Tubes typically have FAR higher rail voltages than SS amps, which therefore gives them more dynamic headroom. Tubed amps also clip in a "harmonious" manner, making them less objectionable when overdriven. This means that a smaller tubed amp can play "louder" than one would think, as it has quite a bit of dynamic headroom and doesn't rip your ears / eyes out when it does clip.
The biggest problem with tubed gear is that it typically lacks current and is bandwidth limited, both on top and bottom. The lack of current is what gives most tubed gear that "round, tubby" bass that many folks dislike. At the same time, this "added warmth" tends to "fill out" many of the leaner digital recordings that we hear. The limited bandwidth up top tends to soften the treble response, making hard, bright and edgy digital sound smoother and more listenable.
Like any other amplifier, you want to look for a design that utilizes very large core transformers. With tubed gear, the output transformers are as important ( maybe moreso ) as the power supply transformer. As such, look at the build quality of the amp more than the actual power rating, as most tubed power ratings are "bunk" anyhow. If you applied the same standards for measuring distortion in tubed gear as one does to SS gear, most of these tubed amps would be rated at about 1/4 to 1/3 of what the manufacturer advertises.
There are obviously designs / products that fall outside of these basic recommendations i.e. output transformerless ( OTL ) designs, digital power supplies, etc.. These are all non-standard designs, so the "rules" are apt to be different for these products. In such cases, proceed with caution and just make sure that you're dealing with a reputable manufacturer. Sean
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PS... Buy more than what you think you need, as running tubes quite hard on a regular basis also means replacing tubes on a regular basis.
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What the specific test results that D. Edwards linked to tries to point out is that many ( NOT all !!! ) tubed designs typically sound "richer, fuller & airier" because they are generating TONS of harmonic distortion.
These harmonics tend to make "sparse" sounding digital recordings sound more natural, hence their ability to not only "warm things up" due to the aforementioned lack of bass control / current output, but also to "breath life into" these recordings. They do this through emphasizing all of the harmonic overtones that may / may not have been present originally.
If you look at the Halcro vs the Yamamoto's spectral content, the Halco produces the primary signal and is -100 dB's down by the second harmonic. The Yamamoto on the other hand, is only about -40 dB down. This means that the Yamamoto is producing 60 dB's more output than the Halco at this point, let alone WAY more output across the entire spectrum. It does this at any given time or amplitude. Bare in mind that the Halco is doing this at 100 watts of output whereas the Yamamoto is at a watt or less !!!
Quite honestly, this is NOT a very fair comparison, but it does make a point. Using a poorly designed and possibly misadjusted ( read the text ) SET tube amp to compare to a high negative feedback SS amp just isn't fair or right. Maybe as an example of what is possible on both extremes, but not as a general example. Most designs are going to be somewhere between these two extreme examples.
On top of that, i really don't think that anyone shopping for something along the lines of the tubed Yamamoto ( rated at 2 wpc ) is going to be comparing that to the Halcro. They are completely different design approaches producing very diffferent sounds and system compatability issues. Whereas the Yamamoto might sound "larger than life" due to all of the spurious harmonic distortions added, the Halcro tends to sound thin, sterile and lifeless. Most of that is probably due to use of too much negative feedback.
Whereas the Yamamoto is a prime example of why some people refer to tubed audio gear as "distortion generators", some would say that the Halcro is a prime example of why some categorize SS gear as being "unmusical, lifeless and sterile". While one could be said to be "more technically correct than the other, it all boils down to system synergy and personal preference. After all, the bottom line is building a system that you can listen to and enjoy the music without being fatigued, annoyed or distracted by the gear itself. Some do that with tubes, some do it with SS. Some use a combo of the two, looking for the "added harmonic richness" of tubes with the speed, focus and authority of SS. Whatever you choose to go with, just make sure of one thing. That is, it makes YOU happy, as YOU are the one that has to use and listen to YOUR audio investment : ) Sean
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There is NO reason that an SS amp couldn't make use of very high rail voltages like that of a tubed amp. I've mentioned before that the amp with the highest rail voltages and greatest current capacity would be the most universal in application, so long as the rest of the circuit was fast and "sounded good". Sean
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PS... Whereas tubes are primarily a "voltage device", transistors are a "power device", hence their ability to double down when properly designed. Having the extra current capacity doesn't necessarily make up for the extra voltage capacity that most tubed designs bring with them. |
Forgive me for generalizing, but most tubed amps SUCK in terms of their power bandwidth. This also means that their transient response sucks too, hence the amount of low frequency tilt and rounded square waves that are seen in most test reviews. Yes, there are units that don't follow this trend, but they are a small portion of what is available out there.
The Atmasphere amps, as far as i know, are the fastest mass produced tube amps that i'm aware of. As such, one can see why Ralph would take umbrage at these statements. He doesn't want his products "lumped in" with those that i am criticizing.
The higher the rail voltage, the less likely the amp is to clip. So long as the amp can deliver the current needed, the voltage doesn't sag and larger peaks can be delivered without hesitation or distortion. This maintains a higher level of OPERATIONAL headroom than an amp that can deliver the same amount of current, but with a lower rail voltage. After all, musical peaks are voltage driven, not current driven.
