DIY Pass Labs First Watt F5 amp impressions

I recently acquired a well- built DIY First Watt F5 amp for the cost of the parts, which is the understanding among those that build the designs that Nelson Pass has made available to the DIY folks.  I have been using a McCormack DNA .05 Deluxe amp and a Don Sachs 6NS7 tube amp to drive my Spatial Audio M4S Turbo S Hologram open-baffle speakers, which are ~94 db efficient.  I have raved about the McCormack amp on this forum, but swapping the F5 for the McCormack amp has been a  sheer revelation.  I have had dozens of tube and class A/B amps my system over the past 4 decades, as well as a couple of very good class D amps recently, but the tonal accuracy and 3-D representation of music with this F5 is simply stunning.  It is offers a glimpse into the inner details of the music.  I do not have the superlatives to describe the musical enjoyment of the addition  this amp provides to my system.  I am put off the heat his amp generates -- similar to that with a tube amp with a quad of KT88's, but it is a fair bargain for the most ruthlessly revealing sound reproduction I have ever heard in my system.  My regret is that I waited so long to have a Nelson Pass-designed amp in my system.  If your speakers are quite efficient, there are several variants in the First Watt line of amps that might be a great match for your system.  Mark at Reno Audio is the guy to help you sort out the best amp for your needs.
Cheers, Mark

I can relate to the F5, I built the F5 and the F5 turbo version from the kits that were sold on Diy Audio.But know I have a Diy version of the Sit 2 but mine is class a push pull using Nos Sony Vjeft's and produces 40 watts into 4ohm.I also own the production version of the F7 and It's very nice as well.They all run hot and that is just the nature of the beast,but the sound qualities make up for that as you are finding out.

I'm like you I was kinda enamored with the latest Class d amps but after trying several of the latest they don't have all the sonic qualities I desire.

Anyone know if there is a version of the SIT-1 monoblocks available that will put out more watts than the production version? I'd be interested in these if there were a pair that put out 40 wpc or thereabouts....
We evidently traveled the same path with the Class D amps. I do think they have a lot to commend themselves, but after listening to them for a few weeks, something was missing.  The lacuna of sound is only apparent by comparing it to other amps. Interesting other SS amps you have.  As I mentioned, I have admired Pass gear just like I do fancy car that drive by, but this oppty to get a well-constructed F5 ranks as the all time audio score of my life.  

I swapped my McCormack amp back in place of the F5 after the post above and immediately had a frown on my brow!  It is just two-dimensional compared to the 3-D presentation of the F5.  Folks on this forum might note that I have never had a piece of gear that I did have something good to say about -- very little gear now days sounds bad, I am literally awestruck with the sound of my speakers with the F5.  

In terms of the heat generated by the amp, it is rather like having big tube amp... but never having to bias it or replace pricey tubes.   Cheers. 
No,they would be push pull topology like my stereo version to have 40watts into 4 ohm.

The Production Sit1 mono's are single ended and only put out 10'watts.

I also own the F7 and It's still being produced as far as I know.


I'm glad your are liking the F5,Maybe class d will be there someday.

All the First watt amps are very special and I really like mine also,I will be comparing them to a Linear tube audio Zotl40 paired with a MZ2s preamp,I've only had it for a few days but I like what I'm hearing so far.

kdude66 would love to hear the details of your time with the LTA equipment in comparison with the First Watt stuff. I'm interested in the LTA stuff as well.
I’m not surprised that the Mc Carmack sounds 2 dimensional vs the First Watt amplifier in direct comparison. I just happen to believe that lower power simple circuits running exclusively in Class A offer superior sound quality. Tube or transistor, both excel with this approach.

Hi Kenny I also look forward to your listening impressions of the well regarded  LTA ZOTL amplifier compared to the  First Watt amplifier.

I have not heard the F5, but, I have heard their J2 model in my system (a loan from a friend that I had for two weeks) and liked it very much.  It delivered the liveliness, expansive and enveloping soundstage and good low-level dynamics that is usually exclusive to tube gear, particularly, low-powered tube gear.  The only thing that was sort of "solid state" about the sound was the slightly hard edge to the initial attack of notes.  I also found the music to be engaging and held my interest in a way that I don't experience with other solid state amps (they somehow sound lifeless).  The J2's sound MUCH better, to me, than any Pass amps.

I have also heard a friend's DIY project amp that is a First Watt design.  I believe it is one of the SIT amps.  I heard it in his ultra high sensitivity system.  That amp, too, is a winner.

