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CJH

Moon 330A and the first 5 watts of Class A

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Posted (edited)

Hi all.

 

I am trying to understand the behaviour of the Moon 330A amplifier and the first 5 watts of class A amplification before changing to class A/B. I am thinking of a possible purchase but want to understand what this means for me as this is a HUGE investment for me.

 

My speakers have a sensitivity of 84dB spl (2.83V, 1m) and are 8 ohms. From googling it an 8 ohm speaker at 2.83V, 1m specification is the equivalent of 1W at 1 meter. I also see that the resistance varies at different frequencies, but for now I will work on 8 ohms to keep things relatively simple.

 

I know this is quite technical and there are many, many things that need to be taken into consideration, but as a general rule of thumb would my thinking below be reasonable?

 

5 Watts into an 8 ohm speaker of 84dB spl (2.83V, 1m) will produce a 7dB increase in spl resulting in 91db at 1 meter.

 

With a decrease of 6dB per the doubling of distance we get:

 

85dB at 5 watts at 2 meters listening distance.

81.5dB at 5 watts at 3 meters listening distance.

79dB at 5 watts at 4 meters listening distance.

 

That means that for a Moon 330A amplifier that runs class A to 5 watts, at a distance of three meters I will be able to get about 81.5dB spl before it switches over to class A/B. 

 

Grammar warning below! Forgive me...

 

The other question I have is, if I set the volume of the amplifier such that it outputs 5 watts and do not change it, will the amplifier wattage output change as the dynamic range of the music changes? Would I have to set the volume to be at 5 watts for the loudest part of the  music to stay in class A mode?

 

Any guidance and comment on my thinking is valued, as well as comments on anyone who actually owns a Moon 330A amplifier.

 

Thanks.

Edited by CJH
Remove the word "pure" as it is causing off topic debate.

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Posted (edited)

It's the upper mids highs you should be looking at for the Class-A below that B is fine if well designed amp, as crossover distortion "is said" to be detectable by the ear in the upper mids/highs.

But here's something for you to ponder over, say a violin is being bowed in it's upper frequencies and it's within the 5w of Class-A and sounds sweet as a nut, but then a low organ note comes in, would that violin stay in Class-A sweet as a nut, or is it now Class-B and tearing your ears out off, with the organ and is nothing in Class-A, where does the 5w of Class-A go??   

 

Cheers George 

Edited by georgehifi

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5 hours ago, georgehifi said:

It's the upper mids highs you should be looking at for the Class-A below that B is fine if well designed amp, as crossover distortion "is said" to be detectable by the ear in the upper mids/highs.

But here's something for you to ponder over, say a violin is being bowed in it's upper frequencies and it's within the 5w of Class-A and sounds sweet as a nut, but then a low organ note comes in, would that violin stay in Class-A sweet as a nut, or is it now Class-B and tearing your ears out off, with the organ and is nothing in Class-A, where does the 5w of Class-A go??   

 

Cheers George 

 

Sorry George - I'm not trying to stalk you (this being the 2nd post of yours I have just responded to) ... but once again, I don't comprehend your post.  "Where does the 5w of Class-A go?"

 

AIUI, the amp was outputting <5w when just the violin was playing.  So the amp was in "Class A" mode.  Once the organ comes in (I presume you mean with bass notes - not treble notes), the amp is outputting >5w.  So it is now in "Class B" mode - it's no longer in "Class A" mode.  The "5w of Class A" hasn't magically "disappeared" ... it has simply shifted into Class B, due to the suck of power by the organ's bass notes.  And if it's a good-sounding Class AB amp (as mine are) ... the violin would certainly not be "tearing your ears off".

 

Andy

 

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I’m not 100% sure why you’re asking these questions @CJH

Is it for technical understanding or something else?

I read it that you want to listen in class A.......could be wrong though 🤔

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Posted (edited)
12 hours ago, andyr said:

I don't comprehend your post

Simple

 

There's nothing better sounding in hiend audio than a good recording (eg: RR  Scheherazade) of a violin being bow gently in it's upper frequencies using good class-A amp into ESL's or Plasma tweeters. Put in a Class-B amp and it all turns to ****   

 

It's a quiz for all with 5w class-A  A/B amps, what happens to the sweet class-a sound solo sound of the gently bowed violin because it only used up to 5w all class-a, then an organ comes in playing in underneath "in the same space in time", does the violin remain Class-A sweet or does it go into Class-B edgy sounding, because to organ is now using 50w and sending the amp into class-B

 

Cheers George 

Edited by georgehifi

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People seem to be assuming that amps can segment their outputs between Classes A and B.  I would have assumed that once an amp reaches its Class A limit, it switches to Class B.

