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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
whitestix
kalali's avatar
kalali
1,645 posts
 
Love to hear all the excitement. How does the in-production F7 compare to out of production F5?
kdude66
1,198 posts
 
kalali,

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.

Kenny.
kosst_amojan
2,095 posts
 
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. 
kdude66
1,198 posts
 
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.


Kenny.
kosst_amojan
2,095 posts
 
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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