Past projects: Pass Labs F5

One of my most successful builds (no doubt thanks mostly to the genius of the designer 🙂 ) is a Pass/Firstwatt F5 class A amplifier.

The F5 design is the work of legendary amplifier designer Nelson Pass and the design was first published in 2008 – mine was built a couple of years later. It’s a low-powered class A design that puts out roughly 25W at full tilt – and around 180W of heat at idle! The amp is a very simple design, but it sounds very good and it has been immensely popular. There must be thousands of DIY-versions of these playing all over the world and supplementing the 100 original Firstwatt-built models sold for retail (read the 6moons review of that here)

For my build, I used good quality “clone-boards” from ebay, not because I like that but because that was all that was available when I started the project. The components are all sourced from more reliable sources than ebay…The one exception to that is the softstart which is a premade unit from ebay as well. This one is working fine, but I have had another one literally blow up in front of me so one needs to be a little careful here…

The chassis is a 4U/300mm “Pesante Disspante” model from modushop and the heatsinks stay around 25-28C above ambient which is fine. The transformers are custom-wound by which I have had very good experience with for other projects as well.

The sound of this thing is incredibly transparent and with a big, big soundstage (I like that…) and when you first hook it up, you can scarcely believe that something so simple can sound like this. The bass is also very good as long as you don’t push the amp to far, not quite as controlled as the class D amps I have but with more “body” and “presence” to it. In comparison to the ICEpower125ASX-based amp that is otherwise my normal reference, the general signature of the F5 is a little warmer and “fuller” whereas the ICEpower sounds “lean” in comparison. This extra “warmth” really makes my Elacs (that are a touch bright at times) sing.

Right now the F5 is on “summer holiday”, simply because it emits too much heat to be used when it is warm outside, but in the winter I enjoy it immensely 😉

16 Responses to Past projects: Pass Labs F5

  1. Peter Hyldborg says:

    Jeg var sgu nødt til lige at lægge en hilsen.
    Jeg har aldrig før stødt på nogen, hvis “udvikling” i den grad ligner min egen.
    Jeg har selv haft “The bastard”, jeg har ICE 125 monoer, jeg er igang med Retro fra Twisted Pear og glæder mig over din positive beskrivelse af den.

    Desuden har jeg Hypex HG400 monoer og forforstærker baseret på THEL moduler…de er gode og rimelig billige.


    Peter, Odense

  2. theslowdiyer says:

    Hej Peter,

    Tak for din kommentar, det er lidt sjovt at vi sådan har samme smag 🙂 Lad mig vide hvad du synes om Retro’en når du er færdig med den.


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  5. Joe A says:

    Nice casework. Is it commercially available? If yes where did you get it from? I’m planning to build my own Pass F5. Thanks.

    • theslowdiyer says:

      Hi Joe,

      Thanks for your comment. The case is from in Italy. I have used a 4U x 300mm deep version, but going to 400mm is of course better – it does get quite hot. If you are US-based, I’d recommend that you look in the store as they ship cases from the US. The back panel is custom made by Schaeffer AG in Germany (aka Frontpanel Express in the US).

      Hope this helps!


      • Joe A says:

        Thanks for your quick reply. I am in Europe so I can contact them directly. Thanks for your advice. Regards, Joe A

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  12. Steve Oltman says:

    Hi, is there any chance that you can find the ebay seller for the pcb? Thanx.

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