Proton D1200 Power Amplifier Review

Proton’s D1200 power amplifier features a Dynamic Power on Demand (DPD) circuit design that gives it an extraordinary dynamic-headroom rating of 7.3 dB. Proton’s DPD system was first used in the 40-watt D540 amplifier, and in the D1200 it is responsible for some of the highest power outputs we have measured from any amplifier.

The DPD circuit is designed to allow an amplifier to deliver many times its rated steady-state power for short time intervals. Many musical waveforms can place such demands on an amplifier. Conventional amplifiers can deliver their rated power for extended periods and can usually develop a slightly higher output for short intervals.

Such short-term output is the subject of the EIA dynamic-headroom specification, which is based on a 20-millisecond tone burst repeated twice per second. Typical amplifiers may have a dynamic headroom in the range of 1 to 2 dB, corresponding to power outputs 25 to 50 percent greater than their rated values. In order to deliver several hundred watts of short-term power to a speaker, such amplifiers must be capable of high continuous power outputs, and they are correspondingly large, heavy, hot, and expensive.

Proton’s D1200 is a moderately powerful amplifier, rated for 100 watts continuous output into 8-ohm loads from 20 to 20,000 Hz with no more than 0.02 percent distortion. In addition to its normal power supply, the D1200 has a second one that can supply a much higher d.c. voltage to the output transistors when required. Normally the amplifier operates only with the main power supply, but when the signal approaches its clipping level, the second power supply is connected to the output stages by fast electronic switches. It is designed to supply a higher voltage for a short time, up to about half a second, after which the voltage gradually decays back to the normal operating value. After the transient peak is gone, the second power supply completely disconnects and recharges to await the next peak.

The effect of the DPD circuit is to give a 100-watt amplifier the ability to deliver very high power outputs for several hundred milliseconds instead of merely the 20 milliseconds called for in the dynamic-headroom standard. The major advantage of this approach, compared with conventional amplifiers, is that the power supply, heat sinks, and output transistors do not have to be able to deliver an unnecessarily high continuous power. The resulting economies make a DPD amplifier the sonic equivalent of far larger and more expensive conventional designs. Nevertheless, there is no sign of skimping in the Proton D1200, which boasts two toroidal power transformers (apparently for the two channels) and a number of very large filter capacitors.

In addition, the D1200 is rated very conservatively. It is protected against short-circuits by relays that shut it down in such an event, and it has no current-limiting circuits that could prevent it from driving low-impedance speaker loads (or that could cause distortion under certain load and drive conditions). In combination with the DPD system, these features give it some extraordinary power ratings, ranging from a clipping power of 110 watts into 8 ohms to a dynamic power of 1,500 watts (for 20 milliseconds) into 2 ohms! Moreover, the D1200 can also be operated in a bridged mode, becoming a mono amplifier with double its stereo power ratings.

The D1200 is finished in black, with clear plastic over its two large power meters. The meters’ logarithmic scales span a range from a few milliwatts to many hundreds of watts (into 8 ohms), and the pointers have a rapid response time to follow program peaks. Insulated binding-post speaker connectors, input jacks, and a stereo/mono switch are in the rear of the amplifier. The D1200 measures 16-1/2 inches wide, 14-1/4 inches deep, and 4-1/4 inches high and weighs 33 pounds. Price: $599.

Lab Tests

The exterior (especially the top) of the Proton D1200 became quite hot during its 1-hour preconditioning at one-third rated power, but its temperature did not increase beyond that during subsequent testing. The DPD system made clip-ping-power measurements somewhat awkward. Since the initial clip-ping-power level was so much higher than the continuous value, the waveform would appear to clip and then collapse. By gradually increasing the drive level until the onset of clipping, backing off from that point, and waiting for the waveform to stabilize, we made the necessary measurements without difficulty.

The D1200’s output clipped at 155 watts per channel into 8 ohms, 175 watts into 4 ohms, and 350 watts into 2 ohms. We measured the dynamic power response of the amplifier with bursts of 20, 100, and 200 milliseconds repeated twice per second. The 8-ohm output with a 20-millisecond burst was 593 watts, for a dynamic-headroom measurement of 7.73 dB. The power increased to 1,187 and 1,800 watts into impedances of 4 and 2 ohms, respectively. During the longer bursts the available power was lower, of course, but even at 200 milliseconds the 8-, 4-, and 2-ohm power readings were 478, 700, and 1,055 watts. It appeared that only after the signal duration exceeded 1 or 2 seconds did the power output fall to its steady-state value.

The 1,000-Hz harmonic distortion was between 0.0025 and 0.005 percent from 1 to 130 watts into 8 ohms. The 2- and 4-ohm readings were in the range of 0.008 to 0.003 percent from 1 watt to their clipping levels. With both channels driving 8-ohm loads, the distortion was typically about 0.003 percent from 50 to 2,000 Hz at power outputs from 10 to 100 watts, and the maximum reading was 0.024 percent at 20,000 Hz and 100 watts.

The sensitivity of the amplifier was 0.105 volts for a reference output of 1 watt, and the A-weighted output noise was —78.6 dB referred to 1 watt. The slew factor was 2.5 (a 100-watt sine wave became triangular at 50,000 Hz). The amplifier was stable with reactive simulated speaker loads. Its frequency response was flat within +0, —0.2 dB from 20 to 20,000 Hz, falling off to — 3 dB at 260,000 Hz. The amplifier apparently has an infrasonic filter (not mentioned in the specifications) that reduced its response by 1.5 dB at 15 Hz, 10 dB at 10 Hz, and 31 dB at 5 Hz.

Colored LED’s on the front panel accurately signal the onset of DPD circuit action and flash at peaks of 3 and 6 dB above rated power. The dimly lit meters cannot be read at any distance from the amplifier, and their indications were consistently about 40 percent low. They responded to burst signals so rapidly, however, that with our 20-millisecond test signal they read only 3 dB lower than the amp’s steady-state value.

Comments

The Proton D1200 comfortably surpassed virtually every one of its specifications that we tested, and in such important respects as dynamic power output it performed far beyond its ratings. There is no doubt that the D1200 is one of the most powerful amplifiers made for home use despite its nominal “100-watt” rating.

This is also an uncommonly attractive amplifier. Many powerful basic amplifiers are ungainly, featureless blocks of black metal, but the D1200 is a component that almost no one would mind having on display in the listening room. The weak illumination of the meters unfortunately diminishes their usefulness, though the led indicators for DPD action worked well. Operationally, the D1200 is as silent and unobtrusive as one could hope for. It emits no fan noise (another benefit of the DPD design), it does not hum or buzz, and it sounds every bit as good as it looks.

About the sound . . . well, since I do not subscribe to the popular notion that good power amplifiers sound “different” from each other, there is not much I can say on that subject. Still, when we were playing some CD’s with exceptional dynamic range through this amplifier, the impact of the high-level transients—which often lit the 6-dB DPD light and produced meter readings of more than 400 watts— was nothing short of stunning. Not too many “100-watt” amplifiers can match that performance.

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