Nelson Pass F5 Class A Stereo Amplifier

By jason, April 5, 2012 on 9:53 pm | In projects | 2 Comments

F5

Another project in the books. This one is my first class A power amplifier build. The circuit itself is actually relatively simple. And the casework wasn’t bad either truth be told. But I’m getting ahead of myself.

The design is from one of the godfathers of the DIY HIFI community, Nelson Pass. He runs a DIY website as well as a commercial and a sort of hybrid enterprise that sells retail items but also provides schematics and articles so the enterprising DIY’er can “roll his own.” He’s also active on the diyaudio.com site and chimes in from time to time.

The specific design I chose is the F5. It’s a push-pull Class A amplifier, utilizing JFETs and MOSFETs in a very simple two stage complementary circuit. It spits out about 180W (most of it in heat) and is one of the more popular designs from Mr. Pass.

For my build, I sourced the PCB from tech-diy.com and most of the little bits from mouser. The heat sinks were from antek and the rest of the chassis was made up of scraps actually. I had spare 1/4″ aluminum for the bottom, African mahogany for the front panel, plywood for the rear, and plexiglass (from an old document holder) for the top. I did buy some 1/4-20 bolts and some new RCA jacks and speaker posts. But all in all, I think I only spent about $300 all told. Some have been known to spend that much just on the front plate!

In any case, pictures are where the fun is so here you go.

The trafo, rectifiers, and first pair of caps that make up the separate PSU enclosure. I used a 4-pole Neurtrik Speak-On connector for V+/G/V-/earth ground:
F5
F5

Here’s the enclosure for the PSU. Made from more scraps:
F5 PSU

Here are the amp PCBs coming to together mounted on the heat sinks:
F5 boards and sinks

Here’s the view of the front/bottom with the mahogany and aluminum holding the heat sinks together:
F5 Amp

And of the rear/bottom:
F5 Amp

In mockup mode:
F5 Amp

The beginnings of milling the rear:
F5 Amp

All put together with the plexiglass top:
F5

Shot of the front with the powdercoated aluminum letters I splurged on:
F5

Stacked on the PSU:
F5

It sounds great and doubles as a space heater! I got it biased up to about 580mV and the DC offset is essentially zero on both channels.

Some sketches I made on the plane on the way home from a business trip:
F5 sketches
F5 sketches

2 Comments »

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  1. Looks good ! Just a few questions though, What size are the heat-sinks and how hot do they get ? Next could you provide a little more information about your remote psu ? Thanks

    Comment by Jeff — December 19, 2012 #

  2. Heat sinks don’t get too warm. They’re the standard 10″ width from Par-metal and are 5″ high. I haven’t measured, but I can hold my hands on them indefinitely which typically means they are well within safe range. The remote PSU just has the trafo, rectifiers, a pair of caps, and the IEC/umbilical. They’re all mounted on an 1/4″ aluminum plate that is held in a 1/4″ groove in the front and rear members. Does that help?

    Comment by jason — January 7, 2013 #

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