[minima] Re: Impedance matching audio amp and filter
- From: Richard Hayter <richardshayter@xxxxxxxxx>
- To: minima@xxxxxxxxxxxxx
- Date: Thu, 26 Feb 2015 15:30:33 -0600
Hello Franz
with reference to your inquiry visa vis an audible variable audio filter.
May I recommend a circuit from WB7AEI.
My build and measurements of this circuit can be found on my blog as
follows:
http://zs6rsh.blogspot.com/search/label/Audio%20Filter
I inserted this filter after the audio pre-amp in a W1FB DC receiver since
in that position it will have less impact on the Rx noise figure.
This filter is very narrow. I mainly use it for CW. However it should be
possible to widen the response quite easily.
Let me know what you think?
73 Dick N4HAY (ex ZS6RSH, Z21BK, G4ZYK)
zs6rsh.blogspot.com
On Tue, Feb 24, 2015 at 5:03 PM, Franz DX <dd4wh.swl@xxxxxxxxx> wrote:
> Dear Joe, Allison, José,
>
> thanks a lot for your suggestions!
>
> After another few hours of experimenting, I could reduce the hiss a
> lot with a simple RC filter. I also reduced the gain of the audio amp,
> which was way too high. But I couldn´t manage to reduce the hiss to an
> acceptable level (acceptable for long hours of listening). I also
> experimented with different resistors in order to deliver a proper
> termination of the product detector: 50 ohms definitely did not work
> (audio very low and thin). It seems to need >1K.
>
> The best solution in the end was to reactivate the original Minima
> Audio preamp and Audio power amp without an RC filter. That is really
> an excellent amp with very very low noise and a perfect frequency
> range for ham and BC radio! Thank you very much, Farhan, for that nice
> circuit!
>
> It also works very well in my mixed setup:
>
> Minima ver2 FST 3253 mixer - original 20 MHz crystal filter - Joe
> Roccis B-CAS IF amp with manual gain control via Pot - Minima ver1
> product detector - Minima ver1 Audio preamp & Audio PWR Amp
>
> That setup has (much) higher sensitivity than Minima ver1 and I do not
> use the audio gain control any more, because I now have the manual IF
> gain control from the B-CAS IF amp.
>
> What I will do next is to build an active variable audio lowpass
> filter and experiment with that. (I also plan to build Joe´s AGC
> circuit with the TL974, but that has to wait until the opamps arrive:
> my first attempt with NE5532 was a disaster ;-))
>
> Some questions have remained:
>
> - I am not sure where to place an active audio filter. Directly after
> the Product detector or after the preamp before the audio pwr amp?
>
> - What kind of termination does the Minima product detector need?
>
> - Has anybody got a link to a good website or schematics for an active
> (variable!) audio LP filter with single power supply with the NE5532
> or TL974? Or maybe a more general intro to these things?
>
> Best wishes from Dresden,
>
> Frank
>
>
>
> 2015-02-24 15:25 GMT+01:00, "José A. Amador Fundora"
> <amador@xxxxxxxxxxxxxxxxxxxxxx>:
> >
> > It is not a matter of matching, it is a matter of "filter termination".
> >
> > The response of an LC filter depends critically on its terminations.
> > PSpice is a good tool to evaluate it by means of simulation. Real life
> > may be a little more complicated, a model is better as it resembles
> > better to reality, but a complex model is harder to nearly impossible to
> > simulate.
> >
> > I have used Active Sallen Key low pass filters using gain ~ 1 (actually,
> > 0.999...) using transistors, which does help to combat tirednedness by
> > high frequency audio noise where there is no desired signal, as the
> > crystal filter on the input end has removed them. Any audio bandwidth in
> > excess of the filter bandwidth leads to that problem. I used an extra
> > active audio filter in my old Swan 700 using a Chebycheff LPF with three
> > sections of a TL084, and cutoff on 3.0 kHz. The fourth section was a
> > high impedance input buffer. It did alleviate the HF noise quite a bit.
> >
> > All "simple" SSB radios are plagued by SSB demodulation of desired
> > signals, and DSB demodulation of noise, worsened by AGC. A wideband high
> > gain IF amplifier with a crystal filter at its input end may do it
> > easily. A better solution is using a "simpler" crystal filter at its
> > output end to stop DSB noise, or resorting to an image reject (other
> > sideband noise) demodulator which is fairly more complex. SNR with those
> > solutions may be better, which is what counts for digging out the weak
> > ones, which often are other QRP stations.
> >
> > My advice is using an active LPF after the demodulator, is the simplest
> > and fairly effective way. It has no termination issues and can be quite
> > small. It is also quite inmune to stray fields.
> >
> > 73,
> >
> > Jose, CO2JA
> >
> >
> > On 23/02/2015 15:26, Franz DX wrote:
> >> 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
> >>
> >>
> >
> >
> >
> >
> > 50 Aniversario de la Cujae. Inaugurada por Fidel el 2 de diciembre de
> 1964
> > http://cujae.edu.cu
> >
> >
> >
> >
>
>
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