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Multi-band Low-pass Filter Kit for unfiltered amps

Discussion in 'Amplifiers' started by eagle1911, Apr 4, 2011.

  1. eagle1911

    eagle1911 Active Member

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    To start this thread let me state that I am on a SERIOUS budget when it comes to my radio hobby, so I'm always looking for ways that I can get top-shelf performance (or close to it) from inexpensive gear.



    Right now I've got QRP capability on the HF bands. I need an amplifier, but the FCC type-accepted options are more expensive than I'd like. Because of this I've been looking at many different options, mostly broadband cb-type amps.

    I've found that RM Italy makes a decent amp for very little cash comparatively. Since the core push-pull transistor configuration is pretty much the same across almost all solid-state amps, the brand isn't overly important. What is important is the bandwidth. RM makes several simple amps that cover 80-10m, which was ultimately what sold me on the KL-300p and KL-400. I have examples of each, and I've upgraded the KL-300p to class AB as can be seen here. The KL-400 is class AB from the factory.

    Now that the amps are class AB, the only thing left is to provide a good mutli-band low-pass filter system. I decided that I would like to have a stand-alone low-pass filter box that I can put in line with ANY 100-150W amplifier I might own or build.. I don't plan on exceeding that power level until I can afford a legal-limit amp from a mainstream manufacturer.

    I originally was going to build a set from scratch based on plans found in the ARRL book "RF Amplifier Classics", but then I found this sweet little kit: Kit - Low Pass Filter Module 100W Relay Control 160-10M It's affordable and very easy to build thanks to extremely detailed instructions. Virgil (K5OOR) is very good with customer service and provded fast answers to the few questions I had.

    Using a simple single-pole 6-position switch it is possible to switch between 6 bands, covering 160-10m. 12VDC must be supplied for the relays, but the current draw is negligible.

    I've assembled and tested the circuit, it works great. Here are a couple pics of the completed PCB:

    [​IMG]
    [​IMG]
    (I'll post pics of the entire completed box when I finish the build tonight or tomorrow)

    Just thought I'd share this in case there are other ops out there that are interested in adding high-end features to basic CB-type amps. These additional features (bias modification combined with band-specific low-pass filtering) provide a MUCH cleaner signal than class C CB boxes on their own.. This is better for everyone involved. The station op gets a low-distortion signal on SSB and AM, and the rest of us get less splatter and interference on the bands.
     
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  2. Kamikaze

    Kamikaze Active Member

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    Nice project. In addition one can pluck bandpass filters from dead transceivers.
     
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  3. Moleculo

    Moleculo Administrator Staff Member

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    Cool! That's the board / kit that's from the HF Packer 100 watt amp kit, right? That's a good idea to make it stand alone for your own use on any 100 watt amp. The finished project will be nice, I'm sure. Show us the pics when you're done!
     
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  4. eagle1911

    eagle1911 Active Member

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    Thanks all!

    Moleculo: yes, it is the LPF assembly for the HF Packer 100W amp. It seemed ideal for what I want to do with it, plus from the looks of it it will handle a bit over 100W.

    Kamikaze: I've actually been all over ebay for a few weeks looking for a harvested LPF unit but I've not had any luck. The only one I found and won was sold out from underneath me by an unscrupulous Brit. Part of me doesn't mind though, since building one, either from scratch or a kit, is pretty rewarding.

    I will post pics of the finished item very soon!
     
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  5. Captain Kilowatt

    Captain Kilowatt Professional Amateur Staff Member

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    That's a great idea and it looks to be a decent filter. Glad to see that you realised that it takes more than just a filter to make a class C amp sound decent on the bands.Good work on the conversion to class AB. (y)
     
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  6. KM3F

    KM3F Active Member

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    If you want to follow the letter of the law there is more to this than just the harmonic filtering.
    The devices used should have low intermod distortion and not be driven more than about 80% of the available output.
    You must know that 12 volt devices don't have much head room before RF compression becomes an issue.
    This means the input vs output curve begins to flatten above about 80% of max output.
    This results in distortion and adds to IMD at voice peaks.
    The RM units are not known to be all that good.
    You get what you pay for.
    Go to the QTH.Com website and look at th e" FCC approved list" of amplifier mfgers. The RM brand is not there.
    Good luck.
     
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  7. mackmobile43

    mackmobile43 Jock Supporter

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    I wonder if davemade made the list of paid advertisers there?
     
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  8. eagle1911

    eagle1911 Active Member

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    Erm yeah.. k thanks. (y)

    In fact, I am aware of all of that, but as an amateur radio operator in the US, FCC type approval of the equipment you use in not a requirement. It is only a requirement for equipment that is entered into commerce. Imagine what a damper it would put on experimentation and DIY if it was.

    I am also keenly aware of the specs for the "devices" used in all of my amps and I drive the circuit accordingly. I'm not looking for more than 120-130W PEP from these amps in the extreme case.

    What's interesting to me is that there is always some guy there ready to bash RM.. this is made even more interesting to me when I consider the fact that a push-pull pair of SD1446s at a given voltage with appropriate feedback and AB bias will perform very similarly whether assembled by a company with FCC accepted products or by me in my shop.. Both of the amps that I mentioned above are simply a push-pull pair and biasing. RM's is no different than any other at the core. There are some subtle enhancements that can be made to the circuit that aren't found in these, like LCR feedback to flatten the gain curve across all bands, but these are not earth-shattering. In fact, I've added inductance to the feedback network on my KL-400 since I do enjoy those DIY mods.
     
