World Premiere!
Antique Sound Labs Monsoon DT MKII Tube Amplifier
I could listen to this amplifier for days on end...
Review By Ron Nagle
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  The dictionary defines a Monsoon as a wind system that reverses direction seasonally and causes wet and dry climate conditions in Asia. I cannot tell you what is happening in Asia but I can tell you the Antique Sound Labs (ASL) Monsoon DT MKII in my living room is certainly warm and dry. My Monsoon, or rather Monsoons, are a pair of mono block power amplifiers each with twelve vacuum tubes per chassis, that is a total of twenty-four radiating and glowing glass globes. If you take a broad overview you can divide power amplifiers into two major classes, Tube and Solid State. In the world of tube amplifiers there are generally many more sub classifications than you will find in solid-state devices. Tube amplifiers can be grouped according to the type of circuit design, the type of output tubes employed and the class of operation. Why is that? The answer is that all of the preceding factors separately or in combination can markedly affect the way these amplifiers reproduce music.

Understand that even in the world of tube loving audiophiles there are even finer gradations of taste. On one hand there is the purist triode faction believing that old style directly heated tubes like the 2A3 and 300B powered amplifiers convey a more true to life sound. This type of circuit topology is controversial because it harkens back to the very beginnings of tube amplifiers. I believe that these vintage design amplifiers would have been a distant memory if not for the love and care lavished on them by Japanese and Chinese audiophiles. Contrasting this is a more modern class of amplifiers using beam power tubes like the EL34, KT88 and the widely used 6550. The five-element pentode family of tubes is valued in part for their superior delineation and greater frequency extension.

Today the majority of Hi-Power tube amplifiers use the efficient five-element pentode design and the EL34 is one of these. The ASL Monsoon DT MKII is uniquely designed to optimize every thing that is good about the EL34 tube. Note that the ASL Monsoon DT MKII is an improved second-generation amplifier. The original (pre 2007) Monsoon used KT88 pentodes for power and did not have a digital bias meter and did not have a separate standby switch and there was no provision to switch between pentode and triode operation. This very much-changed MKII version incorporates the circuit design used by another highly regarded Antique Sound Labs Amplifier called the Hurricane.

The Subject
Moving right along, the ASL Monsoon DT MKII monoblock amplifiers price out at $4,000. I have no doubt that if they were made in the United States they would cost nearly three times that price. They employ an Ultralinear design invented by David Hafler and Herbert Keroes of the now defunct Acro Products Company. Also called Distributed Load Amplifiers for those curious. The Ultra linear concept is fully described in their 1952 US Patent Application 2710312, go to www.uspto.gov.  The amplifiers arrived in two double walled cardboard boxes each cushioned with Styrofoam slabs. Each amplifier requires an application of muscle to move them each one weights 57 pounds. I unpacked them and set up on the floor between my speakers. It was immediately after that when I ran into my first spot of bother, namely the owner’s manual. The manual is very obviously a translation from Chinese into English and while I have experienced translation problems like this before this book is a hallmark of bad composition. As you open the Owners Manual the first sentence in the setup instructions reads like this.

Step 1. Open carton: "For avoiding vacuum tubes damaged, please don’t make carton being upside down". 

Just below that instruction is a diagram of the amplifier that has very small hard to read numbers corresponding to a list of the functional parts. It is not just that you have to read each line more than once there really is simply not enough information in this manual. At the Divergent Technology web site the specification given for power is 120 wpc the owner’s manual lists this as 100 wpc. I have seen this disparity before and I believe this is because the line voltage in China is different than in the United States. The rear skirt of the chassis has an IEC socket three speaker binding posts that will accommodate 4, 8, and 16-Ohm speakers. Additionally you will find a power line fuse and connectors for balanced and unbalanced inputs from a preamplifier and a toggle switch to select between the two. There are four transformers on the chassis of each monoblock amplifier. They are specified as an output transformer, low power transformer, high power transformer and a power supply choke.

When you unpack you will find the tubes stored under the amplifiers wire tube cage. The twelve tubes are packed separately each tube in a numbered box. Each box has a small paper tag with a number that corresponds to a tube location. Unfortunately I found that two of the tags had fallen off. On the left side of each chassis you will see a nine-position switch used to select one of the eight bias potentiometers. Each of these potentiometers adjusts bias for one of the eight EL34 tubes. The bias setting potentiometers are divided, four on each side of the chassis. Here is a little hint passed along by the distributor that should have been included in the manual. If you need to replace the EL34 power tubes you can simply insert each tube one at a time and adjust the corresponding bias potentiometer, simply repeat this procedure until all eight tubes are installed. The very same procedure will allow the substitution of two other closely related tubes, the 6CA7 and the 6L6. The specification is to adjust each tubes bias at 350 mille amperes as displayed on the front panel digital meter. Up front the driver tubes are 6SN7, there are four of these in each mono block. This tube is an ultra reliable Medium-Mu Twin Triode that was used in early television sets primarily as a vertical deflection amplifier.

