PX-25 tube amp with Merlin VSM speakers?

I love my Merlins and am mulling over an amp upgrade. I've heard of people driving Merlins with low powered SET and OTL amps, but, for example, the Art Audio PX-25 amp is only 6 WPC. Is that enough juice?

Thanks for any insights.
IMHO, Merlin speakers are tube friendly not SET friendly. When I owned Merlins, I liked them best with 50+ tube watts.
I had great success running VSMs (SE) with the Cary 300b mono amps in a small to medium sized room. Those amps were almost 15 wpc, so I can't predict the results you'd get with your proposed combination, but that system was a wonderful marriage of VSM and low(ish) powered SET amps.

Neglecting room effects, it can be calculated that at a 10 foot listening distance 6 watts into two 89 db 8 ohm speakers will result in an SPL (volume) of 90 db. That will be loud enough for most listeners with most recordings, but it will certainly not be loud enough for many listeners with recordings having particularly wide dynamic range (such as many classical symphonic recordings).

Although many and probably most pop and rock recordings are compressed to dynamic ranges (the difference in volume between the loudest and the softest notes) of less than 10 db, lots of music in other genres can have dynamic ranges of 30, 40, or even 50 db. A 30 db dynamic range means that 1,000 times as much power is required for the loudest notes as for the softest notes. A 40 db dynamic range means that 10,000 times as much power is required for the loudest notes as for the softest notes.

Also, I would by no means count on the 6 watt SET as being able to sound its best when putting out close to that number of watts.

Finally, I note that the **minimum** power recommended at the Merlin site for your speakers, for a tube amplifier, is 17 watts. Unless essentially all of your listening is to dynamically compressed recordings at moderate or low volumes, why "upgrade" your amp to something that is likely to disappoint on at least a few (and perhaps many) of the recordings you listen to?

IMO. Regards,
-- Al
Excellent response Al , very informative .
Is there some type of formula , or better yet a chart that will offer up demands for different configurations ?
Thank you .

Merry Christmas
Yes, very informative and I also appreciate the specific experience mentioned by others. In looking at various options it seems like I might be better off looking at a 300B SET amp. I'm also curious whether Bobby P. has tried his Merlins with 300B and/or PX-25 amps. My room is fairly small (18X12 with 8' flat ceilings) and 90db would likely be enough for most of my recordings (rock, blues and jazz mostly) nearly all of the time, but I'm relatively OCD about these things and during times when I wanted to listen to things fairly loud, I would probably spend time listening to see if the amp was straining rather than enjoying the music.
Thank you Saki.

I wouldn't be surprised if there were online calculators for figuring this kind of thing, but I'm not specifically aware of any. I just do the calculations myself, using a scientific calculator that can do logarithms. If you don't have one, the calculator that is built into Windows (under "Accessories") can be set to scientific mode via its "View" menu.

These are the steps:

1)Compute how many db greater than 1 watt the amp's specified power rating is, based on the relation db = 10log(P1/P2), where "log" is the base 10 logarithm.

In this case 10log(6 watts/1 watt) = just under 8 db.

2)The OP's speaker is rated at 89 db/1 watt/1 meter. Therefore 6 watts will produce 89 + 8 = 97 db at 1 meter.

3)Add 3 db for the second speaker. (If identical signals are received by both speakers, i.e., if the image is centered between them, and if the listener is perfectly centered, I believe that a 6 db increase would result. But it is safer to count on a 3 db increase, reflecting twice as much power being delivered in total compared with the power being delivered to one speaker). 97 + 3 = 100 db at 1 meter.

4)Calculate the reduction in SPL at distances greater than 1 meter as 20log(1 meter/distance in meters). The example I described of a 10 foot listening distance corresponds to 3.05 meters. 20log(1/3.05) = approximately -10 db. 100 - 10 = 90 db, as I indicated.

Some caveats:

1)For planar speakers, such as Maggies and electrostatics, SPL will decrease at a significantly slower rate than this as distance increases. The 20log(1/distance) figure applies to box-type speakers.

