Class D Amp

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terrybooth
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Class D Amp

Unread post by terrybooth » Wed Aug 13, 2014 9:01 pm

Given that there is some talk about these, I thought I'd start a thread.

I have one; the sort with a built in DAC and USB connection. The idea was to base a small system around it. It sits there idle at the moment. My 'other' system has a 40 year old Jap amp in it. Maybe I ought to resurrect the Class D again and see what I think (again).

One thing I may also try is the class D amps that makers of DAC boards for the Raspberry Pi promise. There are a couple in the pipeline but nothing had been put into production the last time I looked.
Pioneer PL71/DL103/ Phono2/HiFiPi/P90SA/TIS/CubixPro two-up

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Dr Bunsen Honeydew
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Re: Class D Amp

Unread post by Dr Bunsen Honeydew » Wed Aug 13, 2014 9:06 pm

1968 The Sinclair Neoteric - the first commercial Class D amplifier made.

Image

Why should I want one?

When it launched in April 1968 , the two-inch-high, 60-watt Neoteric made most other integrated amps look like WWII tanks. Hi-fi folklore even tells the story of Danish design legends Bang & Olufsen paying Sinclair a visit on launch day to marvel at the Neoteric’s slimline steel chassis and rosewood front panel. Its retro-futurist aesthetic also caught the eye of Stanley Kubrick, leading to an appearance in A Clockwork Orange.

What to look for

Unfortunately, you can’t afford to be that choosy. Despite tons of interest and plenty of orders the Neoteric ‘60s design complexities made it difficult to produce in large quantities, which means they’re rarer than a Frenchman’s steak. If owning an original isn’t your top priority, give Iain Sinclair a call. Word from the man himself is that an updated reissue would be considered, if there was enough interest.

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Dr Bunsen Honeydew
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Re: Class D Amp

Unread post by Dr Bunsen Honeydew » Wed Aug 13, 2014 9:13 pm

In the class-D amplifier the active devices (transistors) function as electronic switches instead of linear gain devices; they are either on or off. The analog signal is converted to a stream of pulses that represents the signal by pulse width modulation, pulse density modulation, delta-sigma modulation or a related modulation technique before being applied to the amplifier. The time average power value of the pulses is directly proportional to the analog signal, so after amplification the signal can be converted back to an analog signal by a passive low-pass filter.

The purpose of the output filter is to smooth the pulse stream to an analog signal, removing the high frequency spectral components of the pulses. The frequency of the output pulses is typically ten or more times the highest frequency in the input signal to be amplified, so that the filter can adequately reduce the unwanted harmonics, reproducing an accurate reproduction of the input.

The main advantage of a class-D amplifier is power efficiency. Because the output pulses have a fixed amplitude, the switching elements (usually MOSFETs, but valves (vacuum tubes) and bipolar transistors were once used) are switched either completely on or completely off, rather than operated in linear mode. A MOSFET operates with the lowest resistance when fully on and thus (excluding when fully off) has the lowest power dissipation when in that condition. Compared to an equivalent class-AB device, a class-D amplifier's lower losses permit the use of a smaller heat sink for the MOSFETs while also reducing the amount of input power required, allowing for a lower-capacity power supply design. Therefore, class-D amplifiers are typically smaller than an equivalent class-AB amplifier.

Another advantage of the class-D amplifier is that it can operate from a digital signal source without requiring an digital-to-analog converter (DAC) to convert the signal to analog form first. If the signal source is in digital form, such as in a digital media player or computer sound card, the digital circuitry can convert the binary digital signal directly to a pulse width modulation signal to be applied to the amplifier, simplifying the circuitry considerably.

Class-D amplifiers have been widely used to control motors, but they are now also used as power amplifiers, with some extra circuitry to allow analogue to be converted to a much higher frequency pulse width modulated signal. Switching power supplies have even been modified into crude class-D amplifiers (although typically these can only reproduce low-frequencies with an acceptable level of accuracy).

High quality class-D audio power amplifiers have now appeared on the market. These designs have been said to rival traditional AB amplifiers in terms of quality. An early use of class-D amplifiers was high-power subwoofer amplifiers in cars. Because subwoofers are generally limited to a bandwidth of no higher than 150 Hz, the switching speed for the amplifier does not have to be as high as for a full range amplifier, allowing simpler designs. Class-D amplifiers for driving subwoofers are relatively inexpensive in comparison to class-AB amplifiers.

The letter D used to designate this amplifier class is simply the next letter after C and, although occasionally used as such, does not stand for digital. Class-D and class-E amplifiers are sometimes mistakenly described as "digital" because the output waveform superficially resembles a pulse-train of digital symbols, but a class-D amplifier merely converts an input waveform into a continuously pulse-width modulated analog signal. (A digital waveform would be pulse-code modulated.)

jammy395

Re: Class D Amp

Unread post by jammy395 » Wed Aug 13, 2014 9:27 pm

That's a nice looking box. :clap:

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Re: Class D Amp

Unread post by _D_S_J_R_ » Wed Aug 13, 2014 9:31 pm

I'm not in the slightest suggesting that this design philosophy is the next big thing, but I think it fair to say that it's now possible to move the problem areas well out of earshot.

The original manufacturers of AVI's electronics in the 90's have been doing interesting things here. Click on link and take the 'Products' tab -

http://www.alnerhamblin.com/

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Re: Class D Amp

Unread post by Dr Bunsen Honeydew » Wed Aug 13, 2014 9:38 pm

_D_S_J_R_ wrote:I'm not in the slightest suggesting that this design philosophy is the next big thing, but I think it fair to say that it's now possible to move the problem areas well out of earshot.
Sorry :naughty: due to design parameters you cannot change this will never happen. It is cartoon / caricature music.

brit-pop
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Re: Class D Amp

Unread post by brit-pop » Wed Aug 13, 2014 9:54 pm

Are you saying they work well in subwoofers. I notice REL are using them now.

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Re: Class D Amp

Unread post by Dr Bunsen Honeydew » Wed Aug 13, 2014 10:02 pm

It is sampling rate which is one of the big problems. It is very difficult to get the PWM (pulse width modulation) fast enough for full audio frequency and when it does you get phase anomalies which CANNOT BE REMOVED!!! So it is very simple to build a Class D for a sub woofer as they only have to go up to at most 1.5khz.

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Re: Class D Amp

Unread post by _D_S_J_R_ » Wed Aug 13, 2014 10:10 pm

Dr Bunsen Honeydew wrote:
_D_S_J_R_ wrote:I'm not in the slightest suggesting that this design philosophy is the next big thing, but I think it fair to say that it's now possible to move the problem areas well out of earshot.
Sorry :naughty: due to design parameters you cannot change this will never happen. It is cartoon / caricature music.
Deep respect Doc. That's not what I heard today. The sound was more NVA-like in the best musical sense than any other amp I'd heard up to now.

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Re: Class D Amp

Unread post by Dr Bunsen Honeydew » Wed Aug 13, 2014 10:15 pm

As I said you should have taken the AP20 with you.

People get taken by the open window of Class D, but only when you hear the anomalies removed as you would with a quick swop to the AP20 would you recognise it unless you are a musician. Gromit would spot it straight away. It times wrong.

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