Audio Space 6M-300Bpp 300B Push-Pull Monoblocks
In this episode, my Adventures In High Efficiency takes me into the realm of amplifiers.
As you are well aware, there are scads of low(er) powered amplifiers on the market today. For this article I've chosen a relatively high powered piece of gear, the Audio Space 6M-300Bpp. As the name implies, this pair of monoblock amps are a Push-Pull design and use 300B's as the output tube. According to the manufacturers specifications, this pair of amps puts out 21 watts per channel. In the world of low power amps, that's huge wattage.
According to Alfie Lew of Gini, United States distributor of the Audio Space line, says "Audio Space with its full name Audio Space Acoustic Laboratory, Ltd. is a Hong Kong company that was founded back in the early 1990's before the British turned over Hong Kong back to China in 1997. It still headquarters in Hong Kong, which is now a self governed Special Administration Region (SAR) of China. Audio Space's manufacturing plant known as Corilex, is a separately registered business located in the Southern part of main land China. In Europe, Audio Space collaborates with a European company and have established a line of tube gear marketed in Europe under the name of Duntonic."
The basic design of the 6M uses the 300B in Push-Pull configuration. After opening up the amplifiers it appears that the circuit topology is the standard ultra-linear. The input driver tubes chosen are the 6SL7 and the 6SN7. The good thing about the choices of driver tubes is that these tubes are readily available as new production or if you wish to tube roll, new old stock. Bottom line, your choices for tubes are very plentiful and relatively affordable depending on which route you take.
In talking to Alfie Lew he states that Audio Space get the crème de la crème of the Shuguang 300B tubes. They actually hand pick their tubes from the production line before Shuguang labels them with their own label. The Audio Space tubes are AC matched using a 1k frequency then hand picked to (closely) match mu (the tubes amplification factor). I use a pair of the Shuguang 300B tubes (supposedly lesser quality than the Audio Space tubes) quite often in my Welborne DRD's. I'm very happy with the sound of the Shuguang tubes and they regularly alternate time with my TJ Mesh Plates. They are just a tad brighter than the TJ's with just a bit less midrange fullness. They are a very nice sounding tube.
The 6SL7 and 6SN7 tubes are also of Chinese manufacture also from the Shuguang plant. Though you can get a ‘different' sound from NOS 6SN7 (and 6SL7), the Chinese tubes get the job done fairly well. I personally find them a bit brighter and harder sounding in my system than say my vintage Sylvania's but they aren't a bad sounding tube by any stretch. Although rolling tubes is not a must, to fine tune my system for this article, I used my Sylvania's. My Lowthers don't need any help in the treble department. The Sylvania's to me, just sounded more ‘right,' so I used them.
The potted transformers (power and output) used in the 6M are custom wound for Audio Space. In addition, it appears that the choke used in the power supply Pi filter is custom wound for Audio Space also.
Power rectification duties are accomplished via solid state design using a standard bridge rectifier. The power supply is a CLC design with ample filter capacitance. Power supply filtering is provided by a pair of very nice sounding Nichicon NX series 300mF caps. The rest of the electrolytics in the Audio Space amps are also Nichicon's.
As an aside, the main power supply caps do not have bleeder resistors. Normally this isn't an issue unless you are an idiot and stick you bare hand inside the amp and then ground yourself (like I did). You soon remember that no matter how long a cap sits with no power on it, the caps may never completely discharge (until you touch one of them... zzzzzzZIT). One of these days you'd think I'd stop being over confident inside amps and I'd wear my linesman gloves that are rated to 6000 volts. Oh well, that'll learn me... (again).
