panteragstk

panteragstk

Audioholic Warlord
We have a local night club in town, on their ads in the entertainment periodical, they always tout "Soundsystem by QSC".

I suspect that QSC likely did the work at a discount, with the catch that it must always be advertised in that manner.

Just an FYI, and that's the only place I've ever seen this.
This could also be because QSC makes very nice speakers as well as amps. If their whole system is QSC I can say it will sound quite nice.

Lots of movie theaters use QSC amps and processors too. Very reliable and well known brand in the pro industry.
 
P

PENG

Audioholic Slumlord
Class AB describes a topology whereby the amplifier runs in Class A at low output power then switches to Class B at some point typically based on the bias point of the output devices. It's not exotic by any means, but yes, it does run in Class A under specific circumstances. Exactly at what output this occurs is up to the amplifier designer; it's relatively rare for the amp to operate in Class A at relatively high power but it is not unheard of.

However with most consumer-grade amplifiers the switch to Class B happens at a fairly low output power point. Taking the maximum output of the Rotel into consideration, it's extremely likely that it does not run in Class A for long ... too much heat, and difficult to support very high output power.

The reason this is the case can be summed up fairly easily ... pure Class B sounds terrible (although not as bad as Class C would).

At the crossover point (where the output devices switch halves of the waveform through the zero crossing point) a Class B amplifier will have crossover distortion into the tens of %. So Class AB hands off this critical point to a Class A topology (which is capable of zero crossover distortion), making Class AB a high quality topology suitable for High Fidelity use.
No, not all class ABs switch to pure Class B at any level. I suppose some do but I won't buy those. Also, not all class AB amps run in Class A bias even at low level. Some do and those are the ones I prefer, and I do own at least two of those. One supposedly does so up to about 8W, the other up to 20 to 25W.

The thread is long so I am kind of repeating what has already been discussed in previous posts, not just by me, but by at least 2 other knowledgeable members.
 
Y

yepimonfire

Audioholic Samurai
I don't know where you're going with that but has nothing to do with the avr limiting anything.
The AVR isn't limiting anything on the analog side, I'm just speaking in reference to digital headroom. A signal cannot exceed 0dBfs, therefore, there is absolutely no way you're going to get 120dB per channel as a peak unless you listen with the volume knob turned to +15dB. While 16 bit has a dynamic range of 96dB IIRC, the dynamic range between average and peak level is only about 30dB, with a minimum of 60dB fast response C weighted during quiet passages and a maximum of about 96dB fast response C weighted, with the average level during regular passages being about 80dB. Fast weighting is too slow to account for rapid peaks, and peak levels have been measured to be about 105dB.

It does not matter what the dynamic range is of a recording, if the volume knob is set to a certain setting relative to digital full scale, that's the loudest you're going to get out of a single channel. If you want a 120dB peak headroom on each channel, you'd have to set the volume knob to +15dB, which would be unessecarily loud. I generally find -10dB is suitable for recreating the full reference volume of orchestral music in stereo, with the 3dB gain from dual channels, that's about 98dB full blast, so not far off from a real world measurement.

Sent from my SM-G360T1 using Tapatalk
 
Johnny2Bad

Johnny2Bad

Audioholic Chief
No, not all class ABs switch to pure Class B at any level. I suppose some do but I won't buy those. Also, not all class AB amps run in Class A bias even at low level. Some do and those are the ones I prefer, and I do own at least two of those. One supposedly does so up to about 8W, the other up to 20 to 25W.

The thread is long so I am kind of repeating what has already been discussed in previous posts, not just by me, but by at least 2 other knowledgeable members.
I have no idea what you mean by "switch to pure class B at any level". What class do you suppose they do switch to? Either they have the low eficiency of Class A or the high efficiency of Class B at full output. The Rotel doesn't put out 20 watts, so what do you suppose it is doing then?

Please read some technical documents before you post misinformation. This is fundamental stuff, easily researched.

And then we have "not all class AB amps run in Class A bias even at low level."

