Jul 232009

I have been working on a pair of Pass Labs Aleph Five single-ended class-A monoblock amplifiers for about two and a half years. In truth, it is about 3 weeks of work, it’s just been spread out over a very long period!

The schematic is based on that published in the Pass Labs service manual, with several modifications. Here is the schematic:

Aleph Five Schematic (click for full size)

A few notes are in order:

  1. All the input and output connectors are doubled. This is because I used 1/4″ spade lugs that require two mounting holes. Placing double connectors on the schematic makes it easier to place double holes on the board.
  2. +IN and -IN are for balanced connections. UIN is unbalanced.
  3. R34 and R35 are 10-turn precision potentiometers. R34 adjusts DC offset at the speaker terminals and R35 adjusts the level on the LED VU meter.
  4. Connections PMA and PMB are for a switch that disables the VU meter.
  5. The current source MOSFETS (Q6-8) and the output MOSFETS (Q18-20) are not on the schematic because they are remoted to large heat sinks and replaced on the schematic by the spade lugs to which they connect. Each connection is labeled for gate, source or drain. All of these transistors are IRFP-244’s and should be matched to within 0.01V.
  6. Q1 and Q2 should be matched to each other as well.
  7. R22-25, R40-41, and R64-66 are 3W. All others are 1/4W.

Here is the power supply schematic:

Aleph Five PS Schematic (click for full size)


  1. Filter section is a CRCRC design. The resistors, R1-4, are 50W and are mounted so as to use the rear of the power supply chassis as a heat sink.
  2. TH1 is a thermistor between the circuit ground and chassis ground to limit ground loop noise.
  3. TB and TC are the transformer center taps used to establish the ground level.
  4. +R and -R are the outputs of a 35V 4A bridge rectifier.
  5. Filter capacitors are Panasonic TUP type, 39,000uf 50V.
  6. The transformer is a 30V 500VA toroidal, Avel Lindberg model number Y236802
  7. The switch, rectifier, fuse, current inrush limiters, etc. are all outside the schematic. The switch is a Soft Switch V2 from diyAudio regular Rob Cheng.

If you have Eagle CAD, you can download the schematics and board layouts below. Otherwise, JPEGs of the board layouts follow.

Aleph Five Board Layout (click for full size)

Power Supply Board Layout (click for full size)

Here is the power supply – there are two of these, one for each channel.

Aleph Five Power Supply

Here is the rear portion of the power supply – soft switch on the right, filter caps on the left.

Power Supply Detail

This is the back of the power supply chassis. You can see R1-4. There is a 120°C thermal switch mounted on the inside of the rear panel. The 5-pin connector at the upper left carries power, ground and thermal switch sensing to the main amplifier chassis.

Rear of Power Supply

The center of the front panel is dominated by the power switch. The three LEDs monitor the Fuse, the Power Supply Thermal Switch and the Amp Thermal Switches. There are three of the latter, one mounted on each of the main heat sinks in the main amp chassis.

Front Panel of Power Supply

Love those LEDs!

Power and Status Lights

The main chassis aren’t done yet. Or rather, they aren’t started yet. Here is the testing prototype, which does work and sounds really good.

Amp Prototype

And here are the final revision circuit cards, ready to be stuffed.

Final Revision Amp Cards

My hope is that posting it here means I will finally get it finished this decade. Stay tuned…

Jul 162009

Melissa, Tom and I travelled down to Florida for the launch of STS-127 on Friday, July 10th. The launch was scheduled for the 11th, but it was scrubbed before we even headed out to the cape – there had been 11 lightning strikes near the shuttle the previous evening.

On Sunday the 12th, we tried again and this time we made it all the way to the launch viewing area before the launch was scrubbed for weather once again. The decision was made during the 45-minute hold at the T minus 9 minute point, but it was pretty clear to everyone well before that – we could see dark clouds and lightning throughout the area.

Launch Viewing Area

Monday the 13th was try number three. Once again, we got out to the launch viewing area and all looked pretty good until the end of the 45-minute hold. A thunderstorm cell just to the north was growing and we saw lightning once again just after they called the scrub.

Me (right) and Tom

Based on the forecast, NASA decided not to even try on the following day, and the day after, the 15th, was when we were scheduled to fly home. Of course, the launch went off without a hitch on the 15th! We watched it on NASA TV on our iPhones while at the airport. Our current plan is to try again for the STS-131 in March. Not only will there be less thunderstorms, but the temperature and humidity will be closer to what humans can actually survive.

Camcorder & Telescope Setup

In any case, I got a good test of my camcorder and telescope system for photographing the launch. Remember, the viewing area is seven miles from the launch pad. Here is the test footage, plus a few pelicans that were near the launch viewing area.