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نویسنده : عارف - ساعت ۸:۱۸ ‎ب.ظ روز یکشنبه ۳ بهمن ۱۳۸٩
 

All said, there is much more difference between individual 

designs, whether tube or transistor, than there is between tube 
and transistor designs generically. You can make a fine amp 
from either, and you can also make a lousy amp from either.

Although tubes and transistors clip differently, clipping 
will be rare to nonexistant with a good amp, so this 
difference should be moot.

Some people claim that tubes require less or no feedback 
while transistor amps require significant feedback. In 
practice, all amps require some feedback, be it overall, 
local, or just "degeneration". Feedback is essential in 
amps because it makes the amp stable with temperature 
variations and manufacturable despite component variations.

Feedback has a bad reputation because a badly designed 
feedback system can dramatically overshoot or oscillate. 
Some older designs used excessive feedback to compensate 
for the nonlinearities of lousy circuits. Well designed
feedback amps are stable and have minimal overshoot.

When transistor amps were first produced, they were inferior to 
the better tube amps of the day. Designers made lots of mistakes 
with the new technologies as they learned. Today, designers 
are far more sophisticated and experienced than those of 1960.

Because of low internal capacitances, tube amps have very
linear input characteristics. This makes tube amps easy to
drive and tolerant of higher output-impedance sources, such 
as other tube circuits and high-impedance volume controls. 
Transistor amps may have higher coupling from input to output
and may have lower input impedance. However, some circuit 
techniques reduce these effects. Also, some transistor 
amps avoid these problems completely by using good JFET 
input circuits.

There is lots of hype out on the subject as well as folklore
and misconceptions. In fact, a good FET designer can make a 
great FET amp. A good tube designer can make a great tube amp, 
and a good transistor designer can make a great transistor amp.
Many designers mix components to use them as they are best.

As with any other engineering discipline, good amp design 
requires a deep understanding of the characteristics of 
components, the pitfalls of amp design, the characteristics 
of the signal source, the characteristics of the loads, and 
the characteristics of the signal itself.

As a side issue, we lack a perfect set of measurements to 
grade the quality of an amp. Frequency response, distortion, 
and signal-to-noise ratio give hints, but by themselves are 
insufficient to rate sound. 

Many swear that tubes sound more "tube like" and transistors 
sound more "transistor like". Some people add a tube circuit 
to their transistor circuits to give some "tube" sound.

Some claim that they have measured a distinct difference between
the distortion characteristics of tube amps and transistor amps. 
This may be caused by the output transformer, the transfer 
function of the tubes, or the choice of amp topology. Tube amps 
rarely have frequency response as flat as the flattest 
transistor amps, due to the output transformer. However, the 
frequency response of good tube amps is amazingly good.

For more information on tubes, get one of the following old
reference books, or check out audioXpress Magazine (see the
magazine section of the FAQ for more info on audioXpress).

The Receiving Tube Manual (annual up to 1970)
The Radiotron Designers Handbook
Fundamentals of Vacuum Tubes" by Eastman 1937, McGraw-Hill