I think this is correct!!! But don't quote me!! There is a rule of thumb in coupling capacitors that the period of the lowest freq should be equal or less than 1/5 of the RC time constant of the coupling capacitor and load. 4 mf times 4000 ohms = 16000 microseconds or .016sec. 1/5 of that would be .0032sec inverting that would be 312.5 Hz. There would be a gradual roll off below that and a rapid roll off below 62.5HZ (the freq equal to 1 RC time constant). The resonance with the inductor plays a part in this as well and makes the equations more complex. Resonance will cause an asymmetrical wave shape to a more symmetrical wave shape as the fundamental frequency of the wave nears the resonant frequency of the network. If my calculations are correct the resonate freq of 4 mf and 40 henrys would be about 12.5 Hz. This should be well below any frequency that might come out of Don's mouth. So I would say he is not in fear of having his asymmetrical audio smoothed out by resonance at the modulator output. On the other hand if he were to reduce the coupling capacitor to 1 mf, then the resonate frequency would be higher and some symmetrical stuff might be produced that he didn't say. (That's a way of saying it would not be a true reproduction) In some folks that would be an improvement in listening quality HIHI.
John, WA5BXO -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of John Lyles Sent: Tuesday, August 16, 2005 2:22 PM To: amradio@mailman.qth.net Subject: [AMRadio] coupling cap on the moduation transformer secondary I was just about to say this, Don already responded to it. Besides, finding a compact oil filled capacitor of 2-3 kVDC rating is much simpler if you use a few uF. You don't need 10 Hz low end response, thats wasted power since most receiver/speakers won't reproduce that. Also, the capacitor is just another source of stored energy in your radio, to deal with from a safety standpoint. More is not always better. > Message: 3 > Date: Tue, 16 Aug 2005 03:00:55 +0000 > From: "Donald Chester" <[EMAIL PROTECTED]> > Subject: Re: [AMRadio] AMPX article on mod reactor > To: amradio@mailman.qth.net > Actually, broadcast transmitters use much less than than, and still boast of > flat response down to 30~ or lower. RCA transmitters typically use 1 mfd. > Gates uses 2 mfd. I use 4 mfd in my homebrew rig @ 4000 ohms modulating > impedance. > > 73, Don k4kyv > > ______________________________________________________________ AMRadio mailing list Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:AMRadio@mailman.qth.net
