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Reviewer: Srajan Ebaen
Source: Zanden Audio Model 2000P/5000S; Ancient Audio Lektor Prime; AMR CD-77 [on review]
Preamp/Integrated: Supratek Cabernet Dual; Wyetech Labs Jade; ModWright SWL 9.0SE; Bel Canto PRe3; Melody HiFi I2A3; ModWright LS 36.5 [on review]; Almarro A318B [on review]
Amp: 2 x Audiosector Patek SE; First Watt F3; Yamamoto A-08S; Fi 2A3 monos w. JJ 2A3-40s
Speakers: Zu Audio Definition Pro with Gallo Ref 3 SA bass amp/crossover/EQ; Mark & Daniel Ruby with OmniHarmonizer; WLM Diva Monitor with Duo 12 passive subwoofer, Alto bass amp, Pre/Passive and Bass Controls; Gallo Acoustics Ref 3.1; DeVore Fidelity Nines [on review]; Mark & Daniel Maximus Monitor [on review]
Cables: Crystal Cable Ultra loom; Zanden Audio proprietary I²S cable; Crystal Cable Reference power cords; double cryo'd Acrolink with Furutech UK plug between wall and transformer
Stands: 2 x Grand Prix Audio Monaco Modular 4-tier
Powerline conditioning: 2 x Walker Audio Velocitor S fed from custom AudioSector 1.5KV Plitron step-down transformer with balanced power output option
Sundry accessories: GPA Formula Carbon/Kevlar shelf for transport; GPA Apex footers underneath stand, DAC and amp; Walker Audio Extreme SST on all connections; Walker Audio Vivid CD cleaner; Walker Audio Reference HDLs; Furutech RD-2 CD demagnetizer; Nanotech Nespa Pro
Room size: 16' w x 21' d x 9' h in short-wall setup, with openly adjoining 15' x 35' living room
Review Component Retail: $2,500
The X Factor. In today's game, that's what we call gain. What comes in goes out, simply amplified or enlarged X times. If you strap together an active preamp and an amplifier -- the pervasive combo -- you put together two of those factors for X-squared multiplication. Therein lies the rub. Most people most of the time face seriously redundant gain with this scheme. That's why they need to throw most of it away again to not blow themselves and their neighbors out of the weeds. Throwing away gain is called attenuation. Not only is excessive amplification with subsequent deep attenuation silly and wasteful, it often creates noise especially with highly sensitive speakers. The X Factor doesn't just multiply signal. It simultaneously multiplies noise. The majority of audiophiles find themselves battling redundant amplification with severe attenuation. This raises the noise floor and throws out micro resolution. That much is well known though nowhere near as equally well practiced.


The in-series gain blocks of multi-stage amplification circuits and the paralleling of output devices necessary to achieve high X factors additionally tend to sacrifice subtlety and refinement. In other words, less gain can make for simpler circuits which can sound better. Hence the appeal of micro-power triode amps which suffer their very own limitations and restrictions. Because of that and jive on the salesfloor, it is barely appreciated just how much -- or rather, little -- gain is really required with regular sources and normal speakers. It's this question which Nelson Pass, America's preeminent designer of transistor amps, invites us to consider with his newest FirstWatt power amplifier. The F4 is that rare amp whose X factor is zero. It's minus 0.5dB in fact. Hence it doesn't really amplify. But it most certainly conditions the signal.
That conditioning action makes the F4 a power buffer or Class A impedance converting amplifier - no voltage gain, no feedback. Its input impedance is 48/96Kohm single-ended/balanced-mono respectively, its output impedance about 0.2 ohms. The responsibility to swing voltage and create the desired SPLs remains
with your preamp. A standard active preamp should be perfectly sufficient to fill the usual digs with high sound pressures over standard 90dB speakers. Alas, your preamp still couldn't strap to your speakers directly even with a custom cable. Despite being equipped with perfectly adequate X factor to generate all the desired loudness, high output impedance and insufficient current delivery conspire against actually driving the speakers with a preamp. Enter the F4 follower amp with current -- not voltage -- gain.


This amp delivers +/- 20 volts in single-ended mode, double that in mono. Frequency response is 0.1Hz to 200kHz -0.5dB and power consumption is 160 watts. "As you drop below 87dB speaker sensitivity, you will want to consider 100-watt balanced monoblock operation, with a preamp capable of swinging 14 volts per balanced output and having a gain of 20dB+. As your loudspeaker increases in sensitivity, you need less gain and less voltage swing."



This means that your top-class preamp finally gets to stretch its legs. It is allowed to enter a more optimized RPM and torque zone. No longer does it barely get out of first gear. Previously, preamp and amp gain added up to so much loudness that your listening took place well below full source output voltage. With the F4, one X factor is eliminated. With it, all chances to amplify preamp noise or running your preamp below 9:00 on the dial are eliminated as well. It's an elegantly simple though unconventional antidote for 'gain poisoning'.


