>> 01-19-06: Aball
>> No. The sonic benefit is that the entire signal is
>> swung by a transistor (or several in parallel) instead
>> of individual halves of the signal being split up by
>> two different transistors. Since there is no "handing
>> off" of the signal to another transistor, there is
>> potentially less distortion.
I hate to say this Aball - what you have explained above is single-ended operation & not class-A operation!
it just so happens that many/all single-ended tube & s.s. amps are class-A.
For class-A operation the transistor is biased along the load line in such a way that it always has max current flowing thru it whether a music signal is input into it or not. hence the heat dissipation.
s.s class-A amps can be single-ended (Aleph series) or can be push-pull (almost every other model).
In push-pull you will have P & N type transistors but it is not necessary. For example the Plinius SA-250 has N type push & N-type pull. Slightly higher output impedance i.e. lower damping factor but all N types can be matched better.
I'm sure that you know all of this as you are trained as a EE, I think? Probably wrote your post in a hurry or while cramming for an exam the next day?
FWIW. |
The Pass XA.5 series is single-ended, class A, and doubles down going from 8 ohms to 4 ohms. |
Spear11,
It depends on the amplifier; the regulation of its voltage and its current output capacity.
We've been discussing amps that "double down" - those that will double their power
if the load impedance is halved. If we let V be the RMS voltage; then we can use
equations for power that look like the DC equations - but the use of the RMS values
make them applicable for AC power.
For a voltage source, the power P = V^2 / R; or P*R = V^2 = a constant if V is strictly
regulated. If the amp is rated at 200 watts at 8 ohms, then V^2 = 1600 volt^2. For
your 3 ohm speakers, P * (3 ohms) = 1600 volt^2.
Therefore, P = 533.3 watts.
Now this also presupposes that the amp has the current reserves to fulfill this. For
200 watts at 8 ohms; the current is given by P=(I^2)R; so that the current I = 5 amps.
For 533.3 watts at 3 ohms, the current will be 13.33 amps.
IF [ big IF ] the amp has the current reservers to output 13.3 amps, and the voltage is regulated to be the same irrespective of load impedance; then this hypothetical amp
can put 533.3 watts into the 3 ohm load.
Dr. Gregory Greenman
Physicist |
It depends on the amp. In theory if it is a solid state amp we would hope to see 600 Watts @ 3 Ohms if the amp is rated as 200 watts @ 8 Ohms. This generally doesn't apply to tube amps. Not all (in fact few) amps are capable of this kind of power delivery. Don't assume an amp is capable of this. |
Charlot
To keep it simple - the benefit of the Class A amplifier is in it's sonic product! This is one of those things that is ineffable and must simply be heard and compared.
I have been listening to Class A amplification since the late 1990's and have no complaints. In 2005, I built a 1 watt SET that currently supplies my sonic entertainment.
Keep in mind that transistors as well as tubes can run in Class A. In fact, transistors in Class A come pretty close to doing tube-like stuff sonically.
Hope this helps.
Studio1 |
If an amp is rated 200watts @ 8 oms and 400 watts @ 4 oms what is the 3oms rating.
The reason I ask is Im trying to match components with Thiel CS 2.4 speakers I just purchased.
Thanks,
Dave |
Atmasphere, thank you for your explanation. Although I'm no EE I think I understand what you're getting at. I will have to try a suitably high quality and high power SS amp on my panels to see what it does to the treble.
Yada, currently I am using the Odysseys; their midrange is simply excellent, and they have good dynamics, but (as mentioned) they are not the equal of my previous ribbon speakers in the topmost treble. (Overall a much more musical experience; it's the midrange that counts the most!) |
Calanctus, Your speakers are a good example of what I was talking about. ESLs in general, regardless of their particular impedance curve, want to see the same power for a given sound pressure regardless of what their impedance is at a particular frequency.
In effect, they are driven by power.
