So when do we get to see pictures and specs on this new unit? It's not listed on the products page yet.

------------------
Matthew J. Hill
matt@idsi.net

I asked this for the 755 too, so I might as well add it here too. ( 5 + 1 + 1 = ... 7 ! )

How big is the transformer? How much total capacitance? How much dynamic headroom?

May be very interested in this product. I currently use a Marantz MA700 to power an SVS CSultra. Wondering what the 4ohm would be? Dimensions of the unit and specs would be nice to know too.

[This message has been edited by werner52 (edited November 22, 2002).]

I would be almost certain that it uses a swtiching power supply, given the narrow profile (1 rack height) of the amp. Such power supplies behave quite differently than conventional ones. I would guess that the dynamic and continous power are about the same.

Class D or H? I hope not...

I don't know what class H is, but I doubt Class D as those are subwoofer amps. I have the impression that these were meant to be the extra two channels for people who bought a 755 and will want, or have, 7 speakers.

It undoubtedly is something like that - one thing fer sure, it ain't no class "A" circuit in a 1 3/4" high chassis dissipating 200 watts! Probably even class "B" would dissipate too much static heat. It almost has to incorporate some type of auto-ranging output scheme. The amp is undoubtedly designed to be 'stackable' in a rack, which would place a premium on power efficiency. Kind of a nifty concept actually, that type of circuit can be quite good, if designed well. But then, hell, I use vacuum tubes........

[This message has been edited by soundhound (edited November 23, 2002).]

A little birdy told me that a 200 is about 20 lbs, and does have a toroidal transformer in it (although very "mashed"!). That would imply AB, right?

I don't know anything , but class H to me means "cheap receiver".

(My Equitech 2Q, at 2 rack units high, has a 45 lb transformer in it. So 1 rack unit, ~20 lbs, could work.)

Having a big transformer would mean that it has a standard linear power supply, but there is still the possiblilty that it is run in something other than class A/B. It is pretty hard to build an amp that small and dissapate the heat, especially if it's going to be stacked. There's always the possiblilty of an itty-bitty fan in there somewhere too. I had a pro amp that was 1 rack unit high at one time, but it was only 100 watts. We'll have to wait, I guess for more info.

[This message has been edited by soundhound (edited November 23, 2002).]

I suppose a torroidal power transformer(s) can be made to fit in a single rack height chassis that has the needed power capacity. Also, some bipolar output transistors such as the MJL21193,4 complimentary pair, the MJL2115,6 complimentary pair and the MJL3281A, MJL1302A complimentary pair can operate with low bias current and have low distortion.

Maybe a thermally activated fan can take care of the large heat loads. If there is one, I hope it is not too noisy.

Paul

------------------
the 1derful1

Well, came in this morning and see the specs are posted, sounds great, where are the Pics of the rear? what turn on modes does it have?, Audio Sense?,Voltage Trigger?,Alway's on???

To answer your question I have taken some text from the "about" section of the Model 200 web page:

"...in general use we’ve made it easy for you to use the Model 200 by providing two ways to turn it on:

A trigger jack is provided for automatic turn-on when the Model 200 is used with products such as our Model 950 preamp/processor or Model 1050 receiver that output a low voltage control signal. For use with non-Outlaw products (SHAME ON YOU!), the Model 200 also features a “music sense” circuit that automatically turns the unit on when an audio signal is present, and turns it off within ten minutes after the audio signal stops."

Also, there is a mistake in the information that will be corrected shortly. The Model 200 is 1 rack unit high making it 1 3/4 inches tall.

Shame on me ? Scott it's not like that I'm the only one that uses other manufacturer's equipment, would like to get a look at your rack at home...Oh and by the way, We (Niles) make a device that will take that low voltage, low current trigger output and give you A volage that you can actually use, to trigger screens, lifts, etc.etc...Just a thought... Have a great weekend everyone, Peace!!!

From the 200 page:

Amplifiers are at the core of Outlaw’s history and they are the foundation that built our company. The Model 200 may be small in size, but its specs are outsized and truly worthy of the Outlaw name. It features innovative, fan-free design, a low-profile torrodial transformer, and high quality components throughout. Preliminary specs are as follows:

Power Output: 200 watts @ 8 ohms, 20 Hz ­ 20 kHz, <0.05% THD 300 watts @ 4 ohms, 20 Hz ­ 20 kHz, <0.05% THD Signal to Noise: 100 dB (unweighted) Input Impedance: >10K Ohms Gain: +27dB (1.7V sensitivity) for full output Input Connectors: 2 RCA Jacks Output Connectors: Five-Way binding post Power Consumption: 600 watts maximum/<3 watts standby Weight: 18 pounds Dimensions (H/W/D):1.5” x 17” x 11.5”

So no fans....

