[minima] Re: Impedance matching audio amp and filter
- From: allison <ajp166@xxxxxxxxxxx>
- To: minima@xxxxxxxxxxxxx
- Date: Mon, 23 Feb 2015 20:39:02 -0500
Frank,
Several things...
That amp has a fairly high input impedance quick estimation is higher
than 3K.
A filter that peaks in the 4-5K range will accentuate hiss as that is
heard better there.
If the detector is not loaded properly it may also generate noise or
propagate it.
I try to avoid ferrite loaded inductors at audio frequencies due to lack of
shielding and also its internal resistance can produce low Q filters.
They can
work but care with models of perfect vs real parts.
I also build in a low pass in the amplifier itself. No sense having a
an amp flat
from 10-100khz when I need a hard ceiling of not more than 4.5Khz maybe
less.
Usually I do that with RC and some Miller capacitance (base/collector or
gate/drain).
Selection of coupling caps and maybe emitter bypasses if use are sized
to roll off
200hz and down in particular more strongly below 100hz as that takes out
hum
and other low frequency noises both local and on the other guys signal.
Allison
On 02/23/2015 05:35 PM, Franz DX wrote:
> 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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>>
>
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