If an amplifier driving speakers like this puts out a different amount of power into different impedances, the effect is a shift in tonality. This is why transistor amplifiers (in general) tend to be very bright on ESLs, as their 'constant voltage' characteristic causes them to put out a lot of power into the low impedance of the ESL at high frequencies. Some speakers (for example B&Ws) are designed to expect a 'constant voltage' amplifier, and so will have flat response in the room. This is much trickier with ESLs!
In essense, there are two paradigms competing in high end audio today- the 'voltage paradigm' and the 'power paradigm'. No-one talks about this!- but we see it all around us:
The Voltage Paradigm is the reigning test and measurement paradigm. It is only concerned about voltage, and so when measuring an amp or speaker, voltage is the only thing considered. 'Constant voltage' output refers to an amplifier that puts out constant voltage regardless of load (contrary to morbious' comments, this does not require power supply regulation to accomplish this- merely enough feedback). Such an amplifier is regarded by these rules to be 'load impervious'.
The problem lies in the fact that negative feedback runs counter to how our ears detect volume (higher odd orders being the key- these are enhanced by negative feedback). Additionally there is the question of whether or not a speaker is driven by voltage or power, and of course the answer is power. So the 'power paradigm' says that the amplifier will have little or no feedback to reduce the odd ordered harmonic that the human ear dislikes, and that the amplifer will produce (or attempt to produce) constant power into all loads. This, BTW, is very nice for ESLs, horns and magnetic planars. Amps that fall into this category are SETs, some OTLs, transformer coupled push-pull tube amps and a very small number of transistor amps with zero feedback. Such amplifiers typically have higher output impedances, and nearly all will be unsuccesful at the goal of constant power (in fact many designers of such gear may not even acknowledge that they even *have* such a goal).
A parallel controversy is the subjectivist/objectivist debate. Roughly, the power paradigm is more subjectivist and vice-versa, but this is *not* by any means cast in concrete!
The power paradigm has its roots much further back than the voltage paradigm (1920s), and only seemed to resurface in the last decade or two. The obvious proliferation of advanced SETs (and tubes in general), horns, single driver full range speakers and the like are an indication that no-one has all the answers. |
Atmasphere,
Careful here. The ability of the ss amp to "double down" is
really a function of the regulation of the power supply in
conjuction with its power reservers. This is not really
negative feedback.
Additionally, it's the feedback on the gain stages for the
signal that can have a deleterious effect on the sound
quality.
One can design a zero-feedback gain stage that is fed by
a well-regulated power supply that has, in essence feedback.
However, the sound of such an amp won't be compromised.
The only drawback is that the amount of headroom in such
an amp is not dynamic. As you pointed out, it doesn't
increase in power for short transients.
However, I'd wouldn't count on the dynamic headroom in any
case - I'd size the amp so that the continuous power level
meets my needs.
Dr. Gregory Greenman
Physicist |
I hate answering my own questions but after several attempts to get Klipsch to answer with regards to my Palladium 1Ks running 125 watts in class A (and threatening to call Mondial directly). I finally got a response. A completely unqualified yes its absolutely the truth. I still find it kind of hard to believe given the amps' weight (each monobloc is about 70 lbs) and temperature, but that was a major company going on record by e-mail that it is true. No mention that you need to be playing them into 8 ohms but I'll save that for another day. Thanks for your patience and to you other secret Palladium owners out there in audioland now you know. |
Calanctus, I use Martin Logan clsiiz and Krell 350mcx. What a coincidence to this thread topic. What speakers do you use? I was wondering the same thing as you after reading this thread! |
Atmasphere, thanks for your informative comments. But I'm not clear on why it is unacceptable for some speakers to have an amp that increases power into a lower impedance load. My Martin Logans are superb, but among their very few weaknesses is the fact that they roll off the topmost treble...and the impedance in the treble is 1 ohm or less. Surely an amp that can deliver more power into this tough load has the potential to provide better treble performance...? |
Atmasphere, thank you. I was aware of the above. I think your point about amps increasing power not necessarily being a good thing is well taken. As you well know as an amp may increase its power out-put into lower impedances, it will also decrease its power out-put into higher impedances. As you pointed out with some speakers that vary a great deal in impedance load this will have a rather negative effect. On the other hand speakers with steady low impdeance loads will benefit from such an amp. Once again its about using the right tool for the job at hand. |
Unsound, just so you know, Class A has nothing to do with push-pull vs single-ended. Also, a Class A amplifier's peak power will always be the same as its continuous power, IOW the 'headroom' rating will be 0db. An amp that has more that 0db headroom is likely an AB amp.
