Stereo Amplifier Power Specifications
- From: "Bill Evans" <bevans@xxxxxxxxxxx>
- Date: Thu, 23 Feb 2006 09:55:13 -0600
Back in the late 1970?s and early 1980?s when I was purchasing quality
stereo amplification equipment for various projects, I became quite familiar
with the techniques used by manufacturers to overstate the power output of
their amplifiers ? specifically statements presenting total instantaneous
power, peak power, or other such number-inflating techniques. I had thought
that the practice of requiring power output in RMS to be specified at a
specified distortion level had put an end to that type of power inflation
technique and provided serious buyers with an appropriate yardstick for
comparison.
In those days, amplifiers that were capable of 50 Watts RMS per side or
higher were relatively beefy with massive heat-sunk power transistors, heavy
power transformers, filter chokes and substantial electrolytic capacitor
banks to meet the RMS power requirements. I still enjoy a number of
amplifiers from those days and appreciate their ?clean? power delivery,
particularly in the bass range.
While I recognize that advances in power supply design, such as switching
mode, and newer amplifier designs, such as classes G, H and D, have afforded
the ability to eliminate significant weight and size from amplifiers, I?m
concerned that somehow the RMS rating criteria is somehow being bypassed in
current stereo and home theatre products.
For example, I recently trialed a name-brand stereo amplifier that was
specified at 100 Watts per side into 8 Ohms. I really didn?t need a new
amplifier, but couldn?t believe the price for what was claimed to be
equivalent to my 15-year old Akai, which isClass G and rated at 130 Watts
per side into 8 Ohms and 100 Watts per side into 4 Ohms. The name-brand unit
was about ¼ the size and weight of my Akai, which cost five times as much 15
years ago.
When I connected the name-brand unit to my Altech "Studio Monitor" 8 Ohm
speakers and played a favourite CD, I immediately recognized that this unit
was dramatically underpowered, particularly in the bass range ? probably
doing no better than 10 Watts in terms of what I associated with RMS power
performance.
When I returned the unit (at the speed of sound) to the big box store audio
expert, he emphatically stated that I needed a ?high-current? amplifier,
pointing to a much more expensive / expansive name-brand unit that was in
the same, size, weight and price range as my Akai and Pioneer units.
Still adhering to the belief that Ohm?s Law is applicable to the
specification of power, and assuming a fixed speaker impedance, I could not
understand the relevance of ?high current? to getting better sound out of an
amplifier with the same 8 Ohm RMS power rating, but no ?high-current?
designation. For a given power output, if the current is lower, the voltage
must be higher, per Ohm?s Law.
I note seemingly incredible claims being made for home theatre
systems ?seven channels at 100 Watts RMS output ? ?simultaneously?, as
promised by a ?sales engineer? in a professional audio shop that I recently
visited. I can?t believe that amplifiers of this size, weight and price on
offer at this shop could possibly deliver that type of relatively
undistorted RMS power simultaneously from five channels, let alone seven.
How are the manufacturers and sales agencies getting around the old RMS
power specification ?equalizer?? I note reasonable specifications for IMD
and THD associated with these current power claims, so what am I missing???
Comments would be very much appreciated.
Bill Evans
.
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