sound of class d amps vs class ab

killerb

im running my sr124-dvc with a zapco c2k-3.0 putting 660w @ 2 ohms. i think i need more power. i got a good deal on a massive audio p1500.1 putting 950w @ 2 ohms. its a d class amp. how is the sound quality with class d amps compared to class ab? i know polk makes class d amps so i guess they should be alright. i heard some good things about massive, has anyone else heard anything about them?

hondahirny

Class D amps are more efficient, but have more distortion. At the low frequencies that subs operate, this isn't much of an issue. This is why many sub amps are class D. You'll probably be happy with whatever you get, as long as it's a decent name brand of amp and the power level is well matched for your subs. I'm running pretty much polk everything, with the subs/amp rms wattage matching. Sounds pretty good. Some say you can never have too much power....

MacLeod

You can easily have too much power!

A bigger amp is a good think in that youll have more headroom but make sure you set it properly cause a bigger amp also makes it more likely youll overpower your sub and fry it.

hondahirny

I was just joking about the too much power thing. My 180W RMS from the amp going to each sub is doing a fine job caving my head in, if i want it that loud.

killerb

yeah, i make sure its set right. that sr12 should be able to handle 950 with no sweat since each coil can handle 700w rms. the new amp is a monster, 1200w rms into 1 ohm and 1500w at 14.4v. massive audio is supposed to have very good build quality. check them out.

MacLeod

The SR12 is rated to handle 700 RMS TOTAL, not per voice coil.

You hook 1400 watts up to it and youll likely have a very nice and unique door stop.

bknauss

hondahirny wrote:

Class D amps are more efficient, but have more distortion.

This is true for a bad design. Since Class D and switching technologies have been hacked at furiously for a while now, this isn't nearly as true as say... 10 years ago. But someone could also make an AB very distorted if they were a schmuck and didn't know how to design it.

Greg Peters

hondahirny wrote:

Class D amps are more efficient, but have more distortion. At the low frequencies that subs operate, this isn't much of an issue. This is why many sub amps are class D. You'll probably be happy with whatever you get, as long as it's a decent name brand of amp and the power level is well matched for your subs. I'm running pretty much polk everything, with the subs/amp rms wattage matching. Sounds pretty good. Some say you can never have too much power....

Correct me if I'm wrong, but aren't the new Alpine PDX series amps full range Class D amplifiers? They were pretty well recieved, too- well within typical Class A/B specs for distortion etc. with good sound quality.

AltoMobile made some full-range Class D amps a while back, and they were supposedly pretty clean too.

Class D is usually more efficient than a Class A/B, but when you get into full range Class D's the price climbs up there on the better products...it's not that they all have more distortion, but that they have to put more technology and more expensive parts into one to achieve specs that Class A/B amps can make (at a lower price).

Where Class D amps become a good idea is in systems with limited electrical capacity, where an all-out high current (and low efficiency) set of amplifiers would take too much toll on the electrical system to produce the desired output.

MacLeod

Yes, the new Alpines are Class D. Xtant also makes Class D full range amps. The 1.1 are 100x1. Theyre also $350 a pop so if youre wanting to bi-amp your mids/tweets with them, youd be dropping $1400!!! Yikes! Class A/B for me!

1996blackmax

Eclipse also makes full range class D amps. One of the good things about the Alpine amps are their small size. You could cram those things into pretty tights spots.

killerb

polks white papers on page 17 has continuous power rated at 1400w for the sr124-dvc which i have. the sr124 is 700w continuous. they have the same voice coil, but they have two of them so half the power goes to one and half to the other. the alpine pdx amp isnt full range the response is 20-200hz. also the only full range digital amps i ever saw are class t amps made with the tripath chip. i would only go class d on my subs, class ab for everything else.

Greg Peters

killerb wrote:

the alpine pdx amp isnt full range the response is 20-200hz. also the only full range digital amps i ever saw are class t amps made with the tripath chip. i would only go class d on my subs, class ab for everything else.

Though the technology in the PDX amps is called "Icepower", it is indeed Class D in the case of the the four and two channel (full range) amplifiers as well.

http://www.alpine-usa.com/en/products/product.php?model=PDX-4.150&lang=en&tab=F
Alpine Electronics of America, Inc.

http://www.alpine-usa.com/en/products/product.php?model=PDX-2.150&lang=en&tab=F
Alpine Electronics of America, Inc.

