Component Reviews

Consonance M400S EL34 Monoblock 40w Push-Pull Vacuum Tube Power Amp No.2

Published on February 1, 2005

No. 2 [No. 3] [No. 1] •   January-February 2005

Consonance M400S EL34 Monoblock 40w
Push-Pull Vacuum Tube Power Amp

SRP: $1995 pr.

Consonance M-400S tube monoblock amps

Opera Audio Co. Ltd.
Beijing, China
US Importer/Distributor:
Nat Distribution/Quest for Sound
650 Cedarbrook Road
Southampton, PA 18966

Control Functions: Power on/off toggle at rear
Inputs: RCA left & right
Output Impedance: 4 or 8 ohms, user selectable
Output Power at 1 kHz for less than 1%THD: 40 x 2
Bandwidth at -3dB: 6Hz to 60Hz
Signal to Noise: 90 dB
Input interfaces: 1 group (RCA)
Input impedance: 100k
Power consumption: 90w x 2
Tubes: EL34 x 4; 6922 x 2; 6189 x 2
Dimensions: 355 L x 200 H x 150 W mm x 2
Weight: 11kg x 2 (Net); 22kg (Shipped)


Opera Audio is one of the many Chinese manufacturers rapidly gaining a foothold in the world of high end audio. They have several different series of components, but this monoblock amp is part of the M-Value Series, which in addition to the M400S offers the M-100 Plus EL34 integrated amp, the T-1 12AU7 Line Stage Preamp, and the M500S Single Ended monoblock amps using 300B tubes in parallel. In their other lines Opera also has two stereo SACD players, a turntable, vacuum tube tuner, integrated amp and preamp, a phono stage and two headphone amps.

I became acquainted with the M400Ss when I mentioned to Albert von Schweikert that although his VR-2 speakers I was also reviewing were not yet completely broken in, I was concerned about a hardness in the treble that I was hearing. He suggestion I might like to try bi-amping the speakers – relegating my Parasound solid stage amps to the woofer section and powering the front and rear tweeters only with a much lower-wattage tube amp. He recommended the Consonance M400S.

I went ahead and requested a pair of the monoblocks for review. They came extremely carefully packed, in separate, much larger boxes that as you can see by the specs above ended up doubling their weight. I had not dealt with vacuum tubes for many years. I once had a fine Counterpoint hybrid preamp but that was some time ago and my last experience with tubes involved a phono preamp that picked up both hum and a local FM station and quickly went back to the maker. However, upon disconnecting the jumper bars on the VR2s and hooking them up to power the tweeters I heard a lovely enhancement of the high end and most of the hardness disappeared. (The rest went away after I had fully broken in the speakers, added lead shot to them, and eliminated the Outlaw Bass Manager and all the additional cables I had in the circuit.)

Using Amps Fullrange on Speakers

After enjoying them a while in this setup, I was rereading the VR2 manual concerning impedance and power requirements of the speakers. I was surprised that it suggested as little as 20 watts was sufficient to power them fullrange. I knew the dictum that a watt of vacuum tube power is more powerful than one watt of solid state power, so I thought I would try running the speakers fullrange with just the tube monoblocks. I reconnected the jumpers and did so.

I found the choice of 20 to 50 watt or so basic tube amps to be very limited. There appear to be many integrated amps of this type however. Most tube power amps available today seem to be in only two areas – the absurdly low power 9-watt single-ended wonders so popular with the SE/horn crowd, and huge and outrageously expensive 150-watt or 200-watt tube amps.

The results with the 40w monoblocks were astonishing. I felt my entire system had taken on a new and much more musical quality than when either my former very inefficient Celestion minimonitors or my present VR2s were powered from my 220 watts-per-channel Parasound amp. Strings took on a much more realistic timbre without any of the hardness heard via most solid state gear. Piano reproduction, which is supposed to be better with solid state, sounded to me (and I’m a pianist) more natural, more dynamic and there was more of the sense of the strings’ vibration than just the general impression of the piano as a percussion instrument. It was just easier on the ears.

And this was before I had installed the spikes on the speakers, put any cones under the amps, any rings or collars on the tubes, connected any accessory AC cables, or installed the new bi-wire speaker cables I had on order. I was using some standby speaker cables sent by a small manufacturer for review. (They had sounded considerably worse than the test cables I had made up from Lowe’s 6 gauge stranded wire as per the instructions in Bound for Sound. So I decided it best to not review them at all.) And yet everything was sounding great.

Theory of Operation

I’ve had to learn some of the little additional tasks required of tube aficionados. One was the biasing. The M-400Ss have RCA jacks in front of the two EL34 tubes on each monoblock, and a little lock-collar screw just behind each tube for setting the bias. The jacks are unusual – the standard approach is to provide two little insulated holes in which to jab the prods of the multitester – sort of like an electronic Dracula had been at work. I had to go out and purchase a multitester to check the bias of all 4 tubes (the small tubes don’t require this and neither do the single-ended Opera amps). I found the factory settings to be accurate on all 4; they are -35V sustained biased voltage.

The monoblocks are push-pull Class AB mono power amps with little global feedback. The signal goes first to the 6922 dual triode acting as the input stage, then to the ECC82 dual triode acting as the drive stage. The ECC82 drives the big EL34 output tubes – one matched pair per channel. The EL34s are transformer-coupled to the load, which in this case is the speaker. The final tubes in push-pull amps are always in pairs – two or four, etc. The Class AB design means that much greater efficiency than strict Class A is achieved by having one tube’s grid driven until its plate current cuts off completely, then the other tube takes over and handles the power output. This can result in greater distortion unless carefully designed, and requires some negative feedback. But it also runs cooler.

