After looking at the links to all the tubes at the top of this forum, and after reading the thread about the DIY tube amp that can be found here:
http://www4.head-fi.org/forums/showt...threadid=17155

I am starting to have some questions about tube vs solid state amps.

I am pretty familiar with how SS amps work (and their limitations), but know almost nothing about how tube amps work.

That being said, why is it that most solid state amps have very high power ratings compared to tube amps? For instance, the last SS amp that was reviewed in Sterophile mag was the Perreaux R200i, which is rated at 200 Watts/channel. I don't know if this was RMS or not, but for this discussion, it doesn't matter.

On the other hand, it appears that most of the amps that use tubes have relatively small power ratings. For instance, the DIY tube amp from the above thread has 8 Watts/channel, and the tube amps I have looked at in the Tubes site faq seem to max out at around 20 Watts, yet seem to drive loudspeakers just as well as the Solid State Amps.

Why is this?


BPRJam

Quote:



I think that's because, in general, solid state power is cheaper/watt and many *buy into* higher the power, better bs.

the other reason, I can think of is that ever since transistor amps became popular, more speakers today have lower sensitivity and/or tougher load than before. Many amp manufactures need to keep up with the demand for powerful amps to drive those speakers.

Those high power output ratings on paper mean nothing if they are not supported by large enough transformers. Also, when you look at any power output, make sure @ what load. Some company list their power output at 4 ohms instead of 8 ohms to make the number larger than actually it is.

There are few companies who offer smaller power SS amps. Notably from 47lab's Gaincard and thier power supply.

Quote:



there are also few tube amps with he-man power. ARC, VTL (Wotan at what? 600 watts in triode/1200watts in tetrode??? )

they are push-pull design unlike the 8 watts tube amp you mentioned. Single-ended tube amps use just one ( or sometimes it's paralelled at 2/channel ) output valve for each channel, thus, lesser power than push-pulls which use more than 2 valves per channel.

You really can't isolate power rating from speakers they are driving. It all depennds on what load they are carrying. For instance, 1 watt of power can drive the speakers with sensitivity of 100db or more to ear-splitting level with no sweat.

Remember, in olden days, all movie theatres used 300B single ended triodes at maybe 10 watts of power to fill the entire theatre. ( they used horns normaly )

In general, high-powered SET are very rare. The highest I have actually heard used a pair of large ceramic tubes ( these don't even glow ) and they ouput were about 100 watts. KR audio, I believe, also have high-powered paralelled single-ended power amps at.. maybe around 100 watts or so?

Class A valve amps are extremely linear in a sense that they swing more voltage .

Maybe, this is why valve amps sound quite often more powerful than solid state at lot less *power rating*.

kuma, how can listing the wpc into 4ohm make the power look larger than it actually is? It's very important how much power is put into 8-4-2-1 you want an amp that can push 4ohm loads. Preferably an amp thats stable to 1 ohm. Most good SS amps will double the power as the load goes down because the loads become more difficult to push.(needs more power) It's not deceiving at all to tell the rated power at 4 ohms. The trend in speaker building has taken an upswing towards building speakers which present a 4 ohm load to amps. Amps that are powerful into 4 ohms are going to be powerful into 8ohms. But alot of amps that can push 8ohms have trouble once it gets to 4 and below.

Tubes are high voltage and low current devices . Soild State devices are low voltage and high current. Current is what drives the loudspeaker. This is a simplistic answer why solid state has the big power ratings and tubes small ones.
Tubes also have high output impedences and require a transformer(in most cases) to lower the impedence to drive the loudspeaker.This high output impedence can alter your speakers frequency response (for better or worse) and makes it hard to predict how they will sound with your current speaker. Solid state can drive them directly, and has a low output impedence that will not alter the speakers frequency response.
Tubes can drive a lot of speakers but not low sensitivity and low impedence types, which are the bulk of whats currently available. They dont have enough current, unless we are talking about a huge and very expensive tube amp. Of course there are exceptions I am sure, but I dont know what they are. Tubes generally are going to require high sensitivity / high impedence speakers.
Tubes have a more pleasant, ear friendly type of even order distortion when they get near their limits; solid state generally has odd order distortions that are very unpleasant to the ear, even in small amounts.
Generalizing again, tubes have more body because of their distortion spectrum noted above than solid state;this results in a tube amp actually sounding subjectively louder than an equivalent solid state unit of the same or even higher power rating.
I am sure there are more differences between the two, but I have to go to work now , so I will let someone else add to my list. Hope this answers most of your questions.

