.
NuForce believes that uncompromising real world music reproduction can only be achieved by having:
High Bandwidth
NuForce™ Analog Switching Amplifier provides a ruler-flat response from 5 to 100,000 Hz. Only a handful of very expensive high-end linear amplifiers have a bandwidth of 20-100,000Hz. Most linear or digital switching amplifiers hardly achieve 20-20,000Hz of bandwidth.
Low Distortion
The majority of today's amplifiers provides a good number, typically less than 0.1% of Total Harmonic Distortion (THD), when measured at 1W using a 1000Hz sine wave. The manufacturers of these amplifiers do not tell you that at full power, the distortion of their amplifiers could be 10 to 100 times worse, and at higher frequencies, their distortion increases further. NuForce's patent pending technologies actively cancel out the distortion at every cycle of operation. As a result, NuForce's analog switching amplifier has the same low distortion characteristic independent of and indifferent to the output power level or audio frequency.
Unique close-loop design
Unlike the Class-D amplifier, NuForce's analog switching amplifier does not require a sawtooth waveform for modulation, but a proprietary naturally occurring modulating signal. In conjunction with taking the signal at the loudspeaker terminals thereby eliminating all distortions, NuForce's natural switching signal does not add noise into the system. NuForce™ amplifier also does not suffer from the 180-degree phase shift caused by the output filter.
The high bandwidth and unique close-loop system provide very high forward gain, well beyond the audio frequency range, at up to 10 MHz. All non-linearities are hence greatly reduced to achieve an operating power bandwidth as high as 150 kHz, while at the same time maintaining a consistently low distortion across that frequency spectrum.
I think for the tech heads there is some faqs explaining the workings of their technology
http://www.nuforce.com/faq/faq.htm
What is an Analog Switching Amplifier? What's the difference between NuForce's amplifier and other digital amplifiers?
NuForce's switching amplifier is a drastic departure from conventional approaches to switching amplifier design. Most class-D amplifiers use a fixed sawtooth waveform to modulate an audio signal, and suffer from the 180-degree phase shift of the LC reconstruction filter which would normally cause a feedback from the load to the error amplifier to oscillate unless phase compensation is used. That compensation network drastically reduces the amplifier bandwidth to below the corner frequency of the LC reconstruction filter. Thus most class-D amplifiers have low bandwidth and high distortion due to limited gain of the phase-corrected error amplifier at audio frequencies.
NuForce's amplifier technology is based upon the principle that a power oscillator can be modulated by an audio signal so that it produces an amplified audio signal obtained with a reconstruction filter, without the bandwidth limitation of a fixed frequency carrier-based conventional PWM control. It uses analog modulation technique and close-loop control systems. Therefore NuForce refer to its audio amplifier as Analog Switching Amplifier.
What are the problems with Class-D digital switching amplifiers?
Digital Switching Amplifiers (commonly known as Class-D) have been around for years. Nevertheless, it is nearly impossible to engineer a conventional Class-D amplifier that handles the full requirement, 20-20,000Hz, for full-bandwidth music reproduction. A Class-D amplifier works by utilizing a high-frequency sawtooth waveform to modulate the music signal (to learn more about how Class-D amplifier works, click here). The constant presence of the sawtooth waveform, which is very high in frequency spectrum and its inevitable frequency jittering, can mask or corrupt low-level music signal. The output filter designed to filter out noise and overtones caused by the sawtooth waveform adds a 180 degree phase shift to Class-D output stage, causing possible instability and adding distortion due to its own inherent non-linearities. Additionally, the output filter presents frequency-variant output impedance that can interact with a speaker's complex impedance. Variants of Class-D amplifiers with the addition of Digital Signal Processor claim to improve music reproductions. However, because of their lack of close-loop design, especially from the speaker's terminals, spurious interaction between the speaker's complex impedance and back-EMF with the amplifier's resonant output filter can result in harsh sound reproduction. The fundamental flaws of conventional Class-D amplifiers remain unresolved.
http://www.nuforce.com/faq/faq.htm
What is an Analog Switching Amplifier? What's the difference between NuForce's amplifier and other digital amplifiers?
NuForce's switching amplifier is a drastic departure from conventional approaches to switching amplifier design. Most class-D amplifiers use a fixed sawtooth waveform to modulate an audio signal, and suffer from the 180-degree phase shift of the LC reconstruction filter which would normally cause a feedback from the load to the error amplifier to oscillate unless phase compensation is used. That compensation network drastically reduces the amplifier bandwidth to below the corner frequency of the LC reconstruction filter. Thus most class-D amplifiers have low bandwidth and high distortion due to limited gain of the phase-corrected error amplifier at audio frequencies.
NuForce's amplifier technology is based upon the principle that a power oscillator can be modulated by an audio signal so that it produces an amplified audio signal obtained with a reconstruction filter, without the bandwidth limitation of a fixed frequency carrier-based conventional PWM control. It uses analog modulation technique and close-loop control systems. Therefore NuForce refer to its audio amplifier as Analog Switching Amplifier.
What are the problems with Class-D digital switching amplifiers?
