Re: Re: Mod for more AF gain on K2

Posted by Mike S-8 on Oct 23, 2004; 5:19pm
URL: http://elecraft.85.s1.nabble.com/Mod-for-more-AF-gain-on-K2-tp371029p371034.html

At 11:00 PM 10/22/2004, wayne burdick wrote...
>I tame the noise by putting a resistor/capacitor network in series between pins 5 and 8.

Perhaps a "real" LM386 has more noise. Noise in the NJM386B I used measured ~20 mV p-p in the absence of signal (AF pot at min, measured at the speaker). That's 46 db down from full output, just barely audible via either speaker or headphones. This noise is quickly concealed by other rig noise (preamp off, dummy load connected) as soon as you turn up the AF just a bit, except for some higher frequencies which don't get masked and are only noticeable with headphones. Putting the 380 back in, I measured 8 mV of noise, about 8 db better than the 386. That difference contrasts favorably to the 11 db difference in gain I observed between the two.

I then installed an RC network with the 386, as mentioned above (2.7K/.01u, copying what's in the KX1). It didn't change the level of the overall noise significantly, but it did pretty much remove the noticeable higher frequencies from the output.

All in all, not a bad change - trading about 2 db in maximum volume for about 11 db in gain. Since I socketed this and built everything piggyback on the 386, it's a simple chip swap to revert to stock condition.

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