[TenTec] Re: TenTec Digest, Vol 14, Issue 4 : Jupiter and D-104

[Allan Henry Kaplan]

Don't bother with the amplifier and impedance-matching transformer, OM. 
  Jupiter's input impedance is high enough [47K-ohms] to match the D104 
microphone.  Any post that refers to 47K as "low impedance" is way off 
base.

A few words about imedance matching may be of general interest here:
The reason for impedance matching is that a load which matches the 
impedance of the source results in transfer of the maximum power 
available from that source.   This is fairy important when the source is 
  a transmitter and the load is an antenna.  A bad imedance match in 
that situation will at least result in wasting much of your signal 
power, and may damage the transmitter.

When the source and the load are handling low power signals, as is the 
case with a microphone and the transmitter's audio input, an impedance 
mismatch will waste some audio power, but there is no likely risk of 
damaging either source or load.  We consider impedance here as purely 
resistive, just to simplify the discussion and expose the basic principles.

Two cases of low power impedance match present different results, to wit:
(1) when the load impedance is much lower than the source impedance, 
power transfer will be inefficient AND low frequency response will 
suffer because the coupling capacitor in the transmitter forms a high 
pass filter with the (resistive)source impedance;
(2) when the load impedance is much higher than the source impedance, 
the power transfer will be inefficient, and the microphone may sound 
somewhat bassy (depends on the mike characteristics).  The MIC gain 
control can usually overcome the power transfer inefficiency.

D104 is basically high impedance, but I'll just bet the amplifier output 
impedance is low enough so that case (2) applies.  Wire it up and plug 
it in!  You won't break anything, and it should sound OK. Go easy on the 
D104 amplifier's gain if it has a control.  You can use Jupiter's 
MONITOR function with headphones to get a good idea of the audio quality.

Disclaimer:  I am a retired Ten-Tec engineer who uses microphones very 
infrequently, and am much less fussy about audio than many....

73 de W1AEL, Allan in Sevierville, TN.


>    1. Jupiter and D-104 (Kreinberg, David)
> 
> 
> ----------------------------------------------------------------------
> 
> Date: Tue, 3 Feb 2004 10:51:32 -0600 
> From: "Kreinberg, David" <DKreinberg at necinfrontia.com>
> To: "'tentec at contesting.com'" <tentec at contesting.com>
> Subject: [TenTec] Jupiter and D-104
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> 
> Hello group,
> 
> I decided to 'go for it' and purchased a Jupiter this morning. I am very
> excited to see how this little jewel
> operates.
> 
> I have an old D-104 (amplified) mic sitting around and was wondering what
> the opinions were of using this
> with the Jupiter. I noticed an older thread on this that said to build an
> amp and attach an impedance matching
> transformer reversed to match the lower input impedance of the Jupiter.
.........
> 73 de Dave, NR3E