[Elecraft] Mod for more AF gain on K2

[Mike S]

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
5Gnd..Vout
6Vout.Vss
7Vss..Bypass
8Bypass...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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Re: [Elecraft] Mod for more AF gain on K2

[G3VVT]

Another alternative to fit the 386 in the place of an LM380 audio amp in  the 
K2 would be to cut the print on the affected pins and fit insulated wire  
straps to change the print layout. Have not checked the 386 pin out to confirm  
the details, but pin 5 would need isolating from ground on the top/component  
side of the control board and on the underside 4 lines of print to pins 6, 7, 8 
 would need to be cut with something like a Dremel tool. New insulated wire  
straps could then be added to change the print to that required for the  386.
 
My own K2 (serial no. 4168) does not suffer from low audio and is run with  
the AF gain control at 1/4 to 1/3 rotation from minimum for most uses  with 
either the normal internal speaker or a single earpiece headset/electret  mic 
that I use with the K2. Is mostly used for SSB with OP1 filter in use,  only 
resort to using the drum (CW) in extreme circumstances to complete a  contact 
when condition are poor. I would be more susceptible than many to the  effects 
of low audio after loosing most of my hearing on one side after an  accident at 
work and the remaining ear down to telephone bandwidth only.
 
Is the low audio complaint applicable only to early K2's, the effects of  
narrow filters in CW reception or is it a random effect caused by gain  
variation 
of active components?
 
Regards,
Bob, G3VVT
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