Pass F5 Amplifier

The FirstWatt F5 Amplifier. Nelson Pass simultaneously released a series of production units for purchase, and a set of schematics to allow DIY audio enthusiasts to build their own.

As a followup to our speaker building projects, some club members expressed interest in building amplifiers. Our resident guru Chuck B suggested the FirstWatt F5 as a nice entry level project build that yields an excellent sounding amplifier. Chuck had built his own version of the F5 as monoblocks, and participated early on in the F5 builders thread at the DIYAudio website. Don M followed up with organizing and hosting the build sessions for 15 participants. The build sessions occurred during our 2010-2011 season.

The Nelson Pass F5 power amp is one of the most popular DIY builds in audio history. Over 1500 F5 related thread pages exist on the Pass Forum within the DIYAudio website. This presents an exhausting read, but the first few pages cover the design and most questions are answered within pages 600 thru 700. Circuitry, design concepts and measurements are detailed on Nelson's First Watt website. A full review of the commercially available amplifier is reviewed on the 6moons website. It might seem like 25 watts per channel is on the low side for a large and heavy amplifier, but keep in mind that this amp is easily capable of driving 2 ohm loads and has headroom into class AB operation. We haven't measured the output of the standard build, but it looks as if it would be easily capable of delivering over 70 watts per channel into 4 ohm loads.

Two versions of the F5 were offered to builders: The original release as a 25W/ch into 8 ohm Class A amplifer and a higher powered "turbo" version. Most builders opted for the turbo version as the added expense and complication was minimal. Most builders were novice level. Only a few had any experience in DIY electronics projects.

A note about the F5 turbo versions:

The simplest turbo version employs a higher voltage and higher capacity transformer to increase the maximum output well beyond the stated class A levels. We chose to run the power supply rails at +-34V which allows the single pair of output mosfets to produce approx 140 watts per channel into 4 ohm loads at full bandwidth and with low distortion. However, this increase in rail voltage presents an over-voltage situation to the driver JFets. To accommodate, a cascode circuit is employed that limits voltages at the JFets to a safe level. We added cascode circuits to the undersides of our printed boards as elevated and inserted components. Soon after, turbo versions of the circuit board became available, as well as super turbo versions where output Mosfets numbers are doubled or quadrupled and power output capabilities are further increased. An article on firstwatt.com outlines various turbo options.

Chassis parts, transformers, power supply parts, circuit boards and circuit parts were obtained with nice savings from group buying power. We were able to source the increasingly rare authentic SJ74/SK170 Fets and chose the IR version of the suggested output Mosfets.

The first build sessions were dedicated to assembling and prepping chassis and attaching heatsinks. Lots of careful cutting, drilling and thread tapping were required for a professional look. Our heatsink arrangement required us to mount the output Mosfets off of the circuit boards to maximize cooling capacity. An amplifier biased to run 25 watts per channel in class A needs to be able to dissipate a lot of heat! Most builders opted to purchase chassis from Par-metal.com while others repurposed other audio chassis or even made a chassis from scratch. Our transformers were supplied by Par-metal's sister company Antek Inc.

Completion of a pair of chassis showing rear panel components and the 600VA transformer. Note the 1.75" diameter holes cut into the chassis sides to accommodate mounting Mosfets directly onto the heatsinks.

Our power supply configuration was based on a traditional unregulated linear supply, but some of us opted to further lower the noise floor by

employing a CLC, rather than CRC filter circuit. The inductors used are large 12 gauge 2.2mh air core. These have become increasingly costly! Twelve gauge solid copper was used as bus wire. Caps were press fit into holes drilled into laminated mounting boards. We chose to use 47,000uf filtering caps (total of 4) as our standard, and applied 4.7uf 600 volt poly caps to the output side of the bridge rectifiers. Power supplies were tested and measured for low noise. A few more build sessions involved populating and soldering up circuit boards (supplied by vendors via DIYaudio.com) as well as final input and output wiring.

A completed F5 amplifier is shown. The front panel has Ebony veneer applied as well as a power indicator LED. The neutral bus wire is the star grounding point for inputs, ouputs and supply. The Mosfets and biasing thermistors are mounted directly onto the heatsinks.

Units were commissioned by Chuck B. A variac was used to gradually increase the F5's to full operating voltage. Biases were set and units

were allowed to idle for 15 minutes to reach full operating temperature. Bias settings were then fine tuned and adjusted to keep DC output

below 20mv. Every unit built by the original crew worked A-OK from the get-go. One unit suffered from oscillation and required a quick repair. Luckily, this was the first unit commissioned and it revealed oscillation sensitivity along the Mosfet gate leads. We quickly corrected that situation on all F5's by removing the gate resistors from the board and soldering them directly onto the Mosfet legs.

After some experimentation with feedback and gain structure, I've concluded that the feedback/gain arrangement suggested by Mr. Pass

is about perfect. Reducing feedback to increase gain is nice for adding life to smaller scale music, but suffers a bit under complex music.

Increasing the feedback beyond the suggested rate results in an amp that is a bit more clinical and dry in nature. In any case, the amplifier sounds wonderful. It offers a holographic soundstage and neutral response with no veiling. We are using these amplifiers to drive all sorts of speakers from high efficiency horn systems to large Magneplanars.

A few club members have built other Pass DIY designs including Aleph and A75 amplifiers, Pearl phono stages, the latest F6 amplifier, B1 buffers as

well as B3 gainstages. Special thanks to Nelson Pass for designing and publishing this project. It yields superb results.

Don M