++++++++++++++++++++ See also Headphones +++++++++++++++++++ Reply-To: From: "Sverre Holm - LA3ZA" To: "Elecraft at Mailman. Qth. Net" Date: Sun, 30 Dec 2001 19:02:38 +0100 Subject: [Elecraft] K2: more headphone output I have done some measurements and experiments with a smaller value for the headphone series resistors. I paralleled the two 82 ohms resistors with 22 ohms each. I then adjusted my 600 Hz sidetone level for 100 mV rms before the resistors, on my multimeter, in my K2 this is St L = 056. I have done the sidetone modification, so my sidetone is close to sinusoidal. Measurement using a pair of 1970's Yamaha HP1, orthodynamic headphones (quite ineffecient I think - DC resistance 125 ohms): original 56 mV, after mod: 86 mv, increase 3.9 dB. Now I have no noticeable difference in audio level whether I use the headphone jack or the speaker jack in the rear. Measurement using a pair of recent Sony CD player earplugs (MDR-ED136) (DC resistance 17 ohms): original 17 mV, after mod: 48 mv, increase 9 dB. The modification is easy to do after removal of the front bottom cover of the K2 (not the one attached to the RF output transistors). The resistors are located at the front of the RF-board, next to the headphone jack, the pair of 22 ohm resistors are best put on the under side of the board. Happy New Year to all Elecrafters, including the staff at Elecraft! Sverre Holm, LA3ZA Oslo, Norway ----------------- www.qsl.net/la3za -----Original Message----- From: Sverre Holm - LA3ZA [mailto:svholm2 at online.no] Sent: 23. desember 2001 12:43 To: Elecraft reflector (elecraft at qth.net) Cc: 'N2EY at aol.com' Subject: Re: [Elecraft] K2 Audio Question I think the resistors in series with the headphone jack can be reduced substantially. I looked at WB4LVQ page of component data sheets for the K2 at http://www.qsl.net/wb4vlq/, and the LM380-N is said to be "short circuit proof with internal thermal limiting". The resistors could probably be reduced from 82 ohms to say 15 ohms without problem. (I just picked 15 because then a dead short on both headphone outputs will give a standard load of almost 8 ohms) Sverre Holm, LA3ZA Oslo, Norway ----------------- www.qsl.net/la3za N2EY at aol.com wrote on 18 Dec 2001 08:20:59 EST: >But Looking at the K2 schematic, I note that there are two 82 ohm >resistors in series with >the audio to the 'phones. >I think these resistors (R35 and R36) are there to avoid dead-shorting >the audio if a mono >plug is inserted into the headphone jack. They >also knock down the audio level a bit - if >8 ohm 'phones are used, >only about 1/11th of the voltage gets to the 'phones. >Would there be any ill effects in reducing the value of these >resistors, or removing them >entirely? Will the audio output chip be >damaged by a dead short on the output? ++++++++++++++++++