Recently, I had the great fortune to get in on a group buy of sorts, for a modified version of the Starving Student Millet Hybrid amplifier. The SSMH started life as a simple but ingenious hybrid (tube and solid state) design by Pete Millett, in which he used the heating element of the tubes to put a load on the MOSFETs of the amplifier, cutting complexity and cost. Originally, it was a very basic amplifier with a budget-minded soul, built into whatever enclosure was handy with point to point (P2P) wiring. You can see the original build at his site here, and the forum thread following it’s history here.

DIY forum user Dsavitsk then improved on the design with some slight modifications to the circuit. He also developed an equally ingenious way to mount the amp in an enclosure – a PCB would be developed and attached to the sliding top of a standard Hammond enclosure, allowing the tubes to be mounted to the “underside” of the PCB and stick out through the case. This would eliminate the P2P wiring that caused many problems in DIY builds of the amp – problems with grounding and bad connections had plagued countless hobbyists.

Collaborating with Tom Blanchard of Beezar Audio, the two developed PCBs and set the project in motion. Tom took on the arduous task of organizing the procurement of the increasingly rare 19J6 vacuum tubes, the custom-machined Hammond cases, and all the other various parts needed to develop kits of the SMMH.

I was lucky enough to get in on this undertaking and purchase a kit, and after a couple months (during which the cases were machined), I recently took delivery of the kit and quickly put it together.

The ease of construction of this amp is a great testament to the amount of work that Tom, Dsavitsk, and originally Pete put into this design. I had the amp completed from start to finish in less than 5 hours, with no serious problems. With me and DIY projects, that’s unheard of. Tom made it extremely smooth with his excellent instructions. I flipped the power switch, watched the glow of the tube heaters and LEDs come to life, and plugged in my headphones and source. I wouldn’t call myself an Audiophile, but the sound of this amplifier far outranks anything I’ve listened to, and blows my own basic DIY amps out of the water. Plus, it looks amazing on my desk. Glass tubes? How quaint!

For those looking for a great audio project with that old-school tube feel, there are still PCBs and tubes available, with a chance that more custom cases will become available. [EDIT] As of February 2010, the supply of 19J6 tubes has been nearly exhausted and no more PCBs or kits based on this tube will be produced unless a stash is found in the corner of a warehouse somewhere. The possibility of doing this build point-to-point still exists if you want to buy the tubes in small quantities at high prices, and there are modifications that can be made to use this design with more common tubes – see the SSMH website or the Head-Fi thread for details.

Even if this amp seems like too much of an undertaking, I highly recommend dabbling in DIY audio – a great place to start is the classic CMoy headphone amp in a mint tin. Tangent has some of the most complete instructions here.

Un-Disclaimer: I’m in no way involved with Beezar audio or anyone else mentioned in the post; this was just an awesome project that came together through the power of the DIY community.

Top View of the USB Power Shield v2.0

• Smaller Size – cost is determined by size, so I brought it down to the bare minimum – just enough to cover the Arduino pins.
• Power Planes – Added a 5V plane on top and a GND plane on the bottom to increase the reliability of the design, allow higher current, and also act as a bit of a heatsink
• Thicker Traces – the traces in v1.0 are only 8mil, which is pretty small and is really only good for about 200mA. Some devices powered here may need more than that. To support the USB spec of 500mA per device, I’ve bumped up all the traces to 16mil. There’s plenty of space for traces anyways.
• Remove the Vin LED – unnecessary.
• Flush caps – leave space so the caps can be bent down, so another shield can be fitted on top of this one. The regulator is also mounted on the edge to allow bending it down, although in many application it will need a heatsink so they may not clear even when bent down.

And here it is built:

Parts needed:
Keep in mind not all parts on the board are needed. For instance, if you’re powering a single pair of lights (or anything running at <300ma) you can just use the Arduino's built in DC-in with voltage regulator (USB power alone is not going to cut it). If that's the case, you can leave out the 7805 regulator, the 100uF capacitor (keep the 10uF), and the power diode D1. You also don't have to have the power indicator LED or its resistor RLED. And if you're only controlling one device, you only need one relay and one USB connector. All in all, you can get the parts cost for this board down to about $4 at the minimum configuration. If you want to go all out, here's what you'll need:

2x G6JU-2P-Y-DC4.5 Relays @ ~$2.85 each
2x USB A connectors @ ~$0.45 each
1x 7805 5V Regulator @ ~$0.37
1x DC-in 2.1mm Jack @ ~$0.63
1x 100uF 25V capacitor @ ~$0.03
1x 10uF 25V (10V ok) capacitor @ ~$0.03
1x 1N4004 standard Diode @ ~$0.04
1x 450-500 ohm resistor @ ~$0.04
1x 6x6mm pushbutton switch @ ~$0.24
1x 5mm LED (any type/color is fine) @ ~$0.08
1x 28+ pin row .1″ male header @ ~0.11

Total parts cost ~$8.17

You’ll also need a heatsink for the regulator if you plan on drawing more than ~200ma from it, those can range from $0.30 to $2, depending on size and complexity. There is a quite large one in the picture above as I was using it to charge my MP3 player, which draws a lot of current.

Stay tuned for an assembly guide. Also, I’ll be selling the few extra boards I have at cost to those interested, details to come soon.

My first google searches ended in disappointment – using the built in Terminal program with Screen was unsuccessful (probably because it doesn’t seem like you can set the baudrate). Update: it turns out that you can set the baudrate with screen using an argument like so – “screen /dev/ttyWHATEVER 115200″. Thanks goes to the David in the comments below. After a little searching, it looked like the few serial terminal applications for Mac were old and outdated, but I did manage to find ZTerm. It’s simple and basic – perfect for use with the bus pirate.

Don’t forget that you need to have already installed the FTDI driver so that your computer will see the device over USB. Start Zterm and choose the correct port in the popup window (if it isn’t already chosen automatically). Then you need to change the connection settings to talk to the Bus Pirate properly. Go to Settings->Connection and set it like this:

Data Rate: 115200
Data Bits: 8
Parity: None
Stop bits: 1

The rest you can leave at the default. [UPDATE] On newer firmware versions you’ll need to deselect Xon/Xoff as per Ian in the comments below.

Hit ok and go back to the terminal window to start talking

Hit enter to start communicating. Entering ? will bring up the list of commands like so:

To test some functionality of the bus pirate, we’re going to measure the voltage on the ADC pin. We’ll do this by putting the Bus Pirate into just about any mode besides HiZ – we’ll choose 1-Wire since it’s simple to set up.

Enter “m” to bring up the mode menu, choose 1-Wire by entering “2″. Hook up some voltage to the ADC pin (see the bottom of the Bus Pirate for it’s pinout). Then enter “d” to read the value on the pin. I connected the pin to the 5V input from the USB line, so it reads 5.1V. You can see that full process in the terminal below.

That’s it for the basics! See the Bus Pirate site for more examples of how to use this nice piece of hardware.