Manufacturer's Specifications
Phono Section
Frequency Response: 100 Hz to 20 kHz, ±0.2 dB; 20 Hz to 100 Hz, +2,-0 dB.
Time Displacement: Approx. 2 µS.
Hum and Noise: Silent at normal listening levels; noise is dominated by thermal noise of cartridge.
Maximum Output Voltage: 9 V rms.
Rated Output Voltage: 2 V rms.
Distortion (All Types) at Rated Out put: Below 0.01 percent.
Input Impedance: MM input fully adjustable; MC type current driven and matches any cartridge impedance.
Line Amplifier Section
Frequency Response: 15 Hz to 1 50 kHz, +0,-3 dB.
Time Displacement: Approx. 8 Maximum Output: Main, ±12 V peak; speakers, ±24 V peak.
Rated Output Voltage: 3 V rms.
Distortion (All Types) at Rated Output: Below 0.01 percent.
Output Noise:-80 dBm, unweighted.
Main Output Impedance: 50 ohms.
Input Impedance: 20 kilohms.
Gain: Main, 20 dB; speakers, 26 dB.
General Specifications
Dimensions: 16 1/2 in. (41.91 cm) W x 2 1/4 in. (5.71 cm) H x 9 in. (22.86 cm) D.
Weight: 11 lbs. (4.95 kg).
Price: $895.00.
MTI is the acronym for Micro-Tech Inc. and when I first began to examine their Model 500 "preamplifier" (in a moment you'll see why I put this in quotes), I thought that there was something familiarly clean and elegant about its front panel de sign. Only later did I learn that the front panel design had been executed by none other than Saul Marantz, who apparently loves the hi-fi business so much that he just can't stay away. Sherwood Scribner, who is more concerned with circuitry than cosmetics, is responsible for the innards of this unit, and, in deed, he can come up with some novel ideas about what a preamp should and should not do.
Clearly, Scribner subscribes to the school of design that wants to minimize signal processing in a control chassis while at the same time maximizing switching flexibility. So, while there are no tone controls to be found on this preamp, it is equipped with a subsonic filter, a pair of tape monitor circuits, tape copy switches, mono/stereo switching, dual concentric level controls, and an illuminated separately mounted power on/off switch.
Program selection is done by means of push buttons for tuner, AUX, phono 2 (MM), and phono 1 (MC). A button identified as "Reduce Gain" does just that and is to be pushed when an adjacent overload light illuminates. A rotary control and three push buttons are used to adjust the resistance load across moving-magnet cartridges and to adjust the loading capacitance from a low of 50 pF to a maximum of 370 pF in seven steps. A helpful table of recommended capacitance values for some popular makes and models of cartridges is included in the owner's manual.
The power supply for this preamplifier is a separate module connected to the main control unit by means of a long multi-pin cable and connector. The power supply contains a pair of a.c. convenience outlets (one switched from the main chassis, the other unswitched) and a pair of fuses: A 10-ampere fuse that protects the electronic power switch and the lower 9-ampere convenience outlet, and a '/z-ampere fuse that protects the line amplifier in the Model 500 itself.
Although the Model 500 is called a preamplifier, it incorporates a line amplifier capable of driving loudspeakers of reason ably high efficiency to fairly loud listening levels. Interestingly, the designers tell us little about the IHF Standard ratings of this built-in amp when it is used as a power amplifier, saying instead that it "sounds like a 50-watt amplifier." I am not quite certain as to why MTI chose to call the 500 a preamplifier instead of an integrated amplifier, which, from my point of view, it certainly is, but I suspect they want the user to regard the amp as a temporarily available feature, suitable for use with speakers until a more powerful basic amp is purchased.
Circuit Highlights
MTI suggests that "time alignment" of the entire audio spectrum, the ability of this preamplifier to process or pass all frequencies through it in correct relative time relationship, is as important in an electronic component as it is in the design of a multi-driver transducer system. The company further suggests that such time alignment circuitry has been employed in the design of this system-a claim that we would find difficult to affirm, since we were not provided a schematic as the circuit is proprietary.
The Model 500's built-in moving-coil pre-preamplifier has a current-driven self-matching moving-coil cartridge loading sys tem which does adjust automatically to provide an ideal match for any MC cartridge. The flat amplifier section, which MTI calls the "line amplifier" of this unit, is a Class-A design having high headroom capability and, as mentioned earlier, a speaker output stage which can drive loudspeaker systems having impedances as low as 4 ohms. Low-level signals can be driven over extremely long distances because of the low output-impedance characteristics of the line-level outputs of the amplifier. The MTI 500 has a "Separate Amp" switch on the rear panel which, when pushed in, bypasses the line amp, routing signals directly from the level control to the main-out jacks with no gain. The reasoning here is that most high-level sources (tuners, tape recorders), as well as the phono preamp section of the MTI 500 itself, have enough gain to drive a power amplifier directly. When the "Separate Amp" switch is depressed, the line amplifier's inputs are routed to "Separate In" jacks on the rear panel, allowing the amplifier to function independently of the control functions. In this mode, the line amplifier might be used to drive a subwoofer or it might also be used as a high-pass amplifier in biamplified systems.