An amplifier Class is not an optional designation. Either it's Class A, Class AB, or Class B. If it runs in Class A at the crossover point in a push-pull configuration, it's either Class A or Class AB. If it then switches to Class B operation at some point chosen by the design engineer it's a Class AB amp. Period. Full Stop.

I'll even help; the first Google result of a simple search:
http://www.electronics-tutorials.ws/amplifier/amplifier-classes.html

How many amplifiers have you designed? Feel free to ask me the same question.
 
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P

PENG

Audioholic Slumlord
I have no idea what you mean by "switch to pure class B at any level". What class do you suppose they do switch to? Either they have the low eficiency of Class A or the high efficiency of Class B at full output. The Rotel doesn't put out 20 watts, so what do you suppose it is doing then?
"switch to" was actually your words, imo not the best description there is no switching as such, I simply followed along. I meant they are still bias to operate as class AB. The bias can be controlled such that the amp is bias in AB class full time, or it can also be controlled to in fact operate in class A at low input signal level, and "switch to class B at some point..." as you previously described.

The point is, they don't have to (but yes they could, if the designer choose to do so) switch to class B, they can, and I believe in most cases, the bias are designed for them to be in AB practically full time. In some cases they could also be designed to operate in class A bias and "switch to" class B at some point, but not always, as they can also be designed to switch from A to AB. As I mentioned before, I do own two amps that operates in class A up to 8W (Halo A21), the other up to 20-25W according the the manufacturer's engineer. If you are curious enough you can email Parasound and ask if their Halo amps would switch to class B at some point. I am sure the answer will be "no".

Please read some technical documents before you post misinformation. This is fundamental stuff, easily researched.

And then we have "not all class AB amps run in Class A bias even at low level."
That is not very nice for you to say I posted misinformation, unless you have proof that I did.

As ski2xblack said in post#12:

Verdinut, class A/B amps, by definition, conduct slightly more than 180 degrees. They still do that, even at high power levels (they don't somehow switch to class B operation). Class B is inappropriate for audio purposes.

http://sound.whsites.net/articles/amp-classes.htm
He said basically what I said with much fewer words:D, though I slightly disagree with his point of class B being inappropriate, that's just too black and white.


I'll even help; the first Google result of a simple search:
http://www.electronics-tutorials.ws/amplifier/amplifier-classes.html
Thanks, but I read that article before you even linked it. If I remember right, nothing in that article contradicts what I have been saying so far.

The definitions of classes presented in that article were:
  • Class A: The amplifiers single output transistor conducts for the full 360o of the cycle of the input waveform.
  • Class B: The amplifiers two output transistors only conduct for one-half, that is, 180o of the input waveform.
  • Class AB: The amplifiers two output transistors conduct somewhere between 180o and 360o of the input waveform.

According to the definitions from that article, if the bias is controlled such that output transistors conduct for even 181 degrees, it can be correctly defined as class AB. Now, would you really consider such amp class A at low input signals, with just 1 degree overlap? I supposed you may do so technically speaking, but I doubt any such amps would bother claiming to run in class A at any point in practical sense, I meant like by saying their amp runs in class A for up to 10 mW ouput?

By the way, I decided to re-read that article, and I found the following paragraph that actually made basically the same point I have been trying to make.

"The biasing of a Class AB amplifier output stage is generally adjusted to suit a particular amplifier application. The amplifiers quiescent current is adjusted to zero to minimise power consumption, as in Class B operation, or adjusted for a very small quiescent current to flow that minimises crossover distortion producing a true Class AB amplifier operation."

Note the word "true", that is exactly my point.

Let me summarize by saying there are several scenarios in the class AB design:

1) As you described, one that operates in class A up to a certain point, then "switch to" class B above that level, hence the term "class AB". Note: I don't think this is the popular design.

