Step-by-Step Assembly Guide

You'll probably want to have the schematic handy while building the amp.

1. Add the op-amps

The general rule for populating circuit boards is shortest to tallest, and inside to outside. Since the op-amps are centermost on the board and all but the shortest parts, we'll do those first. If you've never soldered surface-mount chips before, you might watch the third video in my Tangent Tutorials video series before starting.

Be sure to orient the op-amps correctly on the pads. If you solder a dual-channel op-amp backwards, it will usually die very quickly as soon as power is applied.

2. Add the small passives

Install all the short 2-lead components: the resistors, the inductors, and the ceramic capacitors. None of these parts are polar: install them whichever way makes the most sense. I like to align the resistor labels so they face the same direction as the silkscreen labels.

3. Add the larger SMT parts, the LED and regulator

Now you can solder the remaining surface mount parts. Then, optionally add the regulator and the LED. If you want to put the LED on wires, you can put it off for now.

It matters how you orient all of these parts. The stripe on the tantalum capacitors marks the positive lead, while the stripe on the diodes marks the negative lead. The long lead of the LED is usually positive, but check the datasheet to be sure; some LEDs are backwards in this regard. As for the regulator, just align it as indicated on the board's silkscreen layer.

4. Add the big stuff

Install the remaining parts.

The input caps aren't polar. I like to arrange them so the label faces away from the pot, so I can read it. The electrolytic rail cap is polar: the stripe is on the side of the cap with the negative lead, and the positive lead is the longer one. If you can't figure out which way the pot goes, you need to abort this project now, before you get hurt.

5. Test the amp

It is imperative that you test the amp after making any change. With several of the chips popular in this circuit, it is very easy to go from a perfectly working configuration to one that will fry your headphones and/or the op-amps within seconds of turning the amplifier on. Testing reduces (but still does not eliminate!) the possibility of such problems.

I recommend that you test the amp outside of its enclosure, so you have easy access to everything on the board. The first time you test the amp, the easiest way to test it is to solder the panel components to the end of hookup wires going to the board. Make the hookup wires longer than they will need to be once the amp is enclosed. Then when you move on to the next step, you can just desolder the components temporarily, trim the wires to the proper length, and reattach the panel components.

Test 1: If your amp has a power LED, turn the amp on and check that the light is on. Not only does this test the LED itself, some circuit problems will result in the LED being either dim or completely off, even though the LED itself is correctly hooked up.

Test 2: Check for excessive DC offsets. First, measure DC millivolts from IG2 to OG. (IG2 is just a dedicated test point pad that's connected to the ground plane, just as IG is. You may hear people say you should test between IG and OG — it's the same thing.) OG is easiest to measure at the snout of the output jack. This should be a very small value: perhaps 10 mV at most. If that's okay, measure DC millivolts from OG to OR, and then OG to OL. These values will be higher, but as long as they're within a few tens of mV, you're okay. Some headphones may tolerate 100 mV or more, but even then, you still want low offset to keep distortion low.

(If you're using bipolar-input op-amps you will get higher DC offsets than you may be used to if you've built other circuits on this site, such as the PIMETA or PPA. The goal here is to avoid excessive offsets, not to try to achieve the extremely low offsets you get with FET-input op-amps.)

Test 3: Check the op-amp temperatures. When idling, the chips should not be so hot that you can't keep your finger on them. Some of the chips (particularly the AD8397) will get quite warm, but not truly hot, in normal operation. When under heavy load, some of these chips may get burn-your-finger hot and still be operating within normal parameters, but distinguishing that case is for the next test. For now, you just want to be sure that the chips aren't immediately overheating. If they are, the amp is either oscillating or the chips have been killed somehow, so you need to look into issues like the bypass caps, the component you're using the in L1 position, etc.

Test 4: Check the amp's current draw. The procedure for doing this and its interpretation are more complicated than I want to go into here. Please read through my Basic Troubleshooting for Headphone Amplifiers guide for details on this and other troubleshooting steps you can take if the amp is not working.

Test 5: Plug a source and some headphones in, and make sure the volume control adjusts the music volume smoothly. If you hear odd noises or the pot doesn't do what you expect, go back through the tests above, and also through the signal tracing steps in the troubleshooting article. If you can't get the problem to happen without heapdhones plugged in, try plugging in an extension cable instead. A simple unterminated cable a meter or two long can be sufficient to cause problems.

This must be the absolute last test after any change. If you test first with headphones, we reserve the right to make fun of you in public.

6. Case it up

Now for the hard part. :)

If you're going to use a mint tin, I suggest making the pot mounting hole slightly larger than necessary, so it's easier to angle the board into position. Desolder the panel components attached to the hookup wires that were used for testing the amplifier. Tilt the board into position, then install the panel components, battery holders, etc. Finally, solder the hookup wires to the panel components, doing the joints toward the bottom of the case first and working your way up. Once this is done, getting access to the board again will be somewhat difficult. You can make your job easier, at the expense of neatness, if you leave some slack in the wires.