I unpacked my ol’ headphone amplifier and tried to find the wall wart AC adapter for it.\u00a0 The jack is labeled 12VDC, center negative, so I looked through some boxes for the adapter that matched up.\u00a0 I found one and plugged it in, turned on the amp, and heard Hum-Bub, Hum-Bub, Hum-Bub…\u00a0 The amp was motorboating on both channels, with a lot of hum.\u00a0 Hearing that both channels were the same, I knew that the problem was common to both channels.\u00a0 I’m thinking that the problem is that it’s the wrong AC adapter.\u00a0 The amp needs 12V at a couple hundred mA, so I connect it up to an HP variable regulated power supply.\u00a0 The amp is dead silent, no hum, no motorboating, and works great.\u00a0 I now know that the problem is in the power coming from the AC adapter.<\/p>\n
I decided to change\u00a0 the AC adapter.\u00a0 The adapter I changed to puts out 16VDC no load, and 14V when on the amp.\u00a0 That’s a little high, but ok for the amp.\u00a0 I connected it up and still heard Hum-Bub, Hum-Bub, Hum-Bub…\u00a0 So I clipped a 2200 uF capacitor across the DC input, and the hum was reduced dramatically, but the motorboating was still there, just slower.\u00a0 I added a second 2200 uF, and I got less hum and slower motorboating, but it wasn’t eliminated.\u00a0 I’m thinking that I’m headed in the right direction, but just not enough.\u00a0 So I decide to put a 10 ohm resistor in series with the +DC lead.\u00a0 That helps some more, but it’s still doing Hum-Bub, Hum-Bub, Hum-Bub… just a lot weaker and slower.\u00a0 I’m just not making enough progress.<\/p>\n
I figured an easy solution would be to use a 7812 three terminal voltage regulator IC.\u00a0 The problem is that it needs a few volts from input to output to keep working correctly, and the adapter is putting out only 14 volts under load.\u00a0 There is nothing to spare, and when the AC power line drops lower, the\u00a0 IC will drop out of regulation.\u00a0\u00a0 I could use a LM317 adjustable regulator IC, and set the output for 11 volts.\u00a0 But the headphone amp was really supposed to have 12VDC.\u00a0 Then after a little thought, I came up with the idea of using a capacitive multiplier.<\/p>\n
Capacitive Multiplier<\/strong> – This circuit consists of one transistor (typically a power transistor since it handles the current for the whole circuit), two resistors and an electrolytic capacitor, typically hundreds or a thousand micofarads – I chose 1000 uF.\u00a0 The two resistors are typically 10 to 1 ratio – I chose 3300 and 330 ohms.\u00a0 The 330 is connected from collector to base, and the 3300 is connected from base to ground.\u00a0 This biases the base at a few volts below the collector.\u00a0 This transistor is a common collector or an emitter follower, where the emitter follows the voltage on the base.\u00a0 The capacitor is in parallel across the 3300 ohm resistor.\u00a0 The filtered power is taken from the emitter.\u00a0 The 330 ohm and capacitor form a low pass filter, drastically reducing the ripple from the rectified and already filtered DC.\u00a0 A small amount of current is used by the base, and the much higher current from the collector passes through to the emitter, which follows the very filtered and very low ripple voltage at the base.\u00a0 It’s called a capacitive multiplier because the thousand microfarad capacitor looks like it is multiplied by the current gain of the transistor, which can be fifty or a hundred times or more.\u00a0 The 1000 uF capacitor can do the same filtering as a 100 thousand microfarad capacitor.\u00a0 The penalty is that the transistor drops a few volts, so the output at the emitter is a few volts lower.\u00a0 In my case, this three or so volts drop makes the output 12.3 volts, which is just about perfect for the amp. The other advantage is that if the AC adapter’s output drops, the voltage divider keeps the voltage drop the same, and the output drops some.\u00a0 The amplifier is class A and the current is fairly steady so a small change in voltage will not matter.\u00a0 If I wanted to maintain a constant output voltage, I could have replaced the 3300 resistor with a 12V zener diode, and the output would be kept at about 11.4 volts.\u00a0 But the amp doesn’t need that much regulation.<\/p>\n Get it together<\/strong> – I assembled the above parts inside of the case, and mounted the transistor to one of the existing screws.\u00a0 The 1000 uF capacitor dropped the hum to almost inaudible, and the motorboating stopped.\u00a0 But the slight amount\u00a0 of hum that I could still hear was eliminated by putting a 470 uF capacitor across the 1000 uF, for a total of 1470 uF. \u00a0\u00a0 The amplifier works great, and the circuit will now work on just about any 12V unregulated (16V no load) wall wart that I want to use.\u00a0 I could also use a 12.6VAC, 1\/2 amp power transformer along with a full wave rectifier and filter capacitor, but the AC adapters are easily obtained from surplus places such as MPJA.com, All Electronics, Goldmine Electronics, etc.<\/p>\n Back to experimenting, and now I can listen to my music!<\/p>\n","protected":false},"excerpt":{"rendered":" I unpacked my ol’ headphone amplifier and tried to find the wall wart AC adapter for it.\u00a0 The jack is labeled 12VDC, center negative, so I looked through some boxes for the adapter that matched up.\u00a0 I found one and plugged it in, turned on the amp, and heard Hum-Bub, Hum-Bub, Hum-Bub…\u00a0 The amp was <\/p>\n (Read More…)<\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[8,74],"tags":[83,84],"class_list":["post-3918","post","type-post","status-publish","format-standard","hentry","category-audio","category-electronic-components","tag-capacitive-multiplier","tag-ripple-filter"],"_links":{"self":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/3918","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=3918"}],"version-history":[{"count":14,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/3918\/revisions"}],"predecessor-version":[{"id":3931,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/3918\/revisions\/3931"}],"wp:attachment":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=3918"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=3918"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=3918"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}