Possible applications for today's specialty amplifier beyond standard speaker drive are headphone drive; passive bi-amp drive where the F4's existence at unity gain doesn't require a bass leg attenuator; active biamping with the Pass Labs XVR1 or an equivalent active crossover directly into the amps; and as an add-on power booster and impedance converter for low-power SETs where the F4 takes its input from the SET output. As an unrepentant appreciator of the micro-power SET genre, the mere promise of running my 2wpc Yamamoto A-08S 45-based SET into speakers of more ordinary sensitivity than my 101dB Zus and 97dB WLMs through the mediating action of the F4 was a mighty tease. My Supratek Cabernet Dual preamp has up to 26dB of gain. Its X factor uses four different bands via transformer secondaries selected by a rotary rear panel switch. With the F4, I sensed a heady promise. I might even drive the low-efficiency Mark & Daniel Ruby or Maximus Monitor through the Yamamoto. If I needed more mano-i-mono muscle, Nelson would dispatch two units so I could report on 100-watt power buffer mode. Should this work, it'd be eff to the fourth power - as in why hadn't anyone thought of a clever amp like this before. Well, not for nothing does Nelson Pass enjoy his unrivaled reputation as perhaps the most creative and inspired solid-state amplifier designer currently working.


Before we consider sonics, a reminder. The F4 soars or sinks on the strength of your preamp. More specifically, it rides on the preamp's gain structure - in relation to your speakers, your room size and your desired volume levels. Nelson has driven Quad ESLs with the F4 and surprised himself and their owner with the results. Naturally, passive preamps are out. If your speakers are of low efficiency, your preamp shouldn't be of the low-gain 6dB variety either. And if your speakers are unconscionably hard of hearing, you might need two F4s for balanced/bridged operation to really tear it up: "My Lowther DX55s (93dB/watt) mounted in Alerion enclosures adequately fill my 11,000 cubic foot listening room with about 5 volts (the equivalent of a 4-watt amplifier). I can achieve good levels with headroom to spare with my phono stage or DAC feeding an X1 preamp, which has a maximum of 14dB gain. If I lived in an apartment, I could get complaints from my neighbors."



"As a stereo amplifier with single-ended inputs and outputs, the F4 will deliver up to 25 watts into 8 ohms with a damping factor of 40. It will do 50 watts into 4 ohms and as a monoblock amplifier with parallel inputs and outputs, it will do 100 watts into 2 ohms. As a monoblock amplifier with balanced inputs and outputs, the power output rating is 100 watts into 8 ohms at 0.5%, and the damping factor is 20. Maximum output current is 5A and noise is 60uV unweighted. Input capacitance is very low to be copacetic with tubed preamps or tube-powered sources with variable outputs and the amp is largely indifferent to source impedance. At ordinary listening levels, the amplifier operates Class A and the distortion is 2nd harmonic in character, rising or declining in linear proportion to the output power. The amplifier has a direct-coupled input and output, with a -0.5dB rolloff around 0.1 Hz and 200kHz. It does a clean square wave at 100kHz. The combination of a simple Class A circuit operated without feedback and the excellent objective performance gives us a superb sounding amplifier. The low distortion, bandwidth extension and high damping result in midrange clarity, treble detail and control on the bottom end. While these are available from most good solid-state amplifiers, the F4 also brings depth, imaging, midrange warmth and top end sweetness. Overall, it is one of the best sounding amplifiers. If you can live with unity voltage gain in your amplifier, it is possibly your best choice."


The amplifier can be run in three configurations - stereo; mono balanced from a balanced source; and mono parallel for more current and less distortion (driven by jumpering pins 2 and 3 of the XLR input with the provided U-shaped insert while one of the now paralleled single-ended RCAs gets driven from the source). The amp runs its heat sinks at about 25 degrees Celsius above ambient temperature and a 1-hour warm-up period for best sonics is normal. "I started out thinking that this would be a sleepy sort of product - it sounds really good but of course has no voltage gain. I've been taking it around locally to all the usual suspects though and discovered that most people have enough gain in their systems to do a good job. On Saturday I had it out driving a pair of Quad 989s and I thought their owner might cry when I had to take it home. These amps do require 14+dB of preamp gain depending on the source level and the preamp ideally will swing 14 volts. In any case, the nice thing about FirstWatt is that it can turn on a dime and I think I'll also take this circuit and add a voltage gain front end to it for a subsequent product."



The dime-turning comment of course refers to the kitchen table format of FirstWatt whereby Nelson Pass personally hand-assembles each and every amp. So far, these offerings have remained limited to 100 units each. Afterwards, Mr. Pass enters the schematics into the public domain for DIYers. A chief impetus for FirstWatt is as outlet for a wildly creative mind. It gets bored with the few circuits the mainstream Pass Labs company can package for long-run commercial amps. Nelson probably has one new circuit variation crop up daily in that noggin of his. And twice so on Sundays. "As I release these FirstWatt things, the products I already have naturally suffer for attention. I guess one of these days I'll have to decide if I really want to sell product or just unveil an endless series of novelties. Such problems..." FirstWatt is first and foremost an adventure of pushing boundaries, there at the edge of common usage where special applications require special solutions which no big commercial vendor could afford to address simply for lack of solid demand and sell-thru numbers.


