FIRST WATT

CLASS A POWER AMPLIFIERS

Radical intelligence ... a type of thinking

 

"Nelson Pass is that rare type of audio-engineering maverick who measures and listens with equal facility. He's not wasting his time and your money trying to cram a thousand crap watts into a marketing department's bling box ... my experience with the latest First Watt amps suggests that these are among Nelson Pass's finest achievements"

MADE IN THE USA

FIRST WATT SIT-3 POWER AMPLIFIER

first watt sit-3 .promo_photos_v2_x2.jpg

FIRST WATT SIT-3

 

Single-ended / single-stage Class A

SIT - Static Induction Transistor Topology

 

 

Maximum output

18 watts @ 8 ohms

30 watts @ 4 ohms

Input Impedance

200 Kohms

Gain

11.5 dB non-inverting phase

Damping Factor

30

Frequency Response

-.5 dB @ 10 Hz, – 3dB @ 50 KHz

Output Noise

50 uV unweighted 20 - 20 Khz

Power consumption

150 watts

Weight

32 lbs

Dimensions

17” W   16” D  6.75” H

Warranty

Parts and labor for 3 years, not covering shipping costs or consequential damages.

Handmade in the USA


$ 4,000

"The SIT-3 is the very latest example of single-ended / single-stage Class A amplifiers using the SIT (aka VFET) power transistor exclusive to First Watt. It is preceded by the successful SIT-1 and SIT-2 and very much follows their technical philosophy and construction.

 

There is a key difference in the SIT-3, which is that the Static Induction Transistor (SIT) that forms the heart of the circuit is operated in an entirely different mode, what is known as Common Drain Mode.

 

By way of explanation, any gain device with three pins can be used in one of three modes. For the Field Effect Transistor (FET), they are Common Source, Common Drain, and Common Gate.

 

The channels of the SIT-1 and SIT-2 consisted of a single SIT operated in Common Source Mode in which (conceptually) the signal comes into the Gate and appears amplified at the Drain pin, but phase inverted. The Source pin is grounded. The amplification with Common Source operation is both voltage and current, and the phase inversion is corrected by reversing the output terminals.

 

The SIT-3 goes in another direction, using Common Drain operation, where the signal goes into the Gate pin and comes out the Source pin and the Drain of the Fet is grounded (literally attached to ground). This mode only provides current gain – the voltage gain is provided by a high quality voltage step-up transformer which takes the input signal from a preamplifier (or other device) and boosts the voltage. It has no phase inversion.

 

In both approaches, the SIT does a good job of amplifying the signal without feedback, but Common Drain operation delivers the amplification with much lower distortion and noise and also a much better damping factor for the loudspeaker.

 

The trade-off is the addition of the input transformer, but I think you will find the compromise there is small with respect to the sound quality achieved.

 

Common Drain has the same simple spectral distortion character that graced the SIT-1 and SIT-2, and allows similar control of the amplitude and phase of thesecond harmonic content, but at a much lower distortion figure.

 

If like me you find 18 watts is enough power, then I think you will find that this is a really delightful amplifier."

Nelson Pass

First Watt SIT-3 Transparent Background

"I’ve come away from this reviewing experience with the distinct impression that Nelson Pass and First Watt have upped their game again by designing yet another eminently musical and enjoyable amplifier.

 

In the right system, I believe the new SIT-3 can provide a nearly perfect balance of open, dimensional, enjoyable harmonic beauty coupled with sufficient extension and control to keep matters well in hand at the frequency extremes. I was particularly impressed with the satisfyingly robust sound I got from the SIT-3 driving my large (and expensive) passive monitors while also coaxing superb performance from my budget-minded Zu speakers.

 

First Watt has had a long history of providing a very favorable sound quality to price ratio, which I have found to be seemingly ever more favorable with each subsequent design Nelson Pass puts out.

 

There must be something in the secret sauce over there at Sea Ranch. Or … maybe Nelson is getting a little bit of help from his “friends”. Has anyone thought to check the backyard for debris from that crashed flying saucer?"