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    Last edited: Apr 5, 2011
  9. dudmuck

    dudmuck Active Member

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    I need to wonder how much harmonic problem really exists.

    Has anyone ever heard signal from somebody transmitting on a lower band than you're receiving? I'm sure it can happen, but has it really happened, or is it common.
    HF Ham bands were selected so that most harmonics on lower bands will fall on the upper bands.

    I would suspect, myself, that 3rd order IMD would be a bigger problem than 2nd order intermods.
     
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  10. eagle1911

    eagle1911 Active Member

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    I would agree with that actually, and from what I know that can be vastly reduced by not overdriving the amp..
     
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  11. 5R8GQ

    5R8GQ Member

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    WTF does BANDWIDTH have to do with harmonics??? :oops:

    And like the guy said, RM Italy CB Amps like the KL Series,
    the problem is IMD, not harmonics. They are CB JUNK pure and simple.
    HLA Series not so bad, but still advertised pwr output is at overdriven devices.


    You are fooling yourself if you think your "Quick and Dirty Mod"
    will clean up IMD.


    Look, I have this K5OOR LPF filter, OK.
    It's a cute TOY!
    It is nice for what it is, for 60 watts ICCS SSB,
    but do NOT plan on using it even one watt over 100 watts, IF THAT.


    Use it ONLY AT 100 watts PEAK on SSB, IF THAT.

    Do NOT use it on heavy duty cycle modes, even 50-60 watts PSK31.

    Those little crammed togther toroids on thin gauge wire get
    VERY HOT, and
    the relays are just too small for the job over time.


    Also, BE SURE to check your input SWR into the K5OOR Filter Board,
    especially on the low bands. Many have had problems there.


    I understand restricted budgets,
    But this is NO EXCUSE TO USE CB JUNK ON THE HAM BANDS.
    We see this more and more as, especially in USA, the Code is eliminated, the
    Technical Exams are Dumbed Down, and more and more CB'ers pour into hobby. .
    :bdh:

    CB'er in hobby is OK...AS long as they they don't bring their Old Wive's Tales
    about electronics and amps, and their "pretend knowledge" to the hobby
    .


    Give up this project and build a real amplifier.
    What is American expression?
    You cannot build silks handbag out of pig's ear?

    73, Ken 5R8GQ
     
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  12. GnG8d

    GnG8d Well-Known Member

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    ^^ This is funny.
     
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  13. RatsoW8

    RatsoW8 Supporting Member

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    I assume you've done testing of the RM amps to support your rant or have access to data? I believe the RM amps are becoming quite popular with European hams and if they are run per spec are very usable with clean output. I have an old KL-203 with some QC issues which were easlily repaired but their latest offerings seem to be well built. And yes , they are an affordable alternative if your looking to add an amplifier.

    I use an AL-811 with my yaesu FT-950 but an RM mobile amp isn't out of the question for mobile use with an HTX-100.

    By the way, when did the FCC add a requirement for IMD specs in amateur amplifiers?
     
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  14. Kamikaze

    Kamikaze Active Member

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    One thing is for sure. 10 meters is going to be a real zoo from now on.
     
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  15. eagle1911

    eagle1911 Active Member

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    Man I just love it when people post like they've got you all figured out just by reading about some things you've built. I've been a ham since 1993. I'm no spring chicken.


    Are you ok man?

    I think we need to call a Doctor or something for this guy.. somewhere, in front of his computer, 5R8GQ just had an aneurism.

    It must really suck to wake up every day feeling that strongly about a freaking HOBBY. A true embodiment of someone who needs to get a life.

    Obviously anyone who doesn't want to do the mod, doesn't have to.


    Now I'm going to ask that you please do us all a favor and rather than rant endlessly in rainbow colors, explain in detail why you feel the way you do.

    Please be sure to touch on the following points:

    Explain in detail why, if you in fact read my whole post, you feel that I was connecting bandwidth to IMD or any other type of distortion in some way?

    Explain in detail why you think that I am trying to over-drive the amp past 100-120W PEP when I specifically said that these amps WILL NOT make more than that without far too much distortion?

    Explain in detail why you feel that adding bias to a correctly driven class C solid-state amplifier will have no effect on the cleanliness of the amplifier's output signal?

    Explain in detail why you feel the need to point out the obvious power limitations of the 100W PEP LPF that was graciously provided to me by K5OOR when he has already done so quite clearly on his website WITHOUT being a complete ass I might add?

    In 12 point Times New Roman, and all black please. Thanks.

    And finally, please send me money so that I can buy cool ready-made gear like you rather than experiment and learn as I have been doing.

    IMO you should just be happy that I'm trying very hard to keep my signal clean, while learning as much as I can about this stuff. You can take your self-righteous ranting and shove it straight up your ass my friend.. just for the record.

    Gotta love them Trolls.. Kamikaze, you know what I'm talking about don't you.. You know, the trolls that talk shit but have zilch to back it up.. Trolls like you and 5R8GQ here. You guys should form a little club.

    I await your schooling with bated breath! LOL!
     
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    Last edited: May 6, 2011

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