 Thanks for you keen observations.  I would agree that there is a slightly hard edge to the leading attack of notes with F5.  
I too await a report on the LTA amp from Kenny.  I seem to recall Julius Futterman might have been a early designer and user of this circuitry. 
Pardon my seemingly confusion but genuine curiosity in this thread.

I’m assuming "DIY" is "do it yourself". However, when I check the first watt FAQ page there is a question specific to Do you really build them yourself? to which the response is Yes, until this last year, when my son Colin and nephew Sean began helping with assembly. I’m assuming that means Nelson himself is building the amp.

Can the OP or someone else please clarify if the first watt amps are something Nelson Pass or the end consumer is building? Thanks.

Both,the production versions of first watts and the "Diy" versions that Nelson gives his blessings to the Diy community.Different models were available as kits on the Diy audio website store.

I own a production version of the F7,and my Sit2 is a Diy version but differs from the production version in that mine is class a push pull and uses Nos Sony Vfet's that were made available in matched pairs,The pwr supply,case and the heat sinks are the same,It just doesn't say first watt on it.

I hope this clarifies your question.

I have heard all the first watts,except the F1,and I have owned the F3,J2,and the F7.I have built the F5 and the F5 turbo version from kits and my Sit2 "Diy" I bought used from audiogon.


The F5 is a little bit aggressive from the upper midrange to the highest frequencys and does tend to have a slightly hard edge to the leading edges of notes.It's basically a very picky amp that needs the best components upstream of it.

This Offcourse is highly system and listener dependent.

Pass Labs sells factory-built First Watt amps, as well as panoply of Pass Labs amps.  A great source is Mark at Reno Hifi.  Mine was built by a guy who took the plans that Nelson Pass has generously made freely available for DIY folks.  This fellow then sold it to me for the "price of the component parts."  That is, he did not build and sell it to me as a commercial endeavor for profit, which is antithetical to the intent of Nelson Pass in making the First Watt designs available to DIY guys.   

I will hasten to add that if you are DIY-inclined person, the build of the F5 can clearly be done for a fraction of the cost of a factory-built version and is consistent with the desires of Nelson Pass.  The rub comes when folks use his detailed designs to build his amps for commercial profit.  Reselling such a First Watt as I acquired at the "cost of the component parts" is copacetic, so far as I know.  Others can clarify this understanding.
@kdude66 and @whitestix 

Thank you. Your response does clearly answer my inquiry. And in fact, after checking some more, I failed to take note of which further explains DIY.
I'm wondering if anyone has Tried the High Output version of the F5.... The F5 Turbo.  It outputs more power,  or maybe the F6, wondering how they would compare.
Mark,  You've got me itching to fire up  the xa30.5  here tomorrow.  Maybe the cool mornings  here the last 4 days or so  in Oaktown will accommodate me. .  Its like my Pass amp just sits there draped in the t-shirt dust cover and patiently awaits -- thinking hmm.   he's got those Emotiva monos on a lot lately-- he must be just be so distracted or preoccupied with something.     lol.   
I agree Mark at Reno HiFi is pleasure to deal with. I have a J2 powering a pair of M3 Turbo S Spatials (just sent in for Triode upgrade). Very happy with the results and looking forward to the 16ohm version. 
Update:  I earlier stated that the F5 runs as hot as a tube amp with four KT88 tubes.  That assertion is clearly wrong.  While the F5 runs much warmer than class A/B amps I have had, it generates nowhere near the heat generated by a robust tube amp.  My HK Citation II toasted the self above that in which it resided, but with the F5, the upper shelf is unaffected.  In conclusion, the F5 runs much warmer than a class D or class A/B, but far cooler than a tube amp.  Whitestix

Are you still liking the sound and can you tell us any particulars

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'll bet you are getting very eager to complete the project and fire it up in your system.  I wish I had auditioned a First Watt amp 5 or 6 amps earlier as I would have saved a lot of dough.  Let us know how your masterpiece sounds when it is up and running.  
Very nice and fun to build your own amp,
I built both the F5 and the turbo version from the kits from the diy store but I wasn't quick enough to buy the nos Sony Vfet's that were available as matched pairs to build the push pull version of the stereo sit.

I wish you the best in sound,

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. 
I can relate to you being nervous I was as well.

I work at a major airline as a Avionics technician for 29 yrs now and I work on some pretty complex stuff but It's not like the satisfaction I get when I build a Amp or do mods to my gear.

My best advice is triple check every little detail of the build including every solder joint.Be sure and be careful and take your time when it comes time to set the proper bias,allow proper warm up time and stabilization periods and make small adjustments until you reach the proper bias and remeasure after a period of time.Make sure you have very minimal DC at the outputs.