 

PTA - Parttimeaudiophile

John Richardson |  July 2018

FIRST WATT J2 POWER AMPLIFIER

First Watt J2 Amplifier_photos_v2_x2.jpg

FIRST WATT J2

 

Single-ended / two-stage Class A

Signal JFET + Power JFET Topology

 

Maximum output

30 watts @ 8 ohms / 15 watts @ 4 ohms

Input Impedance

100 Kohms

Balanced Input CMRR

-60dB @ 1 kHz

Input Sensitivity

280 mV = 1W , 1.4 V = rated power

Gain

20 dB non-inverting phase

Distortion

0.025% @ 1kHz

Damping Factor

20

Frequency Response

-1 dB @ 4 Hz, 100 KHz

Output Noise

100 uV unweighted 20 - 20 Khz

Power consumption

200 watts

Weight

32 lbs

Dimensions

17” W   16” D  6.75” H

Warranty

Parts and labor for 3 years, not covering shipping costs or consequential damages.

Handmade in the USA


$ 4,000

"The J2 is a stereo power amplifier 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. In the last few months, advances in Silicon Carbide (SiC) technology have 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.

 

For over 10 years First Watt has been pushing the design envelope with simple high performance Class A amplifier circuits, and when a better transistor like this comes along, it can mean getting the same distortion performance with a lot less feedback, or lower distortion with the same amount of feedback. The J2 does amplifier does both,and that helps make it a better sounding amplifier than the best of its predecessors.

 

With a device that behaves a little like a tube, it is natural to consider popular tube amplifier design topologies. Single-Ended Class A tube amplifiers are not very powerful, and their measurement numbers are not exceptional, but there is no arguing that they have strong musical appeal to much of the audiophile population. The J2 amplifier uses a classic JFET differential input stage followed by a single power JFET transistor. This power JFET is biased by another JFET in what is known in tube circles as a “mu follower”.

 

I use a slightly simplified schematic of the J2 circuit. It could be a classic tube amp, except that the P channel input JFETs would have to be fabricated from anti-matter. The single-ended Class A output stage is “second harmonic” in character, and it uses about half the feedback of a comparable MOSFET circuit but with half the distortion and twice the bandwidth.


The J2 power amplifier is simple, clean, and measures astonishingly well. It achieves a sound which is warm and relaxed, combining precision and detail without sterility. With a pair of sensibly efficient loudspeakers, it will give you a toe-tapping experience that other solid state amplifiers do not. The design is extremely reliable and will never need adjustment."

Nelson Pass

FIRST WATT J2 Rear.png

"The First Watt J2 by Nelson Pass has been written about here and there, but it seems that everyone who's written about it has been in a giant a hurry to say, "It's a solid-state amp that sounds like a tube amp." But when I experienced the J2 in my own system, I felt that this generalization was both inaccurate and inadequate. With loudspeakers of benignly high impedance, the First Watt J2 tracked complex music signals in ways I have rarely experienced.

 

The J2's extraordinary ability to keep close and intimate with complex music, such as early Nina Simone and Klemperer's Bach, allowed my humble reference system to display the music's fundamental chi in a singularly engaging way. Very few power amps can do this.

 

I used to sell $90,000, 25Wpc tube amps for a living. Currently, I use an extraordinary $4450, 22Wpc tube amp as my reference, so believe me when I say that the beauty of the First Watt J2 has nothing to do with the tubes-vs-solid-state debate. The J2 rises high above that dispute between cults. Most likely, the First Watt J2 preserves music's vital chi because it's a simple, high-bias, class-A, single-ended, low-feedback design, created by Nelson Pass—one that has no need to impress the mainstream masses or drive 4 ohm speakers.

 

Nelson Pass is a chi master because, like me, he's a member of all audio tribes. "There is no such thing as a perfect amplifier," he believes. "All audiophiles and their associated equipment have specific needs, but in each case there is such a thing as a best amplifier—the one that makes you happy." The First Watt J2 is, in my view, a best amplifier: Every day, it made me and my favorite loudspeakers extremely happy. Highly recommended."

