F2 Initial Listening Impressions

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Having owned an F1 for many months and loving it, I have had about a week to listen to the F2. The F1 has spent many hours on Terry's Abbeys and Nelson's recommended DX55s. So far, the F2 has powered the DX55s. How do the 2 amps compare on the DX55s? While I like the F1 on the DX55s and love it on the Abbeys, I find that the F2 better complements the strengths of the DX55s. In a simple 0.5 ft^3 box with 10 uf, 6,6 ohm RC network, the DX55s are fast and transparent with no hint of aggression or shout. They give a very natural sound that is neither forward or recessed. The frequency rolloff is slow enough that all but the lowest base can be heard. My overall take so far is that this combination justs gets out of the way and lets the music play. Rock makes me want to dance, and female vocals are to die for. Classical captures the leading edge of piano, the space surrounding John Rutter and the Cambridge Singers, and the theads of large symphonic works. I am not implying that this combination is the last word in audio. The weakness in my system is a Sony XB940 SACD/CD player. In the next few months, I hope to take delivery of a modified Denon player from John Tucker. My point is that I my not have pushed the system to its limits, but even with modest equipment the F2 and DX55s really make music. Thanks Nelson.
The one and only
Joined 2001
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Like I need to be thanked by the man who preserved the

This is a scan of an original one-of-a-kind photo of the very
first transistor, signed by Bardeen, Shockley, and Brattain.
Brattain's signature has become very faint over the years,
but you can just make it out.


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F2 on Lowther PM2A's

I have spent some time listening to the F2 on the bigger, 8" Lowthers rather than the DX55. So if you want my impressions, here they are.

Steve's notes about the characteristics are correct. I could wax for a long time about how these things sound like great SET's, but you get the idea. They combine speed & articulation among the very best I have heard, but also act to greatly flatten the response of the PM2A's in Medallion cabinets as an extra added prize.

The F2 seems to lose a little in the bass compared to the F1, and has much less power into my 15 ohm drivers.

Of primary importance is that they simply play more music. Recordings (usually redbook CD) that were dry on the F1 have flesh and blood put on by the F2. F1 worked fine with vinyl and SACD, so it was not a problem with the F1, just that the F2 is able to make more stuff enjoyable. And playing music is were it is at.

Although I am not one to much care about imaging, the F2 does a great job. Very good depth and localization.

PM2A's are not as neutral as the DX55, having a lumpy frequency response in comparison. In return they give a wonderful sense of dynamics and flow to the music. The F2 allows them to breath and make music. And that is the best thing I can say about any amp, it gets out of the way of the music and let's you simply enjoy. Last night I felt, for the first time, like I had a seat in the recording venue. Strange experience for the first time, and thoroughly enjoyable.

If you have a full range/wide range driver do not let this amp slip by, it is really something special. Your only excuse would be if you have a high impedance driver and wish to wait for the F3.
I hate agreeing with Jon all the time, but can I help it if he's right. All kidding aside, I wanted to add a few more comments about the F2.

Yes, the F2 sounds like the best of the SETs I have heard. I didn't say that before becase to many people that statement translates to a bloated, warm midrange with no frequency extremes. This is not true for the best SET designs which extend good sound into the frequency extremes. But SET types talk about which tube is best (at the moment), who is the best maker and year of manufacture.

Enter the F2. This amp delivers more of the "you are there" quality like SETs without having to worry about shocking the kids, frying the pets or which tube is best.

How does the F2 do this? I don't know. The tech types have many theories ranging from Fourier colored sprectrum to circuit topology (balanced vs single-ended etc.). But over the years I have heard some wonderful amps that "break the rules". These amps share the common trait of being created by talented designers. The F2 stands before you as perhaps the latest example.

Ok, I admit it. "Being there" and "communicating emotional content" are my auditory goals. Not everyone shares these goals and that's OK. My ancient greek philosophy professer often quoted Epicurus, "About taste one can arque forever."

About the F2's output power. It is 5 watts into 8 ohms and more into 4 watts. Since the damping resistor is internal at 16 ohms, I have not head a drop in my DX55s ouput power as compared to F1 since I removed the 47 ohm damping resistor. So those of you with 8 and 4 ohm speakers can play LOUD if so desired.