2) Bias to operate with the output devices (push/pull) conducting for more than a half cycle, but very slightly, such as between 182 degrees to 190 degrees (just as an example, the designer will choose), hence class AB by definition, because the conduction angle is > half cycle < full cycle. Note: I believe this is one of the more popular way, I would assume lots of avrs such as those by Yamaha, D&M's class AB design works this way, just my educated guess though.

3) Bias to operate with the output devices conducting for full cycles at low signal levels so that the amp actually operates in class A up to a few watts or even higher, practically enough to be considered as an amp that runs in class A up to a point and then in class AB above that point.

Examples of such amps: Parasound Halo line, Emotiva XPA-1L


How many amplifiers have you designed? Feel free to ask me the same question.
I won't ask you the same question because I prefer just open minded conversations in less challenging tone. To me this is just a hobby and I come here for idea exchange opportunities, fun, and sometimes also opportunities to help each other by sharing knowledge. I am a practicing EE, and I understand the basic theories in the design of audio amplifiers very well but I have not designed one myself, did build a couple long time ago. I have done a lot of reading, naturally including text books (still have them) in university. I have communicated with those who do it as a living as well as a professor friend who does it as a hobby. One can "design" amps, but that does not mean one understands all the theories behind.

If you read my posts with open mind as I did when I read yours, you may see that we just have a very slight disagreement on the way class AB amps operate in the bias. I accepted for the first part of the way you describe class AB in the first sentence of your post#58, but I take partial exception only on the part "switch to" class B at some point...." because you implied it can only be that way. In fact, that is just one way, there are other ways as I listed above, and you too can do more Googling for further details.

For short version, one just have to read ski2xblack's post#12.
 
slipperybidness

slipperybidness

Audioholic Warlord
@PENG Just 2 cents from me concerning this comment:

though I slightly disagree with his point of class B being inappropriate, that's just too black and white.

Yeah, it is pretty clearly understood that a TRUE Class B amp is NOT appropriate for audio applications. The problem is that you really don't get the exact 180 degrees for the push or pull stage, so you have to bias slightly towards class A to get that AB bias that you need.

Crossover distortion is inherent in a True class B design, and crossover distortion would be incredibly noticeable, probably even painful to hear.

Amplifying other types of signals would likely be fine for class B operation, but not audio signals.
 
P

PENG

Audioholic Slumlord
@PENG Just 2 cents from me concerning this comment:

though I slightly disagree with his point of class B being inappropriate, that's just too black and white.

Yeah, it is pretty clearly understood that a TRUE Class B amp is NOT appropriate for audio applications. The problem is that you really don't get the exact 180 degrees for the push or pull stage, so you have to bias slightly towards class A to get that AB bias that you need.

Crossover distortion is inherent in a True class B design, and crossover distortion would be incredibly noticeable, probably even painful to hear.

Amplifying other types of signals would likely be fine for class B operation, but not audio signals.
That's why I qualified my point with the word slightly and about the black and white thing, because of my ongoing exchange with johnny2Bad, who seems to be saying class AB amp has to switch to class B at some point, see his post# 58 and 64. He already claimed that I posted misinformation so I am trying to be extra careful:D when quoting others out of context (my bad on that..).

So yes, I fully agree with you point regarding the crossover distortion inherent in the "true" class B design.
 
Verdinut

Verdinut

Audioholic Spartan
From what I read about Class AB amps, I haven't seen any reference to the switching possibility from a Class A to Class B at some point in such amplifier configuration. Anyway, there shouldn't be a switching to Class B cause we would be back to the starting point with crossover distortion, and a manufacturer wouldn't be able to sell the product for obvious reasons.

A Class B amplifier to which the designer adds a bias to eliminate the crossover distortion becomes by definition a Class AB amp.

IMO, there may be a few exceptions, but most Class AB amps operate full time with the appropriate bias to eliminate the crossover distortion and such design combines the advantage of both Classes A and B, less distortion (Class A) and more efficiency (Class B).
 