 

Stereophile

Herb Reichert |  September 2016

FIRST WATT F8 POWER AMPLIFIER

First Watt F8 Open _photos_v2_x2.jpg

FIRST WATT F8

 

Single-ended / two-stage Class A

Signal JFET + Power JFET Topology

 

 

Maximum output

25 watts @ 8 ohms

15 watts @ 4 ohms

Input Impedance

100 Kohms

Input Sensitivity

530 mV = 1W , 2.6 V = rated power

Gain

15 dB non-inverting phase

Distortion

0.015% @ 1kHz

Damping Factor

40

Frequency Response

-0.5 dB @ 10 Hz, 200 KHz

Output Noise

< 100 uV unweighted 20 - 20 Khz

Power consumption

170 watts

Weight

32 lbs

Dimensions

17” W   16” D  6.75” H

Warranty

Parts and labor for 3 years, not covering shipping costs or consequential damages.

Handmade in the USA


$ 4,000

Originally created almost 6 years ago, 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.

The simplified schematic : 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, andQ2 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.  And if you don't like that - well, it's only at 0.012%

Nelson Pass

FIRST WATT F8 Rear _photos_v2_x4.png

" The F8 is no regular case, so required a suitable load. Børresen’s 01 monitors and Boenicke’s W11 SE+ weren’t ideal for this job, on the contrary to Martin Gateley’s sound|kaos Vox 3afw with its high efficiency and inherently resolving sonic profile. These two qualities had already proven very useful during the Pass Labs INT-250 assignment.

 

A FirstWatt F7 at my disposal was topologically closest so an excellent sparring partner. The Thöress DFP’s two analog outs connected to both amps fed from the same power bar.

Two Less Loss C-MARC power cords and Audiomica Laboratory’s interconnects made today’s battleground as even as it could be. Just one quick speaker cable swap was all it took to hear my amp of choice. A/Bs this easy are rare.

 

I’d been living with the F7 since early 2016. Although this specialty amp doesn’t play often, it flourished into a keeper just like the AMR DP-777SE, Boenicke W5 and Bakoon AMP-13R. In time I might change every component I own including the most expensive, but none of these four. Each has something special which to my knowledge isn’t achievable for less and/or available in a more compact frame.

 

The F7’s uniqueness is in its sonic elegance. It allowed me to understand that, if matched to a fitting load, low-power amplifiers geared towards richness and generous harmonic content can also be fast, spacious and resolving. By finely combining traits I deemed mutually exclusive, the FirstWatt not only went beyond, but taught me a valuable lesson. It opened my ears. The FirstWatt F7 was the first product by Nelson Pass I was lucky to audition. Since then this list grew bigger to include Pass Labs HPA-1, XP-12 and INT-25 reviewed just recently.

 

They all featured the same unmistakable sonic backbone; a fully mature voice tailored meticulously and tastefully to avoid excess. This sound doesn’t shock by extreme chunkiness or zinginess. It prioritizes complex tonality. It’s vivid, expressive, sensual and sophisticated, and is completely free from shoutiness, grain or any harshness. These qualities imply deep class A bias, but there’s still an execution that’s very much organic, moisturized, breathing and open eschew any stuffiness. Images occur naturally from within a black canvas instead of being irradiated by extra shine from above. Such fine blend is a rare treat but its relaxed quite casual feel is the specific feature that never ceases to impress me most about the Pass Labs sound. There’s no need to focus on anything specific, things just happen all at once.

 

Unsurprisingly the F8 fit that profile to perfection. Uniform sonic results regardless of product type or price implied next-level engineering know-how especially with performance as described but also raised my expectations. Prior to the F8’s delivery I had suspected that unmistakable Pass aroma to effectively go beyond the usual class A charms. I wasn’t wrong. The newcomer too was admirably smooth, tonally loaded, spacious, soothing, euphonic and very coherent. Just as the companion models I’d heard before, it scored very high on tonal complexity, pleasant heft, articulation, insight and pristine backdrop from which everything blossomed without effort. Images outlined and separated with inviting moisture but also served music as a whole instead of dissecting individual sounds suspended in air. The F8’s grand clarity poured down into my ears without stealing the thunder of other aspects.

