Proper Use of the Preamp
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Hi all,
I have K2 #402, and am planning to upgrade it someday. But I was using it last night during the QRP-L fox hunt, and in its present state this K2 seems to be quite exceptional listening to weak signals (or static) on 40 meters. For most of the time, when I had actually located both of the foxes, they were in the noise most of these times. I finally got both by waiting until propagation got momentarily marginally better. My strategy was that I figured that propagation would either get better (50% chance) or worse (50% chance) and it was no use transmitting until I could copy most of what the fox was sending. Why send when you can't hear the fox give your call sign back?(HINT!!). Anyway since I was using all the performance my K2 could give me listening to mostly static, I tried putting the preamp off. It is usually on all the time. What I found was that I seemed to hear the fox better over the static with the preamp off, but the volume was too low. With the preamp "on" I had plenty of excess audio volume, but with the preamp off I didn't have enough volume I felt necessary. With the preamp "on" I also tried putting the af volume on full and then reducing the rf gain slightly. This actually helped but I sensed that using the rig with the preamp off and having a little extra volume would be significantly better. So I wonder if I need more af gain in the rig or maybe a little amp on the way to the headphones. Or maybe I am imagining things and somebody with more experience can set me straight. Thanks, Steve, W2MY/5 --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.786 / Virus Database: 532 - Release Date: 10/29/2004 _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
RE: Proper Use of the Preamp
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Unless you are using a very tiny, inefficient antenna (like a small loop or
very short whip) on 40 meters, there's no reason at all to use the preamp, Steven. The purpose of the preamp is to improve the noise figure on the higher-frequency bands, not to make the receiver louder. It does that, of course, but that's a secondary benefit. The issue is that the 'atmospheric' or 'background' noise on the higher bands - usually above 15 MHz - can be so low the internal noise in the K2 receiver could mask signals. The easiest way to tell if that's true is to turn the preamp OFF, tune you K2 to an empty frequency so all you hear is noise, set the IF filter for the narrowest bandwidth you might use, and disconnect your antenna. If then background noise level drops when you disconnect the antenna, you do *not* need the preamp. The sensitivity of the K2 is entirely limited by the background noise. You need to use the narrowest bandwidth because the noise the receiver picks up is less the narrower the bandwidth. That's exactly why for a given power a CW signal will be stronger at the receiver end than an SSB signal, everything else being equal. The SSB signal needs a wider receive bandwidth so the receiver picks up more noise along with the signal. Your comments about low audio suggest another problem. The K2 is designed to minimize current drain because many of them are used with internal batteries. The designers included a relatively low current audio output I.C., but it should have enough output to "rattle" the phones or speakers on any band without the preamp! If both the speaker and phones are anemic, check your AGC action. Some K2's have overly aggressive AGC action that causes the AGC to limit the maximum volume. Look under "Builder's Resources" on www.elecraft.com or on Sverre's excellent site at http://tinyurl.com/4msw4 for more info about testing your AGC action. IF the speaker is plenty loud (none of them are loud enough to use in a noisy vehicle, but they are loud enough to be objectionable to most ops in a room on outdoors in a park, etc.) but the phones is weak, try a different set of phones. The K2 is designed for use with typical "computer phones" or "walkman phones". They have a higher impedance (about 30 ohms I think) than the typical "hi fi" headphones. There are some resistors you can remove to improve the gain for lower-impedance "hi-fi" phones. That mod is also described on the above sites. You spoke of finding the copy of weak signals better by turning down the RF gain. A great many of us ... Er.... "older tymers" spent the first years or decades at a radio never using the AF gain. We controlled the output using the RF gain control. CW receivers didn't use AGC then either. The reason is what you found. Signals often seem to stand out better if the gain is controlled by the RF instead of the Audio gain. The K2 receiver is excellent, and the differences between using the audio gain (and AGC) and the RF gain are slight. Still, many of us prefer to use the RF gain on CW especially. Note that on the K2 there really is *no* "RF Gain" control as the term is used on older receivers. Originally, the RF gain meant a gain control that adjusted the gain of the first RF amplifier at the antenna input in the receiver. Since the K2 doesn't use such an amplifier (unless you turn the preamp ON), it adjusts the gain of the IF amplifier and calls that the RF gain control. But the effect is still much the same. Many of us like the "clean" sound we get when digging for a weak signal that way. One thing it does is that using the RF gain effectively disables the AGC. The AGC will tend to "fill in" the gaps in the CW signal by turning up the gain during breaks in the transmission. This tends to make the noise sound as loud as the signal when it really is not that loud. That is especially true if you are using FAST