[Freo-1]

Recently came across an amp design from Papa Pass that peaked my interest:





This little gem is a Class A zero feedback amp.  From the owners manual:

"  The First Watt M2 is a two channel audio power amplifier delivering 25 watts per channel. It uses JFET and MOSFET transistors operating in Class A mode without negative feedback so as to produce a more musically life-like sound."

" The M2 is a simple circuit whose output stage consists of a pair of complementary power Mosfet followers biased into pure Class A mode. These devices are the current gain stage of the amplifier, very linearly following the musical signal produced by the voltage gain stage."

"Actually, this is the easy part, and I've done it many times before."

"The voltage gain stage which precedes the output followers is where you usually have the most problems with distortions of various sorts. This is because the mode of operation required (common-cathode, common-source, or common emitter) is the least linear, requiring both voltage and current gain. There are numerous approaches, but it usually boils down to either accepting higher distortion or more complexity."

"The M2 is different. It uses a passive voltage gain device, an auto-former. Like a transformer, an auto-former is a magnetically coupled circuit which changes the ratio of voltage and current while keeping the same amount of energy. In this case, it steps up the voltage by a factor of 5 in exchange for delivering 1/5 the current at the output. Since the input impedance of the output stage devices is very high, this is an equitable exchange."


I think this a somewhat unique and refreshing approach to obtaining the "tube sound" characteristics advantages without the limitations known with tubes. 


Here is the explanation of the circuit design from the manual:

"  So here we have a no-feedback Class A FET power amplifier with transformer voltage gain. Does it have any similarities to no-feedback Class A tube amplifiers with an output transformer? Yes and no.
It does have some of the qualities that SET aficionados look for. The auto-former does bring its own signature to the sound, although in smaller doses than we usually experience. The midrange has a lush, warm character that I associate with tubes and transformers, but subtly so. Part of this is due to the limited bandwidth of transformers, and part of it is the lack of feedback artifacts."

"On the other hand, there is considerably less harmonic distortion than with no-feedback SETs, and if you really do like that distortion then the M2 might not be what you want. Also, there is more power than usual and a relatively high damping factor, so the M2 is different in that regard."

"The M2 strikes a balance in the performance space between the extremes of big behemoth solid state power amplifiers and little tiny single-ended triodes."


I've always been a big supporter of Nelson Pass designs, and his support of the DIY community as a whole.  This little gem has just enough power to push a wider variety of speakers than most of the low power SET designs. 

Looking forward to trying one out next week.  Will be using the M2 with a pair of speakers that are 90 db/w at 6 ohms.  Should be fun.

If this goes well, may look at building a F5 clone for fun.   Will also look at a similar design with J-FET outputs.

[JoshK]

Interesting.   I'd love to hear it.

[Freo-1]

Interesting.   I'd love to hear it.

I'm going to post my observations after I've put it through it's paces. 

I've always had a soft spot for the "tube sound", but have never been a fan of their limitations.  So, been using class A solid state amps, which sound excellent, but not quite like a well made tube amp.  This little gem is suppose to come very close, and still be reliable and long lasting. 

The other cool thing is that most of the First Watt amps can be cloned fairly easily, and Papa Pass is great about offering help.  He helped me with some issues I had with the bias circuits while resorting a Threshold 400A.  Would up doing a total rebuild on the unit, and could not believe how much better the amp sounded after rebuilding the driver boards with new, high quality audio parts.

[tull skull]

If you don't mind, I think many of us would love to watch over your shoulder as you construct the amp. Well, at least I know I would!  Wadaya say? Can we tag along? Please post progress if possible!

[zeke]

Seems i have read that the J2 sounds more "set like" than the M2.

If that is correct, the J2 is the one i'd like to hear.

[Freo-1]

No worries.  If the M2 sounds as good as I think it will, I'll post pics of whatever First Watt clone that gets built.

Here is a pic of of the re-built 400A:

[Lyndon]

More photos, Freo, more!
I'm with Josh in hoping you do a step by step photo journal of building your amp.
I love my Aleph, still have the boards for a mini Aleph, and the Peter Daniel boards for the F-2?
Lyndon

[Freo-1]

Seems i have read that the J2 sounds more "set like" than the M2.

If that is correct, the J2 is the one i'd like to hear.

I think it's the other way around. (Depends what one reads).   

I'm keen to listen to a J2 as well.  The reason I went with the M2 was power output, as it is 40 wpc @ 4 ohms,  while the J2 is more like 13W at 4 ohms.   

[mcoaggie]

+1 on a build out thread!  Sounds like you know what your doing.

I bought an Aleph clone and its my favorite amp so far.
 
I've been tempted to try to build an F5.  I've never built anything before and its a little daunting.  I hadn't seen this M2 before.

FYI...Just noticed an M2 pop up on the 'gon.

[zeke]

I think it's the other way around. (Depends what one reads).   

I'm keen to listen to a J2 as well.  The reason I went with the M2 was power output, as it is 40 wpc @ 4 ohms,  while the J2 is more like 13W at 4 ohms.

iirc, Mark at Reno hifi told me that  --  been awhile tho -- i could have it bass ackwards ....  lol

[Freo-1]

There are not too many designs available that provide an amplifier with passive voltage amplification and no feedback.

