QED Model 7 Preamplifier Review

The Model 7 preamplifier from Questar Electronic Design (QED) is a compact and versatile control center with functions that go well beyond the usual preamplifier roles of gain, input selection, and equalization. Its tone controls have completely passive circuits followed by an “op-amp” gain stage. The phono-preamplifier section has direct-input coupling from the cartridge. There are two pairs of moving-magnet (MM) phono inputs, each with a lever switch that selects an input capacitance of 100, 200, or 300 picofarads in parallel with a fixed resistance of 49,000 ohms. A fourth position changes the gain and input resistance for a moving-coil (MC) cartridge.

To help minimize hum pickup, QED removed the power transformer from the preamplifier, leaving only the rectifiers and filter capacitors within the chassis of the Model 7. The transformer is a small plug-in unit, similar to the a.c. adapters for calculators and other low-power devices, and the removal of a.c.-line connections from its chassis prevents the Model 7 from having built-in a.c. convenience outlets. All signal switching is done close to the jacks in the rear of the unit by means of remote mechanical couplers connected to the front-panel knobs.

The “Bi-FET” output stage of the Model 7 is claimed to combine a very high signal-output capability with low noise. In addition to its two phono inputs, the Model 7 has two high-level audio inputs (tuner and video). The large source knob at the left of the panel is matched by a similar level knob at the right side. A three-position lever switch is marked off, on, and mute (the latter reduces the preamp output by 20 dB). A red LED above the switch blinks on and off in the mute mode. A phone jack is located next to the level knob.

A three-position vertically oriented lever switch (rec) channels either phono, tuner, or video sources to the tape-recording output jacks, making it possible to record from one source while listening to another. In its center (rec) position the dub switch supplies a signal to both sets of tape-output jacks. In the 1-2 and 2-1 positions the tape decks are connected for dubbing from the first to the second (the selected rec program goes to the input of the first numbered tape deck at all times). The monitor switch connects the source to the preamplifier circuits in its s position, replacing it with the output of either tape deck in the T1 and T2 positions.

The Model 7 has a signal-enhancing feature controlled by a three-position switch labeled E E. (Environmental Enhancement). Switching from the off to the enh position introduces a fixed amount of phase-inverted cross-coupling between the two channels. When the speakers are located relatively close together, this tends to spread the apparent image width, giving a broader sound stage in a small listening room. The manufacturer suggests using the tone controls to modify this effect to suit personal taste. Switching to the sum position retains the enhancement feature but blends the low frequencies into a mono signal below a frequency determined by the crossover switch in the rear of the preamplifier. This minimizes rumble or warp signals that could, in some cases, use up much of the excursion range of small woofer cones. To the extent that the bass-program energy is in phase in both channels, the blending will not cause an equivalent reduction in bass response.

The remaining front-panel control is loudness, with an off position and two active settings marked L1 and L2: Li gives a more or less conventional loudness compensation using a modified version of the Fletcher-Munson equal-loudness contours as the basis for its action, and L2 uses the same basic contours but adds a fixed boost of about 6 dB in the upper bass (about 100 Hz) without increasing the output below 20 Hz. This mode of operation is suggested for small speakers, which can thus be made to give the effect of a strong bass without overdriving their cones.

Not all the features of the QED Model 7 appear on its front panel. The rear panel contains the usual jacks for the signal sources and tape recorders. The main output jacks are shorted out when the front-panel power switch is set to off (this does not remove the power from the preamplifier circuits, which QED recommends be left on continuously by keeping the plug-in transformer in a “live” socket). By grounding the output jacks, the switch silences the following power amplifier as effectively as though the preamplifier had been turned off. Next to the main output jacks is a TIM filter switch whose settings are marked 10 V/μs, 100 V/μs, and 200 V/μs. These refer to the rated slew rate of the power amplifier in use, and when the switch is set to a value no higher than the power-amplifier slew capabilities there is no likelihood of transient intermodulation (TIM) distortion.

