Great topic. Please answer the following similar question too if you'd be so good:
For the same money, would you prefer Class A to 35W/AB to 100W -or- Class A to 10W/AB to 200W?
I have the former and can switch to the latter (same amp biased differently) for free in a reasonable timeframe - but then I am committed (will cost me $200 to switch back). Seems to sound great and slam plenty the way it is, but I wonder what is theoretically best in the opinion of the wise people here.
87db efficient 8 ohm speakers (min impedance 6ohm). |
This link may help - has description of class "A" and "B" amps. click here |
Depends on the designer. Example: Krell designs primarily class A, while Cello use to design class AB. Both are great!
Richard |
Try to find out what's the peak and continous current rating of each. Select the one with highest current capability. Most of the time the high current pure Class A amp will outperforn the low current high wattage Class A/B amp. |
Most amps that are rated for Class A operation are actually AB. They stay in Class A much longer than normal, so they refer to them as Class A rather than stating "VERY rich Class AB". Very "rich" AB amps stay in Class A for anywhere between 8 - 30 wpc ( some go a little higher ) before switching over to Class B operation for power levels beyond that point. To dissipate BIG wattage and run Class A would take MEGA heatsinking and forced air cooling if you were standing on the throttle for any amount of time. You will realize just how much heat you have pouring off of one of these amps as the ambient air temperature of the room will climb quite noticeably.
As to whether it is important that the amp stay in Class A for a long time, some people say that Class B sounds better. While there are technical reasons as to why this would be impossible, my ears tell me that the higher the amp is biased, the better it sounds. Keep in mind that high bias amps typically require very long warm up periods to sound their best and typically perform optimally after being on for quite some time. As such, i don't shut my amps off as i've found them to really "sing" after being on for about 2 - 3 days or so.
As to your original question, unless the circuits were identical and you knew the point that the amp switched from Class A to B, there really is no "best" answer. Sean
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I would have to agree with Audiomax....current drives the loudspeaker. Amperage, not wattage is the key. I have heard some rather gawdawful hi-wattage amps, and have also experienced the fidelity and depth of sound that that a high-current, though lower wattage device can provide.
If you can, try to listen to both amps under identical conditions...same program material, same preamp, same cables, same speakers. That is, if you can... this is not a perfect world. |
Try to find out the output at clipping. There are a lot of amps out there that will drive demanding speakers at > 4 ohms continuous output. The question is: how will it handle the short term dips below 1 ohm? Regardless of impedence ratings (which are at continuous output), all speakers will provide short term loads of under 1 ohm (drum beats, bass plucks, etc).
I just upgraded from a Plinius 8200 (175 Watts into 8 ohm) to a Plinius SA-102 (125 Watts into 8 ohm). While the 8200 is a great amp for the price, the SA-102 is heaven. According to the Plinius US Rep, the 8200 pushes about 1000 watts at clipping and the SA-102 pushes about 4500 watts at clipping! (my numbers might be off a bit, but they are in the ball park) When I installed the SA-102, the difference in slam, speed, bass and overall speaker control was dramatic.
What I don't understand is whether the increased capacity of the class A SA-102 is due primarily to the class A electronics, or are class and capacity independent specs? |
As an electronics engineer, I ditto Sean. There are few only class A amps out there and virtually none will do 100W in class A. The problem is the BJT saturation currents on the load line. The higher the bias, the smaller the swing allowable and hence the sooner the need to go class B, otherwise your THD goes up several hundred percent. Class B is generally twice as efficient as class A so heats the room half as much.