"I saw some very inexpensive tube integrateds at Parts Express, the Nueras. One of these is a SET rated at 7wpc and the other is a PP rated at 35. How bad could they be sez I so I ordered them up. My thinking was that they might make good front ends for F4s. The 35-watter was on back-order and I got the 7-watter first. It was physically impressive, appears nicely made and weighs a lot. It also met its specs. On the downside, one of the specs was the 1-watt distortion figure at 1% at 1kHz. Now this was of particular interest because it had a similar-looking curve to the F2, and when I listened to it on the Lowther Alerions (DX55 in a Mini-Medallion enclosure), I was interested to hear a very similar sound and also a nearly identical amount of congestion with complex material - something you don't get with say, an F1 or F3, at least not at these levels.
"I've run into this before but occasionally you have to be hit over the head more than once. In this case, similarities of an SE tube with feedback and an SE Mosfet without but with the same THD figures brought it home clearly. I have to conclude that 1% or so THD at 1 watt is just not an acceptable figure, 2nd harmonic or not, feedback or not. Anyway, the 35-watt unit arrived and sounds much better in this regard. It's spec'd at 0.1% although I haven't had the chance to measure it. I'm looking forward to spending more time with it, particularly as I've been on a vinyl spending spree the last few months, and I have a lot of stuff to wade through.


The 7-watt amp does well with no load to speak of and when I hooked it up driving the F4s, voilà! Lots of gain and that tubey sound that is so prized. Now I've got them on a pair of PM6As (with Ticonal), biamping to Audax PR380 woofers in big TLs. And it's very good. So I've also got some other FirstWatt stuff cooking..."
FirstWatt is obviously subsidized by Pass Labs. The latter affords Mr. Pass the leisure and economic freedom to run FirstWatt as a turnkey DIY venture rather than assembly-line operation. Creativity is king. Plus, Nelson is very hands-on through the Pass page at DIYAudio. A dedicated thread has been growing there ever since rumors of the F4's imminent release first surfaced. And unlike with earlier FirstWatt models, the full circuit schematic for the F4 has already been published. Folks try to roll their own and trip up on specific circuit junctures. The maestro -- or papa as some call him -- clearly delights in his Yoda role in that forum. He routinely doles out terse nuggets of wisdom to those whose demonstrate serious involvement and truly apply themselves. Anyone interested in F4 circuit specifics and possible variations thereof should log onto that particular thread. For the rest of us, Nelson hand-builds
a completed plug'n'play amp. All we really need to know is how it sounds - and whether our preamp will be up to driving our speakers with its own gain and voltage swing potential, the amp merely handling impedance conversion, current supply and buffering. Unless we insert a micro-power SET in-between. Which is where things could get really unconventional.

In such a booster amp scenario, the F4's high input impedance won't load the driver amp which doesn't see the speakers but only the F4. This may require the insertion of load resistors: "It's my observation that transformer-coupled amps (or actually, any amplifier) has a 'sweet' load value at which they perform best. Usually it's at a higher value than the rated load but I have not encountered a case where it's no load at all. The 47K input impedance on the F4 would be considered no load for this purpose. In my testing here, I ended up with 22 ohms for the Nuera SET, which seemed the best compromise value for distortion and bandwidth. Of course it's dubious that buying a $700 SET and a $2500 follower makes better sense than buying a $3000 SET in the first place so I'm not really pushing that concept very heavily. More likely is that the real benefit with SET amps will come from bi-amping where an F4 is driving the bottom." In that case, the SET as tweeter/midrange amp is loaded by the speakers' upper terminals while tapping its signal in biwire fashion from the SET, the F4's unity gain means that no active crossover is required to level-balance between the speakers' upper and lower inputs. Additionally, it's likely that in such a scenario, much of a SET's sonic fingerprint transfers through the F4 to the speakers to minimize or eliminate the otherwise possible polarization between tubes on top, transistors on the bottom.


"By the way, when you are using the output of amplifiers to drive the input of an F4, it is important to establish what is ground on the source amplifier. On quite a few products the (-) output terminal is not ground. An ohm meter is particularly useful for establishing where the real ground is because this needs to attach to the RCA ground or XLR pin 1 on the F4 or else lots of current tends to flow needlessly." FirstWatt retailer Mark Sammut of Reno Hifi will assemble and sell the necessary spade-to-RCA custom cables for customers with booster or bi-amp applications. For conventional hookups from a preamp's outputs, the F4 of course needs no special interconnects at all and functions like any other amp - except for not being a FirstWatt but ZeroWatt amp. In case you didn't send Nelson happy birthday wishes through his forum, here's what you missed: "Thank you one and all. Your names will be placed on the Protected List and when my amplifiers take over, no harm will come to you."