My last comment is to reminds us of something we all know. An amp is only part of a system, and its sound depents on the blending with the sound of the other components. Fostex drivers tend to sound warm to my ears and lack the a little detail resolution compared to Lowthers. Yet the Fostexs are very musical. Thus the F1 and Fostex drivers are a great combination since each complements the other. Apassgear, I wonder if the F2 and Fostexs may be too much of a good thing? Don't know till I try. Perhaps some of the high resoltion SEAS drivers or compression drivers would be a good match even if they are not full range?

Thanks guys. Those are very clear comments about the F2 (and F1 as comparison) I get the feeling of it. Best solid state SET amp?

Steenoe and myself are working on a “back engineered” F2 based on all the tips that Nelson has shared. See http://www.diyaudio.com/forums/showthread.php?s=&threadid=57515&perpage=10&pagenumber=1

On that thread I have posted my interpretation of the circuit for the F2 which includes a rail regulator but really don’t know if this is part of the original F2 by Nelson.

At the moment we have a layout and are working on the PCB. Following the disclosed F1 I have change some values and input topology. I’m showing an up to date schematic for those of you that might be interested.

Biosonar, looking forward to hearing your comments on the F2+Fostex, this combination will be very interesting for many of us!!!


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Biosonar, looking forward to hearing your comments on the F2+Fostex, this combination will be very interesting for many of us!!!
Excactly, Tony. If that combination is good, we might be looking at a major DIY thing here;) I dont suppose any of the happy (lucky:) ) owners of the F2, would care to take the lid off the amp and post apicture, would you:angel:

The one and only
Joined 2001
Paid Member
Re: F2 on Lowther PM2A's

Jon Ver Halen said:
Your only excuse would be if you have a high impedance driver and wish to wait for the F3.

Yes, Jon has been torturing me about the F3. It is a special
version of the F2, optimized around a 16 ohm load, and so you
can imagine that the voltage and current have been adjusted,
but there is also something else unusual, not seen
previously in the Zen genre. I put some more work into it today,
and it is definitely better if your load is 16 ohms or so. Below
12 ohms, you're better off with an F2. I expect availability in
late August, early September.

Hi, Mr. Pass,

Does F2 has output capacitor or DC coupled at output?

NP's Fseries should be better than SET (single ended tube) because both are current sources (SET makes a current source via its OT secondary windings), while NP's Fseries makes a current source via nothing at all (at least in F1, don't know about F2). Output transformers are not linear. Direct connection are.

People usually comparing tubes SET (eg. 300B) with transistors (Mosfet/Bipolar) and say tubes sounds better. I think it is useless to compare tubes and transistor while SET is current source and transistors amp are voltage sources.

Now, those people can make tube vs transistor comparison one more time :D with equal topology (current sources) offcourse.
Picture of F2 amp interior

Mr. Pass wrote:

"I think www.6moons.com put up a nice interior shot."

They certainly did put out a nice clear picture of the interior. A lot of questions have been answered this way, like what kind of power supply is being used and what kind of components as well.

But I found it easier to locate the review of F2 via Google search than navigating on the .6 moons web site...

tmblack said:
How do the F1 and F2 sound using typical 8 ohm 2 way bass reflex speakers?


I can't give you an answer based on actual listening. However, I expect 2 problems that won't give you the results you are hoping for: the crossover and base response.

Nelson has covered the question of how current-sourced crossovers are different than voltage-sourced crossovers in his "Current Source Crossover Filters" article which can be downloaded off the Articles page of the First Watt site, www.firstwatt.com. While possible workaround solutions of building a current-sourced crossover, or using an active or passive voltage-sourced crossover between the preamp and F1/F2 look attactive, none deal with the second problem: interaction of the Box and BR port resonance with the base driver low frequency (LF) resonance.

With both the F1 and F2, the base driver LF response is extended by adding a resistor in parallel with the driver, resulting in an adjustable LF peak at driver's resonance. (See Dick Olsher's panther and leopard projects at www.enjoythemusic.com for clever examples of this LF extension using a closed box solution.) The problem with a BR design is that the interaction between port tuning and driver resonance is messed up by the change of the driver's resonance characteristics brought about by the current-sourced amplifier and its LF damping.

The upshot is amps like the F1 and F2 can be used with BR designs. One just has to take into account the changed nature of LF response that current-souced amps like the F1 and F2 cause when designing the box. I have a base driver with an Fs of 31 Hz and an effective bandwidth product of 96, indicating a BR design. So I am grappling with this design isssue. Any insights are welcome.
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