[Elecraft] [K2] Mic bias bypass?

[David Woolley]

Out of the box all pins on the microphone connector are open circuit. 
There is an internal jumper block that has bias free microphone audio 
and resistor free 5 volts on it.  If you are seeing 5 volts on pin one 
on the  external connector it is because someone has wired 5v to it. 
There may or may not be a load resistor on that, although if pin 1 is 
audio, one would expect there to be one.

The jumpers are designed to be hardwired, but it is just possible to use 
an IDC connector as a pluggable jumper and there is a third party 
product, currently out of production, that provides plug in, 
configurable jumper blocks.

If the connector is wired properly, other than the bias, you want to 
unsolder the bias resistor from the jumpers.

It is unusual to find dynamic microphones on PCs, but if you do, they 
will normally have a mono plug.  The normal PC microphone socket feeds 
bias to ring and audio to tip.  If you plug in a dynamic microphone, 
bias is harmlessly shorted.  If you plug in an electret, the microphone 
has tip and ring connected together internally.

If you have a stereo plug on the microphone, I wonder if you have 
actually measured the substrate diode forward voltage, rather than any 
resistance.

-- 
David Woolley
Owner K2 06123

On 07/11/15 20:03, Neil Martinsen-Burrell wrote:
> I am wanting to connect a computer headset to my K2.  I bought the K2 used
> along with an Icom HM-12 hand mic that I have used successfully.  I haven't
> opened the radio up to see what the bias resistor is inside of the K2, but
> I measure +5V between pin 1 and pins 7 and 8 of the microphone connector.
> I believe that means that a bias resistor has been connected internally to
> put the bias voltage that the HM-12 needs on pin 1. (As a sidenote, trying
> to measure the bias on pin 6 with my DMM appears to reboot the
> microprocessor, sending the LCD screen back to "ELECRAFT"!!)
>
> I am not sure about the bias needs of the headset that I have, which I
> intend to use for infrequent SSB contesting. Following a recent message on
> this reflector, I measured the resistance from tip to sleeve of the
> microphone plug at 830 ohms. That message suggested that such a low
> resistance would be indicative of a dynamic microphone element.  The
> headset works fine with Skype on a laptop if that means anything.
>
> I've got a mic plug and a female inline 1/8 socket and I will be making an
> adapter for the headset. I want to be able to change from the hand
> microphone to the headset without making internal changes on the K2, of
> course. Ideally, I would like to use the same 1/8 inch to 8-pin adapter for
> soundcard digital modes with the K2.
>
> What should I do about the bias on microphone pin 1? Should I just use the
> headset as-is, risking distorted audio with a dynamic microphone fed 5
> volts of bias? Should I include a blocking capacitor (of what value?) in
> the adapter that I build?
>
> Thanks in advance for any advice you can offer.
>
> -Neil N0FN
>