[Elecraft] K2: more headphone output

Mon Dec 31 16:53:02 2001

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:[email protected]] 
Sent: 23. desember 2001 12:43
To: Elecraft reflector ([email protected])
Cc: '[email protected]'
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

[email protected] 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?