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Well-informed insiders will recognize similarities between the F5 and Mr. Plantefewe's Profet circuit. "This has been discussed before. As I said in the article, the topology is familiar. Complementary feedback pairs aka conjugate complementaries are elementary and go way back. My first exposure was from the Motorola app notes, late 60's -early 70's. But I did write a nice note to Mr. Plantefeve apologizing about the coincidental parts of the design, and I promised to pay more attention to his work in the future. I do like to put a little more distance between offerings. Both my and his circuit use push-pull complementary feedback pairs with voltage gain. That said, the details of the circuits and the performance are different: vertical vs laterals, the amount and arrangement of feedback, the bias values, the power supply values, how the adjustments are accomplished, the power rating, damping factor, gain, and distortion. They do appear to have about the same bandwidth. Also the circuit appears in two different Selectronic products, one a 4-channel amplifier designed to accommodate bridging, and another a monoblock with a chip for a balanced input. I'm sure they are both fine sounding amplifiers."

So much for the small print, intellectual properties and the inevitability of similarities when one is, as DIYAudio poster mrothacher sagely quipped, "trying to push the limits of ultra-simple circuitry. Aren't these coincidences sort of inevitable? I mean, Nelson Pass is just about the last guy on the planet I'd accuse of a lack of originality." Amen.

"The F5 uses what I would refer to as low impedance voltage feedback - as pointed out earlier, the circuit is a CFP complementary feedback pair variant. Maybe it should be called a complementary-complementary feedback pair with voltage gain. CCFPWVG is a snappy acronym, no? The point is, this is routinely referred to as 'current feedback' in the op amp literature but it has been misnamed in this regard." Taking pity on prospective and already committed F5 rollers for lacking any type of audition feedback when none had finalized their own DIY variants yet to post sonic impressions for the fence sitters, Nelson Pass then did something quite out of character. He commented on the sound of his own amplifier, something he usually leaves to others: "The F4 is the closest relative. They both have a clean extended bottom and I consider them almost equivalent, with neither having a distinct advantage. The F4 is a little bit warmer in the mid and might be preferred on simple material at modest volume levels, that is to say a solo vocal or instrument or maybe a string quartet. You could say it's a little more tubey. [Above, biamping with an F3 driving the Feastrex units, X150s on 15" woofers.]

"As the volume rises and the complexity of the material increases, the F5 pulls ahead
by retaining more clarity - specifically less IM type distortion. As a result it does a better job with articulation of instruments. There is a little less smearing. If you have a pair of full range drivers, Lowther PM6A or Feastrex D9nf, the F5 brightens the top end up just a bit, adding a touch of tintinabulation that we all like, although it also heightens the perception of response peaks a little more. In this category, if you find the high efficiency full rangers too bright with the F4 or F5, the F3 is perhaps the more appropriate amp. The imaging is very good with both of these amps in my system, and I haven't heard enough difference to make a decision on that, but the localization is pleasing. The open baffle speakers I'm using already have a big ambient soundstage and would probably do that with any decent amplifier. The F4 is a little more relaxed all around, the F5 carries a little more detail."
With two F4s in residence, would I agree or disagree with the designer's own findings? Here are a few final comments from the maestro before we get to that: "After three years, I am still waiting for a FirstWatt amplifier to fail. There was a case of bad solder connection in the field but the amplifier still worked fine. Other incidents have not turned out to be the amplifier. No doubt one will break eventually, and when it does the failure rate in the field will be less than 0.3%."

"The amp gives its best performance after an hour or more and it likes ordinary 25 degree C room temp the best (I know this because summer arrived early here and my air conditioner needed service). The amp's bandwidth being as high as it is, you want to pay particular attention to grounding and cables. The output ground is generally best not attached to anything other than the speaker, and it's a good idea not to intermix the incoming cables with the outgoing cables. Shielded input cables are also an excellent idea. Litz wire for speaker cables could be a little iffy without termination at the speaker end, although I have tested it into unterminated coax without problems.

"What I'm talking about here is that in my experience, amps with wide bandwidth often find themselves in setups where the layout is careless (or experimental) and we see cases where the output is in some proximity to the input or otherwise can talk to the input through bad input ground connections and such. In those case, the output can be re-amplified and the whole system becomes an oscillator. This is one good reason why at Threshold and Pass Labs we have always limited the response bandwidth of the power amps to 100KHz or so. Above that you start having trouble with crappy wiring. In the F5, that would have meant inserting a capacitor. Which I didn't. But I haven't been able to find a load that will induce instability as long as it's on the other end of a foot of cable so I'm not concerned about that. Regarding the F5's feedback, the circuit's open loop gain is about 35dB, 15dB of which are used for actual gain. That leaves the remaining 20dB for feedback."

To conclude the introductory stuff and reiterate something that strikes me as relevant, Nelson's grip on FirstWatt's always tenuous commercial aspect is slipping badly. The man is giving things away left and right. He seems too excited to share ideas rather than wait until his presumptive self interests -- sell-thru of a new model's production run -- have been properly served. His how-to article in AudioXpress and circuit schematic publication well preceded the F5's formal roll-out. Subsequent posts by him in the F5 thread goad on the audience to get busy with rolling their own. This demonstrates Nelson's present focus. Create and share, create and share, at a pace that outstrips the 'manifestation lag' on his kitchen table. If this continues, Nelson might finally admit to prefer coming up with new circuits, then using FirstWatt to publicize 'em. In which case, these commercial ready-to-go F5s could become an endangered species. That such a turn would completely undermine FirstWatt's ability to function as a profit center of any sort... well, about that Nelson doesn't seem to give a flying fig. That's the height of creative success and freedom. Fade to second act. What's it sound like?

Like a true FirstWatt amp, meaning it comes alive at whisper levels - to a clearly superior extent than my treasured Yamamoto A-09S 300B zero feedback SET. This made me suspect that in admiral Nelson's universe, this was more of what he refers to as a 3rd-order than 2nd-order amp. I was sure I'd find out more soon enough.