I just read a review of the new PS Audio HCA-2 (or something). 2 channel power amp that uses a fancy version of class D. Also has a toroidal transformer, so I guess that doesn't necessarily preclude at least class D...

I just found out that the amp has a Damping Factor of 80,that in my book means,No control over woofers,(not a big power supply,)it seems it would be similar to the new digital amplifiers,inpressive power numbers,but no control;also it reminds me of the old Carver Amplifiers......NO CONTROL....sure it may sound good,but if your amplifier does not have control of your woofers you will eventually damage them even if you do not force your amplifier into clipping.I guess I will just keep my old Adcom 555 working on my woofers.......

I think you are way off on the significance of that damping factor number! Here is an excellent article on the subject:

http://www.studio-systems.com/audiofeatures/novdec99/dampingFactor/damp01.htm

The paper concludes that a damping factor beyond 20 is at the point of diminishing returns. This matches my experience in the area. And you are CERTAINLY not going to damage your woofers or anything else if the damping factor is not 'high enough'.

Damping factor is one of those mis-understood things that tends to get way overblown in it's significance. Kind of like "24 bit at 192Khz", big impressive sounding numbers that are useless in the real world get way more meaning attached to them than they deserve.

Believe me, a damping factor of 80 is VERY good, and one that will effectively damp out the back EMF generated by the motor assembly (voice coil and magnet) of any woofer.

There are some very significant downsides to having insane damping figures too. Damping can be increased by the application more negative feedback. You can take a mediocre amplifier, and give it a damping factor in the _hundreds_ if you apply enough negative feedback! But do you want this? Excessive negative feedback has it's own problems, and can make an amplifier sound terrible. I have a test report from an old hi-fi magazine of a tube amplifier with a damping factor of something like 200. The manufacturer stated that they used some insane amount of negative feedback to achieve it. The report concluded with the warning that the amplifier should be used only with speaker systems 'approved' by the amplifier's manufacturer - OTHERWISE OSCILLATION WOULD RESULT! Somehow I don't think that amplifier sounded very good!

A good design is a balancing act, and the ultimate goal is to make a good sounding amplifier. Stressing one parameter just to make it "look good on the spec sheet" is NOT a good way to design.

A damping factor of 80 is more than enough!


[This message has been edited by soundhound (edited November 27, 2002).]

He wasn't really shaming on you, per se. That's written in to the document under the "About" link on the Model 200. It was just a direct cut and paste.

Soundhound (and others)- Any chance you could do a quick list of pros/cons of say 2 mono-blocks vs a stereo power amp?

Not sure about that damping factor info. My 755 manual lists it at 850 up to 400Hz. This is actually higher than typical. Most of my other amps have been in the 150-300 range. Much over 200 is tough to achieve in practice due to cable resistance. So my guess is because the 200 is from the same heritage, the damping factor (Low Freq) is probably 800, not 80.

My question is why are there 2 RCA input jacks to a mono block amp? Should be just 1 unless they are using an RCA for the trigger jack instead of a miniature. Any clues out there? My only thought is one of them is for auto-on and the other is for no auto-on.

2 monoblocks I believe is preferrable because each channel gets it's own power supply, and a sudden drain on one channel won't effect the other by modulating the power supply. Of course, this is less of an issue when playing typical program material that has a lot of common signal between the channels. There is also less chance of crosstalk with 2 seperate amps.

The only downside I can think of is 2 mono amps might cost more.

[This message has been edited by soundhound (edited November 26, 2002).]

The two RCAs might be for some type of feed through of the signal....

[This message has been edited by soundhound (edited November 26, 2002).]

One advantage of a two channel is actually related to the disadvantage - a big mono signal may be able to 'steal' some capacity from the other channel (the shared supply allocated to the other channel, really) but in many cases the transformer has multiple taps and the PS for each channel is independent. It really depends on the designer and design constraints.

The mono amp is more modular for sure. Charlie, king of the obvious.

Owl, I know he was yanking my chain, I am not that easily provoked,I'm just yankin' his...Gives something to do while we work....

