Re: Rx Radio #ubitx-help

Mike aka KC2WVB <rb5363@...>
 

Yes, the UBitx is a nice piece of engineering. Hats off to the developers.

For myself, I don't listen to AM broadcasts for their music content rather I listen to the BBC and other foreign stations to hear views from abroad. 

A long time ago an EE friend of mine introduced me to reel to reel half speed masters and I was a convert the moment I heard the difference. I still remember how startled I was by what I had been missing. The tapes of course are pretty much a thing of the past so I had to make a choice between vinyl or a digital format such as the Sony/Phillips Super Audio CD. I chose the later and it offers some nice advantages over vinyl in that you can turn compression off and experience the full dynamic range of the music being played. Of course no format will sound good if the sound engineering is lackluster so buyer beware is always a consideration prior to spending upwards to $30 on a purchase.

It's too bad in my opinion that quality in music reproduction has seemed to take a back seat to other interests and it is certainly an area where newer is not equivalent to acoustically better. I am just thankful that there are enough audiophiles to allow quality to still be available.

On Sat, Jun 23, 2018, 5:15 AM iz oos <and2oosiz2@...> wrote:

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.

de ku4pt


On Fri, Jun 22, 2018 at 5:41 PM, Mike aka KC2WVB <rb5363@...> wrote:
Ralph, 
Thanks, you did the calculations so a change is now simply swap out. I knew the filter was narrow but not that narrow. I guess they aimed at between ideal for CW and SSB by selecting as is. 
I don't have the stuff required for guaranteeing success when swapping out surface mounted capacitors. If I did I might make the change. 
I run my audio into the line in on a hi-fi system and can make the AM sound well enough to listen to by making audio adjustments, but I can see where it would be a pain in the arse to listen to the AM off a tiny 8 ohm speaker especially when it's frequency range is limited like it is.

On Jun 22, 2018 4:16 PM, "Ralph Mowery" <rmowery42@...> wrote:
While most AM is set up for a 6 KHz of audio, a side band filter of slightly over 2 KHz will usually allow voice through well enough to be understood.  The music will sound clipped or somewhat distorted as it often has higher audio frequencies than  most taking voices.

The filter in most of the ubitx is really too narrow to suit me as I only operate ssb with it.  I changed out the 5  100pf capacitors in the crystal filter to 82 pf ( I think that is the standard value).  That broadened the filter in my ubitx from about 1700 hz to about 2300 hz.  Makes the audio sound much better for me.

One thing not discussed is that while AM often has the 6 khz filter, it only needs to be about half that when using a ssb filter as you are only using one sideband instead of both .  So around 3 khz would be good for AM.

de ku4pt


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