2)Speaker sensitivity/efficiency is often spec'd based on an input of 2.83 volts rather than 1 watt. 2.83 volts into 8 ohms corresponds to 1 watt, so the calculation comes out the same either way in the case of an 8 ohm speaker (assuming the 8 ohm impedance spec is reasonably accurate). 2.83 volts into 4 ohms corresponds to 2 watts, though, which is 3 db greater than 1 watt. So if the sensitivity of a 4 ohm speaker is spec'd on the basis of an input of 2.83 volts, subtract 3 db from its specified sensitivity to determine the SPL the speaker will produce in response to 1 watt at 1 meter.

3)As I indicated in my previous post, these calculations neglect room effects.

This all may sound fairly complicated, but it's pretty easy once you get the hang of it.

Happy holidays to you as well! Best regards,
-- Al
I am listening to my Art Audio PX-25 play as I type - fabulous, but with horns and not the Merlins. Instead of getting a new a new amp I suggest auditioning several different A/C wall outlets such as Furutech, Synergistic Research, and Oyaide. Very rewarding exercise.
I am listening to my Art Audio PX-25 play as I type - fabulous, but with horns and not the Merlins. Instead of getting a new a new amp I suggest auditioning several different A/C wall outlets such as Furutech, Synergistic Research, and Oyaide. Very rewarding exercise.
I am listening to my Art Audio PX-25 play as I type - fabulous, but with horns and not the Merlins. Instead of getting a new a new amp I suggest auditioning several different A/C wall outlets such as Furutech, Synergistic Research, and Oyaide. Very rewarding exercise.
12-22-14: Dazzlingmd
I might be better off looking at a 300B SET amp.

I've owned the Art Audio PX-25, AA Jota, and Welborne
Labs DRD 300B SET monoblocks at the same time. There is no real appreciable difference between the respective 6wpc and 8wpc of the PX-25 and 300B on medium sensitivity 90db/4ohm Jean Marie Reynaud monitor speakers.

Actually, there was no perceivable difference in power on
my much larger and more efficient 95dB/8ohm Cain &
Cain IM-Bens.

Which is better - PX-25 or 300B? Neither, it's just two
different views of the same sunset. Personally, I'd take
the PX-25. It is one of the most spectacular sounding
tube amps I have ever heard. One of the pro reviewers
coined the perfect term "as if notes are lit from
within". But, the 300B monos were just as alluring
in a different way.

I know a lot of reviews, and users, have claimed in the
past the Merlins and SET are a match made in heaven. I
think it's because of Merlin's fairly benign load of 6.5-8.0
ohms, with the two-way crossover zone at ~16 ohms.

I'd give the used PX-25 for sale here a try and see if it
works well in your sized room and musical priorities. If it
works, you will never hear anything like again. It may
need a new set of PX-25's because of low level hum in
one tube, which may not be audible in low/med
sensitivity speakers. That will set you back $400-500

If not, look for a Art Audio Carissa or Jota
Very nice system Dazzlingmd. Tell me, do you feel the need for a subwoofer using those Merlins? I am currently looking at speakers, and the Merlins are on my list, but I am used to running full range speakers, and my fear is that the Merlins low end performance will dissapoint. I've never owned a 2 way speaker in my main listening room, at 15 X 25 X 9 it is about medium in size, but I do not want a speaker that will require a subwoofer.

I'm also looking at DeVore Nines, Verity Audio Parsifal Encore/Ovations, Reference 3A Grand Veena, and Sonus Faber Cremona's. As you can see, the Merlin's are the only 2 way design that I am considering, probably due to the potential of the BAM. While I am not a bass freak, I do like a solid bottom end.

Thanks everyone and yes, the Art Audio PX-25 amp for sale here is tempting me a bit, although I don't need an attenuator (definitely want to keep using my Allnic L-3000 preamp, which strikes me as a good fit with a PX-25 given the incredible level of detail that the Allnic delivers), and I'm not sure about the price if in part it relates to the attenuator and if I'm going to need a $500 pair of replacement tubes. Mulling it all over, though.