Well I got a response back from Mike at the Outlaws,and it seems like I am going to be corrected,But the truth is on the DEMO,so I made my reservetion,and here is a copy of the response i recieved:Subj: Outlaw Audio
Date: 11/27/2002 2:30:53 PM US Mountain Standard Time
From: customerservice@outlawaudio.com
To: Eurorom2001@aol.com
Sent from the Internet (Details)



Hello,

Thanks for contacting the Outlaws!

The damping factor I sent you earlier was the specification that was
provided to me by our engineers. We saw your comments about an amplifier
with a damping factor 80 not being able to control a woofer properly. Quite
frankly this is just not the case. After your comments we noticed that
SoundHound responded and presented a URL of an article. He did not include
the full article. The full article can be found at the following URL:

http://www.studio-systems.com/audiofeatures/novdec99/dampingFactor/damp01.htm

This article describes what damping factors are all about and we hope that
you've had the opportunity to read it. Especially the conclusions in the
article. This amplifier has been tested extensively on a wide variety of
full range speakers. In particular, it has been tested on passive
subwoofers. There never has been nor would there ever be a problem
controlling any kind of woofer. For years there has been a tremendous
amount of misinformation about this specification and what it truly means.
Specsmanship abounds in our industry, quite frankly your pronouncement in
the Saloon that this amplifier would not control a woofer is just plain
wrong. After reading the article that we have referred you to, we might
suggest that you purchase a pair in evaluation and compare it to what you
already own. You will find out for yourself that the conclusions in the
article about damping factor and the performance in the Model 200 are
exactly consistent. There is no problem.

Please feel free to contact me with any further questions or requests.

Thanks and Regards,

Mike

lier.

So if the damping factor of the model 200 is 80, then it must not have the same ckt topology as the 755/770 or the manual for my 755 is incorrect when it says the damping factor is 850. Could the Outlaws clarify. Thanks...

And I got sucked in...

My marantz MA500's have two RCA jacks. One is for uninverted polarity and the other inverts polarity. You can use either depending on how you want the signal delivered.You can also run these amps in bridged mode and you would connect a signal cable between the inverted input of one amp and the normal input of the second amp. Perhaps the Model 200 operates similarly.

Here we go again. Get on a list and wait to essentially be Beta testers for a product that you have very little info on. They haven't told us very much about this amp. It could be a digital amp. PS Audio's HCA-2 has received some very good reviews but it doesn't seem to be selling well ie. there's quite a number of them for sale(used and new) on Audiogon with discounted prices on this new product. PS Audio has an excellent reputation. I read the aritcle about Damping and it seems contrary to another long article I read in the past on Damping which gave a formula to figure the damping you needed in your individual system. I got the article either here or at the "Spot".
It's hard to imagine how this thing would handle the heat produced if it was AB?? If they designed this to add channels to a home theater then it wouldn't matter as much. But using it in a stereo set-up would require it to have a bit of quality to it.

One person commented on 24 bit 192mhz not having much meaning. I have the 24/192 upsampler board on my cd player and when I installed it, it made such a big difference that I had to get used to my stereo set-up all over again. It was so different that I wasn't sure I liked it. I do now - trememdous sound stage.
Then what are you going to do when the hiss,hum and other noise issues come up?? I want a lot more tech info about the amp before I even go for a trial.

Steve:

I may only be a dog, but what information are you looking for that wasn't provided? The whole damping factor thing is the result of information that WAS provided. All the specs WERE provided. Again, I'm only a dog, but it seems to me that one has to presume it is a good, old fashioned, analog amp, as that is the rule these days. Digital amps are the exception, and if it WERE digital (which I'm guessing it is NOT), you'd have to believe that they would be up front in saying so.

I did I miss something, or are the Outlaws twisting your arm to buy this, or are people buying the products of their own free will?

Other products not withstanding, the Outlaws' track record on amps has been superlative, so I have no reason to doubt the quality of the Model 200.

But what do I know, I'm only a dog.

ARF ARF, says Iggy.

But what do I know,

[This message has been edited by Iggy The Dog (edited November 30, 2002).]

Iggy- They don't need you. They have plenty of "lapdogs" that get excited about getting on waiting lists for months for untested products(ie.950)
Questions such as output devices...what kind,bipolar or jfet...matched?. What is in the signal path...capacitors? What is the power supply like? The damping factor may not mean much but it is much lower than any amp I've ever heard of with that kind of power. What does that mean? Like I said, if you are just adding a 6th or 7th channel then quality might not matter and people won't more specs.

