VSP Labs Trans MOS Power Amplifier Review

The name of VSP Labs’ Trans MOS power amplifier contains key clues to its special features. Trans refers to its circuit topology (a transconductance amplifier), and MOS refers to its output devices (power A/OSFET’s). It is rated to deliver 150 watts per channel into 8-ohm loads from 20 to 20,000 Hz with no more than 0.05 per cent total harmonic distortion.

A “transconductance amplifier” is merely one in which an input voltage signal is converted to an output current signal. This is the basis for the operation of all vacuum-tube amplifiers, and MOSFET’s (metal-oxide-semiconductor field-effect transistors) have similar operating characteristics. Ordinary bipolar transistors, on the other hand, amplify an input current to a larger output current. In an ideal world, there probably would be little difference between the two types of amplification, since all dynamic loudspeakers must be driven by both a current and a voltage (in other words, they are neither infinite- nor zero-impedance devices). But all transistors have inherent nonlinearities and response-time limitations that usually require the use of considerable amounts of negative feedback to achieve the low distortion values attained by today’s amplifiers.

In recent years, much has been said and written about the transient distortions that can occur when very large amounts of overall negative feedback (from output to input) are used in a power amplifier having a limited open-loop bandwidth. One way to reduce these effects is to use nested feedback loops, with lesser amounts of feedback being applied around individual stages of the amplifier, and a smaller than usual amount of overall feedback around the whole amplifier. Properly executed, such a design can provide the low distortion of a high-feed-back amplifier along with the transient stability of a low-feedback amplifier.

In the VSP Trans MOS, the MOSFET output devices provide a tube-like amplifying characteristic (believed by some to produce a sound quality superior to that of a transistor amplifier) while retaining the other advantages of solid-state design (small size, low heat dissipation, long life, etc.). In addition, the amplifier uses nested feedback loops for minimum high-frequency distortion and high stability. The self-limiting characteristic of power MOSFET’s eliminates any need for the usual current-limiting protective circuits, which have been known to have undesirable side effects. The VSP Trans MOS is fully protected, however, by high-temperature sensors that guard against excessive operating temperatures by reducing the signal level and flashing an over temp warning light.

The Trans MOS is a compact but rather heavy amplifier. It measures 14 inches wide 13-1/2 inches deep, and 5 inches high, but much of the width is accounted for by the massive heat sinks on both sides. Perhaps because of its visual compactness, the amplifier feels even heavier than its 40 pounds. On the front are a rocker-type power switch, a green pilot light, the red over temp light, and two amber clipping lights. The rear of the amplifier contains heavy-duty five-way binding-post speaker terminals, the phono-jack inputs, two slide switches, and the line-fuse holder. One of the slide switches selects mono or stereo operation (in mono, the Trans MOS becomes a 300-watt amplifier for a 16-ohm load), and the other connects an active infrasonic filter (rolling off at 18 dB per octave below 20 Hz) into the otherwise fully direct-coupled signal path. The suggested retail price of the VSP Trans MOS power amplifier is $975. It is also available in kit form for $850.

Laboratory Measurements

The exterior of the Trans MOS became fairly warm (though never too hot to touch) during our tests, and it was perceptibly warm even when idling, but the over temp light never came on. When we tried to measure distortion at high-power outputs into 2- and 4-ohm load impedances, the amplifier shut down. To restore operation we had to switch off its power for periods of a few seconds to a couple of minutes (the green power pilot light went out when the amp shut down). This mode of protection was not discussed in the instruction manual, but it was undeniably effective in safeguarding the amplifier without limiting its short-term power output.

The clipping-power output at 1,000 Hz (both channels driven) was 200 watts per channel into 8 ohms and 300 and 400 watts into 4 and 2 ohms, respectively. Using the tone-burst signal of the dynamic-power test, we were able to measure clipping-power outputs of 302, 480, and 625 watts into impedances of 8, 4, and 2 ohms. The 8-ohm clipping headroom of the Trans MOS was 1.25 dB, and its dynamic headroom was a substantial 3 dB.