Capacity on the other hand, has very little to do with class circuit topology. It is dependant on number of BJTs handling current and their ratings. Class B has better current gain than class A but with the right transistors you can do the opposite. Take care. Arthur |
An amp with very high rail voltages that is capable of sustained amounts of high current would be best and most versatile. Some loads REQUIRE both aspects for best results. Such a design requires quite a bit of heatsink area though and is not cheap to produce in terms of transformer and amount of output devices. Don't under-estimate the importance of rail voltage in a power amp and simply look for something that is supposedly "high current". You might find something that sounds good, but it would limit what it could be used with in terms of future speaker purchases. Sean
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If you need the power due to inefficient speakers, lossy room and/or high desired listening levels then the higher powered class AB might be better. If you can easily reach desired listening levels in your room with your current equipment and 100 watts of power then you might prefer the lower powered class A amp. The class A amp will probably use more electricity, require more space (may require different cabling) and create more heat. |
This is all very interesting, but why don't you just listen to the two ? I would imagine that this question cannot be answered theoretically since it depends on how loud you listen, what type of music, and your preference for the type of sound. |
Krell's FPB series amps are designed to run at class A to full power. This might have something to so with why they named them as they did: Full Power Balanced
Richard |
I would go for 200w class AB as long as you have at least 8-10w Class A....If the 200w amp has less than that it will likely sound bright after it blasts through the class A region and say a female vocalist is screaming away at the top of her lungs....We at CTC just finished a huge mono amp which runs over 25w class A and, other than running at over 50 degrees C, it measures beautifully when in the class A region with just 2nd and 3rd harmonics and starts picking up some 5th as it crosses over into past 25w up to 400+w or so.....Lots with tradeoffs in amp design and hard to get everything right so we concentrate on the area at 10w and under where most all the music, other than atomic blasts, will be played..... |
I'd reckon that the biggest question is what is the impedance of the speaker? Since this fellow is deciding between two amps from the same mfr, the biggest diff will come IF the "class A" amp slides into AB because of a low Z load. In which case it might just put the same power into the load as the AB amp from the same mfr.
The circuit is likely to be identical, just the rail voltage will change - assuming the description is accurate, the 100w class A amp *ought* to have *higher* rail voltage than the 200w AB amp -as the load causes the amp to slide into AB the advantage of the higher rail voltage will be more output available than with the nominally higher rated 200W AB amp!
:- )
Of course into a calm 8 ohm speaker - go with the 200 watt AB, IF the criterion is only max output with an inefficient speaker...
It also matters what the output devices are. Mosfetters and BJTs act a bit differently in AB vs. A.
On the other hand, with two amps that use the identical circuits, I'd pick the one with the better power supply regardless... :- )
_-_-bear
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Despite all the well-intentioned responses dealing with the theoretical above, I have to agree with Seandtaylor99. You already have your speakers chosen, so your mission is quite clear: find the amp that sounds best with your speakers. If you've already determined that Lamm is the brand you're most interested in, then you need to try and audition both the amp models in your system. One of the two will no doubt win your preference. Nobody should have to drop the kind of bucks these electronics cost without being able to hear them at length at home first. (You might also pose your question to the respective manufacturers of your speakers and desired amps just to get their takes on the issue, but you'll want to be prepared to give them more info than you've given us, such as room size, ancillary gear and cables, musical preferences, and listening volume preferences.) |
If the FIRST watt is not RIGHT, why bother!!! |
Interesting point Audiomax. Too bad more companies don't print max current. Creek would advertise peak current of over 30 amps I think but never in their specs. I can subjectively agree with Drrdiamond about Krell as their integrated is punchy.
Musical Fidelity now publishes this info:
A3.2 integrated 115 watts: peak to peak output current 24 amps.
A3.2 power amp 130 watts:72 amps
A308 integrated 150 watts: 48 amps
A308 power amp 250 watts: 72 amps
From listening, the higher current gives a crisper sound. Also there is more detail possibly because the high current fully magnifies every small detail.
I can also subjectively say going from N804 to N802 and ATC active 20 to ATC active 100 the bigger speaker presented more detail. Again, I think because of the higher power and ease of presentation that a larger speaker can do.