John, my room is fairly small, but I do not feel the need for a subwoofer. In recordings that are somewhat bass-heavy in my opinion (e.g. Beatles Abbey Road, Stevie Ray Vaughan vinyl remasters-Couldn't Stand the Weather), the bass is full and powerful. In recordings that are somewhat bass-lean (e.g. some of the Doors vinyl remasters) the bass sounds a little lean. In other words, to my ears in my room the Merlins produce tight, musical bass that is appropriately powerful when that's what's present in the recording.

I could see how in a larger room you might need a sub to have appropriately full and rich bass.

If you're near Denver at some point, you should come by and have a listen and see what you think.
Hello Dazzling MD,

Here is my two cents with respect to your question.

I have owned the exact speakers that you have currently and powered them with a variety of amps. I would offer the suggestion that you investigate the AUDIO MIRROR 45 watt SET amps. They most definitely would have enough juice to power the Merlins to whatever levels you might desire.

I have a room that is similar in size to yours at 14' x 20' and the AM's really made beautiful music with the Merlins.

They are basically bulletproof amps (mono blocks) as they are self biasing as well as they will shut themselves down if overdriven. I never got to that point with them but did with a pair of Harbeths that I owned which are less efficient than the Merlins.

The AM's use a pair of 6c33c power tubes (which is what BAT and Joule use) and a 6sl7 for an input tube and 6sn7 for a driver tube. The amps is very receptive to making a substantial improvement in sonics from stock with adding NOS 6sn7's and 6sl7"s. I would also recommend the VT99 as a driver tube which is even better IMO than the 6sn7 but it does require an adaptor.

These amps are hand made in Houston by a gentleman named Vlad Bazelkov. He was great to work with and made mine custom with balanced instead of single ended inputs as well as adding V caps instead his stock caps.

These amps really knocked it out of the park with a huge soundstage and excellent dimension and detail without a hint of harshness. You already have a great front end with the Allnic so the sonics should be spectacular.

I hope that this gives you something else to think about as an alternative to the 6 watt SET you had inquired about.

All the best,

FWIW, I took a long, hard look at that AA PX-25 amp the last time it was up for sale when I was working on my amp changes (ended up deciding to build an Audio Note Kit 1 300 B amp). The seller seems to have dropped his asking price $100 this time around.
I'm sure it's a gorgeous sounding amp. It sure looks beautiful! I'd only suggest you do some research on the price of PX 25 tubes. They are not the most readily available tube out there, and I recall, as you say, that current production tubes (KR's?) were going for around $500 per matched set. So you kind of have to figure that into your cost.
Good luck!
Thanks to everyone for all the helpful information and suggestions. I have a lot to ponder on this Christmas Eve as my journey for perfection continues.

Merry Christmas and happy holidays to all!
One more question, Rebbi. How did it work out with the Audio Note kit? Is it something that requires a lot of technical ability? And what is your impression of the sound?
First of all, great moniker! ;-)
Second, I'm in the process of building the kit right now. I wouldn't say
that it requires great technical ability, per se. Mostly it takes a few good
tools (needle-nose pliers, wire cutter, wire stripper and decent soldering
iron, plus a phillips head screwdriver). Manual dexterity is also a plus,
because you are handling some fairly small parts in some places.
Finally, I think that building a kit like this is like cooking from a recipe. If
you enjoy the process of cooking (laying out the ingredients, mixing
things together, etc.) then you'll be a good cook. Similar idea here. If you
like making things and take pleasure in the process of the build, you'll
take your time and do a great job. If you're in a hurry to get to the end
result, you won't have as much fun and you'd be more likely to make
mistakes that you'll have to troubleshoot and fix later on.
I, myself, enjoy working with my hands in this way and I'm having a blast
building this thing. To be honest, as much as I'm looking forward to
hearing what it sounds like, I know I'll have mixed feelings when the last
part is installed because the build process itself is so satisfying.
For more info, I am actually blogging the build process, with lots of
See this.
Thanks, Rebbi. Very interesting and intriguing story and thanks for blogging it! I'm curious to hear your impressions of the sonics once it's all done.
It's a minor mod to remove the PX-25 attenuator from the signal path.