Also on the rear panel are two sets of outputs marked bi-amp output (lo-freq and hi-freq). The Model 7 contains electronic-crossover circuits for driving two power amplifiers in systems using biamplified speakers. The crossover frequency is set by an adjacent switch at 120, 240, or 400 Hz. The same settings apply to the summed mode of signal enhancement mentioned earlier. Although the hi-freq output is in full stereo, with the low frequencies attenuated, the LO-freq outputs are driven with a summed (mono) program at frequencies below the selected crossover. However, the left and right outputs for the low-frequency portion of the system are driven 180 degrees out of phase so that a stereo amplifier driven from them will produce a “bridged” mono output across its two “hot” speaker terminals with roughly four times the power capability of the same amplifier in its normal stereo mode.

The QED Model 7 is finished in black except for the silver control-panel insert, which has contrasting black knobs. The panel is slotted for rack mounting.

Laboratory Measurements

The sensitivity of the QED Model 7 at maximum gain, for a reference output of 0.5 volt, was 53 millivolts (mV) at the high-level inputs and 1 mV for the phono (MM) input. The A-weighted output noise was below our measurement limit of 100 microvolts, corresponding to better than —74 dB referred to 0.5 volt. The phono (MM) input overloaded at 107 mV at 1,000 Hz and at equivalent levels of 140 to 150 mV at 20 and 20,000 Hz. The output waveform clipped at 6 volts when we drove a standard IHF load of 10.000 ohms in parallel with 1,000 picofarads. The 1,000-Hz harmonic distortion was a constant 0.003 per cent up to 6 volts output. We also measured the distortion from 20 to 20,000 Hz at outputs of 0.5, 2, and 5 volts. At the 5-volt level, the distortion did not exceed 0.01 per cent from about 50 to 5.000 Hz, increasing to 0.025 per cent at 20.000 Hz and 0.05 per cent at 20 Hz. At lower outputs the distortion was even less.

The phono-input impedance was measured as 48,000 ohms shunted by a capacitance of 75, 180, or 275 picofarads depending on the switch setting. The MC input had a 100-ohm terminating resistance. The RIAA phono equalization was very accurate, within ±0.3 dB from 30 to 20,000 Hz and — 1 dB at 20 Hz. There was absolutely no detectable change in response when it was measured through the inductance of typical phono cartridges. On our test sample, the TIM filter switch had its 10- and 100- V/μs positions interchanged. Nevertheless, the — 3-dB upper response frequency was 100, 75, and 50 kHz, respectively, and rise time was 3, 4, and 6 μs as the tim switch was moved down to 200 V/ps.

The tone controls had good characteristics with virtually no effect on the midrange response (300 to 800 Hz) even at extreme control settings. The loudness compensation boosted both low and high frequencies, and there was some low-frequency boost when the loudness compensation was switched in even at a maximum level setting. The L2 setting boosted the response by 6 dB at 100 Hz, with some boost being apparent between 400 and 40 Hz. The sound seemed a bit heavy to our taste, possibly because we had no speakers on hand that could benefit from it. As far as we could tell, the response at 20 Hz and below was not affected.

The biamp crossover curves had 12-dB-per-octave slopes, with the LO output about 6 dB higher than the hi output. This implies the need for an input-level control at the bass amplifier. When we summed the LO and hi signals resistively, the transition between the two levels took place smoothly, with the approximate midpoints of the change being at 230, 500, and 1,000 Hz for crossover frequencies of 120, 240, and 400 Hz. In the enhance mode, each channel was cross-coupled in reverse phase to the opposite one at a — 8-dB level. In the sum mode, the phase reversal between channels was fully effective above 2,000 Hz, with 90 degrees of phase shift at 200 Hz and 0 degrees at 40 Hz.

Comment

The QED Model 7 clearly reflects the audio philosophy of its designers, much of which is expressed in the informative instruction manual.

Overall, there can be no doubt that the QED Model 7 is a very competent preamplifier with some unconventional features. We were especially impressed by its ruler-flat phono response and total absence of interaction with cartridge inductance (to say nothing of its ability to match the capacitive load requirements of just about any cartridge). The provision for a bridged amplifier connection is a good idea and is typical of the way in which the designers of this amplifier have built into it a host of novel features, at least some of which will appeal to any audiophile.

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