 

Although both FirstWatt amps felt voiced similarly elegant and more alike than not, I was also certain that the F8 had to set itself apart in one way or another. It didn’t take long to notice that it did but its most interesting feature took me a while to grasp. It sounded more athletic, muscular, potent and contoured than the chunkier, lazier, denser and more relaxed F7. The difference wasn’t night and day but substantial enough to map the F8 as more mainstream, so more universal versus its stablemate’s mellower thicker sonic footprint. To simplify, the F8 showed some extra bite, snap and articulation at a small cost of roundness and gravity.

 

Let’s just say that I know a very good class A amp when I hear one, which is why at this point I intended to wrap it. Then a previously unnoticed quality revealed itself to change my overall take on the F8. Just as an obese person can’t possibly outrun an athlete, it’s only natural to expect warmer denser amps to be slower than specimens groomed for fitness and articulation. That’s why the F7’s bloomier softer bass response should have been looser/slower than the sensibly tighter more powerful F8. Although I’m far from labeling them fat and ripped respectively – they’re far more complex than that – it was reasonable to assume the newcomer First Watt would’ve been the quicker of the two. It wasn’t. The F8’s unusual behavior caught me off guard. To be perfectly clear, neither the F7 was lightning-quick nor the F8 slow and bloated. These amps weren’t voiced to sound flashy but civilized and wholesome. The F8’s sportier and intense profile was true regardless of the music. However, for quick short bass jabs the F7 projected as more impulsive whereas the F8 played it more relaxed as though it engaged softer suspension and cruise control.

 

At first it was odd to question what I’d heard. Was this by design? Or was I delusional? Then it struck me. The F8/F7’s output impedance respectively of 0.2/0.08Ω mean damping factors of 40/100 into 8Ω. Higher damping promises more woofer control, less damping looser bloomier bass. Of the two First Watt amps at my disposal, today’s loaner expressed lesser stopping power over the Vox 3’s twin woofers. Was impedance the reason? The F8 wasn’t lazy, bloated, dull or boomy. To my ears its bass was very civilized, enjoyable and complete to feel refined, seasoned and authoritative even with this twist. Or perhaps was this a deliberate part of its personality profile and not just a function of how it interacted with my speakers? If so, this amp really does have a personality that is very charming and unique. Here’s where my part ends.

 

Summary

Although I haven’t heard First Watt’s most popular J2 model, after spending some time with its successor I have no regrets. Just as any other known to me amp designed by Nelson Pass, his F8 is as addictive to listen to as it’s uniquely voiced. Considering its stout sonics, overall refinement and asking price, I fail to see what type of a discerning audiophile wouldn’t appreciate it. I also think that one could spend a lot more and get far less in return.

FirstWatt F8 is built to last, stable as a rock and deadly silent, whereas its look reflects prioritized utility and performance reliability. It’s a robust no-frills class A power amp that constantly eats up about 170W to generate lots of heat dissipated via its entire enclosure. As such it’s a specialist machine designed to work with efficient speakers perfectly happy with 20wpc or less, so a proposition mainly for enthusiasts already into its topology and clever elegantly simple circuits that make wonderful sound. On these counts Nelson Pass’ work honestly needs no introduction and today’s product clearly fits that profile.

Although First Watt F8 is a non-mainstream affair aimed at a particular audience, just as any of its kin, it doesn’t take a seasoned aficionado to appreciate what this amp does with matched speakers. I think that its fully adult thoroughly enjoyable sound is universal enough to please various tastes more than well. As for pleasing my own, suffice to say the F8 loaner during its stay at my place was in use far more than its older sibling and any other amp I had nearby. That’s how much I enjoyed its engaging personality and now I’ll miss it."

 

HiFi Knights

Dawid Grzyb |  December 2020

FIRST WATT F7 POWER AMPLIFIER

first watt f7 inside_photos_v2_x4.jpg

FIRST WATT F7

 

Single-ended / two-stage Class A

Signal JFET + Power JFET Topology

 

 

Maximum output

20 watts @ 2% THD / 1W into 8 ohms

30 watts @ 3% THD / 1W into 4 ohms

Input Impedance

10 Kohms

Input Sensitivity

570 mV = 1W, 2.7 V = rated power

Gain

14.5 dB non-inverting phase

Distortion

0.05% @ 1kHz

Damping Factor

> 100

Frequency Response

DC to -3 dB @ 100 KHz

Output Noise

100 uV unweighted 20 - 20 Khz

Power consumption

170 watts

Weight

32 lbs

Dimensions

17” W   16” D  6.75” H

Warranty

Parts and labor for 3 years, not covering shipping costs or consequential damages.

