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88 mH coils for your 1970s Audio Filter Project | QRZ …
https://forums.qrz.com/index.php?threads/88-mh-coils-for-your-1970s-audio-filter-project.624949/
Ed Wetherhold, W3NQN wrote a number of articles in QST, W6SAI and ARRL Handbook, and other publications on passive audio filters -- that used the surplus 88 uH telephone coils from 30 to 40 years ago. ... and placing them …
A Passive LC Audio Filter For Amateur Radio Use
http://www.genebitsystems.com/david/AudioFilter/A_Passive_LC_Audio_Filter.pdf
The passive LC filter is physically larger and heavier compared to other filter implementation methods. This limits its suitability for use in portable applications. In spite of these drawbacks, passive LC filters do have a major advantage. Being entirely passive, they do not introduce the noise and distortion typical of many active circuits.
AN EASY-TO-BUILD, HIGH-PERFORMANCE PASSIVE CW FILTER
http://www.discolodxgroup.cl/documentos/otros/ARRL%202013%20Handbook/ARRL%202013%20Handbook%20Supplemental%20Files/Chapter%2011/Passive%20CW%20Filter%20project.pdf
filters due to increased distortion of the CW signal and the presence of a constant low-level, wide-band noise at the audio output. One way to avoid this distortion and noise is to switch to a selective passive filter that gener-ates no noise! Although the center frequency and bandwidth of the passive filter is fixed
Audio Filter - Zerobeat
http://zerobeat.net/g3ycc/filter.htm
In my GQ-40 I used a two stage elliptical filter : L1 = L2 : TOKO 100 mH (88mH toroids should do as well) C3 : 330nF C5 : 330nF C2 : 100nF C4 : 47nF. A 0.2uV RF signal yields a S/S+N ratio of 3.5 dB in my unmodified GQ-40. With the filter and the same RF signal the signal to noise ratio improved to 10.5 dB.
A very simple and very effective passive CW audio filter
http://ki3u.byethost3.com/Radio-PhysicsDIR/ecomDIR/Simple_passive_CW-filter/Simple%20passive%20CW%20audio%20filter.html
The strategy here for realizing a quick, simple, and effective make-do passive CW audio filter is to splice into the line between the radio receiver's audio output and its speaker a minimal series LC combination having a resonant frequency in the CW pitch range .
CW filter for AF usage
http://www.redwaveradio.com/5_f8422a2a3f412e20_1.htm
The expert on passive filters is: Ed Wetherhold, W3NQN 1426 Catlyn Pl Annapolis, MD 21401-4208 Phone: 410-268-0916 Fax: 410-268-4779 . He sells CW Filter kits and 88mH toroidal inductors. I recently bought a CW filter kit from Ed which cost $22 and seems to have about the effect of the 500Hz CW filter on my old Kenwood rig.
Audio Filters Explained: Low-Pass, High-Pass and Beyond ...
https://www.edmprod.com/audio-filters/
A band-pass audio filter is simply the combination of both a low-pass and a high-pass. Example of a band-pass filter. One common application of a band-pass filter is the telephone effect. This is achieved by setting the center frequency around 2000 Hz as such:
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