Steve:

I have heard from anonymous government sources that the 200 has a good deal of good 'ol class A/B happening in there.

I think it's irrelevant to most people in the real world what kind of transistors, capacitors, resistors etc are inside an amplifier. Just because a manufacturer uses a certain kind of part, and brags about it in spec sheets, doesn't guarantee the sound quality or durability of the finished product. In the end, trying out the product in your home system is the best course of action. Either it works for you in your system or it doesn't.

[This message has been edited by soundhound (edited November 30, 2002).]

I think what Iggy is pointing out is that Outlaw's track record with amps is pretty clean. The 770 and 755 launches were smooth, and their amps have all been well regarded as good "value" amps.

------------------
gonk -- Saloon Links | Pre/Pro Comparison Chart | 950 Review

Well guys I did not want to come across as being unhappy with the Outlaws equipment,however since there is not enough information out there to make an inteligent decision,that is way I opted to give out this information.My past experiences with amplifiers is they must have a higher damping factor to control the woofers,NAD rates them at 200,Parasound at 150,Adcom at 800.Carver Audio at 150,Outlaw at 800,but you do not make a decision based on Damping Factor alone!!You will consider sound,power supply size,peak amp to amp's and some more....I was just not expecting this kind of specifications,but i could be wrong!!!

Steve and all:

Now I'm only a dog, but you humans say that we pooches have good hearing. SO: Yes, while it does matter to some extent what type of capacitors or output devices a product has, or what the damn damping factor is, and so forth, BUT, at the end of the day, once you get past key parameters such as signal to noise and such, isn't it the way the amp SOUNDS that counts?

Steve, you sound as though you want to dismiss anything the Outlaws make just because, well, they're the Otulaws. Prehaps a calmer suggestion would be to look at the specs offered, match them against the price, see if the value sounds right, and then audition the product and see (hear!) if you like it. If you don't return it. If you do, forget about the specs and -- I shudder to suggest this -- listen to the music and enjoy the benefits that the product brings you, rather than worrying about the name on the outside, the parts on the inside, or whether your doggie dish is bigger than mine.

But then, what do I know, I'm only a dog.

ARF ARF says Iggy.

The damping factor for any competently designed solid state amp is more than adequate whether the number is 80, 800 or more. The static measurment numbers supplied as specs do very little to tell you how an amplifier will sound when inserted into a specific system. Only the power output number and how low of an impedance the amp. can drive may have some bearing on the sound if the amp is being driven to it's limit. Also, if an amp has a low-ish input impedance and is being driven by a tube preamp with a high-ish output impedance, there may be cause for concern.

So, if the model 200 is "only" speced at a damping factor of 80, I say OK, tell me something important now.

Paul

------------------
the 1derful1

Soundhound: I disagree with you in that audio/videophiles who visit these forums seek knowledge and want more info than cursory stuff like you get at CC and BB.

Iggy: There's not even enough information for me to want to go to the trouble of trying it at this time(Paul). The price gets me interested so I want more info. The specs on power supply, capacitors, output devices etc will give you an idea of the type of sound you can look for. Warm...Harsh etc.
As far as dismissing everything the Outlaws do...not true. I'm glad they are bringing these products to market to give an alternative to all the overpriced stuff out there. The reason I have Parasound equipment is the quality and value for the money. I am bothered by the marketing technique used by them. Create a lot of interest and demand for a product that's not even available. But it's been successful for them because they have all you followers out there that will defend them till the end. When I want to upgrade/build my home theater in the year 2002, I don't want to get on an indefinite waiting list. I don't mind waiting a couple of months if I know the products coming but who wants to do this (other than Outlaw followers)
If this amp is really "on the way" why isn't there more info on it other than rumors?

Steve in Sterling-- As always, your best bet for getting answers for specific (or technical) questions would be to contact Outlaw directly- especially in regards to new products where available information at this site is still limited. Best...

Hello Fellow Outlaws,

Since so many of you have speculated on some of the more technical aspects of the model 200, here is a bit more information.

