|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
12.202.121.230
In Reply to: Watts are watts posted by ebenai on June 19, 2005 at 09:27:50:
I'm definitly not an expert on the subject but here is where the thinking comes from.First you are correct a watt is a watt, be it tube, SS or digital. So on paper at least a 40wpc SS amp will put out the same amount of power as a 40wpc tube amp.
Where the difference is suppose to be, is when the amps are driven to higher levels. When clipping occurs music will tend to sound harsh, brittle and edgy. Increasing all the more as we get deeper into clipping. When you look at a picture of a soundwave clipping the top is flat or clipped.
When you look at the picture of a tube amp driven to clipping levels the osciliscope picture of the soundwave looks different. It dosen't have the same clipped shape. Clipping also tends to be more gradual. So when clipping occurs with a tube amp it tends to not be as audible and have a more gradual onset. Also the clipping pattern unlike a SS amp with the flattened top does not tend to damage speakers as easily.
All things being equal; the 40wpc SS amp with its clipping patteren will tend to sound harsher and damage speakers earlier then a tube amp.
Whats really happening is the 40wpc tube amp can be driven to higher level before audible distortion and speaker damage sets in. This in effect allows more of the 40wpc to be used before distortion and speaker damage sets in.
This of course is just a general rule. But this is where the thinking one tube watt equals two SS watts.
BW Maxx
Follow Ups:
The differences come into play LONG before clipping sets in i.e. transient response and circuit stability related. An amp doesn't have to "clip" in order for distortion to be produced or the signal smeared. If it isn't "fast enough", the transients lack intensity and the peaks aren't of the proper amplitude. These are both distortions, but are different in the fact that they deal with both the volume of the signal and the duration of the signal.It is a commonly understood fact that MOST ( not "ALL" ) tubed circuits are TYPICALLY a LOT slower than most well designed SS gear. In effect, the slow speed of the tubed gear "saves itself" from damaging speakers that might otherwise be exposed to a higher amplitude and longer duration signal that would be produced by the "more responsive" SS gear. It is also that "slow and round" quality in tube circuitry that helps maintain a more consistent average power, which would otherwise be more dynamic by nature IF the output was linearly tracking the amplitude of the input. Our ears hear that difference in average power vs dynamic power, even though the variances may be neither long in duration or large in amplitude. This is why amps are measured in RMS wattage ( a conservative "average" figure ) rather than in peak power. We are FAR more sensitive to sustained energy levels than we are to dynamic variations.
Obviously, there are some exceptions to the aforementioned generalizations, especially in the tube camp. As an example, Atmasphere tubed amps are quite fast in terms of transient response making them a notable exception, but they lack stability ( due to their somewhat higher output impedance ). As such, they simply aren't a good match for lower impedance speaker loads. If one can mate them with an appropriate loudspeaker load i.e. one that stays above 8 ohms or more, they can achieve most of the benefits associated with the "speed" of SS without incurring many of the drawbacks associated with the "slow & round" tube sound. Obviously, this takes a lot of care and thought to build such a system, but then again, isn't that why we are here to begin with??? : ) Sean
>
You are on the right track, and stability is indeed a concern, but I don't agree that solid-state and tube circuits differ greatly in "speed" if they are made in similar fashions (ie, large chassis, discrete transistors in circuits similar in size to tube layouts).Very small transistors (integrated circuits) can be made to switch very quickly, but these are not suitable for audio service.
The Routh-Hurwitz Stability Criterion applies to any feedback amplifier, regardless of whether it is tube or solid-state. The time delay through a stage depends on the driving resistance and input capacitance, and these are to be determined by the designer (assuming he knows what he is doing). Any amp that incurs more than 180 degrees of total phase shift at the unity-gain upper frequency is unstable. Amps that come close are marginally stable and will suffer from the audible problems you described. There are plenty of marginally-stable solid-state amps.
Atma-Sphere amps do not suffer from stability problems. The high output impedance makes them unsuitable for low-impedance loads, but not because of stability concerns. Paul Speltz' ZERO autotransformers are an excellent method of getting the best sound with Atma-Sphere amps and low-impedance speakers.
One can pick and choose the criteria as to what is or isn't a stable design and where you draw the line, but when the reflected EMF of a loudspeaker modulates the output stage of an amp, that's directly due to a lack of contral via instability. This occurs quite frequently, and not just with high impedance output tubed designs. Sean
>
This post is made possible by the generous support of people like you and our sponsors: