![[Limiter Schematic]](shuntlim.gif)
Last update: 6/3/97
Some mods to the basic circuit.
Just take the 1 kHz CW filter in the first R1 article, and put a bit more capacitance across each inductor! I added .022 uF across C12, and .1 uF each across C14 and C16. This gave me about 30 dB down at 1 kHz, with only about -2 dB at about 800 Hz, where I listen.
It really needs a sharper and higher highpass filter for CW, also. Meanwhile, a quick fix is to use .47 uF capacitors in the highpass filter in place of the 1 uF. A higher rolloff, still not very sharp, but it helps.
The .1 uF cap in the 2N5457 audio muting switch is much too large for good CW QSK -- that is, recovery between code elements at 30 wpm. .02 to .03 uF is more like it!
My version has always shown some transients of about .4 volts peak to peak at the output of the audio preamps when I key and unkey the transmitter. Others have reported the same thing. It seems to be caused by any significant change in the receiver input RF level -- and almost any electronic TR switch will let enough leakage through so this is a problem. This is only a problem when the receiver gain is turned up fairly high, then the transients push the LM387 and succeeding stages into clipping.
Fortunately this transient is MUCH higher than any audio signal level that shows up at this point. So one cure is: a series limiter after the audio filter(s). This can also double as a noise limiter, or a primitive form of AGC to keep the loudest signals from suddenly blasting out your eardrums.
I adapted this from the LOWfer transceiver in April 1994 QST ("Build Your Own LOWFER Transceiver" by David Curry, WD4PLI). It goes after the filter and before the volume control in my version.
Just adjust the pot till the thumps disappear, and/or for the kind of audio limiting you want. It does seem that when it's set for no thumps at maximum gain, there is some loss of audio gain. So there's a tradeoff.
However, this does not apply to the MUCH simpler fix to my portable rig with its miniR2 circuit: in the first NE5532 stage after the volume control, just put a pair of back-to-back silicon diodes across the output-to-input feedback resistor! Because the R2's LM387 preamp used at this point is a different animal, I don't know if this will work in that version, but I'll try it and let you know.