[minima] Impedance matching audio amp and filter
- From: Franz DX <dd4wh.swl@xxxxxxxxx>
- To: minima@xxxxxxxxxxxxx
- Date: Mon, 23 Feb 2015 21:26:37 +0100
Hi Minima specialists,
I am experimenting with different audio filters directly after the
product detector of the Minima (like the three pole lowpass filter in
Minima version 2 consisting of a 33mH choke and two capacitors) and
different audio amps, but I am having problems with hiss noise.
When I use no audio filter and connect the product detector directly
into the audio preamp, the hiss is not existent (except for a tiny
little bit). But when I use a filter like in Minima version2 with a
serial 33mH choke and two capacitors of 100nF connecting to ground,
the hiss is getting really loud and I am also observing a high pass
effect, although it is constructed and meant like a lowpass!
My suspicion is that it has to do with wrong impedance matching,
having a low impedance 50ohm output out of the product detector and a
high impedance input to the audio preamp.
But how can I match these? Using an LC circuit like one at the input
of Joe´s IF amp? I am convinced this is a really fundamental and easy
to solve problem, so I would really appreciate a link or a hint where
to read about a solution.
Thanks in advance!
Also, maybe the problem would be solved if I use an FST3253 as a
switching product detector like Joe planned it?
Best wishes,
Frank DD4WH
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