Class D or AB

[seamonkey]

I've heard some claim that the LittleMark lll and LM 800 sound different. I'm not sure why other than the different power supplies??

Difference probably have more to do with the pre-amp part.

If you took the pre-amp out of one and fed it into the amp of another, with matching the gain, I doubt you'd be able to tell a difference. Otherwise, Markbass botch one of the amp section designs and it has nothing to do with class.

 

[James Simonson]

So why do the small heads tend to give out at high volumes? ... or tend to not be durable. I don't hear of that with nice Crowns or QSC class D amps. What's the difference?

It would be great to see Crown or QSC make a great simple bass head.

 

[BurningSkies]

Its all about implementation. In fact, many of the micros are based around the same 3rd party module. So, it behaves like it does...then its up to the amp designer to figure out how to get the performance they want. That's creating a preamp that sounds how they want it to, and how it feeds the power amp. Its also how they institute any kind of power amp limiting, thermal protection, etc.

My experience with the LMII is that it distorts sooner than you hit the thermal limits. It compresses and limits in the deep lows to preserve headroom and volume. Once I had speakers that had high potential for clean performance at loud volumes, I found that I could easily hear distortion from the amp that was not being heard with traditional cabs.

 

[megafiddle]

megafiddle, you make many good, accurate points. And I'm
with you until your fourth paragraph.



Let's go back to my door sensor example. How do I store
that information as a sequence of codes? You can do that no
more easily than you can store the class D amp data. But the
door sensor uses a digital IC at its core. Can you say that is
not digital? It is certainly not an analog circuit.

Let's revisit Wikipedia and see if we can get some
arbitration on this disagreement.

Wikipedia recognizes Morse Code as a digital system.
Morse Code uses mere "ons" and "offs" modulating the width
of the "ons" and controlling the timing between each. I'll be
damned if that doesn't sound exactly like the controlling
signal in a class D amp.

Good question about the morse code. But I think it all lies in the intrinsic differences
between analog and digital. One characteristic of digital is granularity. Your door
sensor cannot tell if the door is just partially open, or open a little more, etc. The digital
circuitry quantizes the state of the door into either open or closed. That is a true digital
system.

But no such quantization occurs in PWM. The resolution is infinite in theory, a continuum.
It's just a different analog representation of another analog quantity.

Consider Pulse Code Modulation, like used for CD's. You can look at any of the codes and
determine exactly what waveform voltage it represents. Each code represents a discrete
voltage. Now what if you look at a PWM level? What does that tell you? There's not enough
information there. You have to look at it over a period of time. The level itself tells
you nothing.

Have to think about the morse code some more.

 

[James Simonson]

Thank you. I agree.

It seems to me that an old Acoustic head or even a Peavey will just not give out no matter how aggressive the signal. It may sound not so great at those levels but it won't give out. It won't strand you 3/4's into the 3rd set or the end of an outdoor 90 minute.

It's probably just my ignorance when it comes to trusting the specs. I want to see these amps succeed and I don't want to bash them but the Class D amps keep letting me down.

Maybe they just shouldn't make the knobs turn that far?

 

[BurningSkies]

Thank you. I agree.

It seems to me that an old Acoustic head or even a Peavey will just not give out no matter how aggressive the signal. It may sound not so great at those levels but it won't give out. It won't strand you 3/4's into the 3rd set or the end of an outdoor 90 minute.

It's probably just my ignorance when it comes to trusting the specs. I want to see these amps succeed and I don't want to bash them but the Class D amps keep letting me down.

Maybe they just shouldn't make the knobs turn that far?

That's because old amps don't have thermal shutdown controls. What you do first is damage your amp. Then you take it to a repair place and get them to replace bits and pieces of it. Modern amps have limiting and compression schemes in place to limit those situations, then they have a thermal shut-off that triggers the unit to shut down if it gets too hot before damage. Then you just need to restart.

For the record, I ran a LMII for something like 4+ years and ran it at high volume without it ever hitting its limit. I'd guess that the unit the LMII (and others) is based around is sensitive to low speaker impedance, and that's what pushes it to shut down for lots of people. So, that would be dependent upon what cabs you have and what their impedance curve presents to the amp.

 

[megafiddle]

Ok, the width of the morse code pulse does not represent a signal value.
It represents a logic state. The morse code "SOS", could be represented
as 000 111 000. The code 000 represents 'S' and 111 represents 'O'.

The length of the "dots" and "dashes" only matter in relation to each other.
The lengths themselves do not represent the signal as they do in PWM.

 

[James Simonson]

It might be protecting the amp but it's not protecting the music, the vibe, the dance floor or my reputation when it gives away.

I guess it's just the same with a Honda minivan vs. a 72' Nova.

I'd rather have the option to drag race, roll with or without a helmut, seatbelt, a gun, or whatever. Americans like to push on things to see what they're made of and the micros don't seem to fit that philosophy. I've blown several Class D amps. My fellow musicians don't like them on gigs. I am a serious gigging/full time bassist. It's my only job that feeds my wife and 2 kids. I'm in the studio most days and gigs almost every night. As far as I'm concerned, these amps are unreliable. I'd take the slow death and get a fresh when it's worn.