The driver (gain) stage is capacitor coupled to the output stage. The capacitors are metalized polypropylene (MKP), private labeled by Audio Space. The coupling cap values are .22 for those of you keeping score. According to Audio Space, metal oxide resistors are used in high voltage portions of the 6M's circuitry while carbon oxide resistors are used in areas of lower voltage. One item that the designer pointed out about this design is that the 6M's are equipped with an "auto hum balance" circuitry to keep hum noise to under 1mV. Supposedly with this feature, there is no need for a hum balance pot when rolling tubes. The designer believes this to be an industry first, though I can't say for certain. He claims that none of the other Chinese brands have it. Without giving too much away regarding this circuit, it consisted of a cap and a voltage regulator (if I reverse engineered it correctly). It did seem to do the job quite well as the amps were extremely quiet.
The Audio Space 6M uses a 'Class A,' Push-Pull approach. The amplifiers are operated at a constant 60mA bias current as opposed to 30mA to 40mA in a standard 'Class AB' design regardless of signal level.
The internal wiring on the driver stage is done with solid core silver wire with Teflon dielectric. The output stage utilizes standard copper hookup wire. As you can see, the internal wiring is very clean and compact. Critical interconnect wiring is tightly twisted to prevent unwanted transmission of EMF's and the like. The small PC board houses the interstage coupling and the driver tube bias circuits. Everything else within the Audio Space amp sports point to point wiring.
An interesting feature to this amp is the fact that it has switchable, negative feedback. One of the switches on the front of the unit allows you to apply a nominal 3.5dB of negative feedback to the circuit. It's actually a nice feature. In fact I added this modification to my big KT88 monoblocks some time back. This feedback switch allows you to do some additional fine tuning to your music.
Recent converts to the tube realm may find the addition of feedback a little more aurally pleasing since it (essentially) cancels some of the even harmonics that tubes are so noted for. The feedback circuit is said to add some ‘stability' to the overall amplifiers performance. Feedback also increases the amplifiers bandwidth and can add a skoash of damping factor to the total design by lowering the amplifiers output impedance. All in all, feedback need not be a bad thing in all cases. That said, personally I'm not a big proponent of feedback. I'd rather have my music served ‘straight up' just like my scotch, but that's just my opinion so take it for what it cost you.
Another cool feature of the Audio Space 6M's are the fact that you can either use these as a standard amplifier with a preamp or you can plug in up to two sources into the back of these units and use the integrated volume pot on the front of the unit. Switching between the two sources is accomplished by a simple rotary switch located on the left side of the faceplate of the unit.
As far as the cosmetics of the Audio Space amps go, these are very stylish looking. The main chassis is highly polish, heavy gage stainless steel. This means there is no possibility of rusting in a humid household environment. The front faceplate on the review samples is also polished with a raised black border. Just below the gold nameplate is a single, bright blue LED power indicator. The power and output transformers are cased in a glossy metal-flake black pot.
On the back side of the amps is the standard fare of gold plated RCA female jacks. The binding posts are nice quality gold plated brass with knurled tightening jacks.
Here is an item of note. As I read through the manufacturers instructions, I notice that the Audio Space 6M's are warranted for a full 24 month's. That a damned good warranty period for a piece of tube gear. The vast majority of competing manufacturers only offer 1 year. So much for the thought that Chinese gear is (or can be) inferior. A warranty like that gives me warm fuzzies.
Overall, I've found the amps to be very good looking and the build and parts quality to be exceptional.
A Brief Statement On Push-Pull vs. Single-Ended Topologies
To my ears a Push-Pull amp can suffer when the music sine wave is split into two inverted phases and then reassembled at the output transformer. Personally, I tend to hear some sibilance on vocals when amplifiers don't reassemble the two halves of the sine wave properly. When the topology is implemented properly, this issue can be nearly inaudible. With a single ended amplifier, there is no splitting of the sine waves, hence no reassembly issues.
The other item that I notice about Push-Pull amps is the soundstage. When I compare a Push-Pull amp to a Single-Ended amp, the soundstage on a Push-Pull amp is noticeably narrower. They tend to have a tight, focused sound. A good single ended amp has a cavernous presentation. Although the sound extends beyond the outside of the speakers, they still retain very good focus. A single ended amp also sounds open and effortless compared to a Push-Pull amp. These qualities too can be minimized on a quality Push-Pull design.