The Fiddly Bits
Am now ready to go, with everything that should be plugged in is. Virtually all tube amplifiers do not take kindly to having things plugged in and out while they are turned on. First actuate the left switch on the front panel; the tubes light up as you have just applied filament heater voltage. A small blue LED comes on and the digital meters now display the numbers – 002 in red a further indication that the amplifier is powered on. Than count to ten and actuate the switch to the right-labeled stand by. This applies B+ plate voltage to the tubes and approximately 20 seconds later there is a click as the output relay closes. In my opinion the markings on the standby switch are not intuitive. The word standby is written just above the switch and the switch is marked with a vertical line and a circle. When you press the top of the switch with the closed line the amplifier is silent. It would be clearer if the word "On" were printed on the front panel just below the switch. Proceeding now with B+ power applied the number values on the digital meters grow larger as the tubes heat up and finally settle at some arbitrary value as the output tubes reach operating temperature. The procedure is than to turn the bias selector switch from the off position and step through all of the eight positions one at a time. At each stop you adjust the corresponding bias potentiometer with a flat blade screwdriver for each of the eight EL34 tubes.

In practice this procedure is not so straightforward because all of the tubes interact. This is where you get into the Fiddly Bits, as you adjust the four potentiometers on the right chassis and the four on the left you have to go back and forth recheck and readjust several times until you have settled on a average for all tubes that is close to the 350 mille-ampere bias specification. Fluctuations in wall voltage can also affect bias settings so you might have to readjust your settings after you activate the amplifiers.  The owner’s manual cautions the user to not operate the amplifier with the bias set above 400. Now the good news, there is a fairly wide range of bias variation allowed, from 350 up to the maximum of 400 mille amperes. When first set up some of the initial bias readings started out at 800 ma. During the time I had the amplifiers under evaluation I did not detect any change in sound quality attributable to drift in the bias range between 350 and 400 ma. Last but not least this biasing system allows you to use unmatched tubes sourced from different manufactures. You would simply adjust each tube separately to the specification.

Cut To The Chase
The Sound Is not at all what I expected. In Pentode operation the Monsoons sound more like a very nimble very up to date solid-state amplifier but with a little something extra. This something extra is a subtle shading of harmonic presence missing from all but a small number of solid-state amplifiers. The extent of this quality is quite frankly beyond my ability to quantify in writing. It is far easer to say the treble sibilance if there is any is always controlled and never acquirers an edge. At the same time nothing is masked or held back it just comes through in a very natural life like way. The very same quality is in force through out the midrange down and into the lower mid-range. I could not tell you if an oboe or a single key on a piano is out of tune but I am familiar with my reference, the sound of a human voice. It is invariably the subtle shadings of a female voice that tell me what I need to know. Not so incidentally this is where the abilities of an excellent tube amplifier eclipse all but a very few solid-state amplifiers.

Getting very specific I would like to mention my favorite long standing reference CD, Time and Tide Basia [Trzetrzelewska on EPIC EK40767]. When listening to the first cut "Promises", the stage width is about the same but the image depth is greater. The opening sentence "Promises we forget about our promises" comes from a place deep in the center of the soundstage. The next line sounds as though it has moved forward and now it comes from a place just between the speakers it is an effect just like a camera zooming in on an object. This studio recording contains a wealth of aural elements I use to evaluate equipment. It is a very good thing when I can expand the boundaries of my little listening space. I believe that a good stereo system must function very much like a time machine transporting us back to the original performance in the original venue. If I had to nitpick this performance it would have to be even the smaller nits that live on the larger nits. There are sibilant words that I key on, the double "S" sound in the word "Promises" is slightly less apparent. If I were asked the question would I like to hear more sibilant sounds on my CDs, I would have to answer, well not really.

If I had to pick only five songs that I could listen to one of them would most certainly be, "What A Fool Believes" sung by Michael McDonald. I listen to it on a new compilation CD The Ultimate Collection [Rhino R2-73161]. The intricate counter point back beat the accompanying synthesizer keyboard line never fails to get me moving and singing "She came from somewhere back in his long ago." It is infectious and all there spread out on a nice wide and deep stage. Our British cousins would say it is tuneful with nice rhythm and pace. Only on the deep bass does the Monsoon reveal its tube derivation. It may be a transient tad slower that solid state. But for me this is no problem as long as the pitch the power of those 120 watts and the pacing are present, all of which it conveys with ease.

