Tube Power Amp Reviews

Sidebar 3: Measurements

I tested the EAR 890 using both its balanced and unbalanced inputs and its 8 and 16 ohm output transformer taps. The unbalanced voltage gain at 1kHz into 8 ohms ranged from a high 29.8dB (16 ohm tap) to 27.1dB (8 ohm tap), the balanced input offering 0.7dB lower gain than the unbalanced. Both inputs preserved absolute polarity; ie, they were noninverting. (The balanced input appears to be wired with pin 2 "hot.") The input impedance at 1kHz was a usefully high 55k ohms for both balanced and unbalanced inputs, and didn't vary with the position of the volume-control pots; neither did it change with frequency at middle and low frequencies. However, whereas the unbalanced input impedance at 20kHz was a still-high 43k ohms, the balanced input impedance dropped to 10k ohms, which will roll off the top audio octave with preamps having a high source impedance.

It was hard to get an accurate estimate of the output impedance, as the open-circuit voltage fluctuated widely. (I estimate output impedance by looking at the rise in the amplifier's output when a load of 8 or 4 ohms is replaced by the 100k ohms input impedance of my Audio Precision System One.) But by comparing the amplifier's output voltage with a 4 ohm load with that at 8 ohms, it looked as if the EAR 890's output impedance was around 1.5 ohms from its 16 ohm tap and 0.5 ohm from the 8 ohm tap. The change in the amplifier's frequency response due to the Ohm's Law interaction between its source impedance and the loudspeaker's change in impedance with frequency is relatively mild, therefore. Fig.1 shows that, with our simulated loudspeaker load, there was just ±0.5dB response variation across the audioband from the 8 ohm tap, with just more than twice that from the 16 ohm tap (not shown).

Fig.1 EAR 890, balanced, 8 ohm tap, volume control maximum, frequency response at (from top to bottom at 2kHz): 2.83V into simulated loudspeaker load, 1W into 8 ohms, 2W into 4 ohms, 4W into 2 ohms (1dB/vertical div., right channel dashed).

The response curves in fig.1 were taken using the balanced input; note the sharp rolloff above 20kHz, and that there is a slight peak before the rolloff with the higher impedance loads. Correlating with this peak, the small-signal squarewave showed a slight, well-damped overshoot (fig.2). However, the degree and frequency of the ultrasonic rolloff also depended on the setting of the volume control. Fig.3 shows the response from the 8 ohm tap with the control set to 12:00; what was basically a flat response to 20kHz is now 3dB down at 25kHz. To my surprise, the unbalanced input behaved much better in this respect. As can be seen from fig.4, while there was still some modification of the amplifier's ultrasonic rolloff by the choice of output transformer tap, the amplifier's response was both flatter and wider than via the balanced input. As a result, the unbalanced squarewave response didn't show any overshoot (fig.5). It looks as if the relatively small coupling transformers used to implement the 890's balanced input are not up to the superb standard set by Tim de Paravicini in his pro-audio products.

Fig.2 EAR 890, balanced, 16 ohm tap, small-signal 1kHz squarewave into 8 ohms.

Fig.3 EAR 890, balanced, 16 ohm tap, volume control at 12:00, frequency response at 1W into 8 ohms (1dB/vertical div., right channel dashed).

Fig.4 EAR 890, unbalanced, volume control at maximum, frequency response at 1W into 8 ohms, 16 ohm tap (top) and 8 ohm tap (bottom). (1dB/vertical div., right channel dashed)

Fig.5 EAR 890, unbalanced, 16 ohm tap, small-signal 1kHz squarewave into 8 ohms.

Channel separation (fig.6) was better than 80dB at 1kHz, with the L-R crosstalk being about 5dB lower than the R-L. Though these figures decreased to 60dB and 66dB, respectively, at 10kHz, this is still good separation. The EAR 890's unweighted, wideband signal/noise ratio (ref. 1W into 8 ohms, 16 ohm tap, volume control at minimum setting) was only a moderate 75.4dB, due to the presence of some low-level 60Hz and 120Hz hum that I couldn't eliminate by experimenting with the grounding of my test setup (footnote 1). This did improve to a respectable 90.7dB when the measurement was A-weighted.

