Lots of active filters can do the filtering function, but DSP and
switched capacitor filters do have some noisy artifacts left in the stop
band and the audio output amp they require also tends to have broad band
noise. These passive filters put out no noise that's detectable in or
out of the pass band. One of them, the 2.4 or 3 KHz low pass would be
great with hifi headphones to eliminate all excess noise outside the
passband needed for communications. Its eerie to listen to CW with a 400
Hz tone and the 600 Hz low pass speaker filter. Its like a code practice
oscillator for cleanliness and lack of both QRM and noise.
Yup, the low impedance makes the capacitors BIG, but not all that much
bigger than the transformers in my previous design which is on line at:
http://www.geraldj.networkiowa.com/papers/speakerfilter.pdf Once I prove
the design, I just might build it into my speaker cabinets and headphone
jack.
I find the DSP in the FT-857D annoying that when going to CW it switches
from low pass to a narrow bandpass that is narrower than I like, and on
FM it cuts too many highs making a muddy sound that it doesn't do on SSB.
73, Jerry, K0CQ
On 8/13/2010 5:50 PM, K3GGN wrote:
> Jerry,
>
> Why not use a switched capacitor filter?
>
> Dean
> K3GGN
> ~~~
> On 8/12/2010 7:31 PM, Dr. Gerald N. Johnson wrote:
>> Back when I designed that filter, the coils I used were fairly
>> expensive, so I used the varying impedance to allow using the same coils
>> for all the frequencies. Plus large value capacitors were not easy to
>> acquire which made the higher impedance nicer using smaller capacitors.
>>
>> Today its possible to buy large polyester film capacitors for about the
>> price of the transformers and smaller value inductors are as cheap as 18
>> cents each or as expensive as $1.75 each from Mouser and JW Miller so
>> reuse is not so critical. So I have reverse engineered my filters and
>> designed a new set without transformers with a more useful set of cut
>> off frequencies, but they do take capacitors 10 to 47 mfd which are in
>> the class of motor run capacitors or switching snubbers. Tantalum or
>> aluminum electrolytic capacitors won't work without imposing a DC bias
>> and that can make the capacitors into a noise source which we are trying
>> to eliminate from the speaker or headphones.
>>
>> I have chosen cut off frequencies of 600, 1200 (for CW) 1800, 2400, and
>> 3KHz (for SSB, the last for HIFI SSB.) I haven't built them yet so I
>> haven't tested. These are designed for 8 ohms impedance. When driving
>> higher impedance headphones they need to be terminated with a suitable
>> resistor to load them to 8 ohms or else an impedance matching
>> transformer.
>>
>> The filter circuit is the same as my speaker filter mentioned earlier
>> without the transformer and switching the whole filter instead of the
>> inputs and outputs and the capacitors.
>>
>> Three inductors in series, the outer ones are the smaller, the middle is
>> 1.5 times the outer inductor.
>>
>> E.g. for 600 Hz low pass, input and output inductances 4.1 mh, center
>> 6.15 mh. Capacitors from the inductor junctions to ground (2 caps) 47
>> mfd.
>>
>> 1200 Hz, inductances 2 and 3 mh, Cs 24 mfd.
>> 1800 Hz, inductances 1.35 and 2 mh, Cs 15 mfd.
>> 2400 Hz, inductances 1 and 1.5 mh, Cs 12 mfd.
>> 3000 Hz, inductances .82 and 1.22 mh, Cs 9.5 or 10 mfd.
>>
>> Going from the available inductors in the JW Miller pages of the Mouser
>> catalog, I'd use 4700 and 1500 microhenries in series to make 6 mh. 3300
>> and 680 microhenries in series to make 4 mh. 2500 and 470 in series to
>> make 3 mh. 1000 + 1000 to make 2 mh. 1000 + 330 to make 1.35, 1000 and
>> 1500 are stock values. 1000 and 220 to make 1.22 mh, and 820
>> microhenries is a stock value.
>>
>> For 9.5 mfd, I'd use 10. For 12 I'd use 12 or 10 + 2 in parallel. For 15
>> I'd use a 15 or a 10 and a 4.7 in parallel. For 24 I'd use 24 or two 12s
>> or two 10s and a 4.7 in parallel. For 47, I'd consider a 47 (it is in
>> the mouser catalog, but I'm not sure its in stock, might be easier to
>> get from W.W. Grainger or a motor repair shop) or 4 x 12 or 5 x 10 in
>> parallel.
>>
>> To recap the circuit, say for 2400 Hz lowpass: input to a 1 mh inductor,
>> then 12 mfd to ground. From the junction of the inductor and capacitor a
>> 1.5 mh inductor. From the free end of that inductor a 12 mfd to ground.
>> From that coil-capacitor junction a 1 mh to the output pin.
>>
>> I'll write it up more and maybe build some to test the frequency
>> variations from rounding off the component values and using 10%
>> tolerance parts.
>>
>> 73, Jerry, K0CQ
>>
>> On 8/12/2010 4:44 PM, John Graves wrote:
>>> Jerry,
>>>
>>> That is a very interesting article. 10 years ago you could have bought
>>> most of the components at Radio Shack. I just had a quick look at the j.
>>> W. Miller line....Winding toroids seem to be the way to go now. Now to
>>> find that switch you used. Only commenting. What you have said makes
>>> sense to me (a network security person) My Dad was the mostly self
>>> taught audio guy (Thanks to WWII).
>>>
>>> John / WA1JG
>>>
>>> Dr. Gerald N. Johnson wrote:
>>>> One solution I use for really noise free audio is posted at:
>>>> http://www.geraldj.networkiowa.com/papers/speakerfilter.pdf This
>>>> filter has selectable cut off frequencies and being passive and built
>>>> for speaker impedance there is no high frequency noise introduced
>>>> after it by 100 KHz bandwidth audio output ICs.
>>>>
>>>> 73, Jerry, K0CQ
>>>>
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>
>
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