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|30th May 2012, 01:47 AM||#131|
Join Date: Sep 2008
Location: "Space Coast" Florida, USA
First, there is the bit about distortion of tubes versus transistors. However, that argument falls apart when you consider that normal (if not ideal) listening levels should never induce distortion in the amp, if you size the system correctly for the task.
That means the amp should never operate in a zone where it is pushed so hard as to generate audible distortion in the first place. It doesn't matter what topology you choose, it should ever have to be overdriven to get the SPL level required for the job.
So, poof! There goes the distortion myth.
So, where is the tube sound? It turns out there is actually a tube sound, but it comes from a source that you would not expect - your speakers!
Sounds crazy (no pun intended)? Not really. You can clearly get a different sound between a tube amp and a transistor amp (running below audible distortion levels) and the difference has to do with frequency response.
In an ideal world a transistor amp and a tube amp should sound exactly the same, but speakers are not an ideal load and impedance varies with frequency when inductors are introduced to the load.
The root of the problem is the way a tube amp and a transistor amp operate into a load. A transistor amp is a Voltage Drive amplifier and a tube amp is a Current Drive amplifier - well sort of, but for this discussion we will just call it so.
What that means in a nutshell is that a voltage drive amp's power will decrease as the impedance of the speaker rises. The opposite happens when a tube amp sees a higher impedance load. its power output increases.
Now, let's consider the average speaker impedance curve over its operating frequency. The type of base cabinet (sealed versus ported) has some impact, but both tend to have a higher impedance in the bass region, which decreases in the midrange, and rises slightly in the treble region.
What does that mean to the listener? Well, a transistor amp sounds a little sterile because the bass is a bit wimpy and the treble lacks that sparkle because the power drops off as the impedance goes up.
If the impedance curve has some bumps associated with the crossover it further aggravates the problem a little.
A tube amp tends to boost the bass a little as well as the treble and just shines more than the transistor amp, which seems a bit lifeless compared to the boosted tube sound.
There are other factors at work here, but I wanted to illustrate one of the important differences that makes the two amplifier types sound the way they do.
A high end sound system with a really good speaker system (where the impedance is fairly constant) should sound about the same with a tube amp versus a transistor amp. However, most speakers are not a flat line when it comes to impedance and therefore react differently when driven by a voltage drive and current drive amplifier.
Then there is the whole subjective idea of "magic", but I refuse to go down to that level of a discussion.
The bottom line is you should correctly size your amp for the job at hand and make sure your speaker impedance is as flat as possible. Then there is the whole room acoustic thing, but that is another form.
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