"Bill Evans" wrote in message
...
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.
**Just a point of pedantry: "RMS power output" is and always was a
nonsensical term. The correct term is: "Continuous Watts", or just plain ole
Watts. Continuous Watts are derived from the RMS Voltage across a given
load, or the RMS current flowing through that load. There is no such thing
as "RMS Watts".
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.
**In many cases, you are correct. In some, you are not. BTW: There are
really only the following Classes of amplification:
Class A
Class A/B
Class B
Class C
Class D
Class C is not often seen in audio. All the other classes (Class G, H, et
al) are not really about the amplifier at all. They relate to various
schemes surrounding the power supply. Ultimately, the actual amplifying
stages will be one of the usual ones (Class A, A/B, B or D).
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.
**How much (in 2006 Squid) did your old amp cost, relative to your new amp?
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.
**It is a fatal mistake to assume that lousdspeakers are resistive loads.
Your '8 Ohm' speakers may be something entirely different. Here is, for
instance, a speaker which is rated at "4 Ohms":
www.rageaudio.com.au/kappa9.jpg
Note the dip in the bass region.
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.
**Most cannot actually deliver full power from all channels simultaneously.
Most can manage full power from two channels simultaneously.
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???
**There are a bunch of issues you need to examine:
* The impedance of your speakers.
* The ability of your amp to cope with the afore-mentioned speaker loads.
Fundamentally, however, you should know that many crappy amps were built,
way back when. There were quite a few decent amps built too. Nowadays, there
are still decent amps being built and, usually, at significantly lower costs
(in real Dollar terms) than in the old days. A good old amp will always
outperform a crappy new amp.
--
Trevor Wilson
www.rageaudio.com.au