Fig.6 EAR 890, channel separation (R-L dashed, 10dB/vertical div.).

EAR specifies the 890 as having an output power of 70Wpc (18.45dBW). Figs.7 and 8 show how the 890's THD+noise percentage changes with output power, taken at 1kHz from the 16 ohm and 8 ohm output taps, respectively. It looks as if the 890 more than meets its specified power when the load impedance is the same as the nominal value of the output tap. The 16 ohm tap, for example, gives out 79.8W into 16 ohms (22dBW) at our usual 1% THD clipping point, while the 8 ohm tap gives 79W into 8 ohms (19dBW). These graphs also show that the EAR 890 offers excellent linearity below 10W from either output tap as long as the load impedance is relatively high. This can also be seen in fig.9, which shows how the small-signal THD+noise percentage varies with frequency with the 8 ohm output driving loads from 2 to 16 ohms. (The 16 ohm output graph, fig.10, is broadly similar, with slightly higher levels of THD.) But note the increase in THD above 10kHz.

Fig.7 EAR 890, 16 ohm tap, distortion (%) vs 1kHz continuous output power into (from bottom to top at 10W): 16 ohms, 8 ohms, 4 ohms, 2 ohms.

Fig.8 EAR 890, 8 ohm tap, distortion (%) vs 1kHz continuous output power into (from bottom to top at 10W): 16 ohms, 8 ohms, 4 ohms, 2 ohms.

Fig.9 EAR 890, 8 ohm tap, THD+N (%) vs frequency (from bottom to top): 2.83V into 16 ohms, 8 ohms, 4 ohms, 2 ohms.

It is important to note that the absolute level of distortion is never as important as its spectrum. At low levels, the EAR's distortion predominantly comprises the subjectively innocuous second harmonic (fig.11). But as the level rises (fig.10), especially at low frequencies (fig.12), the third and higher harmonics appear. The third harmonic lies at -52dB (0.2%) in this graph, which might be expected to slightly fatten the EAR's bass reproduction. Note also that the low-level 120Hz hum can't be seen in fig.13, though an intermodulation product at 70Hz (the difference between 120 and 50) can be seen on the upper slope of the 50Hz fundamental.

Fig.10 EAR 890, 16 ohm tap, THD+N (%) vs frequency (from bottom to top): 2.83V into 16 ohms, 8 ohms, 4 ohms, 2 ohms.

Fig.11 EAR 890, 16 ohm tap, 1kHz waveform at 1W into 8 ohms (top), 0.053% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.12 EAR 890, 16 ohm tap, 1kHz waveform at 20W into 8 ohms (top), 0.33% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.13 EAR 890, 16 ohm tap, spectrum of 50Hz sinewave, DC-1kHz, at 30W into 8 ohms (linear frequency scale).

Finally, that increasing level of harmonic distortion seen in fig.9 will not be a factor at normal playback levels. But there will be some intermodulation at high levels of high frequencies: fig.14 shows the spectrum of the 890's output while it drives an equal mix of 19kHz and 20kHz tones at 35W into 4 ohms from its 8 ohm transformer tap. (This level was just below visual clipping on an oscilloscope with this demanding signal.) The 1kHz difference component lies at -56dB (0.15%), as do the higher-order sidebands at 18kHz and 21kHz. Note also the sidebands at ±120Hz surrounding the primary spectral components in this graph, suggesting the amplifier is getting close to its limits.

Fig.14 EAR 890, 8 ohm tap, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 35W into 4 ohms (linear frequency scale).

Provided the EAR 890 is used with speakers having an impedance that doesn't drop below half the value of the nominal transformer tap in use, it will deliver respectable measured performance. If in doubt, use the 8 ohm tap, which still has usefully high gain.—John Atkinson

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Footnote 1: The importer was aware of this low-level hum when he supplied the amplifier for review. He noted that it was not typical and had developed somewhere in the sample's travels.—John Atkinson