[Elecraft] Re: Mod for more AF gain on K2
On Oct 22, 2004, at 1:17 PM, Mike S wrote: 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. Mike, I'm glad that this worked for you. However, because of the problem with clipping at high signal levels, I definitely don't recommend that anyone do this unless they're desperate for more audio. The LM386 is a lower-power part and is also noisier than the LM380, partly because of all that extra gain. AF output can be increased in less drastic ways. Here are my favorites: - On older K2s, make the 2nd XFIL modification. This allows you to properly align L34, minimizing loss through the 2nd crystal filter in both SSB and CW modes. - Verify that the band-pass filters and L34 are peaked correctly. - Use Spectrogram to properly align the BFOs as specified in the manual. - Adjust the AGC threshold so that it isn't activated by low-level band noise. (First, turn AGC OFF by holding PRE and ATTN. If the background noise comes up, you may have the AGC pot set too aggressively.) - Add the KDSP2 option, which provides programmable AF gain well above the standard level. - Add the KAF2 option, which also increases AF gain (and it can also be adjusted by changing a couple of resistors). - The gain of the preamp can be increased by a few dB with little overall change in RX performance. Simply change R76 from 10 ohms to about 5 ohms. You can substitute a 5.6-ohm resistor, or just parallel another 10 ohm resistor across the existing one. In fact you can do this live to see what the effect is. - If all else fails, signal trace through the receive chain as described in appendix E. You could be losing RX signal in the T-R switch or elsewhere. Note that you can check your K2's performance easily with an Elecraft XG1 1-microvolt receiver test oscillator. 73, Wayne N6KR --- http://www.elecraft.com ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
[Elecraft] Re: Mod for more AF gain on K2
At 06:42 PM 10/22/2004, wayne burdick wrote... However, because of the problem with clipping at high signal levels, I definitely don't recommend that anyone do this unless they're desperate for more audio. The LM386 is a lower-power part and is also noisier than the LM380, partly because of all that extra gain. I was able to make both clip, but in both cases it was when the volume exceeded a comfortable level. The LM386 was putting out over 1/2 W when it did. I don't see where noise is characterized on the National datasheet, and nothing to indicate that the 380 has a better signal to noise ratio, which is what's really relevant. If more audio gain is needed to get useful volume, amplifying noise is unavoidable. The LM386 gain is adjustable, so if less than a 12 db boost is needed, that too is possible. It's reasonable to assume that a 386 configured for the same gain as a 380 would have comparable noise. I agree with your point that adding gain (or decreasing loss) earlier in the chain is preferred, and offered this as a solution for when that fails to achieve adequate volume, as it apparently has for some users. Low power is relative. I measured 4 VAC (p-p, ~500 mW ) on the 386 and 5 VAC (~850 mW) on the 380 at the onset of clipping, a relatively minor difference and inconsequential given that either level provides more than ample volume with the internal speaker. Going past clipping adds volume at the expense of distortion, but at the levels I observed it would only be useful for listening from across the room (or from the next room), where that may be a reasonable tradeoff. I doubt anyone who can make either clip is complaining about low volume. Mike ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
[Elecraft] Re: Mod for more AF gain on K2
I think we're on the same page, Mike. I just want to make sure builders know there are other alternatives before they jump into a different AF amp IC. Lord knows I've designed LM386's into a lot of rigs (the NorCal 40A, Sierra, SST, K1, and KX1, to name just those which made it out of the lab). I tame the noise by putting a resistor/capacitor network in series between pins 5 and 8. But ideally the gain and NF ahead of the amp would be adequate so that no heroics are required here. Regarding how loud is loud, the place where you need the audio levels that the '380 is capable of is in mobile or high-noise outdoor situations--such as Field Day with a generator nearby. Or in my rickety VW van at 60 MPH ;) 73, Wayne On Oct 22, 2004, at 5:06 PM, Mike S wrote: At 06:42 PM 10/22/2004, wayne burdick wrote... However, because of the problem with clipping at high signal levels, I definitely don't recommend that anyone do this unless they're desperate for more audio. The LM386 is a lower-power part and is also noisier than the LM380, partly because of all that extra gain. I was able to make both clip, but in both cases it was when the volume exceeded a comfortable level. The LM386 was putting out over 1/2 W when it did. --- http://www.elecraft.com ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