AGC action instead of SLOW. The SLOW action delays the 'pumping' action of the gain so as long as the CW station keeps sending the noise level stays down. Still, it's nice to forget the fancy AGC and do it "by hand" with the RF gain control for many ops. It sounds like the first step for you is to find out why you can't get enough audio without the preamp, then you can explore all the reasons to ignore the preamp and AF gain for most CW operating <G>. Ron AC7AC -----Original Message----- From: [hidden email] [mailto:[hidden email]] On Behalf Of Steven Pituch Sent: Friday, November 05, 2004 1:17 PM To: [hidden email] Subject: [Elecraft] Proper Use of the Preamp Hi all, I have K2 #402, and am planning to upgrade it someday. But I was using it last night during the QRP-L fox hunt, and in its present state this K2 seems to be quite exceptional listening to weak signals (or static) on 40 meters. For most of the time, when I had actually located both of the foxes, they were in the noise most of these times. I finally got both by waiting until propagation got momentarily marginally better. My strategy was that I figured that propagation would either get better (50% chance) or worse (50% chance) and it was no use transmitting until I could copy most of what the fox was sending. Why send when you can't hear the fox give your call sign back?(HINT!!). Anyway since I was using all the performance my K2 could give me listening to mostly static, I tried putting the preamp off. It is usually on all the time. What I found was that I seemed to hear the fox better over the static with the preamp off, but the volume was too low. With the preamp "on" I had plenty of excess audio volume, but with the preamp off I didn't have enough volume I felt necessary. With the preamp "on" I also tried putting the af volume on full and then reducing the rf gain slightly. This actually helped but I sensed that using the rig with the preamp off and having a little extra volume would be significantly better. So I wonder if I need more af gain in the rig or maybe a little amp on the way to the headphones. Or maybe I am imagining things and somebody with more experience can set me straight. Thanks, Steve, W2MY/5 _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
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Everyone,
Thanks for the suggestions about my lack of sufficient audio with my K2, something I never really noticed until last night. One thing that rang a bell after reading everyone's input was that last night I also played around with the AGC controls and did not notice anything different with any setting. I have not experienced any pumping or increase in volume by say turning the AGC off. The rig has worked so well so far that perhaps I have not ever noticed that maybe the AGC hasn't ever worked. So I will check the AGC. I would think that the Elecraft team would have designed a "kick ass" kind of AGC, like many of the other quality designed parts of the radio. By the way my favorite head phones are my older Heil phones with mike which I have been using with the K2. To think that this radio easily beats my TS850, and that maybe I can fix something to make the K2 even better! Thanks again, Steve, W2MY/5 --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.786 / Virus Database: 532 - Release Date: 10/29/2004 _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
RE: Proper Use of the Preamp
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The AGC (if working correctly) and the dynamic range of that receiver is
excellent. The great dynamic range is why you can get away with stuffing all those extra microvolts into the front end with the preamp ON without causing other problems. The AGC is typical of most rigs I've seen in recent years. It has good delay characteristics, but it is not a "hang" type AGC that picks an average level and stays there for a certain amount of time after the signal stops before a timer suddenly returns to volume to full level. I guess a lot of ops didn't like hang agc because if you were listening to a real strong signal with the audio up high, when the hang time expired it sounded like someone kicked on the biggest, baddest noise source in town! It was well in to the production of the K2 that it was discovered that minor in-tolerance variations in parts can, in some rigs, add up to an over-aggressive AGC action. Another major feature of the Elecraft rigs is that when something like that is discovered, they figure out a fix that can be retro-fitted to all K2's instead of saying "Too bad, so sad. Sell it to someone and buy our new, improved (and more expensive) model next year..." Ron AC7AC -----Original Message----- Everyone, Thanks for the suggestions about my lack of sufficient audio with my K2, something I never really noticed until last night. One thing that rang a bell after reading everyone's input was that last night I also played around with the AGC controls and did not notice anything different with any setting. I have not experienced any pumping or increase in volume by say turning the AGC off. The rig has worked so well so far that perhaps I have not ever noticed that maybe the AGC hasn't ever worked. So I will check the AGC. I would think that the Elecraft team would have designed a "kick ass" kind of AGC, like many of the other quality designed parts of the radio. By the way my favorite head phones are my older Heil phones with mike which I have been using with the K2. To think that this radio easily beats my TS850, and that maybe I can fix something to make the K2 even better! Thanks again, Steve, W2MY/5 _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
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