Quote:



Tim,

What I meant was a manufacture not always state the load with thier power rating at what condition.
which one *looks* like have more power if a given load was not listed ?
100watt (@ 8 ohms)
140 watt (@ 4 ohms )
I agree they should state not only both loads, how many channels driven and duration. We can't assume given power ratings are measured under the same condition.

Quite often, some mass marketed stuff only list their 4 ohm rating which i found quite deceiving. And most won't double when impednace are halved. And not many amps can support 2 ohm and under. ( very rare )

Even then, I have a solid state amp that can output 360 watts/channel@4 ohms on paper. When presented with 8 ohms load but rudiculously low sensitivity, it ran out of juice since transformer was not big enough.

Or speakers with relatively high sensitivity speakers @93db. because of a wild impedance curve ( shoots up from below 2ohm to 24 ohm at crossover point coupled with comlex quasi 4th order crossover ), most amps clipped at that particular frequency.

It is too bad that there is no universal condition for power rating. It's so confusing.

I don't think it's confusing at all. I think your a little confused maybe?

Quote:



I'm not sure I follow. A Watt is a Watt, provided they are measured using the same technique, and whether they have high voltage or high current, the equation W=I*V should give power values in the same range (as long as a few assumptions are made). Why, then, is there such a power discrepancy between tube and SS amplifiers?

Quote:



This makes sense given that tubes are high voltage low current, and current is what drives the primary speaker coil. What doesn't make sense is why most tube amps have low power ratings and SS amps have high power ratings. If the solid state amp has high current and low voltage, and current is what drives the loudspeaker, then it seems that a lower power rating would suffice to drive a speaker for a SS amplifier. Conversely, it seems that tube amps would need much higher power rating to effectively drive the same speaker. Yet, the trend seems to go the other way. In addition, at least anecdotally, it seems that people on this forum seem pleased with the way their low power tube amps drive their larger speakers.

All this leads to a very confused me. The more information I get, the more things seem a little topsy-turvy.

Can anyone help clarify it for me?


BPRJam

Lets take it out of electric and into normal life.

Tubes are like weight lifters, very strong(V) but not much good in a marathon(I). SS on the other hand are like runners, they aren't very strong(V) but boy can they keep cranking out miles(I). Since like you said P=VI and V=IR... speakers with low impedance can take advantage of the high current(running ability) created by SS resulting in higher power. For high power tubes you need the oposite, high impedance so they can muscle the driver but not be asked to run as much resulting in high power. Problem is high impedance speakers are not what I'd call in abundent supply.

High impedance headphones though

Quote:




Great! Now, were getting somewhere!

This applies really well when we are talking about amplifiers with the same power rating. For instance, if we assume we have 2 amps, one solid state, and one tube, both rated at 100W, we would expect that the tubed amp would like a speaker with a high impedance, and the solid state amp would like a speaker with a low impedance so that the intrinsic capabilities of the respective circuits are properly utilized.

However, this still doesn't answer the question about why (at least in my observations) tubed amps seem to have a much lower power rating than SS amps, yet people use them on their large and inefficient loudspeakers with seemingly good results, even at high volumes.

I'm starting to think that it might be the placebo effect. That this sound is comming out of a tube amp, and must therefore sound really good.

I know that with my small 8 Watt/channel DIY SS amp, that if I drive something bigger than a fairly small bookshelf (89dB, 8ohms) to any significant volume, I can both hear and measure (via a scope) the clipping. I'm at a loss as to how a tube amp can get away with it at the same power rating, especially with low impedance speakers.

Any ideas as to why?