The issues that we face with presenting more of this data are centered on the concern that some who profess to be knowledgeable in this area are actually drawing erroneous conclusions based upon what they have heard from others, often with no practical or engineering experience.So with that in mind, here is a bit more information about the Model 200:

First, we want to tell you that the particular engineer who designed this product for Outlaw has over 30 years of successful consumer electronics design experience. In particular his designs have been used in some of the most famous and well- respected names in our industry. Presently there are two extremely high end companies offering products with his amplifier designs.
The Model 200 utilizes a newly designed (and proprietary), hybrid Class A/B/Class G circuit. It will provide class A/B power up to 80 watts (which covers about 98 % of most listening situations). Above 80 watts the Model 200 will instantly shift into Class G amplification. This approach was chosen because of our requirements that this amplifier had to be quite compact (1 3/4 inches high) In addition we insisted that the amplifier would not use any fans. For those of you who are curious, the transition from class A/B to class G requires just two microseconds (NOT milliseconds),and (we know this question is coming) the transition itself is 100% inaudible. This was key to us in that some other alternative designs exhibit audible side effects. Incidentally at 200 watts the 200 has less than .05 % THD.

For output devices, there are a total of six: four bipolar transistors rated at 15 amps each and two robust power mosfets rated at 40 amps each Dynamically, into 8 ohms the unit will put out approximately 300 watts! In addition, it has at its disposal 20,000 microfarads of capacitance.

If we haven't said it before, the Model 200 will sound not only powerful, but neutral in its character. All in all, this is NOT your father’s amplifier

Regards,

The Outlaws!

[This message has been edited by Scott (edited December 03, 2002).]

My father's amps are BATs

Scott, thank you for the additional info.

Rich

[This message has been edited by RichW (edited December 02, 2002).]

Ha! Bat amps, schmat amps!

My father had a kenner close-and-play.

Not really, but he did buy a Magnavox monstrosity: console with color TV, AM, FM stereo, record changer. THE WORKS in a beeeeeeeeeee-uuuuuuu-teeeeeeee-ful console cabinet with fake (not faux, even tackier than faux) wood speaker grilles.

This was my introduction to the world of audio. This fine piece os @#%%$ made such an impression in me: I just KNEW that there had to be better than this and I started out to find it. The magnavox's video left an even more negative scar/impresion: one that I am just starting to recover from. Where's my therapist?

Oh, well. Back to the model 200.

Scott's latest response poses at least as much questions as it answeres. I clearly understand amplifiction classes A, AB, B and C. Beyond C, I have not kept up with the various classes because so many producs are mis-labeled as to what digital or analog trickery class-du-jour it is. The marketing department had as much, or more, to say as to what class the product would be alleged to be in as did the engineering department.

In regards to the model 200 and it's class AB mode of operation, at what voltage output level does it leave class A mode? I thought this would be a good point to sneak in a serious question.

I think a white paper on the model 200 is in order: to explain what class G is, at least as far as Outlaw Audio is concerned. If memory serves me correctly, essentially class G involves switching to a higher voltage power supply(s) when the output of the amp becomes big enough to require it. I vaguely recall several Japanese companies doing something along these lines in the late 70s to mid 80s. With modern power semiconductors, I would expect the result to be much better: the 2uSec switching speed seems to indicate this, as long as the magnitude of any switching glitches within the 2uSec time period are held to reasonably low amplitudes.

Class D is a switching amp. I don't know what happened to letters E and F. I'm shure that other letters are used, such as H and T, for other classes of amps. Well, anyways, the model 200 sounds like a very interesting product.

Paul

------------------
the 1derful1


[This message has been edited by Paul J. Stiles (edited December 03, 2002).]

VERY interesting. Now the discussions will start instead of "I wonder". And we'll all learn something. I second Paul with some of his questions. I would think that 80 watts of AB power would produce heat - especially if driven hard so I doubt you'd want to stack them.
Oct. 02 issue of Stereophile has an article by Sam Tellig about similar concepts in the PS Audio HCA-2.While it's not the same, it is good background info on these new generations of amps.
Sounds to me like it's an 80 watt amp with dynamic power up to 300 watts like you would get in an old Proton or an NAD amp today.
But it's getting interesting.

I'm just assuming this thing is still relatively stable into 4ohms, right? I don't remember seeing anything mentioned about it. Will it still put out something like 300W into 4ohms, and more dynamically, or is it pretty much maxed out at 300W, or...?

Jason

Hello Steve,

With all due respect, as one of our valued Saloon Members, you are drifting into areas where you are making judgments based upon speculation. This is precisely why we were somewhat hesitant to discuss our technological approach until the unit would also be available for audition. However, for the record:

1) This unit will put out 200 watts at less than .05% THD all day long if asked. It has the capability of providing 300 watts dynamically.

