[minima] Re: Impedance matching audio amp and filter
- From: Franz DX <dd4wh.swl@xxxxxxxxx>
- To: minima@xxxxxxxxxxxxx
- Date: Mon, 23 Feb 2015 22:15:29 +0100
Joe,
thanks for the quick answer!
It´s a three pole LP with C1=100nF, L=33mH, C2=100nF. I modelled it in
Elsie, but the values only make sense (as a lowpass) in an 500 Ohm
environment. Only if input and output are 500 ohms, it´s really a
lowpass. If input is 50 ohms and output 1K, the plot still looks
something like a LP, but has a peak at 2500Hz and does not attenuate
well in the higher frequencies.
Farhan in Minima version 2 used C1=C2=470nF and L=33mH, Elsie tells me
that leads to a nice LP filter, but also only with
input=output=500ohms. That´s why I am confused, because if that LP
filter after the product detector is meant to work properly, it should
be in a 500ohm environment, as I understood it. But I thought the
output of the PD is 50 Ohms? And the input of the Audio PreAmp is
something >1Kohms?
Hiss could well be RF pickup, I use an unshielded choke with a ferrit core.
Best wishes,
Frank
2015-02-23 21:39 GMT+01:00, Joe Rocci <joe@xxxxxxxxxx>:
> 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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