This is all a little off track but hopefully interesting. |
Sean, what is a high rail voltage? What are the typical ranges? My amp runs at 60 volts and I'm wondering if that's normal, low or high compared to others. I love the sound, so I'm assuming it's high ;-)
Great posts already. Much depends on how you listen to music -- e.g., quiet or loud. Much depends on the efficiency of your speakers. An AB design done right can give the best of both worlds. A pure A design can sound fantastic in a great design with high efficiency speakers. |
tank's guys for the input ,i think i will have to listen to both of them in my system before i make a choice
enjoy the music |
Ummm that 72 amp "peak current" thing is a bit, shall we say, "optimistic."
Let's take a look at it. In order to get 72 amps drawn by a load it has to fit into the P = I^2 R formula. So, 250w = 72^2 * R - where 250 watts is the max output of the amp, that gives us a load resistance of 0.048ohms. That's about as close as you can get to a dead short... Ok, ok, so you say that the amp can really put out more than 250 watts? Let's look at what the rail voltage is likely to be. It's rated at that power at 8 ohms, so P = E^2 / R... 250 = E^2/8 or E^2 = 2000 or 44.72volts... that's the RMS value of one rail's voltage. The DC rail voltage is about 1.414 times that or 63vdc. Assuming few losses in the output stage the loaded rail voltage at 8 ohms must be at least that, so let's set it a bit higher to take losses into account... about 70vdc. Now we know something of use. *IF* said amp *could* put out 72 amps for *any length of time* at all, then the max wattage required would be 72amps x 70vdc =
5040watts. Ask yourself how many amps have power supplies anywhere near this size. Even if there is a fairly large power supply bank, this sort of current can only be sustained for an incredibly short time by an amp that does not have a huge power supply. I doubt strongly that the MF line has that much supply in it.
BTW, IF it could handle a 2 ohm load, it would have to chunk out 968 watts, and if a 1 ohm load was there, only a mere 1936 watts. :- )
My Symphony No.1 Amp *does* have a huge power supply. It uses 4kva worth of transformers in the original configuration, along with 500,000ufd of filter caps AND 140 amp Hexfred rectifiers. The amp's power section is fed via 10ga SPC wire. The output stage is modest, using only 6 Hitachi metal case Mosfets per RAIL (12 per channel) each of which is rated at 7 amps continuous = 42 amperes per rail, or 100 watts each = 600 watts max continuous. Hitachi rates them at LESS than that for a 10ms 1 shot with rail voltages above 50 volts! BJTs have similar deratings. This amp is a 180 watt per channel rated unit with 60vdc rails.
This amp weighs 128 lbs., none of which is "filler," it's made from aluminum, not steel. Most of it is power supply components.
You can get more power from a smaller, lighter switching supply, but then you are depending upon the *line* stiffness to control the supply and provide the required current on peaks - there are power factor issues there. Halcro has to deal with that sort of thing...
Anyhow... "peak" anything is a very questionable specification, FYI. |
I've always preferred the approach of
1) listen
2) try to use the theory to explain the differences
rather than the other way around. If you do it in the other order then it's too easy to overlook something that might render your theory inapplicable in this particular situation (unless you're really a very good audio engineer, with detailed knowledge of the makeup of EVERY component).
Plus at the end of the day you have to listen to the thing !
Thanks all for the interesting thread, though. Since I studied basic transistor amp design in college and built a few it's always nice to read some more discussions on the subject. |
Thanks for the lesson in physics Bear. You did a wonderful job on that one : ) Besides that, i can see why you are called "Bear". You must be big as a grizzly to tote amps like that around. Those things had better have some hellaciously well made and anchored rack handles is all that i can say. : )
Ozfly: I would say that somewhere around 45 - 60 volts would be about average for most amps. Some may go marginally higher than that. When you start getting up around 80 - 90 volts or so, i would consider that a high rail voltage. Some of the newer switching type amps actually go up quite a bit higher than that from what i've read. Sean
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Bear, I like the reality check. But Musical Fidelity rates it 72 amps "peak-to-peak output current". Is this some trick to make things sound better than they really are? Like NADs "dynamic power"?
Sure sounds a lot more impressive than their 3.2 integrated's measly 24 amps "peak to peak". |
