Parts Connection/Assemblage ST-40 Amplifier

by Todd Warnke

It’s an extremely canny move by the Chrises at Sonic Frontiers (Johnson and Jensen) to send their new Assemblage ST-40 amp kit to a reviewer. First, being the marginally sentient life forms that we reviewers are, if I can build this kit, anyone can. Second, after investing the time, effort and a burned tabletop soldering the thing together, it has to sound fantastic. Building your own amp creates the same type of bond that you get after spending four (or five or six) years at an institution of higher learning. After undergoing the heartbreak, the all-nighters and the lost loves of a kit amp, you’re ready to paint your face in Sonic Frontiers colors and put on that big foam hand with the index finger raised high and proclaim your work #1. Still, I have to put my own sweat equity aside and deliver a full and honest review. That I will. But keep in mind that once the scent of solder gets firmly fixed in your nostrils, even looking at an amp not built with your hands can bring a sneer of contempt to the lips. Anyone can buy an amp. Real audiophiles make their own.

Divide and conquer

Of all the kits I’ve built or seen, those from Sonic Frontiers/The Parts Connection/Assemblage (for those of you not familiar with the SF/TPC/Assemblage connection, see my review of the DAC-2 kit) are about the best and easiest. All the parts are supplied. The instructions are complete with pictures of difficult procedures (of which there are only a few), schematics and a nice but non-intrusive sense of humor.

Since the ST-40 amp kit is step or so beyond the DAC-2 that I reviewed last fall, I want to be careful to describe the do-it-yourself element as much as the sound of the amp itself. Those of you confident in your building skill can jump ahead to the technical specs and sonic review.

I started the assembly by reading the instructions front to back, twice. Although you shouldn’t, with the DAC-2 you could get away with reading and assembling as you go. In this case, due to the sheer number of parts and steps, you need to read the instructions at least once. After that I looked for logical break points in the kit so that I could assemble it over several nights. Finding three easy rest stops, I elected to built the kit over three nights.

The first night’s work went smoothly. Assembling the mechanical pieces on the board (removing the transformers, hooking up standoffs, and soldering the tube sockets) is easy and a confidence-inspiring way to start. The next step is stuffing the main board. The manual has you installing parts by type: resistors first, then diodes, capacitors, and lastly semiconductors. Take care, as this is where most mistakes will be made, and where they are the hardest to find (more on that later). By the time I went to bed the first night, all the resistors and diodes were in.

The second night I stuffed the caps and the semiconductors (which finished off the main board) and completed the power-supply board. I also finished out the miscellaneous parts—bias-test jacks, AC-power socket, speaker posts and RCA input jacks. By this time I had a great feel for the monotony of the assembly line. If you feel this way, stop, brew a nice cup of coffee (or open a brew, your choice) and relax. Remember, a mistake on the board will take a bunch of time to find. None of this stuff is hard, but the job does reward hard work. A bad solder joint, a part in the wrong place, a missed step, and you’ll know what real frustration is.

The last night saw the assembly of the amp itself. The transformers, placement of the boards (main and power), the wiring and the faceplate. I would suggest that you do this section all at once. There are about 50 separate steps, and unlike parts stuffing, where the accompanying parts list helps keep your place, most of these instructions are textual. Read very carefully, take your time and keep a Zen frame of mind (focus on the process, not the outcome).

When you finish, and before you insert the amp in your system, take some time to admire the job. It really is a beautiful little amp. Robin likes it, and especially enjoys looking at it at night. I don’t blame her. Assemblage has put the amp in a small chassis (15" wide and 11.5" deep) that accentuates the tubes. With the three small input tubes up front, the four EL34s behind them, and the three transformers in the rear, the amp looks clean and balanced. Personally, I like the Henry Ford color scheme (any color you like as long as it’s black), but Robin would like to see a silver option. Still, I’d hate to see them raise the price to bring a silver model out.