As I talk about the sound of the Audio Space 6M Push-Pull 300B's in this article, I'll be making comparisons to what I personally feel should be the benchmark to all other amplifiers, well designed, single ended triode amplifiers.
Before I start describing the sound you need to know how my system was setup for this article. I decided to spend the vast majority of this review using my Lowther PM2A's as my main speakers. If any speaker is going to tell me exactly how an amp is going to sound, the Lowthers will. As usual, the majority of the time I used my 15" Goodman's to fill in the lower frequency regions where the Lowthers can't go. I did run the Lowthers full range and also ran my Goodman's (which are actually full range coaxial drivers) to evaluate the bass response of the Audio Space 6M's.
My source gear is (as always) is my heavily modified Korato KVP-10. My digital is provided via the Bolder Cable modified Squeezebox 3 being fed into the MHDT Labs Paradisea tubed DAC. My turntable is the Opera Audio LP-5 with the Dynavector 507 MkII arm with the DV-20XH. the cartridge is feeding my Graham Slee Audio Jazz Club phono stage. The entire system gets clean power from a DeZorel Audio Reference Senior line conditioner. The amps used for comparative purposes are the Welborne DRD 300B monoblocks and my Handmade Audio 2A3 Deluxe Classic.
Now for how the 6M's sound. The guys from Gini suggested that I go ahead and let the amps break in using the stock tubes. After the 100 or so hours of break in they wanted me to start my listening with the stock tubes before I started tube rolling. As expected the Chinese driver tubes sounded fairly decent on my Lowthers but when I rolled the Sylvania's in their place, everything became more to my liking. I kept the NOS tubes in for the balance of my time with the Audio Space Amps.
Lets start at the bottom and work our way up this go around. Since my Lowthers are ‘low frequency challenged' (that's a nice way of saying they don't do bass for squat), for this portion of my testing, I connected the Audio Space 6M's into my 15" Goodman coaxials. Since I have these speakers corner loaded, I'm getting pretty darned good bass out of them all the way down to the mid to upper 20's.
A couple of my favorite tracks to use for testing bass are Until the End of the World by U2 and nearly the entire album Peace Beyond Passion by Me'Shell Ndegeocello.
When you play U2's Until the End of the World on any stereo, its obvious that the bass is completely overdone. Its muddy, boomy and overall sounds pretty bad. So what better way to find out how the 6M's control what is uncontrollable bass. As I played this track I have to say that I was pretty impressed. I half expected the 6M's to roll over and play dead, but they didn't. Even when presented with an extremely difficult recording like this one, they held their own. Not quite as good as a solid-state amp but that is a horse of a different color. The 6M's kept very good control on my 15" Goodman's.
Bass player Me'Shell Ndegeocello from Peace Beyond Passion has some killer bass tracks on it. The opening songs called "The Womb and The Way" are a true test for bass control and accuracy. I'm happy to report that the 6M's passed with flying colors. The bass stayed tight, fast and clean through the entire song which can act as a real torture track especially at the high volumes I was playing it at.
And of course I used my usual rounds of acoustic bass both plucked and bowed. For the plucked I used some Ray Brown from Some of my Best Friends are Trumpet Players. This is just good music, no matter how you slice it. Ray puts on his usual stellar performance on the bass which shows no signs at all of sounding slow, soft or mushy on the Audio Space 6M. Each note stayed as true and concise as when it was played.
Since I've got Ray Brown's disc spinning, I may as well roll right into the midrange sound of these amps. This particular album can easily be used as one of your reference CD's, the recording quality is so good. In turn, the recorded trumpets can sound extremely lifelike with the right amps and speakers. When I compare these tracks on the 6M's to my Welborne DRD's the 6M's come pretty darned close to the clarity of the DRD's but they do fall just a tad short. Speculating, it's the coupling capacitor. I don't think it's a quality issue as much as the DRD's are direct coupled which eliminates that coupling cap. It's not really a fair comparison but I have to use (what I feel) is my benchmark for clarity. That said, I've listened to enough capacitor coupled amps to say unequivocally that the 6M's do an exceedingly good job on midrange clarity.
