On Wed, 3 Sep 2008 14:00:27 -0700 (PDT), Paul Fletcher wrote: Paul,
I's clear from your email that you don't understand how a ham receiver works. See comments interspersed. >Could you do me a favour please? I'm interested in what happens on the >headphone output if you re-run the tests using a 2.7 or 2.8kHz filter with >the audio tone set low enough in frequency so that the 3rd harmonic lies >within the filter passband (i.e. below 900Hz). First, the 2.7 or 2.8 kHz filters are operating at 8.8 MHz, not at audio. They are 2.7 kHz or 2.8 kHz WIDE at the 8.8 MHz IF. Thus, they have NO effect on the measurement of audio distortion in the manner that Jack is doing it. >I use the Heil phones which have a 200 ohm impedance so if you could use that >a the load that would be great. Another misconception. Headphones and loudspeakers do not have a constant impedance -- indeed, their impedance isn't even resistive! Rather, their impedance varies widely over the audio spectrum. International stardards define the impedance of loudspeakers and headphones as the MINIMUM value of the impedance within their operating bandwidth. See my tutorial about this in http://audiosystemsgroup.com/RFI-Ham.pdf Now, the load impedance IS relevant to distortion measurement -- in general, amplifiers produce the greatest distortion when they are providing the most current, so a lower impedance load would draw more current for the same output voltage. BUT -- different headphones have different voltage sensitivities. That is, it takes more voltage to get the same loudness in one headphone than another. AND, a lower impedance headset will take less voltage to get the same power. See how interwined all this stuff is? The closest that Jack could come to honoring your request is to measure the distortion when actually driving a set of those headphones with a specified output voltage over a range of frequencies (because Z, and thus I, is a variable). 73, Jim Brown K9YC _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
