[AMRadio] coupling cap on the moduation transformer secondary

John E. Coleman (ARS WA5BXO) wa5bxo2005 at pctechref.com
Tue Aug 16 17:29:14 EDT 2005

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: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of John Lyles
Sent: Tuesday, August 16, 2005 2:22 PM
To: amradio at 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" <k4kyv at hotmail.com>
> Subject: Re: [AMRadio] AMPX article on mod reactor
> To: amradio at mailman.qth.net

> Actually, broadcast transmitters use much less than than, and still boast
> 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
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