When I finished, I checked the voltages around the power tubes, slapped the bottom cover on, popped the output tubes in, biased the amp and hooked it up. What happened next is best conveyed in an email I sent to John Stafford, who is working on the Assemblage L-1 kit (the line-level preamp).

"Just got the ST-40 up today. After finishing it a week ago, upon firing it up and checking voltage, I thought I was home free. I biased it up and...nothing. Almost no sound at all. Off come the tubes, off comes the base, open it up. Oops! I missed a ground. Hook it up, put the base back on, replace the tubes and...still nothing. Last Friday I call S.F. Off come the tubes, off comes the base, open it up. We test various voltages. After about 45 minutes on the phone I find a zener diode I put in backwards. Hang up. Crack out the iron, repair the job, replace the base, replace the tubes, fire it up, same exact nothingness (Sartre would be so proud of me). Call them up this afternoon. Off come the tubes, off comes the base, open it up. While I'm on hold I notice that the picture of the completed board has about 6 resistors that I don't remember. Look at my board, sure enough, there are empty holes and numbers. Scan the instructions. No, I didn't miss anything. They never mention those resistors. Look in the bag, there they are. I hadn't thought anything of the extra parts since they included extras with both the DAC-2 and the DAC-2 parts upgrade. Same for the extra holes in the chassis since there are others already in place for the parts upgrade. Vincent comes on the line. I say, ‘Hey, I found it.’ He says, ‘R29 and 31?’ ‘Yep.’ Hang up, and start up the iron. 15 minutes later, sweet, sweet music!"

Both mistakes I made were at the end of that night’s work. I have to kick myself for those. In the case of the ground, I soldered the lug, but failed to screw it to the chassis! As for the zener diode, directionality is obvious, and care will make sure that that mistake doesn’t happen. What can I say except to reiterate that mistakes take time to find and fix? Especially little ones.

And as for the resistors that finally fixed everything, keep in mind that I have serial number ST00011, which means I also have version 1.0 of the manual. Assemblage did a superb job getting out a manual with only one mistake. All kits now ship with a manual that corrects this one small error. As for the folks on the Sonic Frontiers/Assemblage support line, they were unfailingly patient and very knowledgeable. Having run the tech department for a small software firm, I have only the highest respect for the job they did helping me.

How long did it take? I timed the assembly process. Keep in mind that I was being very careful, review and all, and am not the fastest person with a soldering iron. I make quality joints, but not with assembly-line speed. Anyway, excluding the two errors I had to fix, the kit took just under seven hours to build. That was reading the instructions two times before starting and reviewing each section after it was assembled (not very well, obviously).

How hard was it? Moderately. Can you solder? Are you careful? If so, you can do this. And if you can’t solder, spend an hour practicing. That’s about it.

I have four final comments on the kit. First, do not put the LED on when the manual says to. Wait until just before you put the board in the case and seal it up. Otherwise you’ll either break it off or have to exercise extreme care while flipping the board during the last soldering run. Second, the power-supply board is a bear to attach. Get a long screwdriver and be careful. Third, don’t try to use a hobby knife to strip wire. There is a lot of wire to strip, so buy a good wire stripper. Your fingers will be grateful. Fourth, read the instructions at least twice before starting, and then read each section before starting it. And a final comment, when you recheck your assembly, do a better job than I did.

Specs

The ST-40 uses a single 12AX7/ECC83 in the input/driver stage, which feeds a pair of 12AU7/ECC82 tubes (one per side). The output tubes are EL34s. The kit comes with Sovtek EL34WXT tubes, which many people consider the best of new EL34s out there. The rated power is 40Wpc into either 4 or 8 ohms (when assembling the kit you have the option of using a 4-, 8- or 16-ohm tap on the transformer), with 1% THD happening at 55W. Input sensitivity is 1.3 volts, while input impedance is 100k ohms. Operationally, I found the amp to perform flawlessly. The power was ample in either of my normal settings, doing especially well in the office where it performed double duty as a space heater/night light so well that I had a tough time moving it out to the main room.

