This (debatably) monstrous-looking thing is now doing duty as my new desktop headphone amplifier at work. And by amplifier, I mean current buffer with slightly negative voltage gain. My computer’s baby little USB audio dongle didn’t have the guts to push serious bass to my headphones. On an oscilloscope, I was getting a booming 3+ volts output, until I put a 32 ohms load on the output. Then it collapses to a level in the millivolts that I didn’t measure because it was ridiculous how badly it collapsed. This is a simple current buffering stage, to provide a low-impedance output for my ‘phones and high impedance for my computer’s output.
The important part: How does it sound? Like nothing. With the volume turned all the way down, there is no hiss/hum/whine or any noise from my headphones. With the volume up, I hear my music. I’m not the kind of person who swears a mains power cable makes his equipment sound better, I just wanted some more power. This gives me that. It doesn’t sound horrible. This was built entirely with parts I had laying around, so I am into it for $0 plus a few hours of time.
How does it look? Here we find the ugly buffer in its native habitat:
I forget it’s even there. I colored it black and it lives in the shadows between a black monitor, a black drawign tablet and a black monitor. The plant is pothos, Devil’s Ivy.
The basic design is the output section from Elliott Sound Products’ P113 http://sound.whsites.net/project113.htm, modified to suit what I had on hand. The R5/R6 bias resistors were lowered to (If I recall correctly) about 4.9k ohms, to increase bias current to just over 2mA as Elliott advises.
My first choice of output devices was a set of SK1388A/SK1389A PNP/NPN complementary transistors I had matched, but I broke one of them (in half!) when disassembling the amplifier to fix a humming problem. I had the main board from an old receiver fitted with 7 channels of 2SD2390/2SB1560 complementary darlington pairs. http://www.semicon.sanken-ele.co.jp/sk_content/2sd2390_ds_en.pdf http://www.semicon.sanken-ele.co.jp/sk_content/2sb1560_ds_en.pdf so I matched a couple of the pairs and ran with them. I had to install an additional bias diode (again, matched, natch) but these are inexpensive (free, I have a roll of them). The choice of transistors is almost unimportant, as long as they can slew fast enough for audio. You could use almost anything. For this design you want about 350mA maximum current capacity, so a TO-92 chip is going to be too small, but otherwise you’re gonna be good with almost anything.
The bias diodes are 1N4003, nothing special. The capacitors across them are random electrolytics I had laying about. The input and output capacitors are also electrolytics I had laying about, with very-large values because I’m not worried about a specific low-end rolloff frequency, I just wanted to block DC.
Power supply: I wanted about +/-12VDC but I didn’t want something too complicated. I went with a 7812/7912 three-terminal regulator pair. The maximum expected current from this at full-tilt-boogie is <0.5A per channel, so a single set of 1A regulators should be fine for the two channels. There is a 1000uF/0.01uF pair of capacitors on the input and output of each regulator. The giant 6800uF capacitor on one side was installed to troubleshoot a whine, and left because it doesn’t hurt anything. The power supply for the power supply is a pair of 16V laptop power adapters that I had laying around. These hide under my desk.
The design part of this project was pretty straightforward. The hardest part was figuring how to mount it all. Dead-bug style worked. In the end, I got a funky whine noise when one particular wire was too close to another, so I zip-tied them both out of the way (on the giant capacitor because why not). The heatsinks are hot-glued to a piece of very-heavy card stock. Good enough for desktop use, but don’t knock it around too much. It turns out that the way these are glued in place, I am able to stand the whole thing up on one end. I did that and colored black the bottom of the mounting plate and the sides of a couple of heatsinks.
This was going to be prettier when I started, honestly. Aesthetic inspiration was provided by this “Crystal” CMoy Free Form Headphone Amplifier by koogar on instructables.com: https://www.instructables.com/id/Crystal-cMoy-Free-Form-Headphone-Amplifier/ That one looks better than mine by a lot(!) but the use of bare wire as bus stock in addition to dead-bug style mounting of everything was the thing. This wire was the individual strands from a 6AWG power cable, stripped out and straightened.
I learned a lot when making this, and definitely had fun.