VTL 100W Compact monoblock power amplifier Page 2

Regarding the output transformer itself, rather than supply a number of secondary taps, for example labeled "2," "4," "8," and "16" ohms, VTL's David Manley prefers to provide the 100W mono with just one, hardwired to the output terminals and set to a nominal 7.5 ohms impedance (footnote 5). The reason is eminently sensible: with conventional multiple taps, the negative feedback taken from the secondary will only be optimum for one tap; for the others, a small series impedance will exist between the amplifier's output terminal and the point where the feedback signal is derived. The back EMF from the loudspeakers will thus cause an error signal to be developed across this impedance which will then be fed back to the amplifier's input via the negative feedback network. With the VTL arrangement, the loop feedback (about 15dB) is always taken from the most direct place, the output terminal.

The housekeeping circuitry of the VTL 100W Mono is high-quality. The input stage heaters are DC-powered via a full-wave diode bridge from a separate winding on the transformer, this also feeding AC to the output tube heaters. The HT power supply is where the biggest differences between a modern tube amplifier and those of 25 years ago lie. The B+ rail of the VTL 100W Mono, cruising at a high 490V or so, is derived from a full-wave–rectified winding on the transformer and smoothed by four 1000µF electrolytic caps. According to the schematic in The VTL Book and confirmed when I took the bottom cover off, these are connected in series/parallel, which results in a combined total of still 1000µF, but with twice the voltage rating of course. This gives a total energy storage, therefore, of 120 joules; while not quite as high as the 250 claimed by VTL, this is still considerably higher than a typical 100W solid-state amplifier's 32 joules.

Internal construction is based on a single, large, double-sided printed circuit board, which carries all the audio circuitry apart from the transformers, the filter caps, and the latter's associated bleed resistors. Component quality is excellent—coupling capacitors are either WIMA polypropylenes or polycarbonates; shielded Cardas Cable is used to connect the rhodium-plated input jack to the circuit board; and the output tubes are from Mullard in the UK. With the exception of one or two electrolytics which seemed to have been shoehorned into available corners (VTL uses the word "Compact" advisably), and rather more solder resin in evidence on the board than I would have liked, the build quality is excellent. My only gripe is a small one, concerning VTL's choice of output connectors. Though gold-plated, and capable of taking both spade lugs and 4mm plugs, these can only be finger-tightened (though I must admit that the connections didn't loosen at all during my auditioning.)

Sound quality
The VTL 100W Monos were pressed into service for nearly three months of recreational listening (read pleasure) and reviewing service. However, setting up the amps proved a frustrating business in that at first I just could not rid the system of what appeared to be a ground-loop hum on the phono input. I tried establishing the system ground at either or both of the VTLs and floating the preamplifier: there was no reduction in hum. Floating the VTL mains plugs on two-pin ground cheaters didn't help. In the end, it appeared that the problem was not primarily due to a ground loop but was due to the VTLs' AC line transformers throwing out one mother of a hum field which was either being injected into the Krell's low-level circuitry or being picked up by the pickup cartridge. Moving the amps as far away as possible to the preamp's right, on the opposite side from the phono input, almost rid the system of hum, but there was still a slight residual (an inaudible –75dB on the preamp's line inputs, –60dB on the phono inputs, slightly audible in the quietest passages but only just above the Krell's noise floor) which I had no choice but to live with.

This was exacerbated by the power amplifiers' high sensitivity. With CD, the Krell's volume control had to be used in the lower half of its range, 12 noon running the VTLs into clipping. David Manley insists that his choice of a 775mV sensitivity is to ensure compatibility with professional studio equipment; in my opinion, as American consumers are the most likely people to use his gear, he should decrease the sensitivity by 6–9dB or so, perhaps by upping the amount of negative feedback a little. This would both reduce any system hum and noise and allow audiophiles to use their volume controls at sensible settings. (Remember, as pointed out in these pages by PS Audio's Paul McGowan in September, the volume control doesn't increase signal level, it only reduces it, so the least amount of potentiometer track in circuit the better.)

I am sure that some among you will ask why I didn't drop the VTLs' sensitivities either by feeding the Krell into an outboard stereo pot or by inserting, say, 47k series resistors into the interconnects. If the VTLs were to stay in my system, I might consider the latter, but the former I am sure is a no-no. Putting three whole 'nother sets of connections into the signal path (including the pot's wiper/track interface) to solve what, apart from the noise accentuation, is largely a convenience problem, seems like a step back to me.

Enough of the grumbles: what did the VTL 100W Monos do to the sound of my system?