As I moved onto some female vocals, namely Alison Krauss and Union Station, Alison's voice came through with a clarity and presence you don't often hear in capacitor coupled tube amps. Though is wasn't as clean as the direct coupled Welborne DRD's, it was extremely close. The best part was I didn't notice any significant coloration. All too often, a designers choice in parts can lead to an unnatural coloration of the music. That isn't the case with the 6M's. It's obvious to me that the designer experimented with the parts to find that ‘right' combination of naturalness and transparency.
All of the great things that I'm hearing in the midrange transfers directly to the high frequencies. Since Audio Space has found that magic combination of parts and design, the highs present themselves just as cleanly as the midrange. In all my time with the Audio Space 6M's (and a significant amount of time it was), I never once found myself having listeners fatigue. As I mentioned early on, I rolled the Shuguang driver and inverter tubes out for some NOS Sylvania's. In my system and to my ears, these tubes sounded better. The highs came through without a hint of glare or etching. The shimmer of cymbals, the ultra high harmonics of things like triangles lent themselves perfectly to the ‘air' surrounding any performance I listened to.
One of the final things I listen for is sound staging and imaging. As I mentioned earlier, Push-Pull designs can be a bit of a compromise when directly compared to a quality single ended design. The Audio Space 6M is no different. Since it is a relatively standard Push-Pull design (read = not a constant current source), you will find that that the soundstage does remain narrower than that of a really good SET. That is not unexpected in the least by me. In fact, this pair of amps actually does quite a good job of projecting themselves based on the type of design.
My usual test for this is Pink Floyd's "Sounds of Life" from A Momentary Lapse of Reason. The water lapping on the shoreline that is projected by the Audio Space 6M's comes from about 4 feet outside of the Medallion's cabinets. That's pretty impressive for a Push-Pull amp though not quite on par with my DRD's.
When it comes to imaging, these amps do a great job reconstructing the performers on a virtual stage. The performers take on very realistic proportions. The amplifiers effortlessly place each instrument and performer without any significant smearing of the image.
Comments About Speaker Matching to Low Powered Tube Amps
Please, don't let what you're about to read, scare you off because it shouldn't. If you've never owned or operated a low powered, high efficiency speaker combination, these are a few of the things you need to be aware of before you dive headlong into this area of what I consider the best sound on the planet. Look at it this way, if you were going to the Caymans to scuba dive, wouldn't you take some lessons before you strap on a tank, jump in the water and start sucking on your regulator?
All too often I see people write that they didn't get the ‘quality' of sound they expected when they listened to a low powered amplifier. They claim they heard slow sloppy bass, poor image reconstruction or even a tilting of the tonal balance. The trouble with these comments (more often than not) is that they have tried to mate a low powered tube amplifier that has a miniscule damping factor to the wrong speakers.
If and when you decide to get into the world of tube amps, it doesn't matter if it is a Single Ended Triode or some huge multi-tube behemoth like the Audio Research Reference 610T, you must pick the correct speakers to mate to these amplifiers. You can't go out and pick any old set of speakers and expect them to work well with a tube amp. You need to do some homework first.
You need to know that just because a speaker claims high efficiency does not mean that it will work with a tube amp that has a low damping factor. There are a number of ‘high efficiency' speakers on the market that pose too great of load for an amp with low damping factor. Remember, tube amps provide ample voltage at very low current. Solid state amplifiers provide tons of current which (in turn) puts a strangle hold on a pair of speakers.
Certain line arrays and some speakers that are designed using higher efficiency drivers and even some prototypical vented enclosures have been known to give a tube amp fits. And finally, let us not forget, just because a speaker has a Klipsch, JBL or an Altec label, does not mean that it automatically will work with a tube amp.
