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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