Audiophile heaven

Pete Dokter, SparkFun Electronics -April 15, 2013

At Sparkfun, we've received emails asking for 6DOF sensor boards and controller boards to be made available independent from each other. So when we revised the 6DOF v4, we decided to make the separate boards as stand-alone as possible.

So what? What do you do with them apart? Well, the sensor board is pretty obvious: it's a bunch of sensors, so you sense things with it. But the controller boardyou can do just about anything with it. It's not limited to sensing applications by any means. It's an LPC2138 with everything it needs to run (plus Bluetooth connection onboard), free compiler, source code, all in a tight little package ready to go. Not that we don't like Olimex. To the contrary, we love Olimex. But for the example I'm about to present, I needed something smaller and simpler than the LPC2138 dev board. Now to the meaty bits.

One of my extracurricular pursuits is tube audio. Not guitar amps (you can read about those here), but stereo amps. I just think they sound great. And tubes pretty much rule as far as I'm concerned. But before I get too far into this, let me address some of the inevitable questions/comments that my audience may have:

  • Yes, you can still buy tubes. Many places, many types.
  • Yes, solid state tends to be more linear. But sound amplification is not necessarily about linearity. It's about the sound and what's pleasing to the ear. Solid-state technology certainly has its place, but I don't want it in my audio path.
  • Yes, I could almost certainly achieve the same sound for less money and effort if I used solid state. That would be cheating. And it wouldn't be nearly as cool.

Here's what I'm working on:

Where's Waldo? Ok stop. There is no Waldo.

This is the first of many tube amps I hope to build. The high-voltage and heaters transformer is in the upper left. The regulator circuits, both for the high voltage (320V) and heaters for the tubes (6.3V) are at the top middle of the picture, with the hot bits mounted on a heat sink. The audio transformers are top and right. There's also a 5V Condor linear supply at the bottom left that runs the control system (more on that in a bit). The tubes are, right to left, four EL84s in push-pull ultra linear configuration, two 6922s as a gain stage and concertina phase splitter, and two 12AX7s as the input section with bass and treble boost/cut. It makes about 12W per channel, and there's no global negative feedback. It sounds like a million bucks.

So what's the 6DOF controller board for? Tubes run at high voltages. Coincidentally, I'm afraid of dying by electrocution. When something fails in a high voltage circuit it's not usually a trivial event. And I've had a few failures that served as motivation for installing the control system that I'm going to talk about.

Let me say a little something about tube circuits and safety. When there's power to the circuit, I don't put my hands in the chassis. Ever. It only takes one absent-minded moment to kill a guy working on this stuff, so I choose not to take any chances. I'll set up my meters where I want them, make sure I'm not going to be messing with any of the leads by placing them carefully, and power up slowly with a variac while watching the voltages. Or, at least, I power up that way until I know the circuit is stable. When I power down, I disconnect from line voltage and watch all the meters go down before I touch anything. Simple rules to follow. The guy that doesn't follow them is the guy that's going to win a Darwin award.

Here's the basic topology of the amplifier power supplies. As I mentioned, there's a 5V supply that serves the control system and supporting electronics. In theory, that one's always on. Then there's the transformer that supplies the heater voltage and the high voltage. That transformer is switched on by relay, coil current for which is supplied from the 5V. The high voltage is also switched on by a pair of relays (two to effectively raise the relay's voltage rating) on the ground line to the transformer. So nothing can be on if the 5V supply isn't on first, and the high voltage can't be on until the heaters are on first. Cold tubes don't conduct, so you have to run the heaters for a minute or so before applying high voltage.

All of the difficulties I've had with this design revolve around the high-voltage regulator, which supplies 320V at 200mA. I didn't have to do it that way. There are plenty of designs that don't regulate the high voltage; I just didn't want any 60 Hz in the audio. Before I installed the control system, my only means of a quick shut-off was a switch on the variac that I run the thing on. It worked well enough to keep me alive, but it wasn't fast enough to save my nerves. Nor was it particularly helpful in sorting out what failed in the first place because it allowed enough time for everything in the circuit to tank before I could shut it off. And it would send some crazy-loud 60 Hz to the speakers that served to freak me out to biblical proportions.

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