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First Watt Amplifiers

 

First Watt is Nelson Pass' "Kitchen Table" Creative Project

Several years after he started Pass Labs, Nelson Pass began nurturing a side project out of his home. He wanted to explore a variety of amplifier designs in what he thought of as neglected areas - high-end amplifiers that might not fit into the mainstream, but would appeal to a subset of audiophiles. Known as First Watt, it's been referred to in various ways: "mad scientist" endeavor, "kitchen table" creative outlet, Pass' vehicle for "less marketable" amplifier designs. Through First Watt, Pass explored low power, single-ended, Class A amps with an emphasis on sound quality. When he started First Watt, Pass was hand-building niche amplifiers in low numbers without drawing on the resources of Pass Labs. Put a different way, First Watt was a way for Pass to explore designs outside of the mainstream of commercial audio without the constraints of economic viability. It was an opportunity to experiment in the marketplace with products that may or may not ever be popular. First Watt amps are produced in limited quantities.

In the earlier years, Pass would build these amplifiers at home, with help from one of his sons and a nephew. Today, Pass Labs handles the production and marketing for all First Watt amplifiers. Pass devotes about 40 percent of his time these days to First Watt.

In a Q&A on the First Watt website, the proverbial customer asks, "Why would I want one of these?" Pass' answer:

"Maybe you don't. These amplifiers are all out of the mainstream, reflecting quality in simplicity and intrinsic linearity, often with little regard for the usual performance specifications. ... With oddball characteristics and output power ratings of 25 watts or less, First Watt is not for most people. If you have efficient loudspeakers, listen at reasonable levels and are obsessed about subjective performance, then you probably have come to the right place."


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"First Watt pushes the design envelope with simple, high-performance Class A amplifier circuits."

As for the name First Watt, it stems from The Absolute Sound's Dick Olsher, who once said that the first watt is the most important watt. Others have voiced similar sentiments, such as, "Who care what an amplifier sounds like at 500 watts if it sounds like crap at one watt?" With this in mind, Pass decided to explore low power amplifiers with an emphasis on sound quality.

Although First Watt amplifiers are often compared with tube amplifiers, Pass has said they are not designed to mimic tube amps. These amplifiers share some of the characteristics of the better tube products in that they have sinmple circuits with minimal or no feedback and emphasize performance of individual gain devices. In some ways they are better than tubes; in other ways, perhaps not, Pass has said.


Release date: 2009

Sound signature: Warm and relaxed.

Highlights: Natural sound with a stellar midrange. Precision and detail without sterility. First Watt's most popular amplifier.

Caveat: Can be picky about speakers.

The J2 is perhaps the most popular amplifier made by First Watt. It was not intended for more than the usual 100 unit total production. However it was so well received that First Watt broke the rule and it is still made. Still, there will be an end, as the special transistors needed have long since been unavailable and the supply is starting to run low.

The J2 amplifier is rated at 25 watts per channel. It has a two-stage circuit that operates in pure single-ended Class A mode, with signal JFET devices forming the input stage and power JFET devices for the output stage.

JFET transistors have long been recognized as having the highest audio quality of any transistor; the input devices of the J2 are known through the industry as the standard for low noise and linearity, and are found on the inputs of the finest phono stages, line level preamplifiers, and power amplifiers. But for over 30 years robust power JFETs have not been widely available since the short-lived efforts of Sony and Yamaha. Advances in Silicon Carbide (SiC) technology resulted in new power JFET transistors with high voltage, current, and power capabilities – as high as 1200 volts, 30 amps, and 273 watts.

Created by SemiSouth Corporation of Mississippi, these robust new JFETs are designed for very fast high power switching in solar power and electric car applications. However, they also have a very low distortion characteristic that makes them superb for use in linear amplifiers. In apples-to-apples comparisons with comparable MOSFET type power transistors, they can achieve 10 to 20 dB improvements in distortion performance.

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Release date: 2021

Sound signature: Warm and relaxed.

Highlights: Body and weight of the J2 with the spatial detail of the F7 (its predecessor). Lower distortion than the J2. "A simple little class A amp with a nice personality."

Caveat: No balanced output.

After the J2, First Watt wanted to create another amplifier with the SemiSouth SiC R100 power Jfets used in the J2. In 2015, they developed a design with the same output stage but an alternative front end circuit. The prototype was clearly an improvement, but the J2 remained so popular that they decided to wait. As the years went by, First Watt put some more work into it, and then released the F8 in 2021.

The F8 is a stereo two-stage single-ended Class A amplifier using the NOS Toshiba 2SJ74 P channel Jfets and SemiSouth R100 SiC power Jfets for signal gain, plus IRFP240 Mosfet mu-follower current sources, for a total of three devices per channel.

It is similar to the J2 amplifier, but has only one front end transistor instead of six, operated as a current feedback amplifier (CFA) as opposed to the J2's voltage feedback (VFA) differential input. This front end is more consistent with the single-ended approach to amplifier design and yields a purer second harmonic character, less distortion with lower negative feedback, greater bandwidth and higher damping factor.

There are two Common-Source gain stages, consisting of Q1 and Q2, both set to their operating points by current source Q3. Q1 is the Toshiba 2SJ74 Jfet, and Q2 is the SemiSouth Silicon Carbide R100 power Jfet, both parts NOS from the First Watt vault. Q3 is the venerable International Rectifier workhorse, IRFP240. The circuit invites comparison with the J2. Both have two stages using these parts, and the output stage is virtually identical, but the J2 uses 6 Jfets in the input circuit - a complementary parallel push-pull differential stage biased by constant current sources. By contrast the F8 uses just a single Toshiba 2SJ74 P channel Jfet which biased by the amplifier's output.

Five fewer transistors is part of the difference - the J2 input topology is what we refer to as a VFA, or “voltage feedback amplifier.” The F8 would be referred to as a CFA, or “current feedback amplifier”. Both approaches to feedback have their advantages, and there are adherents for both. In this case, the CFA topology gives several advantages - simpler circuit, simpler/smoother transfer characteristic and a little better performance by measurement. Something else is missing as well - no degenerating resistors on the output power Jfet.

The Q3 'Mu follower” biases not only the output power Jfet Q2, but also the input Jfet Q1, and this bias feed is also the feedback signal of the CFA circuit. Both devices operate with the same phase with respect to their characteristics. This congruence helps give rise to the nice “negative phase” second-harmonic character that you can see here:

Comparing the J2 and the F8, you will find quite a bit of similarity, but the F8 boasts modest improvement in several areas. In particular, the damping factor and high frequency response are twice as good, and distortion numbers are lower. This is achieved with 10 dB less open loop gain on the circuit and 5 dB less feedback.


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