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Re: [Amps] Advice needed for SS amp

Subject: Re: [Amps] Advice needed for SS amp
From: Alan Ibbetson <>
Date: Thu, 06 Jun 2013 22:53:15 +0100
List-post: <">>
From: Manfred Mornhinweg<>
>Since the output impedance is about 50 ohms, I'm wondering if a 1:1
>broadband antenna balun might serve for the output toroid? Just a thought.
>Might not be a good idea.
Antenna baluns usually don't have galvanic insulation between input and
output, so you can't use them in an amplifier powered directly off-line.
   But using the core of such a balun, and rewinding it properly, should
be possible.

I am at exactly the newbie stage Manfred advocates: building a 50W amplifier using a pair of one dollar switching FETs, in my case IRF640, and a cheap 48V laptop PSU. I have a prototype more or less working for use on CW on 60M.

I'm interested to know why a common mode choke type of balun is such a bad idea, seeing as I am using one to couple the drains of my amplifier to the harmonic filter. I have 5 turns of RG316 on a core with OD about 18mm, giving a choking inductance of around 17uH. Balance seems fine and this configuration seems much less beset by problems with leakage inductance compared to my attempts at making multi-winding conventional transformers with bits of brass tube and so on.

For "galvanic isolation" (I call it DC blocking) I am using physically moderately large 100nF multilayer ceramic capacitors rated 100V. Of course it's the current rating that matters and I have no idea what it is for my capacitors except it must be enough because they do not get hot.

OK, so the blocking capacitors in my baby amplifier don't have to handle the 5 or so amps of RF in a 1KW+ amplifier, but such capacitors do seems to be available, particularly if you use several in parallel.

Or do really bad things happen at the higher voltages and currents in serious amplifiers that I have overlooked?

73, Alan G3XAQ
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