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Vacuum Tube Logic, better known as VTL, has been in the business of making outstanding valve amplifica-

tion products for around a third of a century. It was back in the early 1980s, in South Africa, that the father and son team of David and Luke Manley produced quality valve am-plification pitched at local professional film and music recording studios. From there,

word spread and soon the Manleys crossed the ‘pond’ and started building amplifiers for the domestic market in Europe. Further success, especially after 1986’s Consumer Electronics Show, saw the company move to the United States where, for the first time, they embarked on full-scale pro-duction on a global scale. The MB-125 reviewed here, however, is entirely the work of Luke Manley, David having left VTL in 1993 to concentrate on his eponymously-named company, Manley Labs, which is now owned and operated by David’s fifth wife, EveAnna Manley. (*See left.)

Keep It SImpleAs the entry point to VTL’s line of amplifiers, the MB-125 monoblocs are almost diminu-tive by way of contrast with the high-end’s almost ubiquitous ‘bigger is better’ ap-proach. Be that as it may, but the unpre-tentious MB-125s are nonetheless built to very high standards. And the amplifiers are as simple as they are elegant. The nicely curved brushed aluminium fascia features a large central smoked glass window—which becomes a viewing porthole to the innards and the glowing glory of the EL34 power and 12AT7 signal valves within.

VTL MB-125 MonoBLoc AMpLifiers

*David Manley died of a heart attack on December 26, 2012, at the age of 73, in Varrennes-Jarcy, France, where he had moved to live with his sixth wife, Veronique.

On TEST

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Other than the window to valve good-ness, the faceplate’s only mar from total barrenness is a power switch that’s flanked by a small blue LED. Equal simplicity ap-plies to the sparsely-populated rear panel where only a single-ended RCA provides signal connectivity, an IEC socket provides the 240V a.c. connection and a very nicely machined pair of binding posts will cater for your speaker cables. A toggle switch provides the option of running the amplifier in either low-powered triode or full power tetrode mode. Experimentation here is advised as the sonic differences between each mode will vary depending on speaker load and character. Generally, it’s thought that for a purer, perhaps more full-bodied presentation, you’d choose triode mode while more punch and dynamics can be had by running in tetrode mode. But as they say, your mileage may vary…

The MB-125s are specified as being ca-pable of 55-watts and 100-watts into 5Ω in triode and tetrode modes respectively. Fre-quency response in both modes is claimed to extend from 20Hz to 20kHz at ±0.1dB. The power specifications derive from an ar-ray of four EL34 valves per monobloc which are driven by two 12AT7s. The MB-125 is quoted as being able to drive loudspeaker loads right down to 2Ω. Input impedance is a high 145kΩ which makes these amplifiers suitable to run with almost all preamplifiers I’m aware of.

Overall, the ‘fit ‘n finish’ is of a very high standard and the units feel substantial

in terms of their weight and their torsion strength. Care should be taken when lift-ing the units as they’re disproportionally weighted towards the rear (where the sub-stantial custom transformers are housed). Although I’ve listed the warranty as being two years, if you register your product with VTL’s Australian distributor, Advance Audio, it will extend the warranty period (on the amplifier only) to five years. The valves are warranted for only six months.

On a side note, VTL is not only about high level signal amplification. The com-pany’s two-box TL-7.5 preamplifier, now in its MkIII iteration, has received universal ac-claim as being of reference level and among the very best available. Similar technologies apply to the preamps on lower rungs, so

very sound one-brand pre/power combina-tions are on offer.

On a size note, each of the VTL MB-125 amplifiers measures 400×300×180mm (WDH) and weighs 22kg.

A perfect mAtch?In the world of audio, where the pursuit of sonic excellence is achieved via both scien-tific and creative avenues, the concept of synergy can be either a blessing or a curse. Insert a new component into an existing system and it could be heaven or hell, with

the former leading to long-term bliss and the latter to an expensive and bumpy seat in the gear-swapping carousel.

VTL’s MB-125 has the reputation, pedi-gree and specifications to work with an almost universal set of speakers but, darn it, they sounded outstanding with the ‘on-paper’ difficult-to-drive Wilson Sasha Watt/Puppies. Here were two simpatico compo-nents that greeted each other, shook hands, and went about producing some of the best sound I’ve heard.

The MB-125s would have to be amongst the sweetest yet most hard-driving valve amplifiers given their moderate power rating (I also run a 750-watt solid-state monster—so we’re talking about gravitating from the sublime to the ridiculous).

For a purer, perhaps more full-bodied presentation, you’d choose triode mode...