Stats

After finishing the kit, the amp broke in in the office system. There it primarily fed a pair of Platinum Audio Studio 1 speakers with a biwire run of JPS Labs Superconductor speaker wire. Sources were a Theta Miles CD player directly into the amp, as well as a Sony XA20ES CD player, also directly into the amp. The interconnect was the new Audio Magic Spellcaster II.

In the main room the ST-40 drove Dunlavy SC-IIIs and Kharma Ceramique 2.0s, both with a biwire run of Nordost Red Dawn. The sources were the same as in the office system. Preamps were my Audible Illusions L-1, a Thor Audio TA-2000 and a Joule Electra LA-100 Mk III. Interconnects were Nordost Red Dawn, Cardas Cross and Audio Magic Spellcaster II. Power conditioning was courtesy of an API Power Wedge 116, while power cords were MIT Z-Cords, Discovery, and Audio Magic. Cones were the versatile Golden Sound DH Cones and Squares, and everything stood on a SoundRack Reference Granite stand.

Time for the school fight song

I can cut this review very short simply by telling you what happened when I put the ST-40 in the main room. It was late in the evening. Robin was relaxing on the couch. Out went the Blue Circle BC6. Fifteen minutes later the ST-40 was playing. I turned the lights out for Robin, popped the Cowboy Junkies’ Studio (RCA 07863 67412-2) into the Theta and retired to the office. An hour later Robin commented, "I like the tubes. They look good in the dark. And the sound, it’s not as detailed as the other amp, but it has a richness that the Dunlavys like. I think I like it better." It would be incomplete to end there, but not unfair.

To flesh things out a bit, let’s look at Robin’s "negative" comment, that the ST-40 is "not as detailed" as the $3700 BC6. First, for $3000 more, the BC6 sure better do some things the ST-40 can’t, and detail is one of them.  For perspective, the BC6 is not an etched or ruthlessly revealing amp; rather, it reveals virtually everything on a recording in an effortless and natural way. In my system, the BC6 sets the standard for detail to which every other amp I’ve heard aspires. So the ST-40 is being held to an impossible standard. But purely on its own merits, the ST-40 scores quite well in the detail department. A visual analogy may help to explain what I mean.

I live just east of Denver (no, not Kansas, it just seems like it sometimes). Driving into town I am usually looking at Mt. Evans, one of Colorado’s 54 peaks higher than 14,000'. On a very clear day, and early in the morning, I can sometimes see the road that leads to the summit. On those days the peak looks like it’s about 10 miles away, when in fact it’s more like 45 miles. The Blue Circle amp has detail just like that: not hyped—in fact, very natural in effect, but supernatural in ability. The ST-40’s detail is more like a standard view of Mt. Evans. I can see the ridges that define the mountain, the trees, the rocky outcroppings, the snow fields. But it is also 45 miles away. Natural in both ability and effect.

As for the richness Robin describes, she’s right. The traditional seductive warmth (physical and metaphorical) of tubes, but not their softness, is a virtue of the ST-40. In either system it was so easy to imagine Star Trek days had arrived, that performers had been transported to my listening room, that I often found myself asking phantom recording engineers for drink orders. Seriously. OK, so that’s a bit of literary license. Still the ST-40 communicates the feel of a real musical event better than any amp less than $1500 has a right to. The pairing of the ST-40 and the Dunlavys worked surprisingly well in this regard. I really could find myself recommending this amp for the SC-II, or perhaps a pair used in a vertical biwire for the SC-III. Still, don’t make too much out of this richness thing.

Much has been made in the audio press of the newest round of Sonic Frontiers amps (the Power 1, 2 and 3), namely that they can come off as lacking excitement. Some reviewers have found that with extended listening this lack of character is an asset. Others have felt the lack of excitement is also a lack of expression. In some ways, and in spite of Robin’s comments about the richness of the ST-40, the Assemblage amp shares a part of this house sound. Its rich presentation is not overdone or highly editorial. In fact, it is extremely self-effacing for a $699 tube amp. At this price point you’d expect a large amount of the traditional tube colorations along to create involvement. Not so with the ST-40. The richness Robin referred to was more of a naturalness, of a relaxed, non-hyped and totally believable presentation. And this is something the ST-40 does very well.