Digital Switching Amplifiers (commonly known as Class-D) have been around for years. Nevertheless, it is nearly impossible to engineer a conventional Class-D amplifier that handles the full requirement, 20-20,000Hz, for full-bandwidth music reproduction. A Class-D amplifier works by utilizing a high-frequency sawtooth waveform to modulate the music signal (to learn more about how Class-D amplifier works, click here). The constant presence of the sawtooth waveform, which is very high in frequency spectrum and its inevitable frequency jittering, can mask or corrupt low-level music signal. The output filter designed to filter out noise and overtones caused by the sawtooth waveform adds a 180 degree phase shift to Class-D output stage, causing possible instability and adding distortion due to its own inherent non-linearities. Additionally, the output filter presents frequency-variant output impedance that can interact with a speaker's complex impedance. Variants of Class-D amplifiers with the addition of Digital Signal Processor claim to improve music reproductions. However, because of their lack of close-loop design, especially from the speaker's terminals, spurious interaction between the speaker's complex impedance and back-EMF with the amplifier's resonant output filter can result in harsh sound reproduction. The fundamental flaws of conventional Class-D amplifiers remain unresolved.
IIRC is this the company that uses "one cycle control" for their switching amps. This is some sort of slope converter. Instead of controlling a counter, the comparator controls a switching output stage.
The whole is just another possible topology that can be good or bad, depending upon implementation (as any other topology). The underlying patents are quite old so it did take them quite some time to get a commercial amp running.
Regards
Charles
The whole is just another possible topology that can be good or bad, depending upon implementation (as any other topology). The underlying patents are quite old so it did take them quite some time to get a commercial amp running.
Regards
Charles
The current breed of nuforce amps does not use any of their patents but is based on a control scheme very similar (if not identical to) UcD. Some good ideas pop up in different minds independently.
The distortion results are also similar. I don't consider a bandwidth (let alone a power bandwidth) of 150kHz or more necessary or even wise. Either you have to use a very high switching frequency or you'll get serious intermodulation artefacts when feeding the amplifier supersonic noise (like playing an SACD).
On the other hand, if I were forced to make an amp with a 150kHz PBW I'd take advantage of the extra bandwidth to get a uniform 0.001% THD across the audio band, something nuforce hasn't done either (the measurements that float around the net show something along the lines of 0.02%). There's still some optimisation to be done on that design.
The distortion results are also similar. I don't consider a bandwidth (let alone a power bandwidth) of 150kHz or more necessary or even wise. Either you have to use a very high switching frequency or you'll get serious intermodulation artefacts when feeding the amplifier supersonic noise (like playing an SACD).
On the other hand, if I were forced to make an amp with a 150kHz PBW I'd take advantage of the extra bandwidth to get a uniform 0.001% THD across the audio band, something nuforce hasn't done either (the measurements that float around the net show something along the lines of 0.02%). There's still some optimisation to be done on that design.
Oops..4000 damping factor equal a lot of feedback, on the THD/Freq plot easy to seen slope with pole around 300hz. It's remind my UcD experience with additional loop on the opamp based integrator, but THD@1Khz was <0.002%, but the sound lost any of the UcD advantages. JP, if i remember correctly, posted here THD/Freq plot of the such UcD modification also with very sexual THD@1Khz, however (AFAIK) they do implement this approach only for subamps, probably JP/Bruno can comment it?
Well I'm going to finish that design just for the sports of it (it's nice if you can bandy around a plot saying 0.0005% @1kHz 100W). I would dread listening to it though.IVX said:
It should, however, also be possible to rearrange the filter poles such as to have a reasonably constant 0.003% over the audio range. That would allow a better combination of low THD and good sound.
Hi Bruno
Some People (Mrs Manger for instance ) are hard to convinve for having bandwidths lower than 100 kHz.
But apart from that, there is something else that makes me curious: How does the UcD react to supersonic noise ?
My feeling says that it is less susceptible than a carrier-based class-d on the one hand - OTOH its small signal bandwidth is larger than the bandwidth of the output filter, making it more susceptible to supersonic noise (i.e. TIM). I am of course aware of the advantages of what you do with this filter dimensionig (better carrier suppression for a given overall cutoff frequency, better load independancy ....).
Regards
Charles
Having a BW extending to 100kHz is only a way to improve phase response inside the audio band. Choosing a Bessel (Thomson) lowpass response for the amp is equally valid, and allows for a much reduced bandwidth.phase_accurate said:
Like you I don't like to extend the small signal band-width too far beyond the power bandwidth. I think we're synchronised here. The UcD amps are quite insensitive to HF noise. You can use them as post filter for a deltasigma DAC chip (ie. skip the usual op-amp based filter).
I agree on that. Unfortunately there are a lot of other unknown lowpass poles in a chain usually. This is a thing that could be optimised in the (reproduction part of it at least) chain without much cost and just some work effort, i.e. including preamp and tweeter response as well in order to achieve an overall Bessel response.
Regards
Charles
phase_accurate said:
Yup it's like the discussion about decimation/interpolation filters in recording/reproduction. In 96k/192k systems people are advocating the use of slow-rolloff filters (the correct kind should roll off between 20kHz and fs/2, not 20kHz and fs-20kHz like some idiots are doing it) to remove pre/post ringing. So far so good. Unfortunately if you cascade a bunch of these you get a sharp rollof filter starting at 20kHz, ringing and all. Obviously (?) the answer is to use standard sharp rolloff filters (ideally brick wall at fs/2) everywhere in the chain and only one slow-rolloff filter at one point in the chain (my suggestion: at mastering).
Try to explain about compound response of a signal chain to some people...
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