{"id":1324,"date":"2016-09-11T16:47:23","date_gmt":"2016-09-11T20:47:23","guid":{"rendered":"http:\/\/www.n4rfc.com\/?p=1324"},"modified":"2016-09-15T12:35:07","modified_gmt":"2016-09-15T16:35:07","slug":"vintage-radios-and-modern-headphones","status":"publish","type":"post","link":"https:\/\/www.n4rfc.com\/?p=1324","title":{"rendered":"Vintage Radios and Modern Headphones"},"content":{"rendered":"

I have a number of \u00a0pieces of “Vintage” Amateur Radio equipment that I have collected, repaired, and use regularly on the air. \u00a0A lot of my time on the air is on CW, or Morse code for the non ham readers out there, and I use headphones to copy weak CW signals from time to time. \u00a0 Vintage headphones are not particularly comfortable to use and are hard to find and every harder to restore. \u00a0So I wind up buying modern stereo headphones to use on my newer equipment and also on these older tube type radios. That creates a problem. \u00a0These older radios are not the quietest things you want to hook to a pair of stereo headphones that have a wide frequency response. \u00a0There is high frequency hiss noises created in the tube circuits and 120 Hz\u00a0hum from the unregulated power supplies. \u00a0Where modern amateur radio equipment has electronically regulated power supplies providing ripple free voltage for the circuits these older radios typically have a transformer with a full wave rectifier and high voltage filter capacitors to try to remove the ripple from the power that feeds the audio circuits. \u00a0\u00a0 That produces a small amount of hum in the speaker that becomes not so small when you connect a set of headphones to the radio.\u00a0 So, what has that got to do with stereo headphones? \u00a0Well, first off they are stereo. \u00a0The older radios are all monaural, so some sort of plug in adapter is required to bridge right and left channels of the stereo phones to the monaural radio. \u00a0Second, there there is the hiss and hum, particularly the hum being a problem for me. \u00a0Modern stereo headphones are designed for very flat frequency response from below 50 Hz to 20,000 Khz. \u00a0I don’t want to be hearing all that directly coupled 120 Hz hum from my Drake 2-B into my new Koss headphones!<\/p>\n

I hit upon the idea of putting a small audio passband filter in a 1\/4 phone plug and build a passive filter for this application. \u00a0I cracked the books on basic filter design and came up with this circuit:<\/p>\n

\"outputfinal\"<\/a>

Click on the graphic to see a larger image<\/p><\/div>\n

A passband filter is comprised of a Low Pass Filter and a High Pass Filter in series between the input side and the output side. \u00a0In this case I selected a low cutoff frequency of 400 Hz and a high cutoff frequency of 900 Hz. \u00a0 I usually tune in CW signals at around 600 to 700 Hz so that makes the filter just right for CW work. \u00a0Since these are R (resistance) and C (capacitance) filters, I wanted to keep the resistor values low in this low impedance circuit to keep the loss of signal down. \u00a0Using the formula:<\/p>\n

\"formula\"<\/a><\/p>\n

I selected 27 ohm resistors and calculated the capacitor values and then rounded them to standard values that I had in my parts box to get the circuit shown above. \u00a0The caps came out to be 6.5 microfarads for the Low Pass Filter and 15 microfarads for the High Pass Filter.<\/p>\n

Not only did I want filter the audio I needed to convert from the typical 1\/4″ Headphone jack that was used back in the day to a 1\/8″ Tip\/Ring\/Shield that is commonly used on modern headphones. \u00a0I hit my local Radio Shack’s parts drawers and found a very old school 1\/4″ plug that had a big fat pull handle on it. \u00a0It would give me plenty of room for my R\/C components inside. \u00a0I did a bit of trimming on the ground side of the plug to give me a bit more room for the components, and everything fit nicely into the handle. \u00a0Here are a couple of pictures of the results:<\/p>\n

\"Finished<\/a>

Finished Filter Adaptor<\/p><\/div>\n

\"Components\"<\/a>

Assembled Components<\/p><\/div>\n

The results were quite good. \u00a0The hiss and hum is reduced by a considerable level making the copy of weak CW signals much better and reduces my fatigue by not having to listen to quite loud hum through the headphones. \u00a0All in all it was worth while project for a afternoon where it was too hot to do much outside!<\/p>\n","protected":false},"excerpt":{"rendered":"

I have a number of \u00a0pieces of “Vintage” Amateur Radio equipment that I have collected, repaired, and use regularly on the air. \u00a0A lot of my time on the air is on CW, or Morse code for the non ham readers out there, and I use headphones to copy weak CW signals from time to […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7],"tags":[],"class_list":["post-1324","post","type-post","status-publish","format-standard","hentry","category-amateur-radio"],"_links":{"self":[{"href":"https:\/\/www.n4rfc.com\/index.php?rest_route=\/wp\/v2\/posts\/1324","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.n4rfc.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.n4rfc.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.n4rfc.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.n4rfc.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1324"}],"version-history":[{"count":11,"href":"https:\/\/www.n4rfc.com\/index.php?rest_route=\/wp\/v2\/posts\/1324\/revisions"}],"predecessor-version":[{"id":1341,"href":"https:\/\/www.n4rfc.com\/index.php?rest_route=\/wp\/v2\/posts\/1324\/revisions\/1341"}],"wp:attachment":[{"href":"https:\/\/www.n4rfc.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1324"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.n4rfc.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1324"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.n4rfc.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1324"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}