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" <[email protected]> To: "Elecraft Reflector" <[email protected]> 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 > ------------------------------------------ > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