First, as any power amplifier should, they did very little. I am not quite talking "straight wire with gain," but their faults were minor. Maybe a slightly forward midrange, along the same lines as that of the KRS2; maybe a slightly looser bass than the Krell KSA-50; maybe . . . actually, these don't matter. The VTLs drove all the speakers with which I tried them with authority. While not quite as iron-fisted as the Krells or Mark Levinson No.20s, low frequencies were astonishingly tight for a tube amplifier. When the music plunges deep—the tolling 32Hz Cs in the central arch of Debussy's La Cathedrale Engloutie (Carol Rosenberger on Delos), for example—the VTLs provide the full measure of weight (yes, even on Celestion SL700s) without losing sight of the notes' harmonic structure.

I would go so far as to say that the combination of the VTL 100W Monos and the Celestions is ideal for the reproduction of piano music. The Julius Katchen Paganini Rhapsody on London Weekend Classics (417 880-2), the Serkin Brahms Concerto 2 with Szell (CBS "Great Performances" MYK 37803), the superb performance from Ivan Davis of Grieg's Holberg Suite on Audiofon (CD72022), the James Boyk recordings, my own analog recording of Ann-Maria Stanczyk playing Chopin . . . one piano record kept leading to another.

This is one hell of a transparent amplifier. The high frequencies were neither granular nor dull, but held in check the slight propensity for the top two octaves of the SL700 to be too forward.

The soundstage was as wide and almost as deep as I have heard. The decoding of the totally synthetic soundstages on, for example, Michael Hedges' Aerial Boundaries (Windham Hill WD-1032) was totally convincing in its ersatz reality. Perhaps the similar natures of the KRS2 and VTL midranges resulted in a more forward perspective than would be desirable in absolute terms, but I didn't feel throughout any of the auditioning that I was getting too much of a good thing in this area. I would have liked to have tried the Mark Levinson No.26 with the VTLs, which is more laid-back than the Krell and might make a synergistic combination with the 100W Monos, but as with the Mark Levinson power amplifiers, Larry Archibald seemed more than a little unwilling to part with it.

Perhaps the only long-term limitation concerned dynamics. Although tube amplifiers are supposed to go louder than an identically rated solid-state design, I kept wishing for just a little more urge from the VTLs with the SL700s when compared with the Krell KSA-50. But as Peter Walker always used to say (and probably still does), "You need to buy a bigger amplifier!"

Oh, and in true Columbo fashion, there was one other thing. During the testing, I accidentally disconnected the input RCA plug on one of the amplifiers while it was connected to a dummy load: as the ground connection is broken first, the result was the injection of high-amplitude garbage into the VTL's input. True to David Manley's warning, the EL34s glowed, accompanied by a sizzling sound, in the split seconds before I could flip the on/off switch. Luckily, there seems not to have been any permanent damage, but the point should be made: While you should never unplug the input lead from a power amplifier while it is turned on, with the VTL designs, you should never, never, NEVER unplug the lead.

To say that I was impressed with the VTL 100W Compact Monoblock is an understatement. My experience of power amplifiers over the last five years has led me to believe that either a solid-state model with a massive or regulated power supply and class-A output stages—the Krell designs, for example, or the Mark Levinson No.20—or a modern, complex tube design such as the Audio Research D250 or M100 were the only guaranteed routes to sonic supremacy. Yet here is a modest-sized, traditionally designed tube amplifier with which I have experienced some of the most moving musical moments in my current system.

And at what must be considered a bargain price! Gordon Holt has often asked in print what the point of buying a tube design is if it doesn't sound like a tube design; it will only be more expensive than a comparable solid-state model. Yet a pair of VTL 100W Monos costs no more than a similarly specified solid-state design, and less than its transistorized competition when it comes to sound quality and audio neutrality.

I can't see that those old bugbears of tube power amplifiers, unreliability and/or frequent and expensive tube replacement, will be factors with the VTL 100W Compact Monoblock. It seems solidly built, and David Manley appears to have been conservative in his design philosophy and selection of components.

Yes, I would like to have had a lower input sensitivity, and the fact that I couldn't quite get rid of the hum on the phono input was somewhat of an irritant. But the fact remains that I was never less than extremely impressed with the sound of this VTL amp.

Highly recommended.

Footnote 5: The output transformer has four 7.5 ohm windings, normally wired in series/parallel to give a 7.5 ohm tap. These can also be wired all in parallel to give a 1.8 ohm tap, or all in series to give a 32 ohm tap. (Though I can't for the life of me think of one modern speaker suitable for driving from this latter tap!) 4, 6, and 16 ohm strappings are also available. These custom versions must be specified in advance by the customer, as the negative feedback network needs to be altered accordingly to optimize the amplifier's overall voltage gain and output impedance.
VTL of America Inc.
4774 Murietta St. Suite 10
Chino, CA 91710
(909) 627-5944