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Positive Feedback ISSUE 28
november/december 2006



SLAM-PPX3 Single-ended Triode Headphone Amplifier - A Review

as reviewed by Max Dudious





SinglePower's name implies a single-ended-triode tube output stage, and Monotheism. I might make the obvious conceit that this model, the SLAM-PPX3, sounds "Divine." Sorry. Last week I wrenched my back stooping for a pun, and I've been unable to straighten up since. I'm pleased to announce the SLAM sounds pretty damn fine fine. Think of it, a single-ended triode design, without output transformers on each channel—with all those silky highs, that smooth mid-range, and all that organ-pedal bass—connected directly to your headphones. Single-ended triode (SET) circuits produce a very seductive sound. I've heard of grown men driving great distances, and suffering extreme hardship just to hear such an amplifier. I might have been one. I think I read of an outraged wife who named a SET 'phones amp in a divorce case, and charged it with "alienation of affection." Maybe on the HEAD-FI website? I'll have to check with Jude on that. SET amps can have that kind of effect on audiophiles who aren't man enough to handle wild relationships with inanimate objects.

A mystic from California's Ojai

Made love to a zener supply,

The thing caught on fire

And burned him entire,

Tush, Scrotum, and all-seeing Eye.

Other headphone amplifiers offer only one damping setting, the one set at the factory, and that's usually a compromise. With the SLAM's variable damping, you can dial up the exact quality of bass you favor, between lean and fat. You might ask, "How does that come about?" My SLAM-PPX3 came with a variable damping factor control they call "Texture," a slick feature. With it you can adjust this wonderfully tubey amp to make it sound pretty tight (or well-damped), for a tubed piece; or pretty loose (or less-damped). [I played the SET SLAM as a preamp with the SET de Havilland Ios amp into a pair of Lowther PM5A systems, and it acquitted itself surprisingly well. After diddling the damping control a while, I was stunned at what an advanced combination the SLAM and the Ios made.]

You might think of variable damping as a device to extract the best bass your headphones have to give. That is, you might use the SLAM with Sennheiser 650s to dial in a tad more bass than they usually offer, or you might tighten up the bass a tad on the Grado GS-1000s. Since deep bass might be a function of the CD player you use, plus associated interconnect cables, plus various headphones, or some combination of all these variables, the SLAM provides a lot of flexibility. Variable damping is a strong point in the SLAM's design. Definitely. And, of course, the unit produces excellent sound.

I have suffered some confusion owing to the language Single-Power uses in its promotion, and actually embosses onto the chassis of some of its amplifiers. On the front of the Supra it says, "Single Power Audio, Inc." implying Single-Ended Triode designs. Since all its tubes are various twin-triodes that are often used in push-pull circuits, Single-Power could have made it easier for prospective buyers (and me) if they had specified "how" a designer can use a twin-triode tube in SET mode. Also on the chassis frontispiece it says, "Pure Class A Triode Headphone amp." It seems almost laughable if it is Class A or AB in an amp that delivers a maximum of 0.5 watt. This amp is designed as a Class A amp through all of its output range; that is to say, the tubes are always "on," even at idle. Since Class A is more of an issue for solid-state devices and circuits, Single-Power could have made it easier for prospective buyers (and me) if they had specified how and why they chose to run Class A in this amp. I don't know if I'm caviling or niggling here. You choose. Maybe someone will write a letter to Pos-Feed's Reverberations section, and explain it to me. I'm at the end of my competence here.

Comparing Sound

In order to give you, gentle reader, an idea of how the SLAM-PPX3 sounds, I thought I'd compare it to some better known headphones amps. At the moment I have three headphone amps in-house: the Grado RA-1 (an elegant, battery-powered, minimalist chip-and-pot design); the HeadRoom Desktop Millett Hybrid Amp (a more complex solid state front end, tube output, wall-wart power supply design); and the SinglePower SLAM-PPX3 (an all-tube, single-ended-triode, on-board power supply design). As you might expect, each has its pluses and minuses, and those are usually most pronounced at the extremes of the audio bandwidth, extreme highs and lows. In the midrange, they sound strikingly similar.

