After using my Dynaco ST-70 and PAS-2 for a few months while working on our senior project, I realized that I was wasting my resources! I had the ST-70 turned down so low that it was just not worth throwing 50 watts of turn-on power out the window when probably less than 0.1 watts were being used for sound. That sets the backround for the Indigo project.

The Indigo project is a very sleek, thin and tiny power amp that uses a high-efficiency 3rd generation class-D power amplification scheme while integrating a vacuum tube preamp stage. The case would be made of glass and would light up by blue LEDs when on. The block diagrams below show some ideas I've had.

Other possible block diagram

The heart of the amplifier is the Texas Instruments TPA3000D1 17-watt mono 3rd generation class-D amplifier IC. This chip is a TSSOP-24 package, which is ~ 1mm smaller in each dimension than an 8-pin DIP component, but has 24 leads! It is very small and takes a brain surgeon's hand to solder! The PCB is only 1" by 1.1" and contains mostly decoupling capacitors. The switching frequencies is around 250kHz, so an output filter is unnecessary! The TPA300D1 has selectable gains of 12dB, 18dB, 23.5dB or 36dB. Unfortunately, TI is not recommending it for use in new projects because it is being phased out. In fact, I can't even order samples any more or even find the datasheet! I haven't found a similar IC on their site yet, but I hope they come out with something good.

The amplifier PCB was fabricated by ExpressPCB.com using their ExpressPCB software download, which is very intuitive to use. I used their MiniBoard service, which gave me three 3.8" x 2.5" boards, so I could fit six amplifier boards on one big one, giving me 18 perfect amp boards for $62-- not bad! See the Indigo.pcb (29kB) file to use my layout. See the Amp_Sche.gif (13kB) amplifier schematic I used in generating this layout. Also see the Indigo_parts.htm partslist that goes along with the project.

The next step was the 5744 tube preamp section. See the preliminary schematic, Sche_5744.gif using the small 5744 pencil tube and the Indigo_parts.htm partslist that goes along with the project. This schematic gives a gain of around 20dB into a 22k ohm load impedance at a bias point of 0.45mA. The 5744 triode has the following characteristics:

• Filament: 6.3v at 200mA
• Plate max: 250v
• Plate Resistance: 500k ohms
• 0.38" tube diameter, ~1.4" long, <0.5" at crimp point.