Handmade in the USA


$ 3,000

The F7 is a very unique power amplifier, a two-stage push-pull JFET/MOSFET topology with fewer parts than any First Watt amplifier to date and incorporating a very interesting balance of very low negative voltage feedback and a little bit of positive current feedback to give an astonishing measure of control over reactive loudspeaker loads.

 

Introduction - Short Story Long

Conceived in 2007, our F5 was a push-pull Class A amplifier employing eight semiconductors and 23 resistors to achieve 25 watts output with good specifications and good sound. You can read about it in detail in both the owner's manual and DIY construction article, both posted at www.firstwatt.com. The F5 received good reviews, sold well and received plenty of attention from the DIY audio community ... as of this writing, the F5 thread on www.diyaudio.comis cresting 3 million views.

 

Like other First Watt amplifiers, it was a limited release, and was discontinued after 100 pieces were built. Subsequently there was a more powerful version released to the DIYers, the F5 Turbo, which offered more power and numerous options for experimentation. The F5 was noted for detail and neutrality, the result of simple Class A operation, wide bandwidth, and a generous amount of negative feedback.  With some material and equipment, the sound was exquisite, but the amplifier also tended to highlight the faults of recordings and the rest of the system.  A poorly engineered record, a second-rate DAC or peaky loudspeakers and the amplifier was merciless.
 

Since then I have spent time mostly exploring other kinds of design approaches to amplifiers, but I always meant to come back to the F5 to see how it could be improved.  Mostly I was looking for two things – better sound and an even simpler circuit.  The desire for a simpler circuit is self explanatory – apart from the aesthetic, I imagine that simpler circuits tend to sound better.  (Because I build them myself, I will add cheapness and laziness to my motivations.)

 

I envisioned a circuit with only four transistors and four resistors – the bare minimum for an amplifier of this type, where all the Fets are operated in Common-Source mode, giving both voltage and current gain.  Two complementary input Jfets drive two complementary power Mosfets, and the output voltage is fed back to the Source pins of the Jfets in what is commonly called “Current Feedback” (CFA).

In such a simple circuit, there are opportunities for improving performance by careful choice of transistors, resistor values, voltage and current values and precise matching of parts.  I built up a number of such amplifiers evaluating both the measurements and the sound, and after a while a very nice little amplifier emerged.  That it (and the F5) resembled Plantefeve's Profet was testament to his fine work. It had most of what was on my wish list :

 

  • Very wide bandwidthLow distortion and noise

  • Large Class A operating region

  • Less feedback

  • No degeneration in the output stage

  • Very low thermal distortion and drift

  • No capacitors or transformers (apart from the power supply)

 

.... Looks like a great laundry list.  I thought the amplifier sounded pretty good, but after a time the consensus was that it was kind of polite, not as musically involving as some other examples.  One area where the amplifier fell “objectively” short was the output impedance.  Most of your “better” amplifiers have higher damping factors like 20 or 100, and this amplifier was only about 5 or so, typical of a simple Common-Source topology and low negative feedback. The damping factor, which is the inverse of the output impedance, determines the flatness of the amplifier response when the load impedance varies and is important to the transient response of reactive loudspeakers, which is just about all of them.

 

You could argue that single-ended tube amplifiers sound good and don't have much damping factor, and you would be right, but I'm not trying to duplicate single-ended tube amplifiers here – the First Watt SIT amplifiers, doing a decent solid-state emulation of Triodes, represent that genre nicely. I'm shooting more for the sound of The Beast With a Thousand JFETs , but with obtainable parts. I tried other FETs, varied the the feedback network values and tweaked the gain symmetry. These efforts delivered a little more warmth and dimensionality, and about a year ago I decided to go ahead and build a small pilot run in anticipation of a product release – after all, it was a perfectly nice amplifier. Still, I was not really satisfied, so I created new printed circuit board artwork, adding cascode operation to the input stage and doubling up the output devices. This version had a little more control, but still fell short. So...

