I ran two LC 3khz audio filters in LTspice. #1 has a sharp cutoff down 44db at 4khz and continues down to 92db at 4.9khz. But it shows a 420* phase shift at 3khz.
#2 filter is down 34db at 4khz and reaches 46db at 4.4khz (that's all the attenuation it has). The phase shift is 250* at 3khz.
How does the extra phase shift of filter #1 show itself in listening?
Evaluating for radio filter.
Thanks, Mike
What are these filters for? They look like elliptic filters given the steep cut off or notch. Such filters are not good for audio except in special circumstances. Why do you want to cut off at 3KHz? Is this to eliminate noise in a poor audio channel in which case audio quality is not good anyway and such filters would make little difference. Or, is this a low pass filter for a crossover or other network where audio quality is important? If audio quality is important, these are not the filters to use. These filters are multi-pole high "Q" and will cause "gritty" unnatural sound in high quality applications. The grittyness is caused by "ringing" that these filters exhibit when hit with an impulse. Do an impulse response and see that there is a lot of overshoot and undershoot or ringing. Such a response is a disaster for audio unless you are making bells. Bells are supposed to ring but not filters. Secondly, the high and varying phase shift will make a nightmare response for a crossover network where the filter works in conjuction with another, high pass unit. The sum of the two outputs in the crossover region will be very uneven or bumpy. As a general rule, never use a higher "Q" or more poles in an audio filter than absolutely necessary to do the job.
Mike, I need more information. What kind of fiters are they (Butterworth, Elliptic, etc)? How many poles? What the bandpass, bandstop attenuation specs? corner frequency?
This site gives the explaination of use.
Thanks, Mike
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As I thought, multi-pole elliptic filters of high "Q." They are the "special circumstance" I mentioned above. Fading SW, AM is not high quality audio under any definition. These filters are used to "clean up" an already very poor audio signal and are appropriate for that purpose. Whether they work very well or not is another matter. They ain't HI-FI and should NEVER be used in your stereo for the reasons I mentioned! These filters are designed to be used with speakers at 8ohms but they have some inconvenient, large components. It is possible to do these filters at low level using op-amps as active components with R's and C's, no inductors. That may be more convenient, simpler and a lot cheaper.
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Ah, yes, I think the OP's posting name, "AM DX," gives a clue. ;-)
As Bob says, implement them with active filters. In fact, if you do, you can re-tune them relatively easily and evaluate which works best for you without having to rewind big coils.
Even better, if you can, implement your filter as an FIR filter in a processor. FIR filters can easily have linear phase, at least, though they likely will have some "ringing" to get the sharp cutoff you probably want. An additional advantage is that it's then easy to change the filter cutoff frequency to suit your needs.
If the stations you listen to are channelized, that is with uniform frequency spacing, it's probably worthwhile to have a filter with a deep null at the channel spacing frequency. For example, a filter with a 10kHz notch is worthwhile for listening to US medium-wave AM broadcast signals, because it kills the "whistle" from adjacent- channel carriers.
Cheers, Tom
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