[Elecraft] KX1 speaker output with superimposed oscillation

[Don Wilhelm]

Paco,

That certainly sounds like the strong station is overdriving the LM386, 
and that the overdrive point changes depending on the load to the 
LM386.  Try reducing the RF gain to see if the condition is improved.

The LM386 characteristics are different than the LM380 AF Amp used in 
the K2, so the fact that your headphones and external speaker work fine 
with the K2 is not really a good indication.

The load to the AF Amp in the KX1 changes as the AF Gain is changed, so 
the fact that you notice the distortion only at the max settings is not 
an indication of a problem.  It may just be that you cannot use the 
RP-HT60 headphones nor your 8 ohm speaker at max AF Gain with your KX1.  
A 4 ohm speaker or an amplified speaker may be the best solution, 
although you could reduce the gain of the LM386 to prevent the strong 
input signals from driving it to the zero voltage rail.

73,
Don W3FPR

Francisco Barros wrote:
> Hi to all,
>
> The scope shows a superimposed high frequency oscillation just on the 
> negative peaks of the audio signal.
> It produces a distorted and annoying audio when the signal peaks, 
> specially when listening a loud station on the external speaker.
> This happens when the volume is close to or at maximum volume. I fails 
> just with Panasonic RP-HT60 20 Ohm headphones and external 8 ohm 
> speaker (both work fine with my K2).
> It works OK with Panasonic earbud type headphone and with an external 
> 8 Ohm load !!.
>