For these two, frequency response is stated as 20-20,000 hz and 10-50,000hz respectively. In the case of the latter, that is better than what most would consider "full range". Not that I'll be buying any (mostly due to the price point).

killerb

dam, i didnt know there were so many full range class d amps out there.

MacLeod

killerb wrote:

polks white papers on page 17 has continuous power rated at 1400w for the sr124-dvc which i have. the sr124 is 700w continuous. they have the same voice coil, but they have two of them so half the power goes to one and half to the other.

Im wagering that is a type-o. The specs on the website AND the specs in the owners manual states 700 RMS and 1400 Peak. We had the same issue with the Momo DVC's when they first came out. Everybody thought they were double the power handling.

the alpine pdx amp isnt full range the response is 20-200hz. also the only full range digital amps i ever saw are class t amps made with the tripath chip. i would only go class d on my subs, class ab for everything else.

The PDX amps designed to power subs have 20-200 Hz frequency response but the 2 channel and 4 channel PDX amps have FR's of 10-50 KHz!

killerb

macleod, your right. i just called polk and they said its a typo. the sub will only handle 700w rms. that sucks because i got the massive audio amp because of that info in the white pages. do you think i should still run the massive audio amp at 950w? or just keep using the zapco? i want some more output, and i dont slam my subs all the time. i dont think it will be a problem, what do you think?

AustinKP

try 'em both. I bet you won't be able to tell one bit of difference if you have the gains set the same.

PoweredByDodge

The saddest part of this whole post is that you're using an amp with a BJT output stage for a subwoofer. It's a friggin sin.

You want to split hairs... your typical N-fet AB amp or NP AB amp isn't going to have much different audio characteristics than a well built newer class D full range(class I, class T, whatever the hell else these proprietary **** are naming it these days)... speaking in terms of overall bandwidth.

As technology gets better - there will come a point (some argue it's already here - honestly I don't know if it is or not) when the class D full range have virtually identical specs with the run of the mill Fet-output AB's.

However, the nicest amps are driven out of BJT's, in my opinion. The specs (across the industry) tend to back up that opinion... then again maybe it's just that the only people that use BJT's are anal about audio and make expensive nice amps --- so you could argue that these people could do just as good a job with Mosfest or, more recently, Gasfets. Then again - you have to ask youself -- if these guru's of audio awesomeness have all these things to choose from, why are they choosing the less efficient, more expensive, and far more difficult to design with piece? ... 'cuz its better.

play amongst yourselves....

killerb

yeah, your right in a way. but im using a vintage carver magnetic field amp for my front stage. i tryed the zapco there but the carver sounds better and puts out more power 75w into 2 channels vs 120w into 4 channels. i have had this carver since the 1990's and still LOVE the sound of it. its all i need for a front stage, powerful and mutli- channel. the zapco is a great amp, but i want to sqeeze a little more from the sub. ill probably have the zapco for many years and put it in another car. who knows. the carver stays forever, it just sounds that good.

PoweredByDodge

old amplifiers are all BJT outputs... so I'd imagine that an early 90's amp has BJT outs... that was an era when quality of sound was more important than quantityof power -- it was a good era

killerb

you are right,i love the oldschool amps!

killerb

well i solved the power problem. i got one more sr124-dvc and will power them with the massive audio amp @1500 watts into 1 ohm. this should be a great combo! ill let you know what happens. no more power problem.

audiobliss

Haha, that is an awesome solution!!

killerb

well, thank you sir.

killerb

well i hooked up the massive audio amp today. all i can say is this sub really likes power. i have 950w going to it and it sounds good and clean. im waiting for my sub box to get here so i can hook up both with 750w per sub. the massive amp is very nice with plenty of power. i like the sound of the d class amp. i cant tell the difference between the zapco and the massive except the massive has a ton more power.

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.

Hokie

Does anyone have experience with the Rockford Fosgate Prime R600-5? It has class A/B for the front/rear speakers and class D for the sub. Seems like the best of both worlds.

MacLeod

Pretty much all 5 channel amps have a class D amp section for the sub amp. Gives you more power, accommodates 2 ohm loads and wont use as much input voltage as its more efficient. Rockford Fosgate is a very good brand. Id feel pretty comfortable running one of theirs.