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Class D vs Class A or A/B

#284
agedhorse said:
So, it really doesn't matter all that much in practice if the amp has a DF of 2000, 1000, 500 or even 200 because that term is so far to the right of the decimal point to be relatively insignificant compare with the other terms.
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Yep. It doesn't really matter.

In such case.

But take a feedbackless tube amp or solid-state amp with current feedback and the damping factor might easily drop anywhere between 50 and 0.5.
 
#285
Duke21 said:
Yes, but after to cups of espresso I managed to read through your (as always) very educating posts! :) you of course should stay away from espresso this late on a Sunday night!
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Absolutely, but I'm off for espresso this morning and to read through a couple of "rather dry" engineering magazines. As a general note, I find myself reading 7 or 8 magazines a month just to stay current with the state of the industry. In some ways, it's even harder than when I was in school so many years ago. In a field like this where technology moves very fast, to stop continuing education is the kiss of death and certainly career suicide.
 
#286
agedhorse said:
I agree with Bob's comments for the following technical reasons...

SLEW RATE: Minimum required slew rate is related to both level and frequency. For definition, a slewrate of one volt per microsecond is the slope of the voltage waveform and it must be larger than the steepest part of the curve at the highest frequency and level that the amp must reproduce.

For a rough back of the napkin calculation, let's use 1 volt per microsecond as the amp's slew rate. This means that the transition from -Vpeak to +Vpeak must occur in less time than 1 uSec. This is 1/2 the period of a full sine wave, so at 1 volt peak to peak, 1 V/uSec slew rate will support 500kHz. (this is not completely accurate because we have looked at the average slew rate of the waveform and not the instantaneous rate of change in voltage or dV/dT, but it's close enough to describe the basic principle).

Now to determine the required slew rate of an amp, we take the required maximum voltage (peak to peak) and the maximum frequency that FULL power is required (for bass, as frequency increases power density decreases). Using full power of say 500 watts "RMS" at 4 ohms (based on RMS voltage) this is 45 Vrms, 63Vpeak and 126 Vp-p.

Let's use 1kHz as the maximum full power bandwidth, the 1/2-period time is 0.5mSec.

The amp must be able to slew at a rate of 126V/500uSec or 0.25V/uSec. Double the power and the required slew rate does not double because of the squared term in the power equation (P=V**2/R), it goes up by the square root of 2 or 1.414, so at 1000 watts into 4 ohms the required slew rate is ~0.35v/uSec. All amps I am aware of have a slew rate a minimum of 10 times this, and most are 100x higher.

When Bob (and myself and other engineers here) say slew rate does not matter, they mean IN CONTEXT of the application. There are some really talented and experienced engineers here that I respect very much, it turns out that all of these folks generally agree pretty closely, they are good resources to learn from IMO. They certainly make me think about topics they bring up. I am also seperating slew rate from gain bandwidth product, slew rate is also a function of GBW product, but slew rate is the easily measurable and visable (audible) end result.

Now, as the application changes, and we need to increase the maximum power bandwidth of an amp, say we increase the maximum full power frequency to 10kHz, the required slew rate will increase to 3.5V/uSec for our 1000 watt example. In practice, it will be a little higher due to the dV/dT considerations, but I am staying with the specific concept and not trying to get too detailed.

At 20kHz, this would require 7V/uSec.

I like to use a design margin of around 5x for slew rate (accounting for the zero crossing dV/dT) so for an amp with a maximum power bandwidth of 20kHz, 1000 watts/4 ohms, I would look to somewhere around 35V/uSec but not be terribly upset if 25-30V/uSec was the best I could do if the trade-off to higher slew rate was lower stability.

Just giving a simple " 'round the coffee table " argument to support Bob's comment, and the more you guys understand what this stuff means in general and WHY it's either important (or not important), the better prepared you are to think through these kinds of arguments on your own. I hope this is useful information to some of you anyway.

I will address damping factor in another post.
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I am a total hobbyist hack so I cam just barely follow your elucidation. I just recall damping and slew rate being big issues back in the 90's. It's kind of weird to see them dismissed as "solved".
 
#287
Calaverasgrande said:
I am a total hobbyist hack so I cam just barely follow your elucidation. I just recall damping and slew rate being big issues back in the 90's. It's kind of weird to see them dismissed as "solved".
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The slew rate and damping factor "issues" were bigger for marketing than engineering back then, more so than today. I remember a magazine ad in the early/mid 80s for a consumer power amp with a rather moderate output voltage swing (I think it was about 75 to 100 watts/ch into 8Ω) and the ad bragged of a slew rate of 67 V/µs. That would suggest that it might also be sensitive to AM and shortwave broadcast signals that get into the input. That was a case of sensible design sacrificed for specmanship. And I've seen some amp designs from less reputable manufacturers that omit an output network so they can publish on paper an impressive number for "damping factor."

Andy's explanations are excellent, and he rightly showed that slew rate is not a useful spec by itself, but is interrelated with maximum voltage swing and maximum frequency.
 
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#289
ThisBass said:
The DF of my class D amp is speced to: 300 at 8 Ohm
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Not worth worrying about IMO, there are other specs. that are more likely to be responsible for the way the amp sounds/feels. I expect that this amp is not rated to drive 2 ohm loads?
 
#290
agedhorse said:
Not worth worrying about IMO, there are other specs. that are more likely to be responsible for the way the amp sounds/feels. I expect that this amp is not rated to drive 2 ohm loads?
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I don't worry about it :)

But indeed it is a 2 Ohm amplifier.

I never run the amp at 2 Ohm so I can't tell something about this.

I purchased this amp as a used item from a prof PA renting agency some years ago.
I kept my focus to the sound but not to any specs. The specs I did not know just this moment.

The charge I had to pay was appro $1000.
I did not know the charge for a new one just this moment but, I was thinking $1000 is a very good deal for any amp that sounds very pretty good like this ....

The amp is out of production in the meanwhile, discontinued 2007/2008.

But it is still a big monster. Very great opening sound performance and superb dynamic.
Even with a moderate amount of DF that is only 300.

Powersoft Digam 5000
 
#291
What is the load that the DF of 300 is specified at? If it's at 2 ohms, it's not a problem at all. If it's at 8 ohms, it could be something that might be audible because that would be a DF of ~75 at 2 ohms.

This is one reason why the spec alone can lead folks to think that something is not what it really is, and marketing folks gladly take the bait and run with it ;)
 
#292
Bob Lee (QSC) said:
You could modulate the supply voltage rails, but its more commonly done by commuting two or more tiers of rail voltages. No regulators necessary, and it can be done with line-frequency or high-frequency power supplies.
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A bit late coming back to this thread. I've seen conflicting definitions of classes G and H, but this is from the Wikipedia article on amplifier class:

"class-G and class-H amplifiers are marked by variation of the supply rails (in discrete steps or in a continuous fashion, respectively) following the input signal."

This is the definition I had in mind when I asked about modulating the supply voltage in class H.

Is this a gray area definition-wise?
 
#293
Is this a gray area definition-wise?
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In my experience,

Yes.


G and H are often used interchangeably and I'm quite sure there is no definitive standard which one is actually which. Even if there was people would still abuse it. AFAIK, the definitions are in reverse in America and Europe.
 

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