I'm currently a matter of hours away from finishing a 50wpc version of the F5 using the standard F5 topology, not the F5 Turbo. (32V rails.) I'm getting very excited to hear what it's like. It's the first amp I've ever built so I'm not rushing to finish it. I'll be finishing wiring the PS tonight and testing it and hopefully begin setting bias tomorrow night.
As far as I know, Nelson personally built most of the First Watt amps himself until recently when construction was transferred to the Pass Labs facility.
I chose the F5 because it's a very simple, straight forward circuit as opposed to being pure current gain or obtaining voltage gain though a transformer or any number of the "unique" designs Pass dreams up. It's going to be powering Focal 936's and I felt a "normal kind of amp" (Nelson's words) would be the best place to start.
The bottom line on why I chose to build one is real simple: How else are you going to obtain a Pass-designed amp for under $1000? You pretty much can't! Some aren't well suited to very reactive loads, like the F4 and J2, but some, like the F5, can be built up to drive a 1 ohm loads all day very reliably. And have human lips ever uttered "That Pass amp sounds like crap!"? I don't think so! |
I got my boards, transistors, and case through the diyaudiostore too. Real good stuff. I'm definitely getting excited to hear it. I think my next project is going to be a B1 buffer pre-amp. Need to dig into that world a bit and learn more though. I'll be back to let you know how it sounds, assuming I don't accidentally reduce it to smoldering rubble. Pretty nervous about that first power up moment. |
The bias is going to have to be something I play by ear on account of the 32V rails. I've got a sheet of copper and I'm debating bolting some sinking made of that to the top sides of the MOSFETs. Last night I learned that the power supply works a lot better with fuses installed in the power input module. Don't know why I just assumed there was one in there. It made me double check a bunch of stuff though! Hopefully tomorrow night it'll be done. We'll see. |
I'm listening to the amp as a write this and it sounds incredible! Very visceral, physical, deep, and richly textured sound indeed! It still needs a bunch of time burning in and to have the bias adjusted up a bit more, but I love the thing already. |
Thank you very much! It does have a brightness to it, but it almost seems like it’s just that much more clear and detailed up high than I’m used to which may be why it sticks out. There aren’t any negative characteristics of the highs except that they’re just there. To be expected of an amp with the specified bandwidth of DC-1,000,000Hz and nothing limiting it I suppose. There’s greater crimes an amp could commit! I’m extremely pleased with it all around! I’ve never heard an amp with this kind of imaging, punch, clarity, speed and delicacy before. It really is like a revelation. It’s definitely going to be a benchmark I judge all other amps by. I’m driving a pair of Focal Aria 936’s with it and it grips them with surreal authority. It seems an excellent pairing given their complementary strengths. I’m truly thrilled to have a rig like this in my living room. It’s a privilege and an honor to be able to hear music like this. I’m also tickled that Nelson Pass called me a "fearless amp builder" for going with 32V rails and yanking out all the current limiting. |
I'm totally loving the thing. The thing runs blazing hot. I'm running about 2.2 amp of bias through it and it idles at 55C on the sinks and about 77.5C on the MOSFETs. Maybe I could eek more out of it, but 2.2 amp of bias on rails that actually measure 34.5V is already pretty bold. Anything more would be diminishing returns. I'm pretty sure I'm a full convert to class A and Nelson Pass designs. |
Yep! And then doing everything he can to get guys like me to build them!
More people should give it a try. It's like they say: the best amp you'll ever hear is the one you built!
There's a lot of upsides to it though. You can't buy amps like that, period. Dealers don't even want to sell you amps that idle at 135F. |
Seeing as how Nelson only constructed 100 examples of the F5 9-10 years ago they're probably as rare as hens teeth these days. The F7 is supposed to be an improved revision of the F5. Never heard one though. If one is of the DIY mindset and has a serious thirst for more power than a standard F5, there's always the F5 Turbo. The F5T can be scaled up to 200 watt mono-blocks and run on balanced input. Of course you'd need an absolutely massive cooling solution for an amp like that, but it's totally possible with this topology.
The reason I was able to push mine well beyond 25wpc is because the chassis I used has bigger heatsinks. It's actually a pretty simple amp to build which is part of the reason I chose it as a first build. Under $1000 for an amp like THIS? I feel like I just robbed a bank, and not only got away with it, but found they dumped a million dollars in unmarked bills in my bag! You can't just go buy amps like this. Every single F5 in the world is hand built with quality parts and carefully tuned in a "time is no object" way. If you ask me, that is what true high end is. Sure sounds like it from my chair! More people should try building one.