 

I think the OP should be more concerned about dynamic headroom than how much Class A he's got. His amp is rated at 125 Wpc continuous into 8 Ohms and, while that seems a lot, he's likely to need a lot more than that to cope with dynamic peaks with speakers that are only 84 dB sensitive to start off with.  Otherwise he may notice harshness that he is blaming on lack of Class A, whereas it's just the amp clipping.

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Posted (edited)
1 hour ago, Bronal said:

People seem to be assuming that amps can segment their outputs between Classes A and B.  I would have assumed that once an amp reaches its Class A limit, it switches to Class B.

 

I think the OP should be more concerned about dynamic headroom than how much Class A he's got. His amp is rated at 125 Wpc continuous into 8 Ohms and, while that seems a lot, he's likely to need a lot more than that to cope with dynamic peaks with speakers that are only 84 dB sensitive to start off with.  Otherwise he may notice harshness that he is blaming on lack of Class A, whereas it's just the amp clipping.

 

Absolutely!  :thumb:  :thumb:

 

6 hours ago, georgehifi said:

It's a quiz for all with 5w class-A  A/B amps, what happens to the sweet class-a sound solo sound of the gently bowed violin because it only used up to 5w all class-a, then an organ comes in playing in underneath "in the same space in time", does the violin remain Class-A sweet or does it go into Class-B edgy sounding, because to organ is now using 50w and sending the amp into class-B

 

Cheers George 

 

 

Of course the violin goes into Class B - as the whole amp is now in Class B (due to the extra power draw of the organ notes).

 

But the violin won't necessarily sound "edgy" when the amp is in Class B.

 

Andy

 

Edited by andyr

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Posted (edited)
2 hours ago, Bronal said:

People seem to be assuming that amps can segment their outputs between Classes A and B.  I would have assumed that once an amp reaches its Class A limit, it switches to Class B.

I can't comment specifically on the circuitry for the amp the OP is considering purchasing, but the classic way a solid state Class AB output stage works is to have a resting current high enough for both sides of the complementary pair output stage to remain on as long as the output waveform doesn't exceed a certain amplitude.  This is Class A operation. For argument's sake, let's assume as an example that Class A occurs if the instantaneous value of the output waveform stays within the limit of plus or minus 2 volts.

If and when the instantaneous amplitude of the output waveform goes beyond that limit, this will involve one side of the output stage ceasing to conduct for the duration of that voltage excursion. (If it did conduct, it would waste power by trying to pull the output waveform in the opposite direction.) For that part of the waveform we have pure Class B operation.

 

To use our example, if the output waveform covers the range from minus 10 volts to plus 10 volts, the parts of the waveform that lie within 2 volts of zero will be handled in Class A mode, but  the parts that go beyond that will be handled in Class B mode.

So in the situation @georgehifihas raised of a quiet violin sound and a loud low frequency organ sound together, the violin sound may be reproduced entirely in Class A mode over the milliseconds when the low frequency organ tone waveform crosses zero, and may be reproduced entirely in Class B mode over the milliseconds when the low frequency organ tone is at its positive or negative peaks.

With good design of a Class AB output stage, the transition from Class A to Class B should be seamless to the human ear. However there is audiophile lore that disagrees.

@CJH, I would encourage you to go to a showroom for a demonstration of a Moon 330A and other amps set up for the same output level into the same speakers, to determine whether you can hear any differences, and if so how significant they seem to be.    

Edited by MLXXX

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84 DB sensitive speakers, or should I say inefficient with an amp thats rated at 125 watts continuous into 8 ohms and running 5 watts class A. WTF, you need to get hearing aids mate to hear the class A segment of your amp. That amp is not class A as far as I'm concerned. Its Class A/B and that means nix if it sounds good.

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Thanks all for your feedback.

 

@Gryffles I am trying to understand from a technical standpoint how the switch over from class A to class A/B works, and if in the end I am just going to be landing up with class A/B amplification anyway. In this case I could save quite a lot of cash and just get a straight A/B amp.

I have a particular interest in the class A sound as I have never owned class A for many reasons, cost being one of them. I am interested if my ears could really hear the difference. As can be seen from my posting, my speakers sensitivity don't lend themselves well to small wattage amplifiers. The hot Australian climate also tends to not encourage high power class A amplification....

 

@Bronal Food for thought. I don't have a new amp as yet, but I take your point that 125W for these speakers may be low depending on the volume levels used with 84dB sensitivity. I tend to listen to music quietly as I live in an apartment where any loud music quickly results in a visit from the body corporate.

 

@MLXXX I might just have to take my speakers with me, but I agree that in the end, its the ear and hairs on the back of the neck that must make the decision, not the specifications on a piece of paper :) Now to find a 330A in Canberra......

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