Johnny2Bad

Johnny2Bad

Audioholic Chief
" ... Class B has no bias current, Class AB has a moderate bias current, and Class A has a high bias current. Class AB push-pull amplififiers are hybrids between Class B and Class A. Class AB run Class A at low power levels, and become Class B amplifiers at output currents determined by the bias. ..."

-Nelson Pass, "Leaving Class A"

http://www.firstwatt.com/pdf/art_leave_classa.pdf

Unlike many who approach the subject, "Papa Nelson" is also a good writer, and explains the concept very well.

My only objection was that it was posted here that one poster doubted whether the Class AB Rotel operated in Class A at any point. Clearly it does.
 
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PENG

Audioholic Slumlord
From what I read about Class AB amps, I haven't seen any reference to the switching possibility from a Class A to Class B at some point in such amplifier configuration. Anyway, there shouldn't be a switching to Class B cause we would be back to the starting point with crossover distortion, and a manufacturer wouldn't be able to sell the product for obvious reasons.
I read a few that did say that, it's the internet you know.. I agree with you, and that's why I said if such class AB amp exists, (and I am sure they do otherwise Nelson Pass wouldn't have said what he said), I will not want one. That is of course unless it has enough output in class A mode for my desktop system.:D By the way, I think Nelson Pass wouldn't like anything else except class A, so you got the point right?
 
P

PENG

Audioholic Slumlord
" ... Class B has no bias current, Class AB has a moderate bias current, and Class A has a high bias current. Class AB push-pull amplififiers are hybrids between Class B and Class A. Class AB run Class A at low power levels, and become Class B amplifiers at output currents determined by the bias. ..."

-Nelson Pass, "Leaving Class A"

http://www.firstwatt.com/pdf/art_leave_classa.pdf

Unlike many who approach the subject, "Papa Nelson" is also a good writer, and explains the concept very well.

My only objection was that it was posted here that one poster doubted whether the Class AB Rotel operated in Class A at any point. Clearly it does.
I was ready to move on, but now I can see that you totally mixed up between the amp I referred to with some Rotel amp you were thinking about? I have never doubted any Rotel amp about anything, quote me please if I did.

Please re-read my post# 45 and see that the amp I referred to was the QSC RMX850a, not your Rotel amp. And I only expressed doubt, I even suggested that instead of assuming, Verdinut should email QSC just to be sure.
 
Verdinut

Verdinut

Audioholic Spartan
Here is a good website on electronics. Amplifiers are well covered.

As for Class AB amps, there are several ways of designing them to eliminate the crossover distortion. The topic is more detailed than that from other sources: www.learnabout-electronics.org
 
Verdinut

Verdinut

Audioholic Spartan
While looking for some info on current dumping amps, I came across a British forum and one poster commented positively about MC2 amplifiers. He was convinced that their Class A/AB design was superior to Peter Walker's QUAD design.
Those MC2 amps are, according to what I can make out, operate with a Class A (voltage amplifier) driving a complementary Class AB push pull output stage. That type of configuration seems to require an AC positive feedback from the output stage (bootstrapping circuit) to achieve a high gain in the Class A driver stage.

For details of a comparable design configuration: www.learnabout-electronics.org

I have been comparing the current draw of the S800 model to that of the QSC RMX850a, and I am amazed at the MC2's better efficiency. However, I suspect that the British product is classed in a definitely higher price bracket.

They have excellent specs: Heavy power supplies providing, for the S800 Model, 475W/ch at 8 ohms and 950W/ch at 4 ohms and 1550W at 2 ohms for example, and very low distortion figures. They seem to be excellent contenders in a powerful studio or home system, but too powerful for my needs.

Has anyone had knowledge about that MC2 Audio , a British manufacturer of pro audio electronics, and some of their amplifiers?
 
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Johnny2Bad

Johnny2Bad

Audioholic Chief
I was ready to move on, but now I can see that you totally mixed up between the amp I referred to with some Rotel amp you were thinking about? I have never doubted any Rotel amp about anything, quote me please if I did.
PENG said: ↑
" ... I highly doubt it runs in class A at all. ..."
 