2) Please do not compare this amp to the PS Audio in that the approaches taken to these amplifiers defer significantly.

3) Some of the Outlaws have extensive experience in NAD and Proton amplifiers. As a matter of fact, one member of the gang is responsible for naming Proton. Working with NAD engineers, this same person developed NAD’s Power Envelope Technology.

That said, there is no relationship between the design philosophy of this amplifier and any NAD or Proton amplifier. We truly believe that instead of trying to second guess (or for that matter guess) what the engineers did, the best judgment you can make would be in experiencing this amplifier for yourself.

Best,

Scott

Hi Scott:

With all due respect, I wasn't making judgements, just guessing and wondering about thigs which goes on all the time in this Saloon.
#2 I stated that "it wasn't the same thing" in regards to the PS Audio. But now we all know.
#3 Proton(Ihave one) and NAD have great reputations - a feather in your cap.

This saloon is all about guessing and second guessing. That's what happens when you give out basic specs and not much more. Sorry if I touched a nerve, but you have given us a lot more info in the last couple of days.
I know anyone can try it and send it back but shipping is not cheap.
I guess(again) I'm the only one interested in this as there doesn't seem to be any other comments out there.
Thanks for the info you have given us.

Steve

Okay, Paul asked about amp classes... I second that request. I know (or think I know) about classes A, B, AB, and D, but I really have no clue about C, G, H, or T, or what other classes there are.

Could someone in the know post (or post a link to) a brief description of these types? It doesn't need to be too long or complex, just a few lines or so on what each type is and what it means to the sound quality.

I'd do some research myself but I don't even know, really, where to begin searching for information on amp types and what I do know is information I've kind of absorbed from others over the years and I'd hesitate to repeat it for fear of polluting someone else's idea of amp types.

------------------
Matthew J. Hill
matt@idsi.net

Matthew I did not take the time to really READ these and could not find anything definitive Like a Home theater primer type article will look more later. I would like to get these down. On a quick search I ran into engineering courses to register for…with out an explanation of the different classes this is the little I found quickly.
http://www.class-d-amplifier.nl/Classes.htm

http://www.eatel.net/~amptech/elecdisc/ampclass.htm

http://www.geocities.com/SiliconValley/2775/clsses01.html

Like SLL, I haven't really read thru this stuff, but the Rane audio dictionary gives a brief rundown of the various amplifier classes here http://www.rane.com/par-a.html#amplifier_classes

Class C is where the output device(s) are conducting less than 180 degress (less than half) of the input waveform. I think class C is mostly used in RF stuff.

Class H is: (longer explanation) Picture a power amp with +/- 50v supplies for the output stage (plus 50v and minus 50v). For a given voltage gain of the amp, the amp can correctly amplify increasingly larger input signals untill the output transistors are fully turned on. The amp. is on the verge of clipping at this point. The output voltage magnitude will be a little less than the +/- 50 volt supplies because the output devices have a minimum resistance and any current passing through this minimum resistance will result in a minimum voltage drop. If the input signal gets too large, the output signal will no longer increase, the output waveform will stay where it is (assuming a well behaved amp with response down to DC and ideal supplies for this discussion) untill the input signal comes down to a value that results in an output signal that is comfortably within the +/- 50v of the output supply. In class G, before the output gets too big, the output voltage is switched from the +/-50v supplies to larger supplies, lets say +/- 100v. Now, when the input signal is big enough to reqire an output of more than what the +/- 50v can supply, the larger supplies take over. If the switchover time is quick and smooth enough, the output waveform is disturbed minimally by the switchover from one set of supplies to the other.

The advantage of this is in the reduced heat produced by the amp compared to one that used the big output supplies all of the time.

The instantaneous heat power dissipated by an output transistor is the current through it multiplied by the voltage across the output transistor at any given instant. If we are using an amp with small output supplies we will produce less heat than an amp with big supplies. Of course, this amp with small supplies will deliver less maximum power to the speaker. If we switch in big supplies only when we need them, we will be producing "big supply heat" only when the output waveform is big enough to demand the big supplies. When we are playing music at a level that does not need the big supplies, we only use the small supplies and only produce "small supply heat".

So, a class G amp, when playing "quietly" produced "small supply heat". When playing loudly, produces a combination of "small supply heat" and "big supply heat" which, at the WORST case (say, producing full output square waves) will be about equal to "big supply heat" but will much more likely be closer to the "small supply heat" example when playing MUSIC.