The only time current comes into play is when speaker impedances require it. Selecting a speaker that maintains a higher than average impedance without any radical phase angles makes for an easy speaker to drive. In such a case, a VERY low current amp with a reasonably high rail can easily get the job done. This is how / why some SET amps, with their miniscule current capacities, can drive some speakers phenomenally well.
When one starts using low impedance loads, long excursion drivers that generate a lot of reflected EMF and / or highly reactive loads, the amp MUST have high current capacity. If the amp doesn't have the current it needs, the voltage sags and linearity is lost.
In extreme cases like this, the amp temporarily "loses control" of the driver diaphragm. The end result sounds horrible, especially with larger, higher mass woofers. In some cases, you can literally hear the voice coil "bottom out". The lack of control from the amp in such a situation coupled with the high velocity movement of the driver mass results in enough kinetic energy to "slam" the driver against its' mechanical limits. Take my word for it, JBL's sound especially bad / scary when this happens.
As far as Class of operation goes, the lower the bias, the more operational headroom the amp is likely to display. That is, all things being equal ( HA HA HA ). Thermal stress lowers maximum voltage and current capacities, so lower operating temperatures are typically a desirable thing. At the same time, the higher levels of bias that generate "bad" levels of heat also display the highest levels of linearity. As such, design decisions and production trade-off's have to be made in order to produce a reasonably priced marketable product. This is why we have more AB amps on the market than just Class A or Class B. They strive to achieve the linearity of Class A at lower levels with the lack of thermal stress / cost cutting of Class B at higher signal levels. Switching amps take this even further, and depending on their design, can offer some very real benefits in several different areas.
We could go on and on and on and on here, but if one does as i suggested i.e. buy more than you think you need, you'll pretty much be covered. Just make sure that it sounds good in YOUR system to YOUR ears. If it sounds good AND measures good, you've probably got one helluva good sounding component and / or system. Sean
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As i've mentioned before, you have to look at ALL of the pertinent specifications on the whole and understand how the results were achieved i.e. specific test methodology used. Singling out one spec on ANY product in the world won't tell you ANYTHING about how well it works in ANY given situation, let alone on the whole or universally. Audio components aren't any different. Sean
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This is simple Ohm's Law. One can try to make it as complicated as they like, but it is actually just as simple as it sounds.
The amount of current needed in the circuit is dictated by the impedance of the load at the frequency that is being reproduced. After that, it's all a matter of whether or not the amplifier can swing enough voltage in time to keep up with the amplitude of the signal as it changes on a dynamic basis.
Since we've already acknowledged that high current capacity is not as necessary as most would think with reasonable speaker loads, the only reason that an amp would clip has to do with running up against a limited voltage capacity and / or the inability of the circuitry to slew fast enough to deliver the voltage required. Both result in clipping, whether it is due to limited headroom and / or a lack of speed.
With high rail voltages, high current capacity, high speed / wide bandwidth and good circuit stability, you can drive any load that you want with reasonably low levels of distortion. If you limit even just one variable in the aforementioned list though, the versatility and performance potential of the circuit is drastically reduced. This dictates more careful matching of the associated load ( speaker and speaker cables ) to that of the performance limited amplifier.
Since all amplifiers are limited in one way or the other to some degree, and no speaker is purely resistive with a higher nominal impedance, some matching is always required. Having said that, the more competently designed the amp, the more consistent the performance that it will deliver, regardless of the load. Obviously, one can go to extremes coming up with radical speaker loads that might embarrass all but the most advanced amplifier circuitry, but as a general rule, these are just that i.e. extremes and not the norm.
If speaking of extremes, anything is possible and specifics must be mentioned if we are to have any type of meaningful conversation. I have strictly been speaking in general terms, as i've not seen any mention of specifics in this thread. Sean
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PS... The fact that Ralph and i are "debating" should tell you folks quite a bit. That is, even though we both find many of same attributes very desirable i.e. wide bandwidth, high speed, reasonably low distortion, low negative feedback, high stability, high bias circuits, etc... there is still plenty of other things for us to disagree about. This is one of the reasons there are SOOOOO many different products out there with different designs.
As can be seen by our responses here, most of this boils down to what the best way is to achieve all of those goals simultaneously without having to cut a lot of corners to get there. |
Not to change the subject, but i wanted to point something out. Ralph aka Atmasphere said: "Since the peak represents a peak in power as well as voltage, current must therefore be peaked also".
This is not always true. When a speaker resonates, it produces a peak in output, even though voltage and current input from the amp are at a minimum due to the high impedance encountered. How intense this peak is has to do with the alignment of the speaker system and how well the amp can control the reflected EMF.
Some speaker designs seek to tame this form of "self oscillation" ( low Q sealed designs, transmission lines, etc... ) whereas others try to exploit it ( many vented designs ) in hopes of getting "free output". That "free output" doesn't really come "free" though, as this type of system typically lacks damping while producing rather inarticulate bass at or near the point of resonance(s).
Whereas "uncontrolled oscillation" is typically frowned upon anywhere near the audible bandwidth in every type of component that i can think of, this seems to be the accepted norm with many vented loudspeaker designs. This has always baffled me ever since i understood the sonic and technical differences in various bass alignments. Sean
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