[ARC5] [Milsurplus] Audio Impedance Matching on USAAF aircraft

[1oldlens1]

Headphone impedance went from very low, maybe 50 ohms, for telephone receiver use, to very high, perhaps 20K ohms for use with crystal detectors. Many phones were rated by DC resistance. For magnetic phones the impedance was something like 5 to 10 times the resistance.  As an example, the well known Western Electric 509W phones were rated at 1100 ohms each DC or 2200 ohms DC per pair, in series, or about 22K ohms impedance at 1Khz.  The famous Baldwin phones were about the same, to meet Navy specifications. But, Baldwin phones were balanced armature with mica diaphragms.  Both of these were strongly resonant at 1Khz.  Later phones designed for aeronautical use, were less resonant and were rated at 8000 ohms.  Still later phones, the type ANBH-1A  were moving coil type, with flat response over the speech range and were 600 ohms.  The same phones were made for commercial use at both 600 and 50 ohms, used for broadcast monitoring. Very similar phones are still made for audiometry.  The ANBH-1 was developed by the acoustics lab at Harvard university.Probably the most thoroughly engineered headphone was the Western Electric receiver fot the 500 type telephone, known as a ring-armature mechanism.  Sent from my GalaxyRichard Knoppow Los AngelesWB6KBL
-------- Original message --------From: Hubert Miller <Kargo_cult at msn.com> Date: 12/30/25  6:44 PM  (GMT-08:00) To: ARC5 <arc5 at mailman.qth.net>, List Milsurplus <milsurplus at mailman.qth.net> Subject: Re: [ARC5] [Milsurplus] Audio Impedance Matching on USAAF aircraft 


8000 ohms seems so old-world, like 1920s radio.
I have some kind of small ‘impedance adapter’. I don’t know where it is right now or the item designation.

But i think it’s nothing but a pot inside.
-Hue Miller





-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/arc5/attachments/20251230/bc6f4e02/attachment.html>