All of the following comparisons were done using a Marantz CD/SACD player, model SA-1151, their second from top-of-the-line player (soon to be reviewed), and an old Sony XA3-ES. This is a model that was typical of mid-fi CD sound ten or fifteen years ago. Briefly, I think the Marantz is quite good, approaching the best available; while the Sony—with the Burson Buffer Amplifier—is, surprisingly, not too far behind. The interconnects were a 36" pair of Ecosse "Myth-Pro" cables I find extremely neutral (also in the queue for full review), and an 18" set of StraightWire interconnects (with a Sony mini plug) I find somewhat sweet.

Remember, closely studying three amplifiers forces the reviewer to look at the comparison as if on logarithmic graph paper, a representation that exaggerates certain sectors, causing the lowest three octaves (20 - 200 Hz), to appear in expanded scale and seem more detailed than the highest three octaves ( 2000 - 20,000 Hz). In other words, what I say from here on is exaggeration for the sake of comparison. I do this to make my impressions accessible to you and to myself. In practice, the differences I'm about to enunciate are rather subtle and most newbies, concentrating on the mid-range, won't hear them. So try to forgive me because, "Writing about music is as difficult as dancing about architecture." And if you've heard me copping out like this before, I apologize. But consider how my gentle readers range in age, how their hearing varies, and how their ears vary in size—any one of which, alone, could make them hear differently through headphones. So if I whine, it is because my task is daunting. I am daunted because no matter what I say, I'll tick-off some percentage of you. Nonetheless, here I go, whine or lose.

Walker's SST

But first, audio fans, a few words about Walker Audio's E-SST, or Super Silver Treatment (Extreme). This product is new and bold, is less filling and tastes great, is stronger than dirt, eliminates ring-around-the-collar, resists stains and wrinkles, and if it persists beyond four hours you should consult your urologist. This is not a contact cleaner, but a contact enhancer. Assuming the pins on your tubes have been recently cleaned (Walker recommends 800 grit or finer sandpaper along the length of the pins.), you apply a thin layer, a really thin layer, of the silver paste on them. This is not a subjective report, like, "the highs were sweeter." My result was a binary one: Where I had intermittent crackles before, causing me to repeatedly jiggle the tubes in their sockets during listening sessions with the amps under discussion, I've had no crackle for six months through various humidity and spells of intense listening. This stuff actually does what is claimed. It is not some chemical compound put up in industrial five-gallon jugs to be diluted and portioned off in fancy bottles at high prices to audiophiles. Lloyd Walker makes this stuff in his garage. I know. I've been to his home and I've seen his "production facilities." For more info about Walker's product line, see his website at

Surfing around, you'll also see the diversity of Walker products. I'm not sure if I believe in using SST on every contact in a system, from the fuse-box forward, but I'll say (in my experience) it does a great job quieting tube pin connections, RCA connections, and loudspeaker cable connections.

A Tale of Three Amps

Now, to the amplifiers. The Grado RA-1, battery powered (hum-free) headphone amp sounds damn good, accurate, and pleasant. It is noticeably softer on the leading edge of transients than, say, the HeadRoom Mini-Stack Amp (a hell of an amp in its own right), if memory serves. Yet, I don't feel any exactitude is wanting. The bass is excellent, able to retrieve texture and pitch down to the deepest registers through the Grado GS-1000 headphones–the 'phones designed on this Grado amp, natch. The mids are all there, but not IN-YOUR-FACE! The highs are all there, if a little soft. It is a pleasant sounding little amp, a bit on the "tubey" side for solid state; and being portable, perfect for listening at the pool, or alone in the den. I still like it a lot despite its romantic tendencies. At its price of $350, it's a winner. If it has one drawback it is that you have to keep a stash of 9 volt batteries around, in reserve. A small price to pay for such a portable, high performing, unprepossessing (The front panel has but one control, volume; the back has an on/off switch), but otherwise excellent piece.