VTL MB-125MonoBLoc AMpLifiers

Brand: VTLModel: MB-125category: Monobloc Power Amplifiersrrp: $8,495 per pairWarranty: Two Years (Six months on valves)Distributor: Advance Audio AustraliaAddress: Unit 8, 509–529 Parramatta Road Leichhardt NSW 2040T: (02) 9561 0799e: info@advanceaudio.com.auW: www.advanceaudio.com.au

Triode & TetrodeMagic midrangeChino not China!

Power consumptionRuns hot

vtl mb-125 mOnOblOc amplifiErS

Power Output: Single channel driven into 8-ohm and 4-ohm non-inductive loads at 20Hz, 1kHz and 20kHz using tetrode mode.

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Power Output: Single channel driven into 8-ohm and 4-ohm non-inductive loads at 20Hz, 1kHz and 20kHz using triode mode.

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LAB REPORT: Turn to page 58Test results apply to review sample only.

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56

The amps exerted impressive control, not only over the twin high-power 203mm woofers, but with the overall sonic presen-tation, especially in the dynamic contrast domain. Demanding musical fare was just a walk in the park for these amps and, much like the many other US-based valve amps I’ve reviewed in Esoterica, the VTLs managed to tread a space where profound detail, resolu-tion and instrumental separation were the bedfellows of warm, full-bodied, yet quite fast and furious sound. Drums and percus-sion, toms and kick in particular, displayed a ‘roundness’ and a vigorous attack with-

out excess bloom or boom—just a more expressive, less-dry presentation than some high-damping solid-state amplifiers, but one just as powerful-sounding and, in this case, more satisfying. In that last sense, the MB-125s sound like far more powerful beasts than their specifications would suggest. And that, surprisingly, was in triode operation, my preferred mode of listening, with the tetrode mode bringing a bit more punch but at the same time losing some ‘bloom’ and magic in the midrange… but only a tad, mind.

And there was plenty of magic in the all-important midrange. Whether vocals or brass, or the lower violin and upper cello notes—and so much more—the MB-125 monos were always resolute and sweet. Rarely have I experienced such a total lack of grain or etch. And again, stressing the solid-ity of the images, the large sense of space, the superb instrumental separation and the surprising dynamic modulation. In my experi-ence, some amplifiers at this price-point exhibit a limitation of some form; sometimes it’s a tad too much bass bloom (possibly as-sociated with cost-savings and compromises in the transformer design) or a colouration or lack of transparency in the mids or highs

(the association there may be related to circuit and component compromises). The MB-125s just played music; always involving, always utterly listenable.

concluSIonIn my opinion, having at least one item in the system chain sporting valve circuitry brings the magic. The crowded Australian market offers plenty of valve amplifiers, and certainly a whole host of budget fare in over-the-top-bling livery from China, but what the VTL MB-125 monos offer is understated elegance mixed with solid engineering and all-Ameri-can manufacturing (in Chino not China).

Few amplifiers have had the impact or had my reference system singing to the triumphant levels as the MB-125s. Call it synergy, within the microcosm of my refer-ence system, or call it all-round potential excellence in the audio world at large. Sir or madam, one thing is for sure; shopping at this price-point and not trying VTL’s MB-125s is totally at your peril because, in the world of the high-end, these VTL MB-125 mono-blocs are one hell of a bargain.

Edgar Kramer

Rarely have I experienced such a total lack of grain or etch ... and there was plenty of magic in the all-important midrange.

Power Output: Single and both channels driven into 8-ohm and 4-ohm non-inductive loads at 20Hz, 1kHz and 20kHz using pentode and triode modes. [VTL MB-125]

TEST RESULTS ON PAGE 58

58

lab rEpOrT vtl mb-125 mOnOblOc amplifiErS

lAborAtory teSt reSultSOne reason audiophiles (and musicians) like valve amplifiers is that they overload very gracefully… so gracefully that it’s difficult to perceive they’re overloaded at all. Whereas solid-state amplifiers go straight into hard clipping at which point they suddenly generate (literally) hundreds of high-level odd- and even-order harmonic distortion components, valve amplifiers clip ‘softly’ and, when they do, generate only a few, low-order harmonic distortion components, many of which sound so pleasant to the ear (being predominantly low-order and even-order harmonics) that it’s difficult (perhaps impossible) to hear they’re overloading at all. This ‘soft clipping’ effect also makes it difficult to measure the exact output power of valve amplifiers, because soft clipping is also difficult to observe visually on an oscilloscope screen, whereas hard clipping is instantly visible. As a result, the power output figures measured by Newport Test Labs were not taken at clipping, but at the level where THD exceeded 3.0%.