Quote:



Tom,

take a look at the specs from this Pioneer receiver, for instance.

The power rating is listed at 100 watts. But, they dont say at what load!

What I meant was it is *confusing* for the consumers when manufacturers do not say at what condition it can output 100 watts.

Their Elite amps' power ratings are listed at 8 ohms and 6 ohms. The fact that they didn't measure it with 4 ohms, is probably because they will clip at that load. • specs

If you don't find this confusing more power to you.

Quote:



Tube amps paired with wrong speakers can sound god aweful. But many folks who like that type of sound, actually. Sounds like an aural *goo*!

Here we go...

Tubes have a different distortion than SS. Its actually quite pleasant, relatively. The low power rating on tube amps is true, it isn't marketing. Its just a preference that at moderate listening levels tubes are more musical. A tube amp rated for 8W won't shake the house down or ever see club use but would be a great mate for an efficent speaker at home in a dedicated(quiet) room or as background noise ambient style.

That said, very little of the gobs of power available in SS amps ever gets used. I know that my listening levels rarely average above 1W. Hold up before you go nuts about how you usually have the volume half way or quarter way, realize that volume control is not linear and that average means average not the power eaten during impact or loud stages of a song.

Now you're probably wondering why so many people strive to have massive kW mono blocks then? Well despite the lack of need for the power, typically a manfacturers best SS design is reserved for the top dog so it can carry a top dollar value

Quote:



Have you heard how loud 8 watts can be with 110db horns?

It can shake the house down just as good as SS counterpart.

Tube amplifiers can be high power but cost increase to a high level !

We had a 200W Mono Block Power amplifiers in Huge size and cost not Cheap ! retail price at US$4,400/Pair. The other more famouse brand like Jadis in simulair output can up to US$20K or more ! It is not affordable for most users.

Power ratings:

Regarding power ratings, these are only a rough guide to the capabilities of an amplifier. The old, no longer mandated but still valid, FTC specified RMS power rating only tested the ability of an amplifier to drive a static 8 ohm load. Loudspeakers and headphones are usually anything but static loads. The impedance of the speaker varies greatly by frequency. Also, the load seen by the amplifier is rarely purely resistive in nature, instead presenting a fairly reactive load to the amplifier that also varies by frequency.

Distortion and clipping:

Regarding amplifier distortion, tube amplifier distortion is predominantly even order harmonic distortion while solid state devices produce both even and odd order harmonic distortions. To the human brain, the even order distortions aren't nearly as unpleasant as odd-order distortions are perceived. Clipping behavior, what happens when an amplifier reaches its output design limit and is no longer able to produce a larger output signal for a given input signal, is also different for tube and solid state amplifiers.

For a solid state amplifier, when driven into clipping while reproducing a sine wave signal, will cut off the top and bottom of the sine wave until it resembles a square wave. This highly modified output signal is rich in odd-order harmonic distortion products. If you have ever heard a solid state amplifier that is driven into hard clipping, you will find yourself diving for the volume control to turn down the awful sound.

With tube amplifiers, when the signal starts to clip, there is a general rounding (compressing) of the output signal instead of the harsh cutoff of the solid state design. When listening to a tube amplifier, it is not always easy to tell when the amplifier is being driven into clipping.

Tube amplifier power vs Solid state amplifier power:

Some claim that a tube amplifier can generally equal the acoustic power of a solid state design rated twice the power output of the tube design. One explanation is that the audible result of driving a tube amplifier into moderate clipping is not always obvious by the way it sounds, so the less powerful tube amp appears to be playing along at similar levels as a more powerful solid state design. When a solid state amplifier is driven into clipping for very brief musical peaks, this is not usually audible. With a tube design you can push the amplifier harder, resulting in moderate clipping, pushing the amplifier maybe as much as 6db (4X) or more beyond its rated power, before the human brain detects that the signal is no longer being faithfully reproduced.