But, your best bet is probably going with one of the higher-powered Art Audio SET amps like the Jota(20-30wpc, depending on output tube) or 16wpc Carissa. Even their PP amps like Concerto or Tempo would be glorious
Agree with Darkmoebius. I had the PX25 for a short while. Loved what it did but was just a few watts short of driving my Zu Definitions to max capabilities. I ended up with the Carissa at 16 watts, and that did it for me.
Anyway, (FYI) there is an Art Audio Diavolo up for sale on that other popular site for $2300.00. That comes in at 13 wpc.
FWIW, I had the Art Audio Carissa which had the attenuator. On it if you opened the attenuator completely it was removed from the signal path. I used it in this manner with a pre-amp. I believe all the AA amps are configured this way that have the volume control.

The Carissa was ont of the best amps I have owned or heard. I wish I could afford to get another one. I really miss that amp.
Ha anyone compared the Carissa to the Diavolo? I've heard great things about the Carissa but don't know much about the Diavolo.
I've never heard the Diavolo but FWIW, I've spoke with Joe Fratus (he used to be the man behind Art Audio line in the US) a couple times and I got the impression that the Diavolo was his favorite amp in the lineup.
Dazzlingmd, check out the reviews at 6moons. They have write-ups on the Carissa, Diavolo, Jota and Symphony.

The late John Potis, one of the best reviewers wether on the Internet or print, wrote a perfect review of the Carissa and described it sound and character beautifully. He absolutely nailed it.

Also, I do believe Brf's comment has some value to it. Good luck with your amp search.
Brf nailed it. Quite simply, the Merlin is not suited to most SETs due to their efficiency. Dazzlingmd, your room is only a little smaller than mine, and my speakers are almost 10 db(!) more efficient, yet I find that its nice to have the extra power if I really want to enjoy the speakers. Take that extra 10db away (equivalent to 10x the power) and its just not going to work that well.

If the speaker requires that you are push an SET past about 20% of its full power on peaks, you will not be hearing all the amp has to offer and likely not all the speaker has to offer either. In this case, because the speaker is a medium efficiency, you really need more than just 7-12 watts to really make it play. The bigger you make an SET, the more the bandwidth is degraded by the output transformer.

The advantage of an SET is that it has distortion that linearly decreases to unmeasurable as power is decreased. When you remove distortion, detail is revealed. This is why SETs are known for their great 'inner detail'. However the flip side of the coin is if you want to get everything out of the amp that it really can do, you should not exceed 20% of full power, since above that level the higher ordered harmonics (to which the ear is very sensitive) come into play. That is why the best SET installations often include horn loudspeakers.

You may encounter people that may try to convince you otherwise, but IME (The Merlin is the most popular speaker used with our M-60 amplifiers) you really do need some power to make the speaker sing, even if only a moderate amount. The speaker is very revealing and has surprising bandwidth; what I am saying here is that the performance of the amp won't be realized neither will that of the speaker if you use a low powered SET on it, unless you are in a very small room.
Interestingly, upon a closer review of the specs for my Primaluna Dialogue Premium amp, I'm currently running the Merlins on 12WPC as I use it exclusively in triode mode (which sounds significantly better to my ears than ultralinear), with NOS Siemens RFT EL34 tubes. That gets plenty loud in my small room, has solid bass energy and drive (although I'm not "all about the bass"--more of a treble and mid-range guy, but musical, tight bass is still important). Frankly, my setup sounds good enough right now that a significant part of me wants to just leave things as-is for a while.

