[minima] Re: Impedance matching audio amp and filter
- From: Franz DX <dd4wh.swl@xxxxxxxxx>
- To: minima@xxxxxxxxxxxxx
- Date: Mon, 23 Feb 2015 23:35:13 +0100
Hi Joe,
Just to mention that my audio amp is the design by SM0VPO:
http://sm0vpo.altervista.org/use/afamp-4w.htm
Best wishes,
Frank
2015-02-23 22:28 GMT+01:00, Joe Rocci <joe@xxxxxxxxxx>:
> 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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>>
>>
>
>
>
>
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