[minima] Re: Impedance matching audio amp and filter
- From: "Joe Rocci" <joe@xxxxxxxxxx>
- To: <minima@xxxxxxxxxxxxx>
- Date: Mon, 23 Feb 2015 16:28:58 -0500
Franz
A lot of folks slip these LC filters into circuits without regard to proper
impedance termination and the result is, of course, distorted passband
characteristics and excessive loss. Having said that, if we know the
termination impedances, there's an opportunity here to design a well behaved
network that not only filters, but also provides a decent amount (>10db) of
passive voltage gain due to the impedance up-conversion. I'll model this
tomorrow morning, but I expect my findings to be what you already know.
-----Original Message-----
From: Franz DX
Sent: Monday, February 23, 2015 4:15 PM
To: minima@xxxxxxxxxxxxx
Subject: [minima] Re: Impedance matching audio amp and filter
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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