How Much Difference Does a More Powerful Amp Make?

06-23-11: Raquel
Bombaywalla:

I was trying to keep it simple to make a point.

ok, thanx for the clarification. I now know where you were coming from.

Most amps, powerful or not, have anywhere from three to five gain stages (very few have two),

OK, if you say so. i will not contest this as I certainly do not know this for a fact. I have not popped the lid of various amps nor studied their schematics to know any better. So, i'll take your statement at face value....

and we all know that using global feedback with such designs makes the amp builder's job a lot easier.

there was never a contest here - we are in full agreement.

I do not agree that feedback ultimately lowers distortion

I think that you should! ;-) any electronics book that discusses feedback will discuss 'feedback effect on distortion' & you will see mathematical equations showing that negative feedback does reduce distortion.
Having said that, in audio where we are talking about human hearing which is very sensitive to distortion components, I tend to agree with Pass' following statement (taken from your post):

Negative feedback can reduce the total quantity of distortion, but it adds new components on its own,...

I believe that you are in agreement with this statement as well.
this might be a better way of stating the distortion, negative feedback relationship.

Raquel - I've never heard either amp that I mentioned but specified S/N=120dB was confirmed by Stereophile measurement and audition of Coda S5. It is dead quiet and very resolving. As for Ralph amps - reviews praise them just for being quiet and very resolving. Putting two tubes in parallel doesn't make it less resolving or noisier than one tube. Part of SixMoons review of Atmasphere MA-1:

"Compared to my BAT VK-75, the Atma-Spheres just offered more of everything: More detail, more extension, more transparency, better soundstaging, greater micro and macro dynamics, more extended and luscious high frequencies"

You assessment of matching output devices is not accurate because it is easier to get average characteristic by connecting many output transistors in parallel. Linearity also improves since each transistor works at smaller current range.
Noise voltage that limits resolution of bipolar transistor is defined by resistance of the Base and resistance of the Emitter that can be reduced by reduction of emitter current (inversely proportional) while Base resistance become dominating reason for the noise. Placing transistors in parallel not only reduces emitter current but also reduces resistance of the Base (in parallel). Placing many bipolar transistors in parallel is well known noise reduction technique in microphone preamps.

As for negative feedback - It is not the complexity that screws up the sound but lack of bandwidth. Limited bandwidth amp delays the signal (shifts phase). When input signal changes fast, feedback loop is momentarily open creating overshoots (or odd harmonics in frequency domain). Our ears are very sensitive to odd harmonics (especially higher order) because loudness clues lie in them. It is called TIM (transient intermodulation) and is known since 70s.

One way is to compromise THD, IMD, DF and make an amp without global feedback but the other is to use just enough feedback to reduce THD to about 0.2-0.5% and then limit input bandwidth (before feedback summing stage) to one that amp had without feedback applied. There are many great amps designed with or without feedback - there is no simple answer. Some of these great amps are very simple some are very complex.

I just realized that Ralph Karsten has the same last name as first name of the Icepower (that I use) designer Karsten Nielsen. Karsten people design good things.

I have a similar consideration. Recently I added a second amp bi wired vertically into my system. Aragon 8002 125 watts. The second amp really helped in detail and yielded a more relaxed sound.
Now I have a chance to get a pair of Aragon Palladiums 400wpc
to drive my Snell CV's. I am thinking this too should make a nice improvement.
Will this extra power yields even better results? I will have a chance to test drive these puppies and they were top of the line @ $5 K. Hmmmmmmmm...

Consider the average wattage level that you will be playing your music at and consider that dynamic peaks can easily demand 10 db increases. In other words, if your music is played at 20 watts average and your cd played is very dynamic, you need 200 watts to play that 10 db peak. The 150 watt is sufficient in this case, but what if your peaks are 13 db higher? Now you need 400 watts of dynamic headroom. Continued clipping on your dynamic peaks can damage speakers. This is why they say smaller amps blow more speakers. Also, constant clipping can change from music that is soothing to music that fatigues. Consider how loud you play your music and how dynamic is the source. Check the dynamic headroom of each amp before you choose.

Every design, no matter how expensive or supposedly uncompromised, involves inherent tradeoffs. What compromise a designer finds intolerable or allowable accounts for some of the differences in products. For example, OTL amps are designed so as to eliminate the need for an output transformer (other designers actually LIKE what transformers do), but, in order to bring down the inherently high output impedance of tubes, a lot of tubes have to be wired in parallel (the high number of tubes is not just to increase output). Other designers might not like having many output devices. I happen to like the way most OTLs sound, compared to other higher power tube designs, so I would say that is a good compromise (the last thing that would come to my mind when hearing a decent OTL is "mucky"--they are the paragon of clarity and speed). In the SET world, there are a lot of purists who would never do parallel tubes to boost power (particularly tricky with SET amps), but, that is one way to increase power (I own, and love a parallel SET amp).

The same thing happens with solid state amps. DarTzeel and several other companies have tried to minimize the number of output devices, and NOT to save money, but for sonic reasons. This company also minimizes use of feedback, at the cost of lower measured linearity, more issues with instability, etc. I have no idea if these design choices account for the sound of their amps, but, I like the sound of the smaller DarTzeel amp I heard. Some designers even find it worth the trouble to avoid using complementary pairs of output devices--there are almost endless number of design choices.

I have NEVER heard an amp that just offered "more of everything" in all systems--and that is just based on my own preference. If you consider that each listener has a different set of priorities, I don't see any single design will every be crowned the undisputed champion.

I suppose there are some speakers that really demand a lot of power, but, as a general observation, I think there are far too many listeners who overestimate the amount of power needed to handle the dynamics of recorded music. Yes, in theory, the dynamic range of a live performance of an orchestra is extremely wide. However, recordings never actually capture that dynamic range;so, if I set my volume level at a reasonable level for softer passages, the peaks will NEVER reach realistic levels and I really don't need a super high reserve of power. For rock and popular music, my own preference is not to listen at "realistic" live volume levels. As I improve my system, I find that I get full sound and enjoyment at lower volume levels. I don't really care about attaining "realism" I go for sound I like (frankly, if my system didn't sound a whole lot better than ANY live amplified venue, I would junk it). As to the example above where the starting point of the analysis was an average level of 20 watts, that is a completely unrealistic starting point for even speakers of reasonably low efficiency. As an average, even a single 86 db/w efficiency speaker, in free field (no room) would be playing at 99 db at one meter (extremely loud). Actually, it would most likely be playing at a lower volume because it would be compressing like mad from overheating.