Class a/b vs D for sub duty?

Meeks32

Hey guys, im in the market for a new/used amp to power my sub. I've always been a fan of the older amps & was looking at some used PPI pc or an Orion "the beast" amp. Im only have a 12" eclipse 8812 right now & may upgrade to a Tc sounds 2000 later on so insane wattage isnt a huge concern to me. I've never heard a class d amp but after reading some stories online, alot of them seem to say they prefer a good a/b amp to a class d. If anyone has any experience with both, I would love to hear some feedback. Size of the amp really isnt a huge concern, but if installing a 1000w a/b amp is going to be a huge drain on my electrical system, then I may have to take a serious look into a more effecient design.

Jstas

Class D is very efficient at making big power. You can get tons of juice for a low charging system commitment. So if you want to go BOOM, that's the way to go.

Class A/B is not as efficient but much more controlled. You can get the same amount of power but it will be expensive both elctronically and financially. However, for most SQ subs, you don't need disgusting amounts of power. I'm getting by just fine on about 200 watts RMS. It sounds good and there is enough boom to turn it up every so often. Then again, that is D-class power.

However, I did have about 170 watts RMS A/B class power on a 10 inch sub in a sealed enclosure in the trunk of my Thunderbird and many people were impressed with the impact that little sub made. It was, of course, a Polk Audio sub and a Kenwood amp. I also powered two Kicker Competition (the old ones that were rebuildable) with an old Profile amp that was pushing about 600 watts RMS mono. That drew about 60 amps. The little Kenwood drew about 35 amps. Comparative D-class could probably get away with 35-40 amps on the Profile and 20 amps on the Kenwood.


So if you want SQ, A/B power with a sealed box sub is a better way to go IMO and experience. If you want big BOOM, D-class and ported all day long.

exalted512

I would LOVE to get my hands on a couple of TC Sounds 3000s...drool...
-Cody

MacLeod

There isnt a difference in power. A QUALITY 500 watt Class D amp will make 500 watts, and a 500 watt Class AB amp will make 500 watts. However, like John said, the Class D will do it with less current draw from your battery because its more effecient.

As far as sound, there is no difference. Class D amps tend to have more distortion but for subs its not an issue as youre not going to hear it.

The way I boil it down is like this: If space is a consideration get a Class D as they will be smaller than an equally powerful AB amp. If money is a concern get an AB as they will be cheaper than an equally powerful D.

Another advantage to running a Class D is that its stable to 2 ohms mono where most AB amps are not. So if you have a pair of 4 ohm SVC subs or a single 4 ohm DVC sub then a Class D would work better.

killerb

i used to run a zapco a/b amp on my subs and now run a d class amp. the dampening factor on the zapco was way higher, but i hear no difference. i would go class d on subs. my massive audio amp has tons of power and sounds great. 1500 watts on a stock hyundai electrical system.

exalted512

class ab will only be cheaper than a comparable class d if youre doing your power at 4 ohms. But if youre doing it at 2 or 1, then class d is almost always going to be cheaper.
-Cody

docsparkss

Sound-quality of Class-D Amplifiers Less distortion than class AB ?

A main disadvantage of class B is crossover distortion as discussed completely by Douglas Self in EWW.
See also http://www.dself.dsl.pipex.com/

In most high quality amplifiers this distortion is reduced by rising the bias current, making the amplifier operate in Class A at low power levels. This reduces the distortion at the zero-crossings but the problem is then shifted to a higher level where class B begins.

Another distortion mechanism of class B is supply line pollution, When we look at the supply current during a sinusoid output signal, it looks like a single phase rectified sine with high harmonics generated at the sharp edges. At higher frequencies the PSU rejection ratio is poor resulting in highly non-linear modulation of the output-voltage. An power supply with zero output resistance at all audio frequencies will prevent this problem. In better Analog amplifiers the housings are filled with caps of 10,000 mF or more per channel to reduce the supply resistance. In class A operation there is no supply problem at all, except the huge energy account. The current drawn is only DC.

Distortion mechanisms in class D :
In a class D amplifier the distortion mechanisms, are complete different. Distortion at zero-crossings like in AB amplifiers does not occur.

A similar but very complex distortion mechanism is caused by the dead time period between the switching periods of both transistors. When both output transistors are in off-state, the output voltage at the transistor-output is dependent of the current in the filter-coil, which is non-linear dependent of the momentary output-level, creating a voltage drop when the output-current reaches a certain level.

By switching on the 2 transistors fast after each other, with a dead time of only nanoseconds, this distortion-mechanism is minimized. In the remaining dead-time, the momentary output voltage is determined by the charging of the parasitic output capacitors by the inductor current. A large parasitic output capacity smoothens this error voltage, reducing the distortion to only low harmonics.

