Need help understanding tube wpc

"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." [sic]

This statement is patently untrue, but is a very common misconception. There are tube amps with bandwidth to 100KHz and there are tube amps with LF cutoffs as low as 1Hz at full power. Some of these are the same tube amps. So bandwidth is clearly *not* the issue.

Similarly, lack of current has nothing to do with this either. Ohm's Law (which is inviable, BTW) reveals that a shocking (no pun intended) low amount of current is needed to drive low impedance speakers to quite high powers! The 'high current' mindset is an outcome of the introduction of large amounts of feedback in transistor amps, which is one of the major reasons that SS amps have more odd-ordered harmonic content than tubes.

In fact the issue of tube vs SS power does have to do with the rules of human hearing- which audiokinesis outlined earlier. Humans are sensitive to odd-ordered harmonics and transistors make more of those than tubes. To get around the problem you have to have a very big transistor amp so you don't come anywhere *near* clipping.

The idea of voltage rails having something to do with this is incorrect also. The voltage 'rails' merely determine how much power the amp will make- tube *or* solid state- it does not describe headroom at all. As a specification, headroom is more a function of the class of operation (class B amplifiers having the *most* headroom), but it turns out that class A amplifiers carry more authority, and for their size tend to behave as if they have 'more power'. What they *really* have is more *usable* power, and that is what this thread is all about- how much *usable* power the two technologies have.

My experience has been that in general, a tube amp will have the same amount of musical *usable* power when it is between 1/10 and 1/4 the power of a transistor amp. Variables that throw this generalization off are class of operation ( for example, a class A transistor amp will have more *usable* power), elegance of construction (don't expect a $500 SS amp to do what a $5000 SS amp of the same power will do) and the like.

Definitely muddy waters!

Watts are watts, whether ss or tube. How loud an amp can play without objectionable distortion (to the ear) is related to how gracefully it overloads. Tube amps are better in this in that they use less negative feedback. If the same excessive feedback was applied to a tube amp as typically used in ss designs, the overload characteristics would be just as bad if the amp was dc-coupled and far worse if ac coupled as most tube amps are. Fortunately, engineers have not been able to apply large amounts of negative feedback in tube amps because phase anomalies in the output transformer won't allow it.

Of course, lower amounts of negative feedback results in higher output impedance of the amp, resulting in measurement specs gurus freaking out.

David Berning

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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gregm

Yes you are correct the equation for how you calculate power is the same. How that power reacts to real world loads is another story. In the end all that matters is how it sounds to the individual and not how some set of equations says it should sound.

11-22-06: Sean “If it sounds good AND measures good, you've probably got one helluva good sounding component and / or system”

As opposed to it sounds good but measures bad then you have a bad system? Sorry, I cannot agree with that. Some of my amps have THD numbers as high as 3%, but can reproduce piano recitals with more realism than anything I have ever heard, and believe me, I am intimately familiar with piano.

Measuring is probably very useful on a production/assembly line, but I have never found any correlation between measured numbers like THD and what the amp sounds like to my ears. In fact have heard amp with excellent THD number that are useful only as doorstops. I know, I own a few amps like those.

There is only one measure that determines how a component sounds like - your ears. In my experience the folks let their ears be led by measurements have the least realistic sounding systems (read, total crap sounding systems) out there. And yeah, I have heard many of them too.

Regards
Paul