The HeadRoom Desktop Millett Hybrid Amp still strikes me as a killer. I don't know how much more performance Tyll and the gang can squeeze out of this design, with their discrete resistor volume control, and their high-performance Class-A mother-board offered as options. I'd like to hear one all tarted up. As it is, the bass is somewhat fatter than through the Grado amp, and the highs are more a bit more forward and more extended, which makes it sound a little like the Grado with a Loudness Contour circuit. That's fine with me. Listening through the Millett, the Grado RS-1 'phones sound better, to me, in the bass (leaner, more textured) than the Grado GS-1000 'phones. For the $600 price, the Millett is an excellent all-purpose amp. I could listen to it for a long time; blues, big band, bluegrass (the new 3 Bs), jazz, and classics. I have already listened to it for long times because from the bass up it is very balanced and neutral, crisp and well defined, with few peaks. It is pretty compact, though it isn't designed for travel or the beach. That is a drawback I'm willing to concede for such an excellent sounding piece that also affords the user the luxury of voicing by "tube rolling" three different pairs of small, low-voltage, twin-triode tubes (designed for use in older automobile radios), provided by HeadRoom. Altogether a terrific piece and good value for money with flexibility built in by tube–rolling, and possibly upgrading the volume control and mother board ...later. Although, with the power supply set up as it is, you don't get to try different AC cords with the piece. Still, it has a lot of voicing capability. I stand by my previous judgment that this is a superior piece, and excellent value for money.

The SinglePower SLAM amp can lean toward either the suave Grado RA-1 amp, or the crisp HeadRoom Millett amp depending on how the variable damping control is set, and which twin-triode output tubes you choose. The damping control is actually an adjustable resistor, similar to those old wire-wound adjustable volume controls except the resistor is part of the damping circuit. I think of the damping factor spec as a measure of the "Braking Horsepower" of an amplifier. The higher the damping circuit is set, the more pressure on the brake pedal; that is to say, the amplifier has tighter control of the speaker and keeps it from running away, motor-boating, or breaking up. This is true if it's a big amp driving big speakers, or a little amp driving a set of headphones. Of course, both the Grado and the Millett have a fixed damping setting.

The Charm of Variable Damping

With the damping pot set a bit high, the SLAM amplifier will keep all headphone drivers under better control across the audio frequency bandwidth. The bass gets tighter (less overhang, or ringing, or false bass known as BOOM), and the highs get cleaner (less splashy on a scope, or less false highs known as TIZZ). Similarly, the midrange falls under better control (less metallic soprano shrillness). I've found in general, with the gear I've used for this review, the higher the damping setting on the SLAM, the more "refined" the sound gets. There is the risk of under-damping and making the sound too floppy in the lows, too splashy in the highs; while over-damping makes the whole chain of gear sound lifeless. It is a question of taste and finding the sweet spot for your ear and gear. The older Sennheiser 600 melds quite well with the SLAM, and I can finally hear why these 'phones were such a favorite just a few years back. With some headphones I like more damping, and with others I like less. A little cream in your coffee, or black? Some sugar, or none? You know what I mean? Damping to taste, or to each individual set of headphones, or each CD, is a nifty feature.

Having said that, the sound of the SLAM is much the same as the sound of a good push-pull triode amp (like the early Fisher A-50) on a set of friendly speakers. Adjusted for optimum performance considering the headphones it's driving, and the engineering of the software, the SLAM can be just drop-dead gorgeous, even if it tends toward a tad overly round in the cellos, and can be a tad soft on bass and tizz control without proper set up. The SLAM makes up for its really slight colorations with wonderfully life-like human voices. On Gilbert & Sullivan's Mikado, (Telarc CD 80284) for example, the spaces between the singers is precise, and the quality of the voices is amazingly spot-on. On Puccini's La Bohème, (London, 470 624-2, 2003), for another example, the singing of the love duets between Angela Gheorghiu and Roberto Alagna (man and wife in real life) is just unlike anything I've ever heard. I mean goose-bumpily, spine-tinglingly great. The next best thing to being there. (I knew a retired judge who used to say, "Panties aren't the best thing in the world, but they're next to the best.")