If you’re comparing the tabulated figures with the manufacturer’s specifications, you should note that Newport Test Labs used standard 8Ω and 4Ω test loads during the measurements, whereas VTL for some reason used 5Ω test loads when compiling its specification sheet, so the figures are not directly comparable… though 4Ω is close enough to 5Ω that any differences between using one rather than the other would mostly be academic. So I’ll start with the 4Ω loads and, using its high-output (tetrode) mode, the VTL MB-125 delivered 81-watts

(19.0dBw) at low and midrange frequen-cies, but managed only 60-watts (17.8dBw) at 20kHz. In its low-output (triode) mode, the VTL MB-125 was measured as deliver-ing 40-watts (16.0dBw) at low and mid frequencies, and 20-watts (13.0dBw) at 20kHz. As you can see from the tabulated results, the power output figures were not substantially different when the ampli-fier was driving an 8Ω load. Although the amplifier falls short of VTL’s ratings, it didn’t fall much short, in fact it was just 1.4dB shy of spec in triode mode and just 1dB shy in tetrode mode (using the result at 1kHz as the reference).

The frequency response of the VTL MB-125 was extended, reaching out to 94kHz (–1dB) and a fraction further (110kHz) if we look at the 3dB downpoint. At the low-frequency end of the spectrum, the VTL was 1dB down at 12Hz and 3dB down at 7Hz. This puts the ‘normalised’ response at 12Hz to 94kHz ±0.5dB, or 7Hz to 110kHz ±1.5dB. Within the audio band, the response was much flatter again, at you’d expect, and as you can see from Graph 5 (ignore the tiny ripples on the traces on this graph, which were the result of some mains-frequency hum creeping into the measurements). Into a standard resistive laboratory load (the black trace on the graph) you can see the VTL MB-125’s frequency response almost tracks the 0.00dB reference line from 20Hz to 10kHz, then rises to be 0.05dB ‘high’ at 20kHz. (The response continues to rise to peak at 50kHz, before rolling off to 94kHz.) This puts the normalised audioband frequency

response of the VTL MB-125 at 20Hz to 20kHz ±0.02dB.

When the VTL MB-125 was connected to a load that simulates that of a typical two-way loudspeaker (the red trace on the graph) you can see that its frequency response was not nearly so linear as into the purely resistive load, but the overall response was still measured at 20Hz to 20kHz ±0.3dB, which is an excellent result. (The vertical scaling of this graph must be borne in mind: it’s quite extreme, with the tops and bottoms indicating +1dB and –1dB respectively.)

Graphs 3 and 4 show the output spec-trum of the VTL MB-125 when driving 8Ω and 4Ω loads at an output of one watt, respectively. As you can see, distortion is very low for a valve amplifier. Into 8Ω loads, there’s a second harmonic at –75dB (0.0177% THD), a third at –80dB (0.01% THD), a fourth at around –98dB (0.0012% THD) and that’s it (the ‘spike’ below 16kHz is a measurement artefact, and should be ignored on all the graphs on which it ap-pears). When driving 4Ω loads, the level of the second harmonic increases to –68dB (0.0398%), the level of the third harmonic to –70dB (0.0316%) and the level of the fourth to around –94dB (0.0019%) and a fifth harmonic appears at around –99dB (0.0011%). These distortion levels are all quite low, as you can see, but also, adding second harmonic distortion is musically the same as adding a second note an octave above, which the human ear finds ‘pleas-ant’ and adding a third harmonic is musical-ly equivalent to adding a fifth above, which

59

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again results in a sonically pleasing result. The noise floor on both graphs shows that there’s considerable low-frequency noise in the output, likely mains-related, which accounted for the VTL MB-125’s relatively high unweighted noise figures.

Distortion at high power output levels was high, as you can see from Graphs 3 and 4, which show output at 55-watts into an 8Ω load (Graph 4) and at 70-watts into a 4Ω load (Graph 5) both graphed using the VTL’s tetrode mode. You can see that into

an 8Ω load, except for the third harmonic, all the other harmonic components were more than 70dB down, so each contributed less than 0.0316% distortion to the total. Into 4Ω loads, save for the third harmonic at around –53dB (0.22%), the first five

0.00 Hz 6000.00 12000.00 18000.00 24000.00 30000.00

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Graph 6: Intermodulation distortion (CCIF-IMD) at an output of 1-watt into an 8-ohm non-inductive load, referenced to 0dB (Tetrode Mode). [VTL MB-125 Power Amplifier]

Newport Test Labs

10.00 Hz 100.00 1000.00 10000.00

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Graph 5: Frequency response of line input at an output of 1-watt into an 8-ohm non-inductive load (black trace) and into a combination resistive/inductive/capacitive load representative of a typical two-way loudspeaker system (red trace) (Tetrode Mode). [VTL]

Newport Test Labs

0.00 Hz 4000.00 8000.00 12000.00 16000.00 20000.00

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Graph 4: Total harmonic distortion (THD) at 1kHz at rated output (70 watts) into a 4-ohm non-inductive load, referenced to 0dB (Tetrode Mode). [VTL MB-125 Power Amplifier]