To be able to get the sonic benefits of "tube like sound" without tubes is a pretty neat achievement.   This amp is touted to be one of the most reliable designs out there.   

The First Watt approach is really cool.  It allows audiophiles to experience different sonic results based on varied amplifier design approaches. 

To me, amps like those from First Watt are testaments to the fact that “amplifiers can and do sound different”, based on a whole host of factors.  There a number of articles written by Mr. Pass that explain the effects of feedback, how to get circuits and devices to operate in their “sweet spot”, class of operation, etc. all of which help to explain how they work, and some supporting information as to why this happens.

Best of all are the articles on how to DIY projects like the F5 turbo.  (very cool).

[Freo-1]

The M2 arrived today.  First impressions are very positive.   For 25 wpc, it seems to have plenty of drive for the speakers (91 db at 6 ohms).

Will post observations after some time listening, but one thing that is obvious:  Anyone who thinks "all amps sound the same" would have to re-think that after listening to this amp for awhile.

[TONEPUB]

Great amp, love mine.  It will drive darn near everything.

[opnly bafld]

[

[roscoeiii]

Great amp, love mine.  It will drive darn near everything.

How inefficient a speaker have you tried with the M2 TONEPUB? Surprised to see "drive anything" in a thread on a 25WPC into 8 ohms amp.

[Freo-1]

Great amp, love mine.  It will drive darn near everything.

Glad to hear it.    So far, I'm really happy with this.


What are your impressions of the M2 compared to the F5?  (I'm assumimg you have you have listened to the F5).

[Freo-1]

After listening to the First Watt M2 for about a week now, let me state that this is perhaps the most enjoyable amplifier I’ve ever sat down and listened to.  This amp is proof positive that “all amplifiers do NOT sound the same”.  With the right speakers, some types of music from this amplifier often sound closer to a live event than one will generally experience with an audio setup.

To be sure, this amp is not for everyone.  It outputs 25 wpc@8 ohms, and 40 wpc@4 ohms.  So, as long as the target speakers have sufficient sensitivity, the amp will play music as loud as most of us would listen comfortably. 

What this amp does right (and not all that many do) is the ability to re-create the entire frequency spectrum without any undue emphasis for any one area of the spectrum.  The bass is extremely natural, and has sufficient heft, weight, and damping to sound accurate, without calling undue attention to it.  The midrange has a clarity and natural harmonic tonal character that VERY FEW amps get right.  Vocals from the amp sound more natural than one generally hears.  String instruments very realistic with the right recordings and low level details are easily heard, but are heard in the correct proportion.  Some equipment can be “too analytical” when one hears more low level detail than one would hear in a live setting.

Where this amp really shines is the upper registers.  The treble from this amp is easily some of the best I’ve heard from any amp at any price.  There is no trace of HF emphasis or solid state “crispness” that plagues many amps.   
I would suspect that may tube audiophiles would think they were listening to a new state of the art tube amp when hearing music played through the M2.  The passive transformer gain stage, which provides 15 db of passive gain, has a lot to do with the sonic signature of the M2.   There is a total lack of glare with the M2.  This is one of the very few amps on the market where the listener will not experience any listener fatigue.  I think the fact this amp is a Class A unit with zero feedback also is a big reason why the amp sounds so natural. 

From a noise standpoint, there is none discernible with the speakers I’m using (91dbw@ 6 ohms).  The amp is very quiet (quieter than most tube amps by a fair bit). 

In summary, this amp is tremendous performer.  It’s easy to see (and hear) why the M2 is popular in the Asian market.  If you are looking for an amp that provides the best aspects of tube amp reproduction, but still desire a more natural bottom end that is the purview of solid state (not to mention reliability), then give the M2 a try.

You will not be disappointed.

 

[brj]

Thanks for the review, Freo-1.

Have you spent enough time with any other First Watt or Pass Labs amps in your system to allow some comparisons?

[Freo-1]

Thanks for the review, Freo-1.

Have you spent enough time with any other First Watt or Pass Labs amps in your system to allow some comparisons?

No worries. 


I was a little late coming to the First Watt camp, as only recently discovered its virtues.  However, now that I’m there, I’ll be staying in that camp for some time to come.

I currently own a Pass Labs X2.5 preamp, and that is what I’m using with the M2. The gain from the X2.5 is set to low.  The source for the music is a Modwright Oppo BD-95 tube unit.  The speakers are Source Technologies 277SE. 

I have heard a couple of other offerings from Pass Labs, but not in an environment that I sufficiently trusted to use for evaluation.

After having the M2 in the system for awhile, am now considering making a pair of F4 amps vice an M5.  The high gain from the X2.5 will work just fine with the F4, and could get 100 watts a channel as monoblocks. 

[Freo-1]

More photos, Freo, more!
I'm with Josh in hoping you do a step by step photo journal of building your amp.
I love my Aleph, still have the boards for a mini Aleph, and the Peter Daniel boards for the F-2?
Lyndon

OK.

Here is a photo of the inside of a Threshold S300 which was restored, and converted to a SA/3. 

It turns out the driver boards for the S300 and SA/3 were the same, and the optical bias model power transformer supports both rail voltage.  So it was a simple matter of using the alternate voltage rails, bump up the bias, and change out the rail fuses, and presto, a 50 wpc Class A is born.