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Multi-band Low-pass Filter Kit for unfiltered amps

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Old 04-04-2011, 08:39 AM
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Default Multi-band Low-pass Filter Kit for unfiltered amps


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:



(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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Old 04-04-2011, 09:21 AM
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Nice project. In addition one can pluck bandpass filters from dead transceivers.

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Old 04-04-2011, 09:25 AM
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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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Old 04-04-2011, 10:51 AM
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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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Old 04-04-2011, 06:09 PM
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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.
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Old 04-04-2011, 08:44 PM
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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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Old 04-04-2011, 08:53 PM
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Quote:
Originally Posted by KM3F View Post
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.
I wonder if davemade made the list of paid advertisers there?
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Old 04-04-2011, 09:16 PM
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Quote:
Originally Posted by KM3F View Post
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.

Erm yeah.. k thanks.

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.

Last edited by eagle1911; 04-04-2011 at 09:29 PM. Reason: In a bad mood tonight lol!

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