[Boatanchors] Headphone Impedance Matching

[Richard Knoppow]

----- Original Message ----- 
From: "mac" <w7qho at aol.com>
To: <dhallam at knology.net>
Cc: "Richard Knoppow" <1oldlens1 at ix.netcom.com>; "Bill 
Cromwell" <wrcromwell at gmail.com>; 
<boatanchors at mailman.qth.net>
Sent: Thursday, December 05, 2013 4:56 PM
Subject: Re: [Boatanchors] Headphone Impedance Matching


> Sounds like your Telex 600 ohm unit simply has a broader 
> frequency
> response than either the speaker or the 2000 ohm 
> headset.....
>
> Dennis D.  W7QHO
> Glendale, CA
>
> **********
> On Dec 5, 2013, at 4:43 AM, David C. Hallam wrote:
>
>> OK, next question about my headphone impedance matching. 
>> I have
>> listened to the audio from my 75S-3 in three different 
>> ways.
>>
>> 312B-4 Collins speaker
>> Telex 600 ohm impedance headphones
>> Lincoln/Telex 2000 ohm impedance headphones
>>
>> When using the Telex 600 ohm headphones and the audio 
>> gain turned
>> all the way down, I hear a distinct hum.  I do not hear 
>> this hum
>> with either of the other two.  Any explanation of why I 
>> hear it only
>> with the 600 ohm headphones?
>>
>> David
>> KW4DH
>
    I've found the same thing with a couple of receivers 
when using low impedance hi-fi headphones. The residual hum 
is not audible using a loudspeaker or higher impedance 
phones but is distinct with the hi-fi phones. Its a 
combination of good low frequency response and high 
sensitivity due to the good match.  On a receiver where the 
phone jack is right across the speaker a set of 8 ohm or 
even 30 ohm phones will be extremely loud and will show up 
any residual hum or noise.  Note that old fashioned magnetic 
headphones are mostly designed to have a strong resonance at 
about 1 khz and very little low frequency response. This is 
true regardless of the impedance. On the same receivers that 
exhibit the hum using my Sennheiser hi-fi phones no hum can 
be heard using 600 ohm ANBH-1A phones. These are moving coil 
head phones designed for use in aircraft and essentially the 
military version of phones made by Perm-o-Flux and sold for 
broadcast monitoring. They do not have as good low end 
response as the Sennheiser phones but are pretty good. The 
difference is the mis-match when connected across an output 
meant to drive loudspeakers.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com