Magnum Dynalab MD 208 receiver Measurements

Sidebar 3: Measurements

After the usual hour of preconditioning at one-third power, the solid-feeling MD 208's heatsinks were hot, but not so hot that I couldn't keep my hand on them. The unit didn't invert signal polarity, and its maximum gain into 8 ohms was sensibly lower than usual, at 36.35dB. The volume control operated mostly in 1dB steps, with an occasional 1.5dB or 2dB step. The unity-gain setting was "24" of a maximum of 50.

With the volume control set to "40," the input impedance was below average, at 9.4k ohms at 1kHz; as a result, some tubed CD players will suffer from early bass rolloff. The output impedance was very low, at 0.02 ohm over most of the audioband, rising slightly to 0.04 ohm at 20kHz. As a result, any frequency-response interaction between the receiver and whichever loudspeakers partner it will be minimal (fig.1). This graph also reveals that the ultrasonic response rolls off above the audioband, reaching -3dB at around 90kHz. This slightly slows down the risetime of a small-signal 10kHz squarewave (fig.2), but the shape of a 1kHz squarewave was essentially perfect (not shown).

Fig.1 Magnum Dynalab MD 208, volume control at "40," frequency response at (from top to bottom at 20kHz): 2.83V into dummy loudspeaker load, 1W into 8 ohms, and 2W into 4 ohms (0.5dB/vertical div., right channel dashed).

Fig.2 Magnum Dynalab MD 208, small-signal 10kHz squarewave into 8 ohms.

The signal/noise ratio was a good 76.5dB (unweighted, wideband, ref. 2.83V into 8 ohms), which improved to 83dB when A-weighted. Interchannel crosstalk was buried beneath the receiver's noise floor in the lower midrange and below (fig.3), but rose gently in the midrange and treble due to the usual capacitive coupling between channels.

Fig.3 Magnum Dynalab MD 208, channel separation (R-L dashed).

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