I try to resist, but I'm fickle some times, and I've found the SLAM is the kind of amp guys fall in love with. At least, it makes the small hairs at the back of my neck stand at attention. Maybe it isn't love, exactly, but it has this physiological effect on my flesh (I remember from my youth) when I listen to music that features instruments that mimic human voice: the cello, the saxophone, most lead guitars, etc. The SLAM really delivers spine tingles and on-end hairs–a welcome facsimile—that creates a greater emotional involvement.

The SLAM has some drawbacks at its $750 list price that Mikhail (pronounced like Kevin McHale) Rotenberg (CEO at SinglePower) has attended to in the products higher up in his line. In step-wise progression he offers stiffer and more robust power supplies and more dynamic tube sets as the models get more expensive. See his website for more info.

There are some caveats. At the loudest levels, with the damping control at about just beginning to bite into the output a bit, when played too loudly through some headphones, tympani rolls and bass drum whams will cause the SLAM to fuzz up. Some of the tube sets alleviate the fuzz a bit, at the expense of the bass output. The higher the levels you like to listen to, the more you'll have to adjust down. As I implied, the stiffer the power supply (more capacitance, more regulation), the easier time the better amps in the SinglePower line will have with reproducing the most taxing passages in the music. But the SLAM is adjustable. You have to tweak the damping control and the volume control a little bit until they optimize for the recording you are auditioning. It will take at least two or three re-iterative tweaks, but if I can find the sweet spot, so can you.

I couldn't help but notice that on the specifications and features page of the SLAM PPX3 ($750) no mention is made of Reservoir Capacitors, but on the Supra ($1600 for the base model) specs page there is specified 7200 µF of Capacitance Reservoir. Similarly, there is no mention of the SLAM's voltage regulation, but to the Supra is attributed a "Highly Regulated Power Supply," a Gold-Point Stepped Attenuator (24 stops), Jensen interstage Coupling Capacitors, OFC Shielded Wire on all point-to-point internal connections, and a Stainless Steel Chassis. The SLAM PPX3 is really a bare bones version of the SinglePower house design.

Tubes and AC Cords

The SLAM is also sensitive to power cords, and the better the power cord, the better it will sound. Using Harmonic Technology's Fantasy AC cord I heard a tad more analytical sound, while through the late Bob Crump's SLVR by TG model (which is not a silver cable, but a tinned stranded-copper, inexpensive) AC cord (yes, he was an engaging rascal) I heard a tad warmer, more romantic sound. The SLAM is the only one of these three amps that allows the listener to voice the piece by rolling AC cables, another nifty option. You might also engage in tube-rolling following the advice on tubes found on the Head-Fi website As the driver tube, I found the Raytheon 6414 a bit tighter than the Ken-Rad 6SN7. And as the output tubes, I found the Tung-Sol "black plate" 5687 tighter overall, and somewhat more detailed than most. The SLAM is also sensitive to CD players; you can drive the SLAM with a forward sounding walk-around portable red-book CD player if it has a line-out source, or a more reserved universal DVD player, or a beautifully balanced CD/SACD player. As your source gets better, the SLAM seems to improve (wider frequency response, better dynamics, more clarity). If you find your portable CD player a little too brash, you can dial in a damping factor correction.

Features and Specifications

On his website, Mikhail has the following lists: Specifications and Features. These will show you that even the least expensive piece in his line is worthy of many first class parts and features.


  • Input impedance: 100K ohm

  • Output Impedance: < 20 Ohms

  • Power supply capacitance: 215 uF

  • Rectification: silicon

  • Power output < 0.5 watts

  • Usable headphone range: 16 to 2000 ohms.