A variation of the class G is where, instead of switching from small supplies to big supplies and back as needed, the supplies, when needed, vary as needed to allow the amp to put out an unclipped wave form. The output supplies track the input signal (above a certain level). The output transistors have enough voltage to produce an unclipped output but not a lot extra to waste as heat. This variation can result in even less heat but it is more complicated and tricky to execute properly.

Paul

------------------
the 1derful1

Nice Link Ellen, I'll have to spend some time in there.

Thanks for the link Ellen. Just in case anyone is interested they also have a great explanation/definiton of damping factor.

Scott

Those guys at Rane have quite the sense of humor. If you've never read the white paper for their Pseudoacoustic Infector, you have to. For more information, look up the term "Belchfire® Series" in their dictionary...

"For more information, look up the term "Belchfire® Series" in their dictionary..."

Old thread, new member, and I'm still laughing.

The Amplifier Section, Class AB and AClass AB and A amplifiers are similar, so we'll discuss both here. Class AB amplifiers have transistors that pull up to the positive rail and transistors that pull down to the negative rail. This corresponds to the action of pushing the speaker cone out and in.

Class AB means that the output transistors do not always have current on them. For example, when the upper transistors are pulling up towards the positive rail (pushing the speaker out), there is no current in the lower transistors. When the output signal swings through zero, towards the negative rail, the output transistor must go through a transition from zero current to a non-zero current. The best analogy that I can think of is driving an old car with too much slop in the steering. As you go from one side of the road's crown to the other, the steering crosses a "dead" zone, and you tend to over-steer. Special temperature compensated bias circuitry reduces this dead zone, known as notch distortion. The figure below shows the output of a class AB amplifier with too little bias and the resulting distortion. Notch distortion increases at higher frequencies and low volume levels. Some modern designs have reduced this type of distortion to very low levels.

Class A means that every transistor is always conducting current. They are very similar to class AB amplifiers, but the bias circuitry is set so that there are very high currents in the output transistors. Because these amplifiers do not have this "dead zone', less feedback is required to achieve low distortion.

A 100 watt amplifier may dissipate nearly 100 watts internally even when there is no audio output. This type of design is impractical in the harsh auto environment. Many class A amplifiers pedaled for the automotive market are not really class A. They are huge power wasters in the home as well.

Input and Driver Stages
The amplifier works this way: A small audio signal is presented to the amplifier's input. Transistors are not linear, which means that the input signal will distort somewhat as it passes through the various amplifier stages. To correct this distortion, a portion of the output is compared with the input. The difference creates a correction signal reducing this distortion. The input stage is a special type, called "differential". It has a + and a - input because it must accept both the audio input and the input from the feedback circuitry. Excess feedback can lower distortion dramatically, but cause instability. Careful design rules must be followed to avoid this instability.

The output of the input stage feeds into the driver stage. The driver stage may use one, two, or three devices. Often this circuitry is referred to "Darlington", or "Triple Darlington". The driver circuit feeds the output stage, which may have two, four, six, or more transistors. The more output transistors, the better. Multiple output devices reduce distortion (requiring less negative feedback) and improve reliability.

Bipolar or MOSFET?
We have seen both MOSFET (Metal Oxide Silicon Field Effect Transistor) and Bipolar transistors used in audio amplifiers. Claims have been made that each is superior. I have seen claims that MOSFETs have a tube ("Valve" for the Brits) sound. This is more folklore. The musicians and their instruments are supposed to have "the sound", not audio equipment! MOSFETs are tougher than Bipolars, and can pull closer to the supply rail. It takes more Bipolar transistors to achieve the same power as a MOSFET, therefore Bipolar amps tend to be more expensive. But, MOSFETs are very non-linear, compared to Bipolars and require much more feedback to achieve reasonable distortion numbers. They are a great choice for bass amps, as low frequency audio is not difficult for a MOSFET. The most expensive car and home amplifiers almost always use Bipolar transistors.

Efficiency
What makes an amplifier get hot? Both the power supply and the power amplifier generate heat. The maximum efficiency of the power supply is nearly 100%. Good power supply designs, with the highest quality components approach 85%. The class AB amplifier efficiency at full power can approach 75%. The total efficiency, including the power supply, can be about 65%. But, efficiency drops at lower power and can typically be under 20%. A class AB amplifier actually runs cooler at full power than it does at half power. Run this amplifier into clipping and it might run even cooler! Where is all this power going? The output transistor is basically a large variable resistor. If the instantaneous output voltage should be 40 volts and the power supply is 100 volts, then 60 volts must be "wasted" in the output transistors. Driving a reactive load (like a speaker) causes the efficiency to drop ever further. This brings us to the other audio classes designed to improve efficiency.