Activating the toggle switch to place the Monsoon amplifiers into triode operation changes the character appreciably. There are several aspects of the amplifiers musical reproduction that are altered. The treble frequencies do not seem to extend quite as far as they did in pentode operation and as a result some of the open airiness seems to be less apparent. Also the low bass now seems to consist of less musical information. Our British cousins would probably characterize this bass as more "plummy" sounding. This is directly attributable to a diminishment of dynamic drive and by that I mean the speed to start and stop quickly. You might say this is to be expected simply because in triode operation the available power drops to 60 watts or half of what it was. But while that might be a factor I have a 40-watt Prima Luna 2 Class A/B pentode amplifier that has more dynamic energy. I can’t say that this triode sound is wrong. Overall it portrays a very pleasant midrange so natural you might even call it organic. It will not offend in any way and for many audiophiles this is what they absolutely prefer to listen too. However my conclusions are based solely on what the Monsoon DT MKII sounds like in pentode mode.

Prefacing Your Predilections
All of my listening was done through my Audio Research SP 9 Mark 3 using unbalanced Wire World Eclipse 2 RCA cables. My source was a three-piece CD system comprised of a Cambridge Audio CD transport and a Cambridge Discmagic D/A. This was up sampled to 48 and 96 kHz with a Pro ART Audio D/IO converter. It was sent on 10 feet of active bias Audio Quest X series cables to both my Aurum Cantus LE2 and four way Onyx Strata Mini speakers. After some careful thought and a consideration of comparable tube powered amplifiers I can say you get an awful lot of amplifier for $4,000 devalued United States Federal Reserve dollars. You get 120 watts per channel, 8 transformers plus 24 tubes on two hand wired metal chassis. You have the added advantage to be able to operate the amplifiers either in pentode or triode mode and optionally use a choice of balanced or unbalanced component connections. Two things I would change if I could wave my magic wand. I would rewrite the owners manual so it would be easier to read. And I would install an automatic bias system so I would not have to mess around with tube adjustments. However in the real world I know an auto bias circuit for two amplifiers would probably add another thousand dollars to the price. Most definitely this is an amplification system for a tube lover. It has long been my impression that of all popular tube types that are available the EL34 sound is right about in the middle between the most detailed and the most harmonically lush sounding tubes. I believe the EL34 sound and the Monsoon DT MKII represents just about the best mix of these polar opposite sonic qualities that I have experienced so far. I could listen to this amplifier for days on end and never grow tired it is so natural and lifelike that you can forget it’s in your system.

So what is the nature of the beast? I can best explain that by putting the ASL Monsoon DT MKII amplifiers in an appropriate setting. Imagine a cold stormy evening you reclining in a comfortable easy chair with an excellent glass of wine. You let yourself be immersed in a music performance, perhaps a string ensemble performing Vivaldi’s Four Seasons. The tonality is clear the stage is laid out in front of you the sound is just a bit warm like the wine. The tension of the day just slips away it is the perfect complement to an evening when life is good. Semper Hi-Fi

Manufacturer’s Reply
This is a last minuet response from the distributor Divergent Technologies just before the review was sent out.  Ron Nagle

Dear Mr. Nagle

Here are some further facts concerning the Monsoon 2 DT amplifiers:

- A clearly written sheet of instructions for biasing and operational suggestions are also supplied normally with each item shipped.

- Monsoon Mk II is optimized for EL34 output tubes and can also be used with other octal-based output tubes such as 6L6 or equivalents.

- It is equipped with 4, 8 and 16-Ohm output taps and better tonal balance and clarity can be achieved by choosing the correct tap for each speaker (Experiment with this regardless of the loudspeakers rated impedance).

- Auto bias is not always a desirable in high power amplifiers. Bias will remain stable after the initial stabilization period and it is not necessary to adjust it often. Bias readings within 5 percent on the calibrated meter is perfectly fine.

- Triode mode will sound more refined and fluid, as the two of the elements in the tubes are shut-off.  Of course the power is reduced in this mode.

- A unique feature of the Monsoon MK II DT is that there are two separate power supply transformers. One supplies higher voltage to the output tubes and the other to the input tubes precisely. Costlier way to design and amplifier but this improves clarity by avoiding interference between the transformers.

- Monsoons also have (0) global NFB with a more appropriate out put transformer for a faster and more dynamic sound.

Kind regards,

Tash M. Goka
Divergent Technologies

Specifications
Type: Monoblock vacuum tube amplifier
Frequency Response: 20 Hz to 29 kHz
Output Power: 120 Watts (in pentode), 60 Watts in triode.
Output impedance: 4, 8, 16 Ohms
Input sensitivity: 350mV
Input Impedance: 100kOhm
S/N Ratio: 92 dB at full power
Nominal Voltage gain: 38.1 dB
THD: 3 percent at 100 Watts
Dimensions: 17 x 14.5 (WxH)
Weight: 57 lbs
Price: $4000 per pair

Company Information
Divergent Technologies Inc. 
342 Frederick Street
Kitchener Ontario N2H 2N9
Canada

Voice: (519) 749-1565
Fax: (519) 749-2863
E-mail: divergent@divertech.com 
Website: www.divertech.com