Upon further review of what I sent you earlier I have to disagree with
myself. I'm certain that any sort of reduced bandwidth should have
nothing to do with the test as I'm assuming they took the measurement at
1Khz. I know that the spec was take this way but have no way of knowing
how the ARRL performs this test. I'm near certain that they don't
inject a signal at the input of the audio amplifier which is what
National did.
Perhaps the ARRL injects a transmitted signal with a 1khz tone in it and
they then measure that. Added distortion could and would be added by
the transmitter and as well the receive circuitry. Even if they simply
injected a signal into the antenna jack. I guess the purest test that I
can think of off the top of my head would be to use the microphone
feedback feature (the one that is used to listen to your transmit audio)
and then to fee the 1khz signal into the microphone or line level input
and then monitor the power out on the speaker jack. Its possible that
this was done but I doubt it as it would not be repeatable on all radios
that they test. That is the wonderful thing about test data when there
isn't a specification of how the test is run readily available. (or at
least I don't know of where to find this specification).
Furthermore there is a possibility that the stage before the audio
amplifier is clipping before reaching the rails of the audio amplifier.
In other words its possible that the previous stage reaches 10% THD and
the amplifier is only putting out the 2.5ish watts when that occurs.
Truth is that without having the test in hand and being able to take the
measurements myself I won't be able to really know where the difference
comes from.
But honestly 2.5 watts should be enough to make you want to leave the
room with most efficient speakers (ESPECIALLY AT 10% THD!!!) :) If this
power level is a problem for you its certainly enough drive to run a
small outboard amp or to run a powered set of speakers. Just make sure
that the amplifier you choose can handle being in an RF environment.
Also you may want to ensure that the amp doesn't become a producer or RF
either. Anyway that's my two cents on the topic and I'm sticking to
it (for now) ;)
~Brett (KC7OTG)
On Tue, 2008-03-18 at 23:11 -0700, Bob W9RFS wrote:
> I did a little more reseach and found the power amp for the speaker output in
> the k3 schematics. The device is a LM4950TS. Here's a link:
> http://www.national.com/pf/LM/LM4950.html#Datasheet
> Looks like 3.1 watts into four ohms with less than 1% THD. Is this correct
> or am I missinterpreting the specs? Thanks,
>
>
> Brett Howard wrote:
> >
> > 2.5 Watts into 4 ohms if you're willing to put up with 10% THD. Figure
> > 2 watts into 4 ohms.
> >
> >
> > On Fri, 2008-03-14 at 20:41 -0700, Bob W9RFS wrote:
> >
> >> Does anyone know how many watts the audio amp is that feeds the speaker
> >> output connector?
> >> Thanks!
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