Krell's sliding class A approach (a similar form of that being first used by Brooks in the 50s if I recall right), allows the amp to run cool at low power. On account of its power supply regulation it should exhibit 0db headroom.
Changing the load impedance changes everything! Contary to popular belief it is not always a good thing that the amplifier power changes as the load impedance changes. There are speakers that are of course designed for this, and there are other speakers to which this behaviour is unacceptable (i.e. electrostatics, horns and full-range high efficinecy drivers to name a few).
This ability of transistor amplifiers has to do with feedback just as much as the fact that they are solid state. The effects of Negative Feedback in an amplifier runs counter to the way our ears have evolved over millions of years. As a result it is one of those things that we should always question. Once you do so *honestly*, a lot of other things get questioned too, but ultimately the right answers also yield better sound. BTW these comments have nothing to do with transistor vs tubes! -although many will think that they do. The underpinning issue is more fundamental. |
Onemug
I agree with you that in the end its the music not specifications or circuit toplogy that we listen to. And one must keep ones eye on the ball when reading and comparing specifications when trying to make a purcahse decision. But it does lead to some interesting conversations and I really enjoy the learning that goes along with it.
Chuck |
Interesting reading, and discussion men. Thank you. Now I know why I love my FPB400cx! The "real" Krells do sound wonderful on every type of music in my system. :-) |
Onemug, may have a point. Bear with me for a moment and forget the "most speakers are damaged due to being under powered rather than to being over powered". What if a speaker manufacturer refused to honor a waranty because one used amplifiaction that was rated within the speaker manufacturers standard 8 Ohm limits but actually exceeded its 8 Ohm rating? |
What caught my eye in the Stereophile measurements was paragraph 3 where they state it's a 935 watt amp continuous, almost 30db. It doesn't put out 1870 wpc into 4 ohm or 3740 into 2 ohms continuous. "However at it's rated power of 600 wpc, which is 28 db, it would hold that rating for all loads from 8 to 2 ohms". I read about many an amp that put out 100 into 8 and 150 into 4. What if that manufacturer decided to rate it at 75 into 8, even though it put out 100? Then he could say it "doubled down" to 150 into 4. Impressive little amp then. Nobody is going to sue anybody if they buy a 75 or 600 watt amp and it puts out 100 or 935 respectively. Gee, I got more than I paid for. But it's the wordage I look at. Doubles its "rated" power is honest but to me, it is what it is, and if its "really" a say 250 watt guy, I'd be curious how close it came to 500 into 4 but most important is how it sounds into 4 or 2 ohms. Some amps start to get a little unhappy down there. If it makes someone happy to believe their amp doubles down that's fine but I don't listen to specs so my criteria is just "does it draw me in". |
Moribus, thank you. That really clarifies it. Am I correct in assuming that power increases by ss amps into lower impedances is not determined by Class of operation? |
Unsound,
The circuit topology of the Krell FPB series is Class A.
Class A amps don't automatically resort to A/B when they
are asked to output large amounts of power.
No - that is a characteristic of Class A/B amps that are
designed to maintain Class A at low levels - and then they
revert to what they really are Class A/B.
However, a true Class A like the Krells doesn't automatically
revert to A/B at high levels.