Fig. 1--Power output vs. THD and IMD, MTI Model 500 preamplifier.
Fig. 2--Distortion vs. frequency.
Measurements
No information is offered regarding the small power amplifier section of this unit, and we were unable to elicit definitive IHF specifications from the manufacturer on this point. Because we did not have a reference distortion level to check against, we decided to arbitrarily assign our own, 0.2 percent THD. For this level of THD, the amplifier was able to deliver 16 watts per channel. A plot of power output versus harmonic and IM distortion for mid-frequency signals is shown in Fig. 1, while in Fig. 2 we have plotted distortion versus frequency for a constant 10 watt per channel output. Maximum output for 0.2 percent THD at 20 Hz measured 13 watts per channel, while at 20 kHz it was 16 watts per channel.
Figures 3A and 3B show plots of the RIAA playback equalization curve. In Fig. 3A the "cursor" has been moved to read the roll-off at 10 kHz (-13.9 dB for the left channel,-13.8 dB for the right channel), which is within 0.05 to 0.15 dB of the precise required value; in Fig. 3B we show the boost at 100 Hz of +13.5 dB for the left channel and +13.5 dB for the right channel, which is within 0.39 dB of the precise required value. In both plots, 0-dB reference was set at 1 kHz.
Aside from quoting frequency response of the preamplifier and line amplifier sections of the Model 500, the makers of this unit supply little IHF Standard specification data against which to compare our own measurements. Using the latest IHF reference levels, we measured a phono input sensitivity, for moving-mag net cartridges, of 0.87 millivolt for 0.5-volt output. In the case of the moving-coil inputs for the same output level, input required was 0.05 millivolt. High-level input sensitivity measured 63 milli volts for 0.5-volt output. Phono overload was 110 millivolts at 1 kHz with the moving-magnet inputs--acceptable but not out standing compared with some of the figures we have obtained of late from the preamp sections of both preamplifier-control units and receivers. The signal-to-noise ratio for the moving-coil phono inputs, "A" weighted, was a very acceptable 72.0 dB for the left channel and 72.1 dB for the right channel. For the high level inputs, still referred to 0.5-volt output, the S/N measured 77.0 dB for the left channel and 76.7 dB for the right channel.
The subsonic filter has a -3 dB cutoff point at 30 Hz and a slope of 6dB per octave. The rated output of the phono section of the MTI-500 is listed at 2.0 volts rms while that of the line-level amplifier is quoted at 3.0 volts. We are not certain what this latter figure means, since the amplifier can deliver far more than that amount of drive through the speaker terminals. This may mean the reference output level when measured via the main-out line terminals of this amplifier, ahead of the speaker stage.
Returning for the moment to the power amplifier section of this "preamplifier," we measured a damping factor of 40 at 50 Hz, referred to 8-ohm loads. IM distortion at the equivalent of 16 watts measured 0.3 percent. The Model 500's amplifier section does have a surprisingly high dynamic headroom. We measured this at around 2.2 dB (referred to 16 watts of continuous power output capability), which means that under short-term musical signal conditions the amplifier could deliver unclipped signals peaking at around 26 watts per channel, considerably in excess of the continuous power capability of the unit. Voltage gain via the main and speaker outputs measured 20 and 26 dB respectively, as claimed by the manufacturer.
Fig. 3--RIAA play back equalization curves, phono section. Note almost perfect
match between left and right channels.
Use and Listening Tests
We did not judge the MTI 500 as an integrated amplifier, but rather as a phono preamplifier/control unit which is, we believe, its prime purpose. The unit exhibited a high degree of audio quality when used with a top-grade power amp and high-quality, time-aligned speakers. The reproduction that we heard from re cords was excellent. The unit is obviously not intended for those who crave a bunch of knobs and lots of signal processing beyond phono preamplification and equalization. If you are of the school that believes tone controls have no place in an electronic component that attempts to minimize time and phase errors and you can afford a preamplifier control chassis that costs nearly $900.00, the MTI 500 does offer an unusual means of entry into the world of audio esoterica. We must be critical of the owner's manual, which, though elegantly bound and certainly longer in its text than most, unfortunately leaves us without the Standard data we really need and with information a bit irrelevant to the potential user. Still, that's a relatively minor flaw and one which we are told will soon be corrected by MTI.
-Leonard Feldman
(adapted from Audio magazine, May 1981)
Also see:
Mission 735 Speaker (Jan. 1996)
McIntosh MX-117 Tuner-Preamp (Jun. 1982)
Mission 776 Preamp (Jun. 1982)
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