If you have any issue those experienced guys on the pass labs portion on the diy site can definitely help you.

Also the amp will need about 200 hrs of breakin time,mainly for the capacitors to sound It's best.

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. 
Let me be the first to say that I am very happy to hear you are enjoying the sound of your First Watt amp build!  I have had a ton of amps in the past 45 or so years and the F5, with a pair of compatible speakers, is  a sheer glimpse of audio nirvana.  I will say that my F5 does have a bit of a bright edge to the leading notes of music --- no amp is perfect of course, but it reveals music in my recordings with a visceral impact that I have formerly not heard.  I reckon after a couple hundred hours of burn-in with your amp, you will be even happier.  Cheers, Whitestix
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.
Very nicely done I hope you enjoy the tunes of your labor,
Quite a bit of satisfaction in building and or modifying your own gear.

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. 
I think you got her as high as you should go Cap't,
There is no substitute for a good class A Amp and Nelson just keeps designing and building them.

Way cool my friend.

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. 
I just returned from hearing a $6K Bryston 4B3 amp in a pal's system and got home to my system with the F5 warmed up and am hearing artifacts to the same music I was just listening to in my pal's system that were absent in his system.  The Bryston sounded excellent, but it didn't provide the inner glimpse of music that the F5 does.  However, I am biased so take that into account.  

I kinda think I got the sweet spot of the First Watt amps with my F5, but clearly their are nuances to all of them, particularly the more robust version of the F5 that our friend has built. At retail prices, the First Watt amps are reasonably priced, but if you are a DIY sort of guy, they would be an affordable way to have some of the best amplification available, so long as they are compatible with the sensitivity of your speakers.  
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. 
Love to hear all the excitement. How does the in-production F7 compare to out of production F5?

The F7 is slightly more forgiving and smoother sound than any of the variations of the F5 that I have heard.

The F7 has the lowest parts count of any of the first watt amps.
Comparing the F7 directly to my Diy sit2,I much prefer the Sit2 for it's liquidity,texture,warmth,midrange body meat on the bones and even more overall smoothness than any of the first watts,and I've heard them all except the F1 and Sit1 Mono's.
The sit2 gets the closest in overall sonics that a good tube amp brings to the sonic table.IMO.

Now comparing my Sit2 to my LTA Zotl40,both driven with the MZ2S.I'ts going to take me a couple days of proper evaluation to sort this out.

All of the first watt amps need to be paired in a quality system,the better you feed them the better they perform.

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. 
Yes speaker matching with first watt amps is very important,
going to be interesting to see the upcoming new ones that Nelson is working on.

I've heard through the Diy community that he is working on a F8 and also a Sit3.

Also pass labs has the Xa-25 out,but I haven't heard that one yet,
might be a good option for lesser effficent speakers.

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. 
Great information!  You clearly know your way around electronics. I am going to attend Burning Amp this fall down the road in San Francisco to meet "Papa" again.  It is funny that you mention the KEF LS50's.  I bought them shortly after they came out and drove them with a 175 wpc Plinius 8200 MK II integrated amp and the amp rather struggled to drive them properly. There is no chance a FW amp would be compatible with them.

However, with OB speakers like my Spatial Audio Holograms, DeVores, Omegas, etc, the FW amps are a great match it seems to me.  Forget about Martin Logans and the like, but a pair of Klipsch speakers would sign with these amps.  You and Kenny wisely pointed out in your posts the compatibility issues one needs to be aware of to get the the best performance from the FW variants.  


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. 
My very simpleminded/layman observation is some amps put out more power as the load is reduced which implies a near constant voltage delivery. Others behave the opposite which seems to imply a constant current source. The former is better for low impedance loads whereas the latter better for higher impedance speakers.This is also evident when comparing solid state and tube designs. I know I’m stating what most folks already know but a good layman view of the amplifier differences and how to find matching speakers.
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".
Unless you are talking about the Zotl40 which has the output tubes directly coupled to the load taking transformers and capacitors completely out of the equation.

DC coupled valves are out there, but they're not at all common. I'm a little curious to try tubes with the F5.
You may want to look at the LTA MZ2S preamp,
I have paired this with several types of SS amps with great results.

Probably going to build something. No idea what at this point. 
Did I miss your post regarding your impressions of the LTA amp you acquired?  Kosst, I am using a Don Sach's 6SN7-based preamp with my F5 and can say that it is a match made in heaven.  I have plenty of gain with my Spatial Audio OB speakers and zero noise.  Mark
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!