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Class D "digital" amps. Opinions on performance

#522
B-string said:
Thank you Bob Lee, BbbyBld. fdeck and other CONSTRUCTIVE posters for your patience.
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Lol. Good ole amps forum! That wasn't a passive aggressive post towards Jimmy was it? ;)

Man, I've just gotten to the point where a micro head sounded like something I would want to buy! (And I did!) While I'm interested in the technology inside them as well as the engineering history that led to them - at the end of the day the amp either sounds and responds in a way that makes me willing to write a check, or it doesn't. I think the experienced engineers here get that, but it's often really hard to get everyone on board.
 
#524
scotch said:
Lol. Good ole amps forum! That wasn't a passive aggressive post towards Jimmy was it? ;)

Man, I've just gotten to the point where a micro head sounded like something I would want to buy! (And I did!) While I'm interested in the technology inside them as well as the engineering history that led to them - at the end of the day the amp either sounds and responds in a way that makes me willing to write a check, or it doesn't. I think the experienced engineers here get that, but it's often really hard to get everyone on board.
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JimmyM is a great guy and hears what he hears. I would not sarcastically label him "Golden Ears" as others have to myself personally. I am encouraging the technical discussion and appreciate the continued effort. :)
 
#526
B-string said:
I just posted above, not pointing any finger in any direction with intent.
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My apologies then! :)

As much as I find this thread interesting, I am concerned that next time I order a pizza all I'm goona hear is "What topologies would you like? Pepperoni, pineapple...?"
 
#527
KJung said:
It is not what you hear that is the issue, but what you are attributing what you here as a causal driver. Your comments comparing the LM to the F1 make this painfully clear. All integrated bass amps sound different. For some reason, you seem to think that class of amp is the primary driver.
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Do the LMII and F1 both have an SMPS? Why yes they do. But thank you for yet another failed attempt to make me look foolish because I have different tastes than you.

I'm out. You all have fun.
 
#530
dmusic148 said:
Don't designers of micros go to certain lengths to ensure they can't ever go into clipping? By using sophisticated compression/limiting schemes? Might this not explain the sag Jimmy says he hears/feels?
I know he said he perceived it even at moderate volumes. Just spitballing here.
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You might be onto something here
 
#531
JimmyM said:
Do the LMII and F1 both have an SMPS? Why yes they do. But thank you for yet another failed attempt to make me look foolish because I have different tastes than you.

I'm out. You all have fun.
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Sorry Jimmy. If I hadn't been on the iPhone app, I would have put a smiley face on my last post. NOT trying to make you look foolish at all.
 
#532
KJung said:
God help us all. So, the LM is class AB and the F1 is class D. So I guess now you are saying that what you are hearing is due to micro amps that are small, but not too small.
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It's the size of the case.

Dedicated exponential neurodendrites collide with the case interior. This spatial effect can be mitigated by enclosing the class D amp in a case the dimensions of an SVT head case. Add some 6550 power tubes and a tube preamp with Baxandall tone shaping and large transformers.

Sit on a Fridge, play loud and enjoy.
Problem solved.
 
#536
dincz said:
I guess "clipping" is really a misnomer in terms of class D amps. But what does happen when you reach 0/100 percent pulse duty factor? What does it sound like?
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That actually would be clipping. Or directly analogous to it, at least. The output signal, after the reconstruction filter, would look like a clipped signal.
 
#537
Bob Lee (QSC) said:
That actually would be clipping. Or directly analogous to it, at least. The output signal, after the reconstruction filter, would look like a clipped signal.
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Yeah, I said misnomer because it's not clipping in the near-literal sense as in a class B/AB amp. So it's likely to sound just like a clipping AB amp?

Is there normally a non-defeatable limiter in there somewhere?
 
#538
dincz said:
Yeah, I said misnomer because it's not clipping in the near-literal sense as in a class B/AB amp. So it's likely to sound just like a clipping AB amp?

Is there normally a non-defeatable limiter in there somewhere?
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Most likely, yes. There might be some sort of feedback arrangement in the modulator to soften the clipping, depending on what the designer came up with.

I can't speak for all class D amps, but in the case of the QSC PL380, currently QSC's only stand-alone class D power amp, there's a defeatable clip limiter on each channel. When it's engaged, it responds only to clipping (actually to any time that the output signal has an error that the negative feedback can't force the amp channel correct, which in most cases would be caused by clipping).
 
#539
I guess "clipping" is really a misnomer in terms of class D amps. But what does happen when you reach 0/100 percent pulse duty factor? What does it sound like?
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If the duty cycle reaches that point, it instantly blows up! One of the complicated parts in a class D amp is the protection that makes it VERY difficult or impossible to go above or below a certain duty cycle

Don't designers of micros go to certain lengths to ensure they can't ever go into clipping? By using sophisticated compression/limiting schemes? Might this not explain the sag Jimmy says he hears/feels?
I know he said he perceived it even at moderate volumes. Just spitballing here.
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There's actually a lot to that and I'm not sure I can talk about it;), but I will say a little about it. I'm not sure what all class D amps do. Certain class D topologies have absolutely horrible sounding clipping, and some sound okay. Peavey has a patented way of creating artificial clipping by manipulating the PWM, but we don't use it on all designs. If you want to design a class D amp for bass guitar use, you need to make sure that you use control components that are not integrated to the point of not allowing access to certain key parts of the control circuit. That's because developers of those components (and modules sometimes) plan on the devices being used for music playback where the audio is compressed to hell. That's one thing that leads to shutdown issues. In my opinion, eliminating clipping 100% is not a good idea, and that's all I'll say about that.
 
#540
BbbyBld said:
If the duty cycle reaches that point, it instantly blows up! One of the complicated parts in a class D amp is the protection that makes it VERY difficult or impossible to go above or below a certain duty cycle.
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What's with that? Help a junior techie understand this. ;) I'd think if the amp reaches 100% duty cycle, it's the same as temporarily passing DC through the MOSFETs, and then it's just up to the current rating of the devices. So, I'm trying to think of what else could go wrong: Perhaps the feedback circuit latches up, or the MOSFETs take an inductive discharge?

I've run Class-D amps to clipping, and Putzeys shows it in his papers. But I was running a resistive dummy load, so it wasn't exactly under live fire. The ones that I've tested, including two bass heads, seem to clip softly and gracefully.

On the other hand, re the general talk about clipping, I've seen both hard and soft clipping in Class-AB power amps. There are published waveforms showing symmetrical hard clipping in tube power amps. Page 7:

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So, I think clipping isn't it. The technical root cause for a "signature sound" of Class-D, if there even is one, still eludes us.
 

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