[Elecraft] fixed audio for K2

[Jack Brindle]

On Apr 23, 2004, at 9:13 PM, <jeff_elecraft@burnshome.org> wrote:

> Receive audio and the side tone comes together at the audio amplifier  
> U9 on
> the control board. Is VOL1 (pin 2 U9) level fixed? If so, could a  
> second
> LM380N-8 be installed on top of U9? Pin 1,2,4,5,7, and 8 would connect
> directly to the part below. Pin 3 would connect to ground. The fixed  
> level
> audio would be at pin 6.

This would not provide fixed level output since it occurs AFTER the  
volume control pot. It simply parallels the audio pot, so it would  
provide essentially the same results as the audio amp now in the  
circuit. I don't think this will provide what you are looking for.

> One concern is the current the extra amplifier will draw when nothing  
> is
> attached to the fixed output connector.  If I understand the data sheet
> correctly, this could be as high as 25mA. Is there another part that  
> would
> be a better choice? Is there a connector available with a switch that  
> would
> power on the amplifier when a cable is plugged in?

The LM380 would probably have a pretty low current drain if it isn't  
connected to an output since it is essentially driving nothing. Be  
careful that the RC circuit to ground is applied or you will see the  
LM380 draw lots of current as it oscillates.


On Apr 23, 2004, at 11:46 PM, David Pratt replied:

> There is a fixed level audio output available at J5. This is a 3-way  
> connector towards the front of the RF board and connected to pins 4 &  
> 5 of the product detector U11 (NE602). The connector is not fitted to  
> the board, put the pads are available for it.  The output at that  
> point is a fairly low level but it could be suitable for your purpose.
>
> Perhaps Eric or Wayne might like to tell us the intended purpose of J5  
> on the K2?

The K2 manual provides a hint as to its purpose. In Section 9,  
"Modifications" the following suggestion is made:

"A low-level audio signal is available at J5 on the RF board (Aux.
AF). This signal could be used to drive an external audio
filter/amplifier, RTTY terminal unit, etc. You'll need to supply
your own connector to get at this signal (3 pin, 0.1" spacing)."

This does not provide the sidetone, but if you really need it, it can  
be tapped and mixed into the amplifier just like the current K2 circuit  
does.

Thanks, David for pointing out J5. I had completely missed it - it  
looks like it could be the right answer to the fixed-output audio  
situation...

- Jack Brindle, WA4FIB
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