Dr. Gregory Greenman
Physicist |
Unless I missed something, everything in these aritcles suggest that the Krell's are capable of maintaining Class A operation as they double down, but, they don't actually make that claim. As for me, I believe they do. Wretched excess? I love it! I may be part of the minority here on Audiogon, but I really like the "real" Krells and thats based on what I've heard not on what I've read. Though it is pretty impressive reading. |
Unsound,
You can also check out the reviews by Stereophile, such as
Martin Collums April 1997 review of the then current FPB-600.
http://stereophile.com/amplificationreviews/829/
In the Measurements segment, Martin addresses the power that
the amp can deliver into low impedances:
http://stereophile.com/amplificationreviews/829/index8.html
"I wasn't able to run my long-term continuous testing at
2 ohms, but compromised with five-second bursts-long by
peak-measurement practice (eg, 20ms). The FPB 600 could
sustain a 29.3dBW level into this load, corresponding to
3.4kWpc-an extraordinary figure."
Dr. Gregory Greenman
Physicist |
Unsound,
The Krell FPB "Class A" series maintain Class A operation
from 8 ohms to 2 ohms.
They have well regulated power supplies that have the current
capacity to deliver the current that low load impedances
require.
Given the well regulated nature of the power supply [i.e.
the ability to put out the rated voltage independent of load ]
and the current capacity to back it up - that's WHY a
Class A Krell will double down.
Dr. Gregory Greenman
Physicist |
Chuck, I think your right about single ended amps being less likely to produce as much power as push/pull, but, when it comes to solid state the differences aren't as great as in tube amps.......
I'd still like to know if those ss amps that are touted as being pure Class A and being capable of doubling down, maintain pure Class A operation as they double down. |
Unsound
I agree and I didn't mean to suggest that single ended amps couldn't produce high power what I mean't, and I may be wrong here, is that they don't produce the power output of a push-pull design e.g. Pass xa versus x series amps. And yes I remember reading the Stereophile article concerning the power output of the XA200 at least I think it was the XA200 falling short of rated output.
In general, and my logic may be off here as well, I think of class A amps (SET vs. Push-Pull) like I think of tube amps (SET vs. Push-Pull). SET tube amps typically have power outputs in the range of 3-50 wpc while push-pull varieties have power outputs in the range of 50-500 wpc. Anyway sure is fun to have these discussions as I usually learn something in the process.
Chuck |
I shouldn't have singled out Monarchy for "confusion" with re: to rated power out-put. One of Pass' amps failed to achieve rated power during a test by Stereophile. |
Chuck, even single ended Class A amps are capable of respectable power out-put. The solid state single ended Class A Pass Aleph 1's power ratings: 150 Watts > 8 Ohms, 300 watts > 4 Ohms, 600 Watts > 2 Ohms. Of course these amps were expensive exceptions to the norm. The less expensive Monarchy solid state SE 100 single ended Class A power amps power rating: 100 Watts > 8 Ohms, 200 Watts > 4 Ohms. The Monarchy hybrid tube/solid state single ended Class A SE 160's power rating: 160 Watts > 8 Ohms, 320 Watts > 4 Ohms. Needles to say I can't personaly verify these quoted ratings. There does seem to be some confusion about Monarchy's most recent 250 Watt Class A/AB hybrid's power ratings. |
Unsound
That would make sense given the power output. I am sure the Gryphon is as well. It is interesting stuff as I have always found circuit design especially for audio interesting. I majored in Mechanical Engineering as I never had a mind for electronics even though I do enjoy it as a hobby.
Chuck |
I'm pretty sure its push pull. Other than Pass and Monarchy I don't know of any other single ended Class A solid state amplifier manufacturers. |
Unsound
I went to krells web site and he states class A topology. It would be interesting to know if it is push-pull, single ended or what specific design they are.