Voltage source vs. current source amplifier design:

Tube amplifiers are usually referred to as voltage source devices, while solid state amplifiers are considered current source devices. However, there are exceptions to this generalization.
Transistor amps that combine the use of feedback with massive ouput stages are considered 'voltage source' amps. The lower the output impedance, the higher the damping factor and the closer the amp comes to being a theoretical voltage source to the load. Tube amps can also be considered 'current sources', but only in a rough sense of the term. They are voltage sources with fairly high output impedances in the case of designs with low feedback. A true current source amp would have infinite output impedance.


The effect of low impedance loads on rated power:

When you connect a low impedance speaker load such as a 4 ohm nominal load, for the same output power, the tube amplifier must deliver 2X higher current than if the speakers were an 8 ohm load or 4X higher current if the speaker load was 16 ohms to produce the same power. The is also true for a solid state amplifier, except that many times the solid state device is better able to meet the higher current demands of the lower impedance load. Tube amplifers are limited in their ability to produce high current by the tube plate current limit inherent in each tube's design. Beyond this limit, the only way to deliver more current is to change output tubes or add more tubes and run them in parallel.

Because solid state amplifiers do not usually come equipped with output transformers (McIntosh autoformer technology aside) as a rule, many solid state amplifiers can deliver up to 2X their rated power into 4 ohms as 8 ohms because their designs are not current limited. Up to the limit of the power supply and/or fuses of the amplifier, this ability to deliver high current into low impedance loads is essentially extra power built in to the design. A 100 watt/8 ohm solid state amplifier that can deliver 2X rated power (200 watts)into a load of 4 ohms is able provide 3db of headroom. Not much of a difference really, but still audible, just due to the design of the amplifier.


Damping factors:

One reason that tube amplifiers don't always work well with certain models loudspeakers is the damping factor of the amplifier. Tube amplifiers typically have lower damping factors than solid state amplifiers, so they cannot control the reverse electromotive force (also known as back EMF) from the speakers as would a solid state amplifier.

The damping factor of an amplifier is related to the output impedance of the amplifier stage. The output impedance of solid state amplifer is very low, only a fraction of 1 ohm, while for a tube amplifier the output impedance seen by the speaker is perhaps 10X higher. A typical damping factor number for a tube amplifier might be 15. Many solid state designs can have damping factors above 150. Beyond a certain point there is no advantage to having a high damping factor but a low damping factor will result in the amplifier being unable to properly control the motion of the woofer driver or panel. This can account for a poorly defined, mushy sounding bass response you heard when using the some tube amplifers with some speakers.

Headphone applications:

Driving headphones is one application where tube amplifiers can many times perform as well or better as most solid state designs. Consider what happens when trying to drive high impedance headphones (some models from Sennheiser and AKG) using a solid state or tube design. Because of the circuit design, many tube amplifiers are readily capable of producing the relatively large RMS voltages necessary to produce the power needed to drive these headphones to reasonably high volume levels. It is here where many tube amplifers really shine.

With high impedance headphones of low or moderate sensitivity, where rated impedances of 120 or even 300 ohms are fairly common and peaks of up to 600 ohms exist, modest tube amplifiers rated at only a few watts at 8 ohms can produce concert hall levels when coupled with even the lowest sensitivity headphone designs.

This same combination of high impedance and low sensitivity leaves many solid state amplifiers unable to perform for the reason that most of today's solid state amplifiers are designed to produce their rated power only into a 4 ohm or 8 ohm load. While many high-end solid state power amplifiers (Levinson, Krell, Rotel, Classe, etc.) have massive power supplies and can also provide the needed wide peak to peak voltage swing needed for high impedance loads, it is rare for a mass market solid state amplifier to be able to handle both low and high impedance loads effectively.

With lower impedance headphones, for example phones rated at 32 ohms, the difference in the abilities of tube and solid state amplifiers to drive this load is not as much a factor as with the high impedance designs. However, when coupled with a lower impedance headphone, the relatively high output impedance of the tube amplifier design means the tube amplifier has a lower damping factor when driving low impedance phones. This might make a difference in the tube amplifier being able to fully control the headphone driver, and can be one reason why this combination can result in poorly reproduced bass frequencies.