-- Richard Knoppow Los Angeles WB6KBL[email protected]
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There was some recent discussion, I think on this list,
about headphone levels on the R4 B RX. Having inadvertently
become a headphone collector recently I decided to see what
sort of levels I got from several pairs of phones. The
measured impedance varied from abuout 27 ohms, connected in
parallel for mono, for a pair of new Sennheiser 280-Pro
phones (rated at 64 ohms per side) to around 25Kohms for a
pair of old Western Electric 509-W and a pair of WE Signal
Corps P-11, also about the same impedance. I also checked a
couple of pair with intermediate impedance (around 10 to 15
ohms), a couple of pair of 500/600 ohm phones, and a couple
of pair of WE phones with about 50 ohm impedance. The output
impedance of the R4-B is around 4 to 8 ohms, right across
the output winding for the loudspeaker.
The results were as might be expected, all the low impedance
phones were very loud and had significant output with the
volume control at minimum. The medium impedance phones (500
ohm) were the next loudest and had some output with the
volume at minimum. The old-fashioned high impedance phones
had little or no output at minimum volume and required the
control to be at about normal level.
I did not expeiment with a low value resistor across the
output for the high impedance phones but it might affect the
frequency response somewhat depending on the stray
reactances in the output transformer.
Conclusion: Its normal to get residual output with
sensitive low-impedance phones. They are close to matching
the output impedance of the receiver. A loudspeaker requires
so much more power that the residual is not noticable at any
normal listening distance.
This all may seem trivial but the question did come up
and I got curious about it.