P

PENG

Audioholic Slumlord
PENG said: ↑
" ... I highly doubt it runs in class A at all. ..."
I already told you that Verdinut and I were chatting about the QSC RMX850a amp, not a Rotel amp. I wouldn't have expressed such doubt about a class AB Rotel amp. It was very clear in my post #45 in which I even quoted Vedinut's reference to that QSC amp.

So please accept that fact and move on.
 
Verdinut

Verdinut

Audioholic Spartan
Peng,

I wrote to QSC Audio in their forum as there is no way to contact them by e-mail.
I received an answer and I was told that the RMX Series of amplifiers operate full time in Class AB. That is indeed acceptable but I am still bothered by the possibility of being annoyed by the fan noise.
Bob Lee, one of their moderators and employee of the company, also mentioned that they didn't have quieter fans as replacement for the supplied ones.

After doing some further checking and reading on the web, it looks like many owners have complained over many years about the fan noise for that line of amplifiers. That line is part of QSC's line of Live Sound products and I assume that they don't bother to look into that problem situation because most owners use that line of amplifiers for live performances where fan noise is left unnoticed.

So, I have decided to eventually get their DCA 1222 Digital Cinema Amplifier for bi-amping the front central channel. It uses a switching power supply and has a quieter fan. It is also a Class AB amp. However, it costs close to twice that of the RMX850a, its power is roughly 0.5dB higher (325W/ch at 4 ohms/ <0.05% THD (20-20 kHz)), but it has several advantages over the 850a:
1. It only weighs 21 lbs as compared to 35 lbs 2. Its depth is only 14" as opposed to an extruding16"
3. Lower THD figures and a SNR >106dB
4. Damping factor > 500

It also only takes a 2U rack space. I figure that because of the difference in the price, it must be manufactured in the U.S.A. and that's also OK as it keeps some Americans at work.

One other interesting product which they have is a 4 channel amp which I consider purchasing in the same DCA line later on for the left and right front channels, the DCA 1824. It features the 4 channels in a chassis, same size and weight as the DCA 1222. Avge Power output/ch: 250 W at 4 ohms.
 
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Verdinut

Verdinut

Audioholic Spartan
I'll be interested to hear your listening impressions. QSC makes a great amp. Especially the series you are looking at.

I am curious though, did you consider the Crown amps at all and if now why?
Well, after doing some checking on the web, and after being told that QSC did not have a quieter fan for the RMX Series amps, I decided to opt for their Digital Cinema Series amp, Model DCA 1222. It's also a Class AB amp but it uses a switching power supply and has a quieter fan. It also takes a 2U rack space, only weighs 21 lbs as opposed to 35, and the chassis is only 14" deep.

It will cost me close to double that of the RMX850a; I figure that this model is built in the U.S.A. and that's OK.

I'll keep you posted on the developments and will eventually inform you of my impressions on its performance. However, it's going to take a little more time because of the increase in cost, as you know.
Cheers,
 
P

PENG

Audioholic Slumlord
@Verdinut Thank you for the feedback. There is nothing wrong with full time class AB, as I mentioned before, lots of class AB amps operate this way, but the DCA1222 with switching power supply also is a good option for sure.
 
Verdinut

Verdinut

Audioholic Spartan
@Verdinut Thank you for the feedback. There is nothing wrong with full time class AB, as I mentioned before, lots of class AB amps operate this way, but the DCA1222 with switching power supply also is a good option for sure.
PENG,

Did you read my post #73 above about the MC2 Audio amps? Have you heard about them or had the opportunity to hear one?
 
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P

PENG

Audioholic Slumlord
PENG,

Did you read my thread #73 above about the MC2 Audio amps? Have you heard about them or had the opportunity to hear one?
Seems interesting for sure but no, have never heard of them before. By the way, won't you consider the Monolith that AH just reviewed? That things really look attractive in terms of $/Watt, if you don't mind the bulk and the weight.
 
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