 

I went back to the original simple design.  On the wall in my lab is a little box with a glass window labeled : DANGER - POSITIVE FEEDBACK

 

I broke the glass. Inside was a single resistor.... And I put that resistor in the amplifier...

 

A little explanation is in order.  The relationship between an amplifier and a loudspeaker is a bit like a dance.  Both sides have their own complexities, but the point is for them to get along well. The amplifier designer, not generally having control of what speaker is used, usually chooses the amplifier as the dominant partner by making it a pure voltage source. In the typical voltage source, negative feedback in the amplifier is used to define the voltage across the loudspeaker regardless of the current through the speaker. This represents the “have it my way” approach to amplification, and large hardware with lots of feedback are good at this. Most loudspeakers are designed around the assumption of a low impedance voltage source... 

So here we are ... The F7, a nice little Class A amplifier with hardly any feedback does not have the brute force advantage.  It resorts to a stratagem that makes the dance a little more like a Tango. Modest amounts of negative feedback are balanced in counterpoint to small amount of positive current feedback, creating an equilibrium where the output impedance approaches zero, improving transient and frequency response. Of course you can achieve a similar effect with tons of negative feedback, but I think this is more elegant and sounds better. For brevity, I will call it “PCF”. Also, I put more capacitance in the power supply and found a clever way to further reduce the effect of high frequency DAC noise and environmental RF.

 

This is a different amplifier.  The diversity of audio taste being what it is, not everyone will prefer it. I presume however that a portion of audiophiles will like it.

 

In case you haven't noticed, I enjoy amplifiers with some personality. They don't have to measure perfect, they just have to sound good.  This is a very simple little Class A amplifier with lots of personality, and I hope you enjoy it as I do."

Nelson Pass

First Watt F7 Rear_photos_v2_x2.png

A bit of tech talk ...

"What is positive current feedback, you ask? Well, a percentage of the output current is fed back in phase with the input so it increases gain but also increases distortion. The final version of the F7 balances a modest amount of negative feedback (14dB) against a bit of positive current feedback (about 2dB) to nudge the output impedance toward zero. It should be kept in mind that the net feedback applied to the F7 is never positive but simply “less negative.”

 

Since the damping factor is normally referenced to an 8-ohm load, a source impedance of 0.08 ohms gives a damping factor of 100. Of course, the same result could have been achieved with a ton of negative feedback. Not only would that have been suboptimal from a sonic standpoint, but the need for additional open-loop gain would have necessitated an additional gain stage.

 

The F7’s “tango” of negative and positive feedback represents a much more elegant solution, which yields a damping factor of 100 in the context of a simple circuit. You might get the impression that a source impedance of zero would be even better since it gives an infinite damping factor, but that isn’t something that is either necessary or even desirable. In the real world, the speaker cable resistance is in series with the amplifier’s source impedance and can dominate the effective damping factor. For example, if a speaker cable with an impedance of 1 ohm were to be used with the F7, the damping factor would drop from 100 to 8.

 

Now to the F7 ...

The F7 should disabuse you of the notion that the ideal amplifier emulates a “straight wire with gain.” That old cliché was probably coined by an electronics engineer whose measurement load consisted of a resistor. In that case, damping factor isn’t a critical parameter. However, real-world loudspeakers represent reactive loads with wild impedance variations, sometimes exceeding an order of magnitude, and the amp’s source impedance directly impacts the loudspeaker’s frequency response. The only way to preserve the neutrality of the amp/speaker interface is to design an amplifier with a high damping factor (i.e., low source impedance).

 

The sonic payoff of a low internal source impedance amplifier, at least for most conventional speakers, is improved transient and frequency response. This is most noticeable in the bass range. A simple test should make a believer out of you. With the F7 powered off, put your ears next to your loudspeaker’s woofer cone and tap it gently with a finger. Next, turn on the F7 without any music input and tap the cone again. You should notice a distinct reduction in the resonant character of the cone. What is happening is that the F7 is short-circuiting the back EMF of the woofer, thus putting the brakes on cone motion.