Second, a tube amp that sports a Push-Pull design, does not automatically mean that the amp has a large damping factor. In order for a tube amp to perform well, the speakers impedance response must be reasonable. Huge swings of impedance at a speakers Fs (resonant frequency) that go much above 40 (or so) ohms or that dip below (say) 4 ohms can spell disaster to the overall sound of the lowly tube amp that has a damping factor in the single digits. Additionally, large impedance swings can take place at the crossover points between the speaker drivers. These too can negatively effect the sound of what would otherwise be a perfectly fine sounding tube amp. When you (improperly) match these type speakers with a tube amp, you are asking for slow, sloppy, vague, undefined sound.
So In The End…
I can say without reservation that the time I spent with the Audio Space 6M 300Bpp was thoroughly enjoyable. The additional headroom this Push-Pull design provided was a welcomed addition to my system.
I found the sounds that came from these ultra-cool looking monoblocks to be very good. The 6M's presentation was clean and clear. Though sometimes lacking in lesser amps, the midbass reproduction coming from the Audio Space amps were more than ample for my Lowther PM2As. The highs were clean and extended without ever coming across as being ‘bright'. For those of you pushing big woofers, the low end is just what you'd expect from a quality Push-Pull design using very good transformers.
When it comes to Audio Spaces choice of using solid state rectification, I'd have to say it is pretty much a non-issue. Although tube rectification has its advantages, a well designed solid state power supply can sound very good, as it does in the 6M. The other plus is you don't have to worry about ‘loosing' a rectifier tube and taking out half of your power supply.
As I look at the surrounding competition of the Audio Space amps, the 6M's fall towards the bottom of the pile when it comes to pricing. Considering the quality of parts, the solid design, the amplifiers aesthetics and finally the sound, frankly, I'm quite surprised. The vast majority of their competition is almost double and in some cases triple their cost. A quick Google search on a "Push-Pull 300B amplifier" brings up some rather large industry names, most of whom are considerably more expensive. Remember in this business, more money does not necessarily equate to better sound. More often it only means ‘different' sound, and as we all know, sound is a purely subjective issue.
When all is said and done, I personally found the Audio Space 6M 300B pp amplifiers a true joy to live with. The slight reduction in soundstage when compared to a single ended triode design didn't diminish my thorough enjoyment of the amplifiers at all. Their resolution was very good. So good (in fact) that I didn't long to put my DRD's back into the system, and that's saying something.
The other item I really enjoyed was the drastically increased headroom their 21 watts gave me. Never once did these amps run out of gas with my Lowthers. As many of you know, when you feed Lowthers some serious wattage, they can really come alive. They show exactly what they are made of. The dynamic peaks and infinite detail I experienced with the Audio Space 6M's will defiantly be missed.
So, if you are in the market for a fine sounding Push-Pull 300B, you likely won't go wrong with the Audio Space 6M's. Properly mate them with the right speakers and you'll likely fall in love with these amplifiers. They are well worth checking out.
Please keep in mind this rating system is used to compare the Audio Space 6M 300B's against absolute perfection, or a money no object PP design. If you see what you think may be a low(ish) score, it's because there are PP 300B designs that are even more refined but consequently cost considerably more. To top that off, if I assign 5's across the board, I've just painted myself into a corner leaving no room for that ‘ultimate' amplifier. You won't see me handing out many 5's. In turn, I feel I need to leave room in the ratings system to accommodate those amplifiers.
Type: Stereo integrated amplifier
Tube Complement: two 300B. one 6SL7 and one 6SN7 per channel
Output Power: 21 wpc
Frequency Response: 20Hz to 20kHz (+/-2dB)
Output Impedance: 4 ohm or 8 ohm
Input Sensitivity: 300mV/LINE IN, 670mV/DIRECT IN
Input Impedance 100Kohm
Signal to Noise Ratio: >87dB
Dimension: 470 x 180 x 195 (DxWxH in mm)
Weight 15kg each unit
Price: $3,690 per pair
Voice: (905) 470-0825