Newport Test Labs

0.00 Hz 4000.00 8000.00 12000.00 16000.00 20000.00

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Graph 2: Total harmonic distortion (THD) at 1kHz at an output of 1-watt into a 4-ohm non-inductive load, referenced to 0dB (Tetrode Mode). [VTL MB-125 Power Amplifier]

Newport Test Labs

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Graph 1: Total harmonic distortion (THD) at 1kHz at an output of 1-watt into an 8-ohm non-inductive load, referenced to 0dB (Tetrode Mode). [VTL MB-125 Power Amplifier]

Newport Test Labs

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Graph 3: Total harmonic distortion (THD) at 1kHz at rated output (55 watts) into an 8-ohm non-inductive load, referenced to 0dB (Tetrode Mode). [VTL MB-125 Amplifier]

Newport Test Labs

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harmonic distortion components were all below –60dB (0.1%), and above 5kHz, were mostly around 80dB down (0.01%) or more. The small spikes (‘grass’) around the fundamental (at 1kHz) and the harmonics indicates that the VTL MB-125’s power supply was working very hard during these (con-tinuous) tests.

Intermodulation distortion (CCIF-IMD) is shown in Graph 6, and you can see that as with almost all valve ampli-fiers, there’s a considerable (–70dB) signal regenerated down at 1kHz as a result of the two test signals (at 19kHz and 20kHz). There’s also a signal at 2kHz (–80dB). However these regener-ated signals are still quite low in level for a valve amplifier, so the VTL MB-125 appears to have less CCIF-IMD than many competitive designs.

As alluded to previously, the un-weighted signal-to noise ratios reflect some noise in the power supply section of the amplifier, with Newport Test Labs measuring overall figures of 54dB referred to an output of one-watt and 73dB referred to rated output. Apply-ing standard IHF A-weighting to these results, to simulate the level of noise that would be perceived by the human ear, you can see that the S/N figures improve to 72dB and 90dB respectively. These are good results for a valve amp.

The square wave tests (see the four oscil-lograms) present much of the data already measured in abbreviated (but difficult to read!) form (which is why square waves are such a useful tool in amplifier analysis). The VTL MB-125’s frequency extension is obvious from the 100Hz and 10kHz square waves, but the 100Hz wave also shows the MB-125 has excellent low-frequency phase accuracy. Particularly impressive is the ampli-fier’s ability to cope with highly capacitive loads (2µF). The small overshoot visible on all the square waves (including the one into the capacitive load!) is due to that peak in the amplifier’s response at 50kHz, men-tioned previously which, however, because of its frequency will be audibly benign.

Mains power consumption will be high, with this single monobloc amplifier drawing more than 100-watts from your mains at all times, and peaking at pulling more than 250-watts if you sustain playback at high volume levels.

Obviously in a system you’d have two amplifiers operating, so you need to double these figures to obtain ‘real’ consumption. The lesson is to switch your pair of VTL MB-125s off whenever you’re not using them. (And using a dedicated 240V power outlet might also be helpful).

Overall, the VTL MB-125 is a very well-designed valve amplifier that delivers excel-lent performance for its genre. Steve Holding

VTL MB-125 power Amplifier - Test resultsTest Measured Result Units/Comment

Frequency Response @ 1 watt o/p 12Hz – 94kHz –1dB

Frequency Response @ 1 watt o/p 7Hz – 110kHz –3dB

THD+N 0.18% @ 1-watt

Signal-to-Noise (unwghted/wghted) 54dB / 72dB dB referred to 1-watt output

Signal-to-Noise (unwghted/wghted) 73dB / 90dB dB referred to rated output

Input Sensitivity (CD Input) 90mV / 546mV (1-watt / rated output)

Output Impedance 0.54Ω OC = V

Damping Factor 14.8 @1kHz

Power Consumption NA / 112 watts (Standby / On)

Power Consumption 115 / 251 watts at 1-watt / at rated output

Mains Voltage Variation during Test 240 – 252 Minimum – Maximum

VTL MB-125 power Amplifier - power output Test resultsChannel Load (Ω) 20Hz

(watts)20Hz

(dBW)1kHz

(watts)1kHz

(dBW)20kHz

(watts)20kHz (dBW)

Tetrode 8 Ω 76 18.8 82 19.1 61 17.8

Tetrode 4 Ω 81 19.0 81 19.0 60 17.8

Triode 8 Ω 38 15.7 36 15.5 20 13.0

Triode 4 Ω 40 16.0 40 16.0 20 13.0

Note: Figures in the dBW column represent output level in decibels referred to one watt output.

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