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The frequency response of the second IF filters of the Ubitx vary from unit to unit depending also where the BFO is set. My unit for example is about 2khz wide filtering out frequencies below 700hz. This it is very good for speech either ssb or AM/CW. In general the less selective/worse the receiver is the better and the less fatiguing is the listening to AM broadcasts. For AM those large ugly portables with HF shine. The Ubitx sounds more a communication or military receiver, at least my unit. In general a 6khz wide filter allow just 3khz frequency response, which is not much more than the usual Ubitx. If the receiver has a passband offset control (PBT) you can possibly recover all the 6khz. In any case most large portable that have no PBT, the filter is so wide you have a more pleasant audio. To have that wide filters in the Ubitx maybe one could remove most of the crystals in the second IF.
Il 23/giu/2018 00:09, "Ralph Mowery" <rmowery42@...
> ha scritto:
I did not really do much in the way of calculating. I do have the equipment to check it our. One way was to feed in a 1000 Hz audio tone into the mic input. Then measure the output power. Then go up and down in frequency to where the power makes a big drop off.
I know that by lowering the value of the capacitors the filter gets wider and going up in value the filter gets narrower. The first attempt was around 68 pf (or whatever the standard value is near that). That made the filter way too wide,so I put in the 80 some and that put it close enough for me.
Even going into a hifi amp it will be difficult to make the music sound very good. If the filter cuts out the highs and lows before it gets to the demodulation part, about all you can do is use an equalizer and run the audio from less than 500 hz way up and the audio above 1800 way up as there is almost no audio outside 500 hz to 1800 hz.
As the ubitx is, it is a great piece of engineering, but it is also easy to make changes to for anyone that has a bit of equipment and know how. Many have published their modifications so that people like me only need to do them.