  • Total distortion < 0.1%

  • Volume control: Noble

  • Input RCA: Gold Plated Teflon insert

  • Output Headphone Jack: Neutrik Standard Chrome Nose

  • Milled aluminum Knob

  • 1/8" thick aluminum chassis


  • 250v Power Transformer

  • Point-to-Point Wiring (no circuit boards)

  • Vishay RN65 Military Metal Film Resistors

  • Ceramic Tube Sockets w/ Gold Contacts

  • Output Transformer-less (OTL) Circuit Topology

  • 6CG7 Tube Compliment

  • Multiple Stages of Filter Capacitors

  • Noble Volume Control

  • Silver-Teflon Wire

  • Silver Solder

  • Powder Coated Black Chassis w/ 360 Degree Ground Plane

  • Filtered and Rectified DC Tube Filament Voltage

If you are new to (push-pull) triode designs and want to see what they are like because you're considering a big tube amp for your main system, you ought to consider living with the SLAM a while before you make a heavy investment. If you have some experience with headphones, but never heard all you expected from your system, you ought to consider the SLAM. If you can pony up the $750 basic price for a High-End entry-level amp that reaches out from the great audio void, you ought to consider the SLAM. There are some power supply modifications available from SinglePower that will up the price of the piece. If you don't mind shipping the piece to Colorado for mods, you can arrange to have a series of mods over a year or so, or you can upgrade with your original order. That all depends on your trousers and how deep your pockets are.

From my notes on the Head-Fi International Meet last April, the sound does improve audibly with each power supply improvement. This process can be essentially a production or post-production modification. Knowing what I know now, I'd have had the modifications done before I took ownership. Hindsight is better than foreskin, if you know what I mean. But that's what you have me for; to act as your trusted advanced scout. Thing is, SinglePower builds each piece to the needs of the client, one off, which means you can jointly design your amp in collaboration with Mikhail over the phone. Together you can work out just which tubes you'd like, how far you'd like the power supply beefed up, and the like.

In Summary

To review; to compare the SLAM with amps I knew well I described two others as briefly as I could. The Grado is a very good sounding piece and offers good value for money. HeadRoom's Millett Hybrid Desktop Amp sounds similar in its midrange, which is crisper, but still very warm and voice-friendly, and the Millett has a tad airier highs and richer lows when compared with the Grado. The Single-Power SLAM has only high quality parts, and sounds as yummy as single-ended-triode amps are supposed to. It has a variable damping control feature that I find very useful, which should make it compatible with just about all headphones (excepting electrostatics) at all but the highest, masochistic listening levels. The SLAM can be made to sound a lot like either the Grado or the Millett with soon-learned expert-diddling of the variable damping control. The midrange has the triode sound, and very nice top and bottom octaves when tuned. I have a feeling the full complement of options might make this more a killer amp. At the moment the SLAM is a design I'm falling for. It can be voiced with various tubes and with various AC cords until it sings seductively. It can be used as a line amp, albeit with only one input and one output, in a big stereo system, so you can experience its voluptuous quality. It has many possibilities, and I think the sound is subliminally exceptional.

One Caveat

If I have one reservation about this piece, it is how it handles crescendo. At first I had trouble with it, and when the big moments of music that test a piece of gear came, it fuzzed up, or just broke up. I was tempted to say the fault lay with the design. But after a while I figured out how to tune the damping control for optimum results. Tuning the damping is like manual gain-riding in the old tape recording days. First, I'd set the gain for the loudest spot in the music and mark it well; then I'd back-off the volume on the whole recording to where the music no longer saturated the tape. With the SLAM, I set for the loudest level I like to listen, and then I increase the damping and volume until it stops breaking up. It may take you a couple of iterations to get it right (a little more damping, a little less volume: a little more volume, a little less damping); but if I can, you can.


With that one caveat in mind, I can say I whole-heartedly endorse the SLAM-PPX3. It is a flexible line-stage piece, with excellent sound, and with the considerable charm of SET (in some ways superior to high output Single-Ended Triode amplifiers in that it only has to develop a half-watt, so it is never stressed). It beckons and makes me feel like I'm two-timing the Millett. I figure if an amp is good enough to test my fidelity, it must be quite fine, indeed. I'll leave you with this: I'm a man torn between two loves.

If you are seriously in the market, check out Mikhael at, or phone him up. Who said, "Not ending a sentence with a preposition is nonsense up with which I will not put"? Mikhael can be reached at 1 (303) 363-6065. Just pussy-foot, or frug over to the phone, and when you dial him up, tell him old Uncle Max sent 'ya.

Ciao Bambini.