Class D
First, let's dispel another myth: Class D does not stand for digital. The input is converted to a two-state (binary) representation of the audio waveform. That's where the similarity ends. This distinction is important because class D doesn't provide the benefits normally associated with digital components.

That being said, class D designs dramatically improve efficiency. Instead of wasting power in the output transistor, the output is switched at a very high frequency between the positive and negative supply rails. If the output is to be zero, then the waveform is at a 50% duty cycle. If the output is to be a positive voltage, then the duty cycle would be greater than 50%. Because the output devices are either completely turned on (no wasted voltage) or completely turned off, theoretically efficiency is 100%. So the audio input must be converted to a pulse width modulated waveform (PWM). The yellow trace below is the output of the amplifier; the blue trace is the PWM waveform. The blue waveform is fed to an output filter, which results in the yellow output waveform. Notice that the output looks somewhat distorted. All of the switching noise and distortion cannot be removed and the result can be seen here. Because of this process of converting the input signal to PWM and converting back to analog, a good deal of distortion is introduced. Conventional feedback like that used in class AB designs is used in these amplifiers to reduce distortion.

MOSFETs are the only choice for class D designs. Most class D designs are useful only for bass amps as they can not switch fast enough to reproduce high frequencies. Some high quality, full range class D designs exist for pro audio, but they are complex with multi-phased outputs.

Class T
Class T (Tripath) is similar to class D with these exceptions: This class does not use analog feed back like its class D cousin. The feedback is digital and is taken ahead of the output filter, avoiding the phase shift of this filter. Because class D or T amplifier distortion arises from timing errors, the class T amplifier feeds back timing information. The other distinction is that this amplifier uses a digital signal processor to convert the analog input to a PWM signal and process the feedback information. The processor looks at the feedback information and makes timing adjustments. Because the feedback loop does not include the output filter, the class T amplifier is inherently more stable and can operate over the full audio band. Most listeners can not hear the difference between class T and good class AB designs. Both class D and T designs share one problem: they consume extra power at idle. Because the high frequency waveform is present at all times, even when there is no audio present, the amplifiers generate some residual heat. Some of these amplifiers actually turn off in the absence of music, and can be annoying if there is too much delay turning back on.

Class G
Class G improves efficiency in another way: an ordinary class AB amplifier is driven by a multi-rail power supply. A 500 watt amplifier might have three positive rails and three negative rails. The rail voltages might be 70 volts, 50 volts, and 25 volts. As the output of the amplifier moves close to 25 volts, the supply is switched the 50 volt rail. As the output moves close to the 50 volt rail, the supply is switched to the 70 volt rail. These designs are sometimes called "Rail Switchers". This design improves efficiency by reducing the "wasted" voltage on the output transistors. This voltage is the difference between the positive (red) supply and the audio output (blue). Class G can be as efficient as class D or T. While a class G design is more complex, it is based on a class AB amplifier and can have the same clean characteristics as well.

Class H
Class H is similar to class G, except the rail voltage is modulated by the input signal. The power supply rail is always just a bit higher than the output signal, keeping the voltage across the transistors small and the output transistors cool. The modulating power supply rail voltage is created by similar circuitry that you would find in a class D amplifier. In terms of complexity, this type of amplifier could be thought of as a class D amplifier driving a class AB amplifier and is therefore fairly complex.

How to Choose?
Regulated or unregulated? Class AB, D, or T?If you're really into a lot of bass, the class D or T may be for you as these amplifiers will produce the highest SPL with the smallest size. If you just want to wake the neighbors, blur your vision, or make a big splash in SPL contests, maybe you just need one of the inexpensive, powerful, & dirty class D designs. Want the cleanest high frequencies? Maybe a good class AB amp would be your selection. Whatever you choose, I hope this information helps you achieve the sound you're looking for!





Amplifier section.


Regulated power supply.


Output and power supply rails.


12 Volts, switched at 100 kHz.


Input comparator and driver stage.


Underbiased AB stage distortion.


The amplifier output stage.


Class D and Class T waveforms before and after the output filter


Class D/T amplifier.


Class G "Rail-Switcher"


Class H modulated supply.