Nelson Pass’ “other” company (Pass Labs being the first) First Watt announced a new power amplifier this week. The First Watt F8 Power amp is considered to be the spiritual successor to the popular J2 amp from the same company.
Unlike many of the Pass Labs beefy, high wattage amplifiers, First Watt products tend to have output less than 25 watts and run a full class A topology. The new F8 power amplifier does indeed round out at 25 watts into 8 ohms via JFET and push pull ingredients. The push from a design standpoint is usually involves a search for good sounding parts, but doesn’t include a high number of extraneous circuitry inside either. Simplicity is usually the approach, and in the case of the F8, an intentional lack of “degenerating” resistors on the output power Jfet.
From the man himself:
“I spend a lot of time building and evaluating basic amplifier concepts with an eye toward producing the highest quality sound with elegantly simple circuits. There is no such thing as a perfect amplifier, but there is a best amplifier for each type of speaker, music, and listener. Each First Watt model is a unique design and is ‘best’ in some particular way. They look alike because they all use the same basic chassis and power transformer. On the upside, it makes it easy for me to test ideas and compare different designs with the hardware being a fixed constant.
The J2 is perhaps the most popular amplifier made by First Watt. Brought to market eleven years ago, it was not intended for more than our usual 100 unit total production. However it was so well received that I broke my rule and it is still made. Still, there will be an end, as the special transistors needed have long since been unavailable and our supply is starting to run low. “I wanted to create yet another amplifier with the SemiSouth SiC R100 power Jfets, so in 2015 I 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 we decided to wait. As the years went by, I put some more work into it, and now finally releasing it as the F8.
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 characteristics and a little better performance by measurement. Something else is missing as well: no degenerating resistors on the output power Jfet.
This is not the transistor amplifier you may remember from the ’70’s. The fundamental of the amplifier output (red) at 1 watt along with a magnified image of the distortion (blue). You can see that it is a nearly pure second harmonic and that its phase alignment is “negative” at both peaks of the fundamental waveform. No spikes or weird looking stuff. And if you don’t like that – well, it’s only at 0.02%.
10 years ago when I began working with SIT circuits, I discovered that there was a consistent preference for this sort of characteristic in amplifiers over a range of distortion levels from about .01% to 1%. Somewhere below that, it was not usually heard – and above that it could be excessive. Not only was second harmonic preferred over other harmonic content, but its phase seems to matter.
I used blind evaluations for the SIT-1, the First Watt amplifier, and on the front panel there was a knob that allowed adjustment of this characteristic in distortion amplitude and phase. I found that listeners tended to cluster on the same setting, which was made the center of the adjustment meter. The F8 has this same characteristic, but down at the lower edge of audibility.
You are probably aware that with First Watt, numbers aren’t everything, but by virtue of the simpler and smoother gain path coupled with slightly better numbers, the F8 offers some improvement upon the character of the J2. What’s missing? The First Watt F8 does not have a balanced input. It also has output coupling using two big electrolytic and one polypropylene capacitor with a roll-off frequency of 1 Hz.
As you compare the J2 and the F8, you will find quite a bit of similarity, but the F8 is a modest improvement in several areas. In particular, the damping factor and high frequency response are twice as good, and distortion numbers are lower. I am pleased to note that this is achieved with 10 dB less open loop gain on the circuit and 5 dB less feedback. “Still, this is not the primary thing – it’s just what I can show you in print. The real merit to the amplifier is in the experience of listening. This is a matter of how compatible it is with the rest of the audio system and the taste of the listener. “This might not be the amplifier for you, but I think it deserves a listen. “You will have noticed, I enjoy amplifiers with a little personality. They don’t have to measure perfectly, they just have to sound good. This is a simple little Class A amplifier with a very nice personality, and I hope you like it.” – Nelson Pass 6/18/2020
The First Watt F8 has an MSRP of $4,000 USD and is shipping now. More information available on the company site.
If you would like to hear more from Nelson Pass, check out The Occasional Podcasts extensive interview with him where he talks about the history and thought process behind both Pass Labs and First Watt.