Chuck |
Chuck, all early Krell's were Class A. Some newer ones (IMHO the only real Krells) are as well, but, use their proprietery sliding bias scheme. I still think these sliding bias amps can make good on their Class A claim. The very early Krells used fans, the later ones used massive heat sinks and the newer ones sliding bias, (still pretty hefty heat sinks as well). |
Aldavis
The attached website (http://sound.westhost.com/class-a.htm) has a pretty good explaination of class A amplification form a lay persons perspective. I believe the Gryphon is designed as a push-pull topology (unsure) versus the Pass which is a single ended topology and depending on ones definition of pure class A the Gryphon may or may not fit the bill. But again I am not an expert on the subject.
Unsound I believe krell is a class a/b design
Chuck |
Wow, I wonder if Ken Stevens would upgrade my CAT amps with that APS technology! Oh yes, the 1970s and their super huge spec sheets. Those were the days when all amps measured very differently but still sounded the same. And now today everything still measures very differently but sounds wildly different. Hmmm, what happened? Man oh man I miss Julian Hirsch. |
I have a question whether or not the amp I own differs from those statements made above. Can someone tell me whether my belief that my amp runs Class A all of the time is correct or not. I have had one or the other of these units for years and always thought such was the case.
I own a vintage (1979) Yamaha MX1000U amp and had the MX10000U prior to that. Yamaha states that their Hyperbolic Conversion Amplification Circuit (HCAC) "eliminates the only serious drawback of previous Class A operation amplifiers. These had to switch to non-linear Class AB operation above a certain load current, causing a slight deterioration in the otherwise excellent sound quality. The HCAC does away with this problem, allowing the Yamaha MX1000U to deliver superior performance constantly, without switching or cut-off, over its entire power range.
Advanced Power Supply Circuitry (APS) power supply is also incorporated in the amp. The problem with conventional power sources was their tendency to produce pronounced voltage ripples during medium to large amplitude music signals, particularly under low impedance loads, together with voltage fluctuations in the power transistors. APS solves this problem by providing an active power source that ensures stable voltage in the final transistor stage under all operating conditions, thus greatly improving power supply to the amplifier."
Minimum RMS Output Power Per Channel:
0.003% THD, 8ohms - 260w
0.007% THD, 6ohms - 295w
0.02% THD - 330w
Dynamic Power Per Channel (IHF):
8ohms -- 450w
6ohms -- 560w
4ohms -- 740w
2ohms -- 960w
1ohm -- 1000w
Dynamic Headroom:
8ohms -- 2.38dB
6ohms -- 2.78dB
4ohms -- 3.51dB
Power Band Width: 0.03% THD 130w, 8ohms -- 10Hz-60kHz
THD Main In to Sp Out, 130w/8ohm = 0.003%
Intermodulation Distortion (rated output/8ohms): 0.003%
Signal to Noise Ratio (IHF-A-Network)
Main In 9 shorted) -- 126dB
Main In (5.1 k terminated) -- 122dB
Residual Noise (IHF-A-Network): 21uV
Thanks
|
Real Krells can double down and down again from their rated power. I'm not sure if they maintain full Class A operation as they do. I sure would like to find out though. |
Czbbcl , that is not a characteristic of class A amps although this may be a limitation of Pass designs. As stated above Gryphon class A amps double output from 150W @ 8ohms all the way to 2400W @ 1/2 ohm (5000W peak). In the real world your home electrical circuit will be the limiting factor with well built true class A designs with HUGE power supplies. - Jim |
I think I read somewhere that this is acharacteristic of pure class A amps in general and that it also applies to tube amps. If I remember correctly, and I am getting old, I believe CJ told me that it was a characteristic of my premier 11a, but I am not sure. The topic came up with CJ when I was asking them questions concerning tube amps and driving thiel speakers. I maybe speaking to much to generalities and not enough to specific circuit topologies.
