I don't see the issue as being "what impedance the headphone are" associated
directly with the concerns. If the HP Z is low [less than source Z] then
the majority of electrical energy will be dissipated across the series R
(internal) to the circuit (and they all have a source Z of several ohms) and
thus the phones won't be loud enough. Should one turn up the level then
exceeding the amp rated output, distortion is the product. If the HP Z is
very high then the circuit won't be able to develop the voltage across the
load to adequately drive the HI Z phones. Again should one turn up the
level, once max voltage is reached, clipping occurs and distortion is
produced. The old Trimm black phones were typically 2K ohms while newer
phones, Sony MDR Series is typically 200 ohms. There are a few 10 to 30
ohms headphones around.
As to not hearing the hiss in the speaker and hearing it on the phones,
there is a very good reason. It is acoustic coupling to the ears. The
phones are very closely coupled to the ears, the speaker is not. Remember
we are dealing with path loss (acoustic power) that follows an inverse
square rule. Secondarily, the human ear exhibits a factor, in which two
signals of equal energy but of two frequencies arriving at he ears at the
same time, will always appear that the higher frequency signal will sound
louder. Hence, CW sidetone vs. broadband noise or click which is a higher
frequency than CW sidetone. Thus a 700 HZ side tone plus a few lower level
harmonics compared to a high frequency broad band hiss with clicks, the hiss
and clicks will be most likely predominant.
Case in point: in a professional environment where I worked, we used
headphones to listen to stereo tape copies to detect signal dropouts. These
dropouts could not be heard on speakers, even though quite loud and in a
very good acoustic environment. Same thing true here with the use of
headphones and sidetone vs. hiss and clicks. The headphones will make the
problem sound much worse than a speaker.
Now to enhance the problem, certain headphones like many of the professional
series have a much wider frequency response than others and some have a
higher sensitivity. These two factors will influence greatly what SPL
(Sound Pressure Level) is produced by the headphones. The impedance of the
headphones is best described as a factor of efficiency of transfer of
electrical energy. But the real secret is transducer coupling to the ears.
Speaker vs. headphones. The tighter the coupling the more transfer of
energy.
73
Bob, K4TAX
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