Here are two versions of the best clipper circuit I could find in a short search of the radio handbooks I have on my, somewhat limited, book shelf. I was trying to find a fairly simple clipper circuit I could use ahead of a good audio filter to help eliminate static crashes and the key clicks from my own QSK rigs, and to provide some of the advantages of AVC for those receivers lacking one that operates on CW. They are supposed to be very effective and are fully adjustable. One must balance three controls in order to get the best use out of these: the receiver audio gain control, the clipping level, and the clipper audio gain. Although an oscilloscope is not essential, it would make it a bit easier. First you set the clipper audio gain control at about center with clipping level at zero. Then you adjust your receiver audio gain for a comfortable level. Then adjust the clipping level on a signal you find to be at that comfortable level so that there is no apparent change in audio quality. Lastly, adjust the clipper audio gain for your chosen level. What SHOULD happen is that all signals appear to be at the same level, and there should be no apparent distortion. Either clipper has a high input impedance, 220k in the tube version, and also 220k in the SS version as shown here. Output impedance in both cases is "low", I think about 1k or less. Any clipper should be followed by a good audio filter of some sort, whether hollow or sand-state. I prefer a filter such as the MFJ or Vectronics sand-state filter because they are so small and they work like gang-busters.
This is the tube version of the Series Diode Gate Clipper. Experimentation (since I first wrote this short article) with an oscilloscope, proved that in order to clip symetrically, the two resistors in the cathodes of the diode MUST be of different values, and are in a mathematical relationship to the 330k biasing resistor. This circuit was taken from a discussion by Richard Rowland beginning on page 61 of the June 1964 edition of QST magazine. Anyone using this or other circuits like it should read that article. The clipper should drive a cathode-follower which has a high-impedance input.
My thanks to T. M. Hamblin, sine HN, VA3HN for bringing this information to my attention.
Any low or medium mu triode will work well as the cathode-follower. 1/2 of a 6SL7, 12AX7, 12AU7, or 1G6, or a 30 works just fine. If you use a 30, you will want to use a dual-diode with a similar filament requirement and change the power-supply accordingly. Likewise for the 1G6.
Here is the sand-state version of the clipper shown above.
The emitter-follower is one I borrowed from Doug Demaw's Solid-State Design for the Radio Amateur then modified a bit. The resistor values may need to be tweaked. VA3HN suggested values of 22K, 33K, and 75K for the resistors associated with the two diodes for the SS version. However, before I received this information, I had bread-boarded this circuit and, using an o'scope and an audio signal generator, had adjusted the values of those three resistors for symetrical clipping. I arrived at values of 200K, 300K and 700K. I have not yet built either circuit for use in my station, but I hope to soon.
I hope you find one of these useful.
Ken Gordon W7EKB