Ok I'm done. I'll sit down and shut up now

 

[JimmyM]

I've heard some claim that the LittleMark lll and LM 800 sound different. I'm not sure why other than the different power supplies??

This may be crazy but I have to agree with Seamonkey on this one and guess that it's in the preamp. I think they sound completely different, but my Micro VR and PF350 sound exactly the same to me, even though the Micro VR's class AB and the PF350's class D.

 

[James Simonson]

BTW, my cab is an Eminence 400 watt 8 ohm 15 with a tiny bit of tweeter. Passive 66' P or Passive 73' J
Amp is flat.

 

[JimmyM]

Thank you. I agree.

It seems to me that an old Acoustic head or even a Peavey will just not give out no matter how aggressive the signal. It may sound not so great at those levels but it won't give out. It won't strand you 3/4's into the 3rd set or the end of an outdoor 90 minute.

It's probably just my ignorance when it comes to trusting the specs. I want to see these amps succeed and I don't want to bash them but the Class D amps keep letting me down.

Maybe they just shouldn't make the knobs turn that far?

I've said many times that just as there was a learning curve on the dos and don'ts of traditional solid state, there is a learning curve with micros, and cranking them to the Bejeesus Belt is one of the things you're just not going to be able to do. If that's what you want, definitely stick with traditional SS and tubes or grossly overbuy micro wattage. That's what a lot of the pro sound companies do that run class D poweramps.

 

[BurningSkies]

It might be protecting the amp but it's not protecting the music, the vibe, the dance floor or my reputation when it gives away.

I guess it's just the same with a Honda minivan vs. a 72' Nova.

I'd rather have the option to drag race, roll with or without a helmut, seatbelt, a gun, or whatever. Americans like to push on things to see what they're made of and the micros don't seem to fit that philosophy. I've blown several Class D amps. My fellow musicians don't like them on gigs. I am a serious gigging/full time bassist. It's my only job that feeds my wife and 2 kids. I'm in the studio most days and gigs almost every night. As far as I'm concerned, these amps are unreliable. I'd take the slow death and get a fresh when it's worn.

Ok I'm done. I'll sit down and shut up now

So, is it you're against SMPS or class D? Because, as discussed micro doesn't equal class D.

My current rig is class D and has never shut down or showed any signs of failing when 'leaned' on. And that includes volumes that are more than sufficient to overpower the front end.

 

[James Simonson]

You're definitely right.

I was burned on a Rev C 7 Pro. Is your's still holding up?

 

[James Simonson]

Bejeesus Belt... That's funny.

 

[James Simonson]

I had the best results with Class AB 20lbs. 500 watts 4 ohm two rack space types.

But that's all I'm gonna say because you cats could bury me pretty quick. I'm just a gigging player speaking from experience that's all.

OK ... Done

 

[JimmyM]

You're definitely right.

I was burned on a Rev C 7 Pro. Is your's still holding up?

I don't have one. I have a PF350 and a Micro VR. And both are still working well, and I have cranked them hard at home before and they do just fine, although never on a gig. But they do run out of headroom earlier than trad SS and tube.

 

[warnergt]

First off, Wikipedia isn't the be-all-end all source, especially when dealing with the finer details.

Second, we've had this discussion before, and again, the engineers among us agree that class D isn't digital. When dealing with varying voltage as class D isn't just using on/off but also time as a parameter of how it works.

I'm an electrical engineer. I say class D is digital.

 

[warnergt]

First off, Wikipedia isn't the be-all-end all source, especially when dealing with the finer details.

Wikipedia is an impartial source. Except for politics and
religion, it's pretty darn good. And I'd say it completely agrees
with me.
If you disagree with Wikipedia, you can take your case there.

 

[warnergt]

Good question about the morse code. But I think it all lies in the intrinsic differences
between analog and digital. One characteristic of digital is granularity. Your door
sensor cannot tell if the door is just partially open, or open a little more, etc. The digital
circuitry quantizes the state of the door into either open or closed. That is a true digital
system.

But no such quantization occurs in PWM.

On the contrary, quantization does occur in class D amps.
It is not quantized to something like 64,000 or 4,000,000,000
levels (probably what you're thinking of); it is quantized to 2 levels.
The output voltage is compared to the input voltage (using
some circuitry for scaling the voltage) to determine which is
greater. The result of this comparison is "true" or "false" -
the discrete results required of a digital system. That result
is used to determine if the output signal should be pumped
more positive or more negative.

This comparison is quantization. Don't believe me?
Check the Wikipedia definition.
"Quantization, in mathematics and digital signal processing,
is the process of mapping a large set of input values to a
smaller set."
I'm not making this stuff up.

 

[SJan3]

Difference probably have more to do with the pre-amp part.

If you took the pre-amp out of one and fed it into the amp of another, with matching the gain, I doubt you'd be able to tell a difference. Otherwise, Markbass botch one of the amp section designs and it has nothing to do with class.

I see. Thanks!