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
>
