As I write this, I’m enjoying some Boston piped through the Robusto Amp. This was a pretty simple, yet satisfying build.

Robusto Assembly

The Verdict

With the bypass switch, I can compare the amp’s sound to the source. So, how does it sound?

  • With crap earbuds, there’s no difference.
  • With slightly better headphones, it sounds a bit “fuller” with the amp, maybe.

Overall, the difference is pretty minor (if it exists at all). I need to try two things: Line level input and higher impedance headphones. Even my good headphones are fairly low impedance.

Lessons Learned

  • The polarity protection diode payed off. While messing with the power supply, I reversed the polarity at least once.
  • I’m not so good on perf boards. Next time, I think I’ll spend a little more time up front laying things out. Also, I think I’ll try not using the component leads as traces, this makes it really hard to get them off the board if need be.
  • Don’t underestimate the time the enclosure fitting and final build will take. Granted, I’m pretty slow, and I don’t have much of a workshop, but this is a big part of the project. Where putting together a simple circuit on a breadboard is fast, fitting everything and soldering the final circuit can take an order of magnitude longer (for me anyway).

I spent a good portion of yesterday doing the final assembly on the headphone amp. All I had left this morning was to wire up tube and pop the top on. I plugged in the power and…

In the words of Isiah Whitlock Jr.: Sheeeeit! The LED was flashing, it’s not supposed to flash.

After a bit of poking around with the meter, I found the problem: I wired the LM317 regulators improperly. The output was shorted to ground. I carefully disassembled enough to get to the bottom of the circuit board and rerouted a few things. Once the LM317 was wired up correctly, it worked fine. I’m a bit surprised that the components stood up to such abuse.

Here’s the final product playing some classic rock:

Robusto Amp

Amp Front, with Jack

Yesterday was a productive day on the amp front. I made a little progress mounting components on the front panel. I also made a lot of progress with the electronics.

With everything wired up, albeit temporarily, I was happy when I saw the tube glow for the first time, and even happier when I ran some music through it. The amp sounds great. It has a brightness that others have noted, it’s the sort of profile that goes well with the warm sound of first track I played through it (What if We All Stopped Paying Taxes – Sharon Jones & The Dap Kings).

It’s not perfect. There is a low level hiss, noticeable only when there is no sound. I’ll have to see what I can do to get rid of that. It also picks up some RFI if something like an iPhone is within a foot of the circuitry.

Now I need to work on getting it cleaned up and in the enclosure so the guts aren’t splayed out on my desk.

Amp Enclosure 2

The plan with this headphone amp is to build it on a breadboard first. Partly to make sure it works, and partly because I’m still hesitant to etch my own circuit board.

Since the tube wasn’t going to fit on the breadboard, I was looking for something to attach it to, temporarily, so it won’t flop around. I found a cigar box in my small junk collection, which I think will do nicely. I mounted the tube socket on the cover and soldered in some hook up wire.

Even though it is meant to be temporary, this little cigar box could function as a permanent enclosure. It will need some ventilation holes though.

Amp Enclosure 1