Chuck |
highly biased Class A operation This is probably Mr Pass' way of saying the device is operating in "deep class A". Just the PS is well over the expected 4x the energy output rating fro class A. |
Onemug, Jon recently told me the same thing re: a different Class A/AB Threshold amp. |
Chuck, thanks. It's interesting that Pass suggests that "These are not design oversights or flaws in the XA-200, but rather a condition of the very highly biased Class A operation." I don't recall this "reasoning" from some other Class A amplifier manufacturers. It may even contradict what is commonly percieved as techincally available from Pass' previous designs. |
Unsound
Here is the exact information fronm the pas web site I quoted.
"The XA-200 will deliver 200 watts rms into 8
ohms of impedance. This is the amplifiers class
A limit, driving the XA-200 harder or reducing the
impedance of the load will not convey any additional
power. Driving into a short will not convey any
additional power. These are not design oversights or
flaws in the XA-200, but rather a condition of the
very highly biased Class A operation. However, as
an interesting point of reference, 200 watts driving
a 87dB/1W/1m speaker will deliver a 112 dB (very
loud) average acoustic signal in a 100 cubic meter
room. Transient peaks will be somewhat higher."
Chuck |
Unsound, I remember having a conversation with Jon Soderberg, formerly with Threshold and now a top repairman for the older amps. He told me that on the SA-1 that I was actively pursuing that it would double in power into 4 ohms but that the Class A amount of it would be cut in half. Whatever the Class A power was into my Apogees, it was really sweet. |
Hi Chuck, thanks for the update. I went to the Pass site. I didn't see anything about power into lower impedances, but, you may very well be correct. There was mention of the XA series not having the current delivery of the X series and that the XA's using only two gain stages like the afore mentioned Alephs'. |
Unsound
As I understand it (from information gathered from the pass web site) the X series (class A/B) does double down but the XA (pure class A) does not. And as someone mentioned class A amps tend to be sweeter more detailed in the midrange.
Chuck
Chuck |
Chuck, I was under the impression that some particular examples of the Aleph amps with minimal gain stages were unable to double down, but, all the other Pass amps were able to double down. I would guess that those which could/can doube down would halve their class A out-put as they did so. |
As "Aldavis" points out, size and heat indeed. At 145 lbs and 16 power tubes each, with a conservatively rated 150w Class A, the CAT JL-3 monos do a wonderful job doubling as floor heaters. |
Hey frankg how do you like the encore ? I've never compared it directly to the signature but I bet it holds up pretty well.- Jim |
Hi Al. The Gryphon Antileon Signature Stereo puts out 5000 watts at 1/2 ohm. The mono's put out 7500 watts at 1/4 ohm.
Frank
P.S. I have the Gryphon Encore. 500 watts at 8 ohms and the first 30 is class A. |
Another aspect to consider is that Single Ended class A amps do not increase their power output in lower impedence loads e.g Pass XA series amps. Which is why you need to consider the speakers efficency in most cases when using pure classs A amplifiers.
Chuck |
If its not real heavy and real hot its not running an appreciable amount of class A power. I'm not kidding about the hot. My amp ( gryphon antileon signature ) gets so hot you can not keep a hand on it for more than a few seconds when its really cranking. It runs 150W pure class A into 8 ohms 300w into 4 ohms 600w into 2 ohms and 1200w into 1 ohm and 2400w pure class A into 1\2 ohm ! To do this it requires 200 lbs of amp/heat sinks. Any amp that isn't very heavy can not dissipate the heat produced from this output. This is an inherant drawback of inefficient class A operation. Beware of "class A" amps with A/B output stages. These are not class A amps. Reguarding the sound , it is up to you to decide. I love it. One thing is for sure. It is uniformly stable no matter what load it sees. Good class A amps have ENORMOUS power supplies. I'd be surprised to hear of a class A\B that doubles down to 1\2 ohm. So are they better or more powerful ? It depends. If you have speakers whose impedence dips appreciably then they are probably functionaly more powerful. If you have lots of room, dont mind the heat or the aesthetics of a really big amp with questionable waf then yes I think they also produce better sound. This is just my opinion. - Jim |
