• Item: Class A Mono Blocks  
  • Type: First Watt F5  
  • Posted: 7 April 2011  
  • Builder: martin  
  • Country: Austria  
  • Comments: 7  

Small, Hot, and Great Sounding: The F5

Layout and Construction
Class-A amplifiers needs some serious heat sinks. I got used ones and had to verify: will these heat sinks be sufficient? I used an website offering online calculations: Frigus Primore. As a pity, it seems not to exist anymore.

The simulation showed that the heat sinks should stay within an acceptable temperature while dissipating 30 watts from one device. The measurements of the finished amplifiers proved the calculations to be fully correct.

For my F5 mono's, a nice and compact design was on my mind. Very often I checked back with printed layouts of the circuit boards. Within the case, the boards are mounted on two levels to save space. Another benefit of this arrangement is that the electrolytic capacitors are in the lower (and thus cooler) part of the enclosure.

The diode bridges have their place on the heat sinks, just next to the large transformer. This makes wiring easier.
The diode bridges and the transformer are shielded by a vertical steel plate because they are quite close to the amplifier output stage. This plate also creates a separate chamber with separate air circulation in order to keep these parts cool.

The vertical orientation of the transformer was chosen mainly to keep the footprint of the unit small. The cases turned out to be really small, 210 mm wide, 171 mm high, 220 mm deep, which is 8.27 x 6.69 x 8.66 inches.

I wanted to create an internal air circulation for cooling of the hot amplifier circuit and thus milled slots into the heat sinks. After several hours of operation, the air temperature within the enclosure is 54° C. The temperature of the heat sinks rises up to 60°C which is 141 °F.

Out of thick copper I milled pieces that were assembled into U-shaped heat spreaders. The amplifier board is attached to these as is the heavy transformer. With this solution it was possible to assemble the amp with one heat sink only. When all the wiring was done I could easily attach the other heat sink. All enclosure parts were CNC milled out of piece of 0.395 inch thick aluminum. CNC machines help keep tolerances in the enclosures very small. All parts of the two casings would be interchangeable.

For my mono blocks I did not change the original schematics by Nelson Pass; www.passdiy.com.
Each amplifier got its own power supply. As this was laid out for a stereo amplifier, it is really oversized. The amplifier circuit itself is not very complicated. Just a few parts are needed and there are no capacitors anywhere in the circuit.

It is good to buy a few semiconductors more and choose the best matching devices, especially for the JFET´s at the input. The circuit boards were produced with the help of engineer Gerhard Koenig in his workshop in the HTL technical school. Thanks’ again Gerhard, it was an interesting lesson!

I performed a long listening comparison test against very expensive gear and it is true: these little amplifiers perform very well!


  • hkoetz
  • 1 June 2011 at 10:12AM
  • Where did you get those great heat sinks?
  • martin
  • 9 June 2011 at 02:46PM
  • These were used parts from a Siemens converter module.
  • BigAmpman
  • 16 August 2011 at 04:10PM
  • I would love 2 buy these monoblocs from U !!! If interested I will give U my email address.
  • martin
  • 20 August 2011 at 05:38PM
  • BigAmpman, I still need them. The original was just 3000 USD, maybe you can find an example..! See here how I made them:
  • mvdventura
  • 11 June 2014 at 06:08AM
  • Hello,

    I´m writing from Spain and I´d like to know if you still have those beautiful amps.

    Please, drop me an email if you consider selling them (mvdventura@gmail.com).


  • TwistedPair
  • 13 December 2016 at 06:36AM
  • DAMN!!! that is such a CLASSY build. Theres a whole lotta love in there...

    Q. DO you have access to more of these heatsinks???
  • sphinix
  • 2 August 2023 at 12:26AM
  • Q which of the siemens convertors did the heatsink come from

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