 

I should hasten to add that some speakers do benefit from higher source impedance and that was the basis for variable damping amplifiers, a hot topic in the 1950s. However, in the October 1956 issue of Radio-Electronics none other than Paul Klipsch advocated for disabling the variable damping feature, which he claimed resulted in the best sound with the Klipschorn and Shorthorn. In his own words: “Put another way, any adjustment of the damping other than for a reasonably low internal impedance of the amplifier has deteriorated the overall response.” He also warned against use of excessive negative feedback, which inevitably leads to instability

 
What makes the F7 so special is its inherent textural sweetness and warm tonality. There are many (too many?) solid-state amps out there that manage to sound smooth and refined yet lack the organic character of live music. The F7, on the other hand, managed to sail through reproduction of violin tone with superb upper-register sheen and transient finesse—a rare feat for any solid-state amplifier.

 

Having been raised on tube sound, I find textural warmth to be a most welcome attribute, though the F7 does not approach the effusive warmth of a classic tube amp. If you crave even more warmth, do as I do and hook the F7 up with a tube preamp. This is not a euphonic amplifier. Relative to say, a Marantz 8B, the F7 delivers far more incisive transients while its command of space is competitive with the sort of 3-D spatial presentation tube amps excel in. The total harmonic distortion has been trimmed to 0.05% at 1 watt. But Nelson points to the distortion spectrum at 1 watt and 1kHz, and the 10dB ratio between the second and third harmonic as being contributing factors to the F7’s musicality.

 

Since just about all pop music is being recorded in a studio and mixed on a multitrack console, sound engineers are given the opportunity to play various games with the final mix, which may include vocal overdubs, canned reverb for individual tracks, pan-potting, eq, flanging, gain riding, and compression. The F7 was able to dig deep into a mix and resolve such trickery without much effort, a tribute to its superb transient control and laser-sharp image focus. The Eximus DP-1 DAC makes it possible to upsample a Red Book 44.1kHz input to 192kHz on the fly. Discerning a sonic change with some amps is not an easy task. That was not the case with the F7 in the chain. It highlighted significant sonic changes, the opening up of the treble range and tightening of image focus being two major ones. There’s no doubt in my mind that this amp is a high-resolution device.

 

I spent some time pitting the F7 against the F3. While the F3 sounded a bit cleaner, the F7 was better controlled in the bass range and displayed more personality. To confess, the F7’s warmer, slightly lusher textures suited me just fine. I have to disappoint those of you who expected the F7 to go down in flames. Much to my surprise, I ended up preferring it over the F3. Make no mistake about it, the F3 a terrific single-ended amplifier. It’s just that the F7 is the more seductive of the two, just as dynamic, and ultimately a more musical performer. And that’s a remarkable finding for any push-pull amp.

 

The F7 is easy to embrace musically. Its sound is sufficiently addictive that I’m finding it extremely difficult to evict it from the reference system. For the record, it’s likely to stay put for many weeks to come. If the F5 can be said to offer a “half-nelson” grip on sound quality, it would be accurate to characterize the F7’s performance as nothing less than a “full-nelson.” Simply put: one of the best low-power amps money can buy."

 

The Absolute Sound

Dick Olsher |  June 2016

Comparison review:
First Watt F8, J2, F7, and SIT3 Power Amplifiers

Steve Guttenberg, Audiophiliac - January 2021

My latest sit down with First Watt and Pass Labs' Nelson Pass  -  Part 1
Steve Guttenberg, Audiophiliac - January 2021

My latest sit down with First Watt and Pass Labs' Nelson Pass  -  Part 2
Steve Guttenberg, Audiophiliac - January 2021

 OTHER PASS LABS COMPONENTS

Trafomatic-Audio-Evolution-Phono-One-3_p

 PASS LABS
XP-12
LINE PREAMPLIFIER

 $ 5800

pass-labs-xp-12.jpg
Trafomatic-Audio-Evolution-Phono-One-3_p

 FIRST WATT J2 ... F8 ... SIT-3  +  PASS LABS XP-12 LINE PRE
 
BUNDLE SPECIAL
$9200

PASS LABS
CLASS A
AMPLIFIERS

PASS LABS
CLASS AB
AMPLIFIERS

PASS LABS
CLASS A
PREAMPLIFIERS

first-watt-logo.png