But now that I have a dedicated, relatively small room, I have been wanting to test out the magic of the SET sound that many people talk about, just to see if things could be further improved. If I had to point to a flaw in the sound right now, it would be that my system is a little bit too brutally revealing of any flaw in a recording, and accentuates distortion that is sometimes present(for example the notorious distortion at about 5:30 into Stairway to Heaven is quite prominent to the point of distraction, both on the vinyl and the digital files), and overall the sound is ever so slightly forward in the treble range, which is noticeable on some recordings where the treble is a little on the shrill side to begin with.

Atmasphere, I have wanted to try one of your amps in my system for a while as well (and loved what I heard from them at RMAF this year), but people tend to describe the OTL sound as very detailed, revealing and neutral, which is what I have now in spades, in contrast to the typical SET sound which is a little warmer in the middle and slightly softer on top. From those descriptions, trying out an SET amp would in theory be a better fit for what I'm looking for.

In a perfect world, I would demo a bunch of amps in my system of course. The Art Audio Diavolo on that other site is tempting, and claims to have special order upgraded transformers from Art Audio, but the reviews of that amp fairly consistently say it's a little forward in the treble department, which is probably not what I need. In going with just the description of reviewers, the Carissa seems to be a better fit for what I'm looking for. And the 6WPC PX-25 seems to be a higher risk proposition, although now that I know I'm currently only using 12WPC, it's still a little tempting.

Oh, the dilemmas presented by this hobby and all the options. And I know, I know...first world problems...
I just rechecked and Bobby recommends 17 tube watts minimum for the VSM's. What about a parallel SET that is still single ended but pumps out more watts? They are said to give up little of the SET magic but may give you the power you need. Or how about Decware's Zen Torii? Not a SET but (according to Decware) possessing most SET virtues, sonically.
Or, talk to Bobby. He lives to give advice on stuff like this. :-)
Thanks, Rebbi, for adding to the dilemma with the suggestion of another amp that sounds pretty amazing from the reviews and people's experiences! :). My gut is that I would be unlikely to change to another EL34-based push-pull amp as that's what I have now (and am very happy with how that sounds), and that I'd be more likely to try something different, but wow, the buzz on that Decware amp is through the roof.
Dazzlingmd, the difference in a P-P triode amp making 15 watts and an SET making 15 watts is that if you want to hear what the SET does, its likely you should not push is past 3 or 4 watts. Otherwise the higher orders come into play.

(The higher-ordered harmonics behave as loudness cues to the human ear/brain system, causing the SET to sound more 'dynamic' than it should be for such low power. This is something you will often see in reading reviews of SETs- its a psycho-acoustic interaction; once you realize that is has to do with how the amplifier makes distortion, its also easier to hear the amp doing it, so in a way I may have ruined it for you by writing this...).

With a push-pull amp this is less likely. So you get a greater percentage of usable power.

With our amps, as they have little or no feedback but are a form of push-pull, they too have a greater percentage of usable power, but are also quite relaxed in the mids and highs, in fact more so than the typical SET because they are lower distortion. Yet they also have more detail, owing to the lack of an output transformer. The combination of our M-60 on the Merlin is quite good, which is why the Merlin is one of the most popular speakers used with the M-60s. What is nice is to have 'highly detailed' in the same place at the same time with 'relaxed presentation'.
I ended up rolling the dice by pulling the trigger on the Diavolo. At that price it was worth a shot and if it isn't an improvement I can sell it for what I paid and reassess. The Diavolo is supposed to have enough power to easily drive any 90db speaker or higher with authority, and mine are 89db in a small room with a flat impedance curve so I'm hopeful that it will be enough power.

It should be here by the end of next week and then an amp shootout will be in order!

Also, if anyone has thoughts on the difference between the 32b tubes that come with the Diavolo versus the 300BXLS that it can also apparently take, I would be interested in the sonic differences.

Thanks to everyone so far for all the helpful information and ideas.
Congrats! Looking forward to your impressions. From my time with the Carissa I think Art Audio amps are very fine music makers. Their build quality and aesthetics are outstanding as well.