There is a warning to never disconnect the speaker while the amp is on, and to always turn on the preamp first before turning on the power amps. The tiny toggle at the rear of the amp might be a pain to reach on each amp if you have the good-looking wood front panel facing forward, but it is perfectly safe to run your amp AC cords to a power bar (no surge protection please) with a single switch to turn both on together.
Rear Panel of M-400s
There are three 5-way speaker terminals. The two hot terminals offer a choice of 4 ohm or 8 ohm output. The small on/off toggle switch is visible next to them, as well as the receptacle for the detachable AC cord.

Triode vs. Ultralinear

There is one other toggle switch on each monoblock towards the front. It is labeled Ultralinear toward the front and Triode towards the rear. Triode mode reduces the output to about 22 watts instead of 40 but tightens the bass end somewhat and gives sweeter, more “tubelike” midrange reproduction. The Ultralinear mode was an invention of David Hafler and Herbert Keroes in l951. It uses tetrodes or pentodes and special taps on the output transformer. These tapes are connected to the screen grids of the tubes, causing them to be driven with part of the output signal. This lowers distortion, and is seen on amps using only certain power tubes, of which the EL34 is one.

I found it extremely difficult to hear a major difference between the two settings on most source material. One is advised to always turn the amps complete off before switching from one setting to the other. I found some decisive source material was to use solo vocals with band behind. After running several of these I was able to identify that I preferred the Ultralinear setting. It sounded more musical, more full-bodied and richer. One user referred to it as having “more slam.” The Triode setting sounded a bit thin and there seemed to be less support rather than better bass support. I was aware of the lower power output. Perhaps with some even more efficient speakers some users might prefer the Triode setting, but I did not.

Tube Deficiencies in Lowest Bass

This brings up the general conception of tubes being the cat’s meow in the midrange but pooping out in deep bass extension and rolling off in the high end. The fault in the lowest end is due not really to the vacuum tubes but to deficencies in the required output transformers, including delays in transient response. This can cause sluggishness in the bass below 50Hz or so – if your speaker goes down that far. I took a major step in switching to the fullrange VR-2s from a system of 5 minimonitors plus 2 subwoofers. I don’t feel I’ve lost much deep bass in the process and find that what I have is much better integrated with the rest of the spectrum. String basses, for example, now sound realistically front and center whereas before they seemed off in some nebulous area that couldn’t be placed specifically. Since my interest is more in the music-in-surround area than home theater spectaculars I find the fullrange speakers (which get down to 25Hz) the way to go. [Some home theater experts recommend the same thing - see WideScreen Review for their "holosonic" standards.] In fact, even with movies in surround there is a greatly-enhanced sense of envelopment because now there is true bass end coming from the surrounds whereas before it rolled off below 70Hz because there was no sub in that area.

I did some comparisons of the deepest bass reproduction between the Consonances and my solid state 220w multichannel amp before retiring the latter. I found that originally there was a slightly tighter and stronger bass end with the solid state powering, however it was extremely close. Since then I have mounted the speakers on their spikes, which makes a major improvement in bass delivery. I have also put a small amount of Dacron fibrefill into the port, which strengthens the lowest bass when used with tube amps. You just push it into the vent by the handful – with your hand if it’s small and with a broomstick if your hand won’t fit. If the bass becomes too tight-sounding you can always pull out some of the Dacron stuffing. The final step will be to switch the factory-provided Chinese tubes to the Tesla E34l blue tubes (I thought going blue meant the tube was shot – shows what a tube newbie I am…) which I am told improves the bottom end even further. Unfortunately they didn’t arrive in time for this review.

Wrap Up

While I’m not as far out of the loop as those who think tubes are totally obsolete technology and not used anymore, I hadn’t had a comparison between tubes and solid state amping in my own system for many years. This one convinced me so thoroughly that I purchased not only the two Consonance amps but ordered a third to power my front center channel. The distributor told me that some customers have purchased five of them to power their entire surround speaker system!

One of the top achievements of the Celestion minimonitors I used for years was their excellent imaging qualities. In spite of this on many recordings when the center channel was unused the left and right speakers did not always provide a very identifiable phantom center image. The VR2s are better at this and I’m not even missing my center channel greatly at the moment with videos (while waiting for the third amp to arrive) because the dialog seems to be coming from the dead center of the screen of my RPTV between the speakers. I was impressed to discover that the Consonance amps improved this phantom center channel imaging even further than did the solid state power. With the best recordings there is an almost holographic soundstage.

What might seem like a wrenching reduction from 220 watts per channel to only 40 really is not in this case. Tube aficionados often say that a good rule of thumb is that one tube watt is equal to two or three solid state watts. Of course at the same time I switched from extremely inefficient sealed-box minimonitors to very efficient transmission line ported speakers. Another disadvantage suspected of tubes is that they are much noisier than solid state power. Although the specs may not be up to the level of solid state – only 90dB S/N in the case of the M-400S – the actual subjective experience proved differently. I always had some noise and hum present at the speakers if you put your ear close to them when using the 220w amps with the Celestion minimonitors, and this was in spite of the Bybee filters I had mounted on them. With the much lower power Consonances and the much higher efficiency VR2s I am hearing dead silence right at the drivers with the volume at a normal setting.

The M-400Ss may not be exactly an audio bargain, but I’m sold on them and they have little competition. The combination of solid state preamplification and tube amplification seems to work just as synergistically as that of tube preamp and solid state power amps which I once had. The M-400Ss are beautifully retro looking and certainly easy on my ears – perhaps they will also be on yours; check them out. The Opera Co. certainly doesn’t puff up their promotional copy about the amps: In their color flyer on the different models they state: “Listeners can expect to hear a classic tube sound which approaches that of a high end system for a price most ‘philes or music lovers can afford.” Hmmm – approaches?

– John Sunier

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