I unpacked my ol’ headphone amplifier and tried to find the wall wart AC adapter for it. The jack is labeled 12VDC, center negative, so I looked through some boxes for the adapter that matched up. I found one and plugged it in, turned on the amp, and heard Hum-Bub, Hum-Bub, Hum-Bub… The amp was motorboating on both channels, with a lot of hum. Hearing that both channels were the same, I knew that the problem was common to both channels. I’m thinking that the problem is that it’s the wrong AC adapter. The amp needs 12V at a couple hundred mA, so I connect it up to an HP variable regulated power supply. The amp is dead silent, no hum, no motorboating, and works great. I now know that the problem is in the power coming from the AC adapter.
I decided to change the AC adapter. The adapter I changed to puts out 16VDC no load, and 14V when on the amp. That’s a little high, but ok for the amp. I connected it up and still heard Hum-Bub, Hum-Bub, Hum-Bub… 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. I added a second 2200 uF, and I got less hum and slower motorboating, but it wasn’t eliminated. I’m thinking that I’m headed in the right direction, but just not enough. So I decide to put a 10 ohm resistor in series with the +DC lead. That helps some more, but it’s still doing Hum-Bub, Hum-Bub, Hum-Bub… just a lot weaker and slower. I’m just not making enough progress.
I figured an easy solution would be to use a 7812 three terminal voltage regulator IC. 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. There is nothing to spare, and when the AC power line drops lower, the IC will drop out of regulation. I could use a LM317 adjustable regulator IC, and set the output for 11 volts. But the headphone amp was really supposed to have 12VDC. Then after a little thought, I came up with the idea of using a capacitive multiplier.
Capacitive Multiplier – 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. The two resistors are typically 10 to 1 ratio – I chose 3300 and 330 ohms. The 330 is connected from collector to base, and the 3300 is connected from base to ground. This biases the base at a few volts below the collector. This transistor is a common collector or an emitter follower, where the emitter follows the voltage on the base. The capacitor is in parallel across the 3300 ohm resistor. The filtered power is taken from the emitter. The 330 ohm and capacitor form a low pass filter, drastically reducing the ripple from the rectified and already filtered DC. 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. 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. The 1000 uF capacitor can do the same filtering as a 100 thousand microfarad capacitor. The penalty is that the transistor drops a few volts, so the output at the emitter is a few volts lower. 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. The amplifier is class A and the current is fairly steady so a small change in voltage will not matter. 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. But the amp doesn’t need that much regulation.
Get it together – I assembled the above parts inside of the case, and mounted the transistor to one of the existing screws. The 1000 uF capacitor dropped the hum to almost inaudible, and the motorboating stopped. But the slight amount 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. 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. 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.
Back to experimenting, and now I can listen to my music!