[Elecraft] Yamaha CM-500 (was Heil Elite thread)

[Paul Christensen]

The 8.2 ohm build-out resistors set the output Z of the K3's headphone amp. 
Although not specified on the LM4811 datasheet, the output Z of the LM4811 
is very low and is only a small fraction of the external build-out 
resistance.  So, I think it's safe to say that the K3's headphone output Z 
is 8-ohms.

Don't try to strictly match headphone Z with the K3's 8-ohm output Z. 
Although the LM4811 is specified to produce maximum power transfer at 
16-ohms, headphone sensitivity can vary across models and brands.  So, it's 
possible that a set of headphones with a specified Z of 32 ohms could 
produce more sound-pressure-level (SPL) than a set of phones with a 
specified Z of 16 ohms.

Headphone driver technology has come a long way since the development of the 
standard 8-ohm moving coil.  Headphones that produce the most SPL per 1 mW 
of audio input power are the most efficient.  But, as purchasing factors, 
sensitivity needs to be balanced with comfort, reliability and price.

Paul, W9AC


----- Original Message ----- 
From: "Dave Hachadorian" <k6ll at arrl.net>
To: "Elecraft Reflector" <elecraft at mailman.qth.net>
Sent: Friday, August 27, 2010 4:29 PM
Subject: Re: [Elecraft] Yamaha CM-500 (was Heil Elite thread)


>> what impedance is the headphone jack on  the k3?
>
>
> ----------------------------------------------------
>>From Lyle on 6/28/2010:
>
> The LM4811 headphone amplifier provides maximum power transfer
> into a 16 ohm load (105 mW), and 70mW into a 32 ohm load.  As
> you increase the load impedance, the headphone driver becomes
> voltage limited.  If you reduce the impedance too much, it
> will become current limited. There are 8.2 ohm resistors in
> series with left and right channel outputs of the LM4811 and
> the front panel headphone jack, and another pair of 8.2
> ohm resistors in series from the LM4811 to the rear panel
> headphone jack.  The resistors protect the headphone amplifier
> from short circuits, and also reduce the effect of reduced
> output in one set of headphones if another set is plugged
> into the other headphone jack.
>
> 73,
>
> Lyle KK7P
> ------------------------------------------
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