This distortion gets measurable (-100dB) when the output current is reaching the peak of the sawtooth-shape idle-current of the output-inductor. This current draws no power when the Q-factor of the output inductor is high. In our amplifier LPC1 this idle-current is 1/3 of the maximum output current (12 Amps) so the dead-time-distortion begins at 10 dB below maximum output. Using the high idle-current for charging the parasitic capacity will also reduce the switching losses.

The dead time is accurately matched to the charging time of the parasitic output capacitors for minimizing distortion and switching losses.

The supply line pollution has of a much more friendly behavior. Looking at the current drawn from the supply lines, its waveform has a low-frequency component, which is the square of the output signal. With a sinusoid as output signal, and a certain linear output resistance in the supply, a signal with the double frequency will be added to the supply voltage. In a PWM output stage with no feedback the output-signal will be multiplied with this supply voltage. A sinusoid output signal will be modulated by its square and thus resulting in pure third harmonic distortion.

It can be easily calculated that an output stage without feedback driving a 4 Ohm load to 80% of the supply voltage (80V), (resulting in 128W) and powered by a poor supply with 0.1 Ohm will result in 1% D3 distortion. If the supply is only buffered with a capacitor of just 3300 mF and the output stage is fed back with a bandwidth-product of 100 kHz, this D3 is reduced to 0.005% (-86 dB) at all frequencies. However when the supply is only buffered with large caps, low-frequency signals can cause second order inter modulation at higher frequencies which is much higher than this calculated D3.

In the LPC1, the supplyline intermodulation is cancelled completely by a patented circuit, making the amplifier open-loop gain fully independent of supply voltage, even at 20 kHz.

Sound of THD
The main explanation for the better sound-quality in this class-D amplifier is that the calculated and measured non-linear distortion mechanisms all result in distortion-components which increase proportional with the sound level. The relatively high THD-number (0.003% or 90 dB at 1kHz, 400W) comprises most low (3rth and 5th) harmonics.

This distortion is masked by the non-linear distortion and compression of most ears and loudspeakers. In contrast, the harmonics caused by crossover in class AB do peak at low output levels, and have a wider spectrum, making the same THD-number much more audible.

Output impedance
A second disadvantage of a switching amplifier is its complex output impedance caused by the output filter.
The output voltage gain is frequency independent (within 0.1 dB to 40 kHz) when the amplifier is loaded with 2 Ohms at all frequencies. But at other loads the output impedance will affect the frequency response.
This effect is minimized by making the inductors in the output filter as small as possible (6 + 2 microH when designed for 2 Ohm).

The poles and zeros of the filter impedance are outside the audioband, and the output inductance causes only a rise or fall of the frequency response (until +2 or -1 dB at 20 kHz) when loaded with 4 or 1 Ohm instead of 2 Ohm. This can be equalized easily. The inductance of loudspeaker cables (especially the expensive ones) is in the same order of magnitude (but not compensated) as the (8 uH) inductance of the filter. For a flat filter response, a constant speaker impedance is recommended.

A class D amplifier with extra feedback from the output of the filter, reducing the output impedance, is in development.


Very old prototype of 1 kW Amplifier,
only the size is real.

docsparkss

Class D Amplifiers are known for their high efficiency and high power levels compared to conventional amplifiers.


Operation
The operation of Class D amplifier is complete different to Class A and AB or B.: In a Class D power is generated by modulating the duty-cycle ratio of a constant frequency square-wave signal from a switching converter. This square-wave or PWM-signal is amplified with logic gates and power-switches. The duty-cycle ratio results in a low-frequency-component (after filtering) with the same maximum power-level in respect to supply-voltage as Class A or B, but with a efficiency of >90%.

Power-efficiency
Well known is the power efficiency of a class B; with a sine-wave at full power it delivers about 70 % of the supply-power, With a square-wave this rises to 100% and a full-power triangle-or saw-tooth -signal results in just 50 % efficiency. These kind of signals are common in AC-line-regulators, Power-supplies and CRT-deflection-units, but not in most music. At signal levels lower than the maximum level, the power-efficiency drops proportionally to zero.
In music signals the ratio between the peak level and the average (RMS-) level is 20 dB or more (the sound from commercial radio stations excepted) so the average power-efficiency off a class B amplifier will mostly not exceed 5-10 percent.

A Class D amplifier has theoretically no energy-loss. Current drawn 90 degrees out of phase with output voltage (by complex loads like loudspeakers near their resonance-frequency) needs no power at all, in contrast to conventional amplifiers.

Mostly used for power-audio is Class G or H because of the higher efficiency than class A or B at an Ohmic load. At a complex load however these amplifiers do still dissipate power greater than output-power, in contrast to class D. Class G and H power amplifiers still need fans for removing heat and collecting dust. The LPC1 can deliver power to 1 or 2 kW without fans.

20hz

my last amp was a hafler MA1 its a a/b class that made my lights dim when it was driven hard I just put in a kicker dx350 dclass mono amp , I think it doesnt sound as smooth and deep but it hits strong and I like not having diming headlights .
I use A/B class amps for the mids and highs .