[minima] Re: Impedance matching audio amp and filter
- From: "Joe Rocci" <joe@xxxxxxxxxx>
- To: <minima@xxxxxxxxxxxxx>
- Date: Mon, 23 Feb 2015 15:39:34 -0500
Frank
I doubt the problem is caused by impedance mismatching. I also doubt that
the problem will be fixed with a different kind of PD. If you post the
values you're using in the filter, I'll look at it in LTSpice just to see
how well it's filtering, but that won't solve the hiss problem.
As far what the cause of the hiss is, it could be pickup of of some nearby
RF by the inductor in the LPF. Unshielded high-value iron-core inductors can
be the source of all kinds of noise pickup. I generally prefer active audio
filters for this reason.
Joe
-----Original Message-----
From: Franz DX
Sent: Monday, February 23, 2015 3:26 PM
To: minima@xxxxxxxxxxxxx
Subject: [minima] Impedance matching audio amp and filter
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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