Posted by
Mike S-8 on
Oct 22, 2004; 9:17pm
URL: http://elecraft.85.s1.nabble.com/Mod-for-more-AF-gain-on-K2-tp371029.html
It seems that some people have problems with low audio out on some K2's. There was a recent thread where I mentioned that it may be possible to easily substitute an LM386 for the LM380 AF amp and get more gain. I'm happy to report that this works FB.
In actual practice, I used an NJM386BD, available from Mouser for about 30 cents.
The 386 has a slightly different pinout than the 380, and also requires an external component to set the gain (the 380 has a fixed gain of 50). Pins 1 through 4 are identical, but pins 5-8 differ slightly:
Pin..380......386
5....Gnd......Vout
6....Vout.....Vss
7....Vss......Bypass
8....Bypass...Gain
As you can see, three of these pins are simply shifted by one position. I installed a socket where the 380 was (an AMP one, I've never had problems with either these or the nice Augat machined ones). I was then able to slightly bend the pins on both sides of the 386 slightly to offset them and allow all of the common pins to be plugged into the socket. This leaves the Gain pin (8), which simply needs to be connected to pin 1 via a 10 uF cap (sets the gain to 200), which was done piggyback style. Even with the socket and cap, there was still plenty of clearance for the KAF2 (and presumably for a KDSP2).
Results? At an AF gain setting which with the 380 produced 1.15 VAC at the internal speaker, I now get 4.00 VAC, an additional gain in power of about 11 db. It gets almost painfully loud with an S9 signal, and plenty loud with a signal which doesn't even register on the S meter. (I didn't have a problem with lack of power before, but now my AF gain goes to "11" HI HI).
If you compare the specs, the 380 can put out more power. In reality, I measured that the 386 begins clipping at about 4 V out and the 380 at about 5 volts, so the 380 only gets about 2 db louder. Having more output power available does no good if there's not enough gain to get you there. The 380 can go louder in absolute terms, but the 386 can get _very_ loud with _much_ smaller signals.
Mike
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