Yes, do tell us how that combination sounds! That's supposed to be a fabulous amp. Enjoy it!
Congrats on your purchase. I was wondering if that was you who made the purchase. I had it in my watch list and noticed it sold the other day. You got it at a great price. Please let us know if the power suites your needs.
For anyone who is interested, I was able to spend some quality time with the new Diavolo this weekend. I had a friend and fellow audiophile over, and we A/Bd the Primaluna with the Diavolo, playing the same suite of songs (some vinyl, some SACD) through the Primalune, the Diavolo with it's stock KR Audio VV32B tubes, and also through the Diavolo with a pair of Emission Labs 300B-XLS tubes.

Overall, this incredibly fun and interesting science/audio experiment/shootout confirmed that the Primaluna amp, particularly with the NOS tubes I was running, is a great amp. And if I were 15 years younger and in the habit of hosting large parties where I would be playing hard rock/heavy metal music at ear bleeding levels, I would keep the Primaluna. But alas, the Primaluna is, as of this morning, for sale on this site.

In short, the Diavolo doesn't "trounce" or "blow away" the Primaluna, but there are noticeable differences, particularly with the 300B-XLS tubes. It would have been a closer call with the stock VV32B tubes. The Diavolo with the 32B tubes had, hands down, the best bass I've ever heard in my system. Tight, full and punchy, just unreal in the bass department. The midrange and treble was very detailed and very nice, but not noticeably better than the Primaluna from the middle up, in sound, imaging or soundstage.

Then we put the 300B-XLS tubes into the Diavolo and listened to all the same songs...

The difference was immediately noticeable and quite striking. I kept hearing myself say "I've never heard that song sound that good." There was definitely a loss in the bass department, but it's one of those things that you don't miss if you don't have it, you just enjoy it if you do have it. And the midrange and treble were silky smooth, detailed, rich and full, and in particular voices and cymbals were more real than anything I had ever heard. I don't know if the Diavolo is truly "neutral," but honestly I think my system was incredibly neutral and revealing to a fault before I got the Diavolo, and the Diavolo with the 300B-XLS tubes warmed and smoothed things to a point where it's just beautiful music that makes me want to listen to more and more of it.

One shortcoming is that with heavier tracks (Led Zeppelin and Rush for example), my system still sounded great with the Diavolo, but when I cranked it up to super loud levels there was noticeable distortion and I had to turn it back down. According to my SPL I could get it to about 90-92 DB without distortion, which frankly for me is plenty, but I have a small room and I can see that the limitation could be an issue for some people particularly in larger rooms.

Tonight I listened only to the Diavolo with the 300B-XLS tubes without any switching or A/Bing, and had to drag myself away after a few hours because I was enjoying it so much.

I think the Diavolo is a world class amp which makes my system sound better than it ever has before. The difference between it and the Primaluna with the NOS tubes is not dramatic, but it is signiricant, particuarly with the 300B-XLS tubes. I look forward to listening to more of my music collection through my system.

Maybe I can be done upgrading/tweaking for a while now? :)
Congratulations on getting a wonderful amplifier. I've
had the good fortune to use many different 300b tubes in
my SET amplifier. My two favorites are the Takatsuki and
the EML XLS 300b. I also have the AVVT SL 32B and this is
an excellent tube, however I find the former two more
natural and involving. I was in your shoes over 5 years
ago in going from a fine push pull amp to a SET. There's
no going back for me for the same reasons you stated in
your post. I gained deeper involvement and more musical
emotion,soul and realism. When I'd reinsert the push pull
amplifer back into my system for comparison the sound
became more mechanical and less fluid relatively
speaking. Simply put, music sounds more live like and
truthful/believable with the SET in use. I understand
your joy.

Very well said. That captures my experience quite well. I was actually very happy with my push pull amp, but there's a certain something with the SET amp that I'm really enjoying. It's not a major difference or anything that I could have ever known I was missing before, but I'm enjoying my music more than ever right now, which is what it's all about of course.

Now if I could just be happy listening and not start wondering what things would sound like if I could try out one of Atma-Sphere's amps, or with the next higher level Allnic phono stage, and so on and so on, forever...