In its distortion characteristics the Trans MOS was distinctly different from most other amplifiers we have tested. The total harmonic distortion was almost independent of frequency and also varied little over a wide range of power output. It consisted of a number of harmonics (we measured up to the fifth, but even higher orders could be seen at reduced amplitudes on our spectrum-analyzer screen) whose amplitudes fell off rather slowly with increasing frequency. But since the amplitudes were typically in the range of —80 to —90 dB, their rms sum was still low. At rated power or half power into 8 ohms the total distortion was typically 0.02 per cent (varying from 0.015 to 0.03 per cent between 20 and 20,000 Hz). At one-tenth power (15 watts) it was between 0.008 and 0.02 per cent, averaging about 0.01 per cent.

At 1,000 Hz, the distortion into 8-ohm loads rose smoothly from 0 0032 per cent at 1 watt (it was below the system noise at lower power levels) to 0.02 per cent between 100 and 200 watts. Driving 4 ohms, the distortion curve was nearly the same, with readings of 0.01 5 per cent at 1 watt and just over 0.04 per cent between 100 and 200 watts. When we tried to measure distortion at 300 watts, the amplifier shut down in about 20 seconds, before the measurement could be completed, although it had not yet clipped. Into 2-ohm loads, the distortion was about 0.05 per cent from 1 to 10 watts, and it was in the range of 0.08 to 0.85 per cent from 30 to 300 watts (at which point the amplifier again shut down). The IHF IM (intermodulation) distortion, using a two-tone test signal of 18 and 19 kHz with a peak amplitude equivalent to 150 watts into 8 ohms, was a very low —93 dB for the 1,000-Hz difference product and —60 dB for the third-order product at 17 kHz The amplifier required an input of 13.5 millivolts for a reference output of 1 watt, and its A-weighted noise level was —97 dB referred to 1 watt. The slew factor exceeded our measurement limit of 25, and the Trans MOS was completely stable with our reactive simulated-loudspeaker loads.

Comment

Perhaps those audiophiles who can detect audible differences between tube and transistor amplifiers would be able to classify the Trans MOS as akin to one or the other. Frankly, since we have never been able to detect such distinctions in the absence of clearly measurable performance differences, we can only say that the Trans MOS sounds very much like any other fine power amplifier of similar output capability and low noise and distortion.

That does not mean, however, that the overall performance of the Trans MOS is identical to that of such other amplifiers. For one thing, the Trans MOS is capable of delivering enormous power outputs to very low load impedances, and it apparently has none of the problems associated with typical protective circuits. In this connection, we noted with interest some studies made in Finland by Dr. Matti Otala and others and described in a paper given at the Audio Engineering Society convention last March in Eindhoven, Holland. The studies suggest that some highly respected commercial (not “exotic”) loudspeakers may under some conditions (when driven by pulses or square waves) draw signal currents four to five times greater than would be delivered to a substituted 8-ohm resistor Dr. Otala has since informed me that he has measured current ratios as high as nine with some speakers. One can infer from this experimental stud} that an amplifier incapable of delivering such high currents (that is, unable to drive a very low load impedance, possibly 1 ohm or so, at a substantial power level) may distort under some conditions even when it is being operated well below its normal maximum output. If this should prove to be a real problem (that is, one encountered with normal music signals), it would seem that the Trans MOS would be able to “loaf along” under circumstances that would severely overtax many comparably rated amplifiers.

In any case, we were strongly impressed by the ruggedness, both mechanical and electrical, of the VSP Trans MOS amplifier. Nothing we could do to it seemed to bother it in the least. In listening tests, naturally enough, it sounded perfectly fine, with a gratifying absence of transient turn- on thumps or other unwelcome sonic intrusions (even without the infrasonic filter switched in).

We do not know how much work is involved in assembling the Trans MOS in its kit form, and the saving is only a modest one, but at the kit price it is competitive with comparable better-known power amplifiers. Even in kit form, the VSP Trans MOS is a fairly expensive amplifier, but considering what it can do and how it is built, it is not at all overpriced.

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