"Sonnova" <sonnova@[EMAIL PROTECTED]
> wrote in message
news:0001HW.C3E9D1940060E902F01846D8@[EMAIL PROTECTED]
> On Tue, 26 Feb 2008 13:16:26 -0800, Arny Krueger wrote
> (in article <S8ydnYg7ytu6H1nanZ2dnUVZ_qelnZ2d@[EMAIL PROTECTED]
>):
>
>>
>> "jamesgangnc" <james@[EMAIL PROTECTED]
> wrote in message
>> news:13s6so0o3qqg69@[EMAIL PROTECTED]
>>
>>>> Surprisingly, these surround receivers actually do put out 100 watts
>>>> RMS
>>>> each for all 5 channels concurrently, or 85 watts RMS each for 7
>>>> channels concurre
>>
>>> If one of those cheap surround sound recievers is putting out 500
watts
>>> rms then I'm betting it's a class d amp.
>>
>> Generally, no.
>>
>> At the 100 wpc level it is still generally cheaper to make a class AB
>> amplfier than a Class D.
>>
>>> And that's why the distortion figures suck.
>>
>> Just because an amplifier is Class D (switchmode) does not mean that it
>> necessarily has high distortion.
>>
>>> A good class ab amp will list continuous rms watts for 8 ohms and 4
ohms
>>> with .1 thd or lower.
>>
>> That is about specsman****p, not usable performance. Yes, a lot of
>> receivers
>> are giving power specs at 0.7% THD and the like. But, if they would
back
>> their power ratings off by say, 10%, then the THD would probably be
below
>> 0.1%. There is hardly any audible difference between a clean 100 watts
>> and a
>> clean 90 watts. For some reason they want to specify that the amp has
>> 100
>> wpc, and they are forced to specify more distortion than most are
>> comfortable with to do it. However, nobody runs an amp flat out all the
>> time, and in normal listening, the 0.7% THD figure is pretty much
>> arbitrary.
> There's also no audible difference between 0.7% THD and 0.1% THD. The
ear
> is
> amazingly insensitive to the AMOUNT of THD especially below about 2%,
but
> it
> can be somewhat sensitive to the KIND of harmonic distortion (even or
odd)
> and where that distortion occurs.
Sort of, First off, amplifiers don't have a physical property called
Harmonic Distoriton or IM Distoriton. The statement "This amplifier has
0.7%
THD" is strictly speaking, untrue. What is true is that the amplifier
makes
0.7% THD under a certain set of cir***stances. Furthermore, the choice of
cir***stances has a degree of arbitraryness to it. If I want an amplifier
to
have 0.7% THD, all I have to do it pick a certain test signal and power
level.
A jillion dollar, jillion watt amplifier that is speced at 0.001 % THD
will
produce 0.7% THD if you ask it to develop too much power. I have a $25
battery-powered headphone amplifier that is ingloriously called a
"Boostaroo" that has about 0.001 % THD if operated below clipping.
My former post was all about how arbitrary the choice of measurement
conditions actually are. Most modern SS amps have about 0.02% THD @[EMAIL PROTECTED]
1kHz
at
most power levels up to a little bit below clipping. Any halfways decent
modern SS amp that measures out be producing 0.7% THD has been pushed into
clipping.
What amplifiers do have is nonlinearity. THD, IM, and harmonic distortion
by harmonic are simply means for measuring or characterizing nonlinearity.
The cause of a measurement of a certain amount of THD or a certain
harmonic
is nonlinearity, and that same nonlinearity causes measurable IM.
My point is that in general, generating IM is more audible than generating
THD. Harmonics generated by a nonlinear amplifier tend to be masked by the
harmonics in the music. However, since music is usually made by many
separate musical instruments, it generally has very little IM. Furthermore
IM tends to produce distortion products that are aharmonic and therefore
not
so likely to be masked by harmonics already present in the music.
Under ideal but still fairly reasonable conditions, as little as 0.1 % IM
can be heard. The nonlinearity that generates 0.1 % IM will also generate
something like 0.1% THD, depending.
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