Whitestix, I completely agree with your assessment of the F5 sound. It presents with a level of purity, accuracy, power, dimensionality, and ruthless neutrality that just staggers and leaves one groping for hopelessly inadequate adjectives to fully sum up the character of the amp. It's one thing to read the reviews and opinions of the amp, but to actually hear what it does is just impressive. Can it really be THAT good? Yeah. It's THAT good.
The extra power is nice on my build, and it probably lowers the distortion a little, but I seriously doubt I'm using all of what it can do. 75 watts of peak class A power with 92dB sensitive speakers? I'm probably just scratching the surface. |
It's also worth pointing out that all the FW amps drive speakers in different ways. I chose to build the F5 because it's a "conventional" voltage source amp with voltage gain. Not all of the FW amps are. Many are current source and some are just strange. Take the F4. It's a pure current source amp with zero voltage gain and it's pretty much useless with insensitive, highly reactive speakers. It would play well with the benign, high impedance load DeVores present. And because it has no voltage gain you either need extremely sensitive speakers or a pre-amp or flea amp that can give you the voltage gain if you need it.
The F3 and F6 get their voltage gain from a passive transformer and they're said to sound a bit like tubes.
FW amps aren't for every listener with every speaker. The J2 was called Stereophile's Amp of the Year, but it was positively gutless driving LS50's.
Nelson does talks at Burning Amp where he discusses many of his designs and they're very much worth watching on YouTube. His 2016 talk was about the F5 topology. He's also discussed SIT amps. |
I think the XA25 is similar to the F7 but with balanced supersymmetry circuits. It's no more powerful than the F5 or F7 though. I've head that it's noticably better too. Of course you're going to pay a lot more for an XA25 than an F7.
I think I read that the F6, F7, and F8 we're going to be revisits to the basic topologies of the F3, F4, and F5. I'm very curious to see what the F7 layout is like. I might be very interested in building one. |
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I’m using my F5 to drive a pair of Focal Aria 936’s. Those are an even more reactive load than the LS50’s and the F5 handles them with plenty of power, speed, and grip. I think it’s the current source designs that really struggle with the crazy low impedance and phase angles some speakers present. The output stage of the F5 provides nothing but current, but it’s totally unregulated. All the voltage gain in the F5 comes from the SJ74 and SK170 input pair, but they provide absolutely no current gain.
If I recall correctly, the outputs in the F6 provide both voltage and current gain. It just depends on the circuit the devices are in. The F5 is specifically designed to provide current to maintain the voltage gain. The F4 is kind of the opposite in that it’ll let the voltage flop around a bit to strictly follow the current gain. Benign loads do well with amps like that, especially high impedance loads. As the amp tries to ram more current down the load, the voltage demands will actually drop. It’s a different way of attacking the drive challenges of big horns and speakers like DeVores.
In all honesty I wish I knew a hell of a lot more. 2 years ago I barely knew how to solder in a capacitor. I LOVE Nelson's designs because though they're simple, there's a lot going on that compliments what other parts of the circuit are doing. A lot of other designers build far more complex circuits to do some of the things features in his circuits do, but he prefers to reduce the complexity and play weaknesses off weaknesses in a way that makes them strengths. That's the real Zen and allure to what he does. It's got a certain organic nature and beauty to it. |
I think those are good statements of fact. Tubes are a whole different world. They have very high output impedance and high voltages. The transformers convert that too low impedance and current. Still, no transformer can achieve the vanishing impedance of many direct coupled designs. Seems to me, as a matter of principle, if you want the amp to have optimal command over the speaker, connect it directly to the active output devices. There are decent arguments to be made that the "tube sound" is at least in significant part "transformer sound". |
DC coupled valves are out there, but they're not at all common. I'm a little curious to try tubes with the F5. |
Probably going to build something. No idea what at this point. |
Believe it or not, the old Marantz 2252 isn’t feeding the F5 a bad sound. I totally recapped the thing about a year and a half ago with the best ELNA and Nichicon I could find. The pre-amp stage is designed to play well with a 47K ohm load which the F5 has. The soundstage is wide and enveloping as well as deep with some decent fluidity and speed. My stack sounds better than the Cambridge Audio stack I auditioned these speakers on. I’m sure it helps that the regulated supply isn’t driving an intermediate gain stage in the amp with anything more than idle current. I’ve got some stacked films for the coupling caps I bought with an order but I just haven’t opened it back up.
How are the highs with that pre-amp? I’m worried a little about that. It’s not bad with the Marantz. That old Marantz sound I guess! |
I'm getting some brightness especially since I played with the distortion. I need to play with that a little bit more and find the right balance of second and third. It's nice to have an amp you can do that with. |