As for the traditional Audiogeek criteria, the ST-40 has nice but not subterranean bass extension. Playing tube games (Tesla E34L and Svetlana EL34s in place of the supplied Sovtek EL34WXT) affected the bass response without really improving it. The Tesla tubes had a warmer, softer, bottom, while the Svetlanas split the difference between the Teslas and Sovteks. With the Platinum Studio 1 speakers, the bass was never an issue. The Dunlavys showed the tube differences very well, but ultimate bass extension and control were only an issue with the Kharma Ceramique 2.0s, due to their nearly flat in-room response to near 30Hz. Still, I doubt that many users of the ST-40 will have true full-range or near full-range speakers.

Treble was extended and natural—virtually grainless as well. Cymbals were metallic and shimmered with superb decay. On occasion, and only when playing more esoteric ambient albums, I noticed a very, very slight rolling of the top end, but keep in mind that this was only with, um, non-standard music (according to Robin, some of it doesn’t quite qualify as music at all).

The midrange was phenomenal. For a $699 amp, the ST-40 shows what every single solid-state amp (that I know of) under $2000 does wrong. Liquid implies a coloration, and while I can’t truly call the ST-40 100% neutral, liquid would be going too far. I much prefer natural. Female vocals were recreated with all their overtones intact. Guitars had wood and string. Pianos had hammers, strings, sounding boards and lids. And all were beautifully placed in my room.

Staging was nice, but not overwhelming. Right to left, the stage reached the edges of the speakers but couldn’t get outside of them. Depth was very good, but the back corners of the stage were slightly soft. With small jazz combos and rock groups like the Cowboy Junkies this was perfect. Throw large-scale orchestrations on, though, and it became a slight issue. Since my taste in classical music is pretty much bipolar—string quartets and "power" music by Mahler, Bruckner and Wagner—this was an issue for me. On the smaller-size recordings, or mono orchestral recordings, I was fully engaged. On the other hand, recordings such as Bernstein conducting the Vienna Philharmonic in Mahler’s 6th, which I use to torture-test amps and speakers (DG D215076, not the best version of the 6th but the first I heard and so far my favorite), had some small problems in the back row—enough to notice, but really not enough to destroy the illusion.

Dynamically, the ST-40 again gives you more than you expect from an amp of its price. Of late I’ve been playing a lot of ambient and trance music. Far from sedate, some of this music, such as Death Ambient by Kato Hideki, Ikue Mori and Fred Frith (Tzadik TZ7207), can stun from blocks away (by the way, this is one of the discs Robin refuses to call music). The ST-40, the few times it ran out of steam, rather than clipping or chopping up, would simply refuse to get louder. Microdynamics were excellent, with the texture of small sounds remarkably well preserved.

Context and conclusion

Context is everything. If you take the ST-40 and put it up against the megabuck tube-amp state-of-the-art contenders, you’ll probably question my hearing ability. As expected, these other amps will capture most of the virtues of the ST-40 and add many of their own. But for $699, the ST-40 offers four things they may not be capable of. First, the ST-40 is an incredibly well-balanced amp. It reaches high, but does not exceed its grasp. Second, this sure sense of balance gives the Assemblage an ease and involvement that is rare at any price. Third, the ST-40 is a true high-end amp that is also a real-world bargain—a steal, in fact. And fourth, the joy of rolling your own is a joy that, at least once, you need in your audiophile life. The ST-40 makes this attainable for everyone. In fact, this last point brings me to the only real flaw in the whole kit. The label on the back of the case says, "Made in Canada." Assemblage needs to add a line that says, "And lovingly assembled by ___________." After you bond with an ST-40, you’ll know what I mean.

...Todd Warnke
todd@soundstage.com

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