The power amplifier
needs to accurately amplify (apply voltage gain to) the incoming signal, and
deliver that amplified signal, fully in tact, to a designated low impedance
load. The power amplifier is not supposed to alter or reshape the input signal
in any other manner, so optimum accuracy can be enhanced when the potential for
error (a.k.a. distortion) is effectively suppressed.
There are, today,
many solid-state power amplifiers that are highly accurate. They replicate the
input signal with extremely low error, so it’s virtually impossible to separate
such amplifiers by merely listening; they’ll simply sound identical.
Distinguishing differences generally involve power output capability, load impedance
compatibility, basic circuit design (and choice of active components), intended
reliability, and target market. One needs to push deeper into the technical
detail to make sensible decisions about benefit-versus-cost aspects; listening
isn’t enough.
Given this state of
modern amplifier excellence, it’s unrealistic to expect equivalent accuracy
from vacuum tube dependent design. The archaic limitations of the triode tube
(Lee DeForest, 1906) are not consistent with contemporary medians. Ruler-flat
power response, near-zero (< 0.1%)
total harmonic distortion (THD) at full rated power, and ultra-low output
impedance (~ 1/10th the Zout of a typical tube amp) is now routine in the case
of most solid-state power amplifiers. Identical measurements made on the very
best vacuum tube models show that they can’t approach the same readouts. For
example, consider THD: Typical “high-end” tube-type audio power amplifier THD
limits are ~ 16X to 50X worse than as specified for a popular “mid-market”
solid-state power amplifier.* (They’re 1% to 3% THD instead of 0.06% max.) That’s
undeniable regression. Of course, tube boosters assert that this shortcoming is
actually beneficial. They claim that vacuum tube amps will thereby render a
warmer, more euphonic (?) sound. Given this Zen-infused perception, tube power
amplifiers can seemingly transcend their intended role and become creative (but
arbitrary) signal processors. This uncontrolled mutation is not consistent with
the initial (accuracy) objective.
I recommend a power
amplifier that can accurately replicate the input signal, and do so with
sufficient power to deliver some +2 dB to +3 dB more unclipped drive
than your speakers can tolerate. A speaker system’s safe operating range is
defined by its “maximum unclipped power input” limit. The application of
some +2 to +3 dB margin for power amplifier output will then assure that your
speakers never see a clipped (potentially destructive) input signal that’s
within the speakers’ safe operating range. E.g.: If your loudspeakers are able
to accept unclipped power input ≤ 80 Watts, then your power amplifier should be
able to provide unclipped power output ≥ 127 Watts (+2 dB) or ≥ 160
Watts (+3 dB). In this example, do take note that the cited power levels may be
appreciably more than many tube-type power amplifiers can produce. Vacuum tube
power amps are wimpy, as well as grossly inefficient.
Apply that “euphonic
polishing” later, at the loudspeaker stage. The speakers’ performance will be
inseparably linked to the acoustic characteristics of your listening room, so
it’s sensible to address both of those issues together—without the need to
compensate for anomalous amplifier sound.
*Parasound’s Halo
A23+ power amplifier ($1,495 list) versus the best from PrimaLuna, VTL, and
VAC.
BG (February 2019)
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