Berning EA-2101 power amplifier Measurements
Following its one-hour preconditioning test, the Berning EA-101 was quite hot to the touch in the region above its 16 tubes. Its input impedance measured just over 91.5k ohms into its unbalanced input, just under 191.5k ohms into its balanced input. I measured zero DC offset in both channels. The Berning inverted polarity, a positive impulse at its input resulting in a negative impulse at the output.
Beyond these basic characteristics, assessing the performance of the Berning involved a large array of measurements, made necessary by the amplifier's flexibility. Its option of balanced and unbalanced inputs was only the start of it; as described by DO above, there were also six different ways to hook up the loudspeakers—normal taps at 0.89 ohm, 3.55 ohms, 8 ohms, and 14.22 ohms, plus the option of 3.55 ohm and 0.89 ohm high-current lash-ups.
The question of balanced vs unbalanced inputs was dispensed with quickly. With the exception of the above-noted measurements, spot checks of unbalanced and balanced configurations revealed them to be either virtual overlays of each other, or so closely aligned as to make showing both of them redundant. The results depicted from this point on, unless noted otherwise, are for the balanced mode.
Table 1 shows the output impedance from all of the possible output tap configurations. This was assessed by noting the voltage rise when either 4 or 8 ohm loads are replaced by an open circuit (in practice, 100k ohms) at a low signal level. The figure shown is the average of the two readings, though it must be noted that there did appear to be some load-dependence of the measured value. The output impedance is rather high in most of the configurations, and rises significantly at the top of the audio band. While the use of the low-impedance taps would seem advisable to minimize the amplifier's output impedance, other limitations which will be seen shortly may not make this the best choice. The high output impedance from the 14.22 ohm tap is alarming, but it will have less of an impact when used with the very high loudspeaker impedances (not all that common these days) for which it is designed. I would expect the sound of the Berning to be very sensitive to the way in which the loudspeaker's load impedance varies with frequency, with the degree of that sensitivity varying with the output tap used. All other things being equal (which, as we will soon see, they usually aren't), you should experiment to find the best match for your loudspeakers.
Table 1: Output Impedance
|Output Impedance in ohms at|
Table 2 shows the variations in the Berning's voltage gain with different output taps into an 8 ohm load. Note that, as the impedance of the output tap drops, so does the gain. Not shown here is the gain from the 8 ohm tap into an 8 ohm load with an unbalanced input. It was almost identical to the gain through the balanced input. Normally this would indicate a balanced input which just has its hot input strapped in parallel with the unbalanced input and its "cold input" grounded, but that does not appear to be the case here. The Berning probably pads down its balanced input by 6dB to give the same gain in both modes.
Table 2: Voltage Gain
|Output Tap (ohms)||Gain (balanced Input) dB|
The Berning's frequency response is shown in fig.1. Here the output was taken from the 8 ohm tap. The high-frequency rolloff present in many tube amplifiers is evident, though the response holds up well at low frequencies. Note the 0.3dB imbalance in channel outputs, as well as a slight difference (of the order of 0.5dB) in the high-frequency rolloff depending on load. The slightly inferior response is that into a 4 ohm load (2W output vs 1W into the 8 ohm load). This is presumably due to the amplifier's rising output impedance at high frequencies. There were minor frequency-response differences when using other output taps; fig.2 shows the largest such difference. Here the response at 1W into an 8 ohm load using the 8 ohm tap (bottom curves) is contrasted with that when the same load was driven from the high-current-configured 0.89 ohm tap (top curves). The latter holds up somewhat better at high frequencies—though we are still talking of a difference of less than 1dB at 20kHz. Note the hint of an ultrasonic peak in the 0.89 ohm plot.
Fig.1 Berning EA-2101, 8 ohm tap, frequency response at (from top to bottom at 2kHz): 1W into 8 ohms, 2W into 4 ohms, 4W into 2 ohms (0.5dB/vertical div., right channel dashed).
Fig.2 Berning EA-2101, frequency response at 1W into 8 ohms from 0.89 ohm high-current tap (top), 8 ohm tap (bottom) (0.5dB/vertical div., right channel dashed).
The response of the Berning to a 10kHz squarewave input is shown in fig.3. The trace shows no overshoot—the slightly tilted and rounded leading edge echoing the high-frequency rolloff evident in fig.1—though there is a hint of ultrasonic ringing in the unflat top to the waveform, which may correlate with the hint of a peak indicated in fig.2.
Fig.3 Berning EA-2101, 8 ohm tap, small-signal 10kHz squarewave at 1W into 8 ohms.
The channel separation (fig.4), a good though not exceptional performance, shows a slight difference between channels, the crosstalk from left to right being slightly worse than that from right to left. Evident here is the usual increase in crosstalk at the higher frequencies commonly due to capacitive inter-channel coupling.
Fig.4 Berning EA-2101, 8 ohm tap into 8 ohms, channel separation: L-R, top; R-L, bottom (5dB/vertical div.).
Fig.5 shows the THD+noise vs frequency of the Berning from the 8 ohm output tap into 2 ohms (4W), 4 ohms (2W), and 8 ohms (1W) (bottom to top, respectively). Note the relatively high distortion levels here, even into the matched load. The THD+noise vs level shown in fig.6 provides additional data. Note that although the amplifier puts out significant power (especially into 8 ohms from the 8 ohm tap), the distortion reaches a relatively high level at fairly low output power.
Fig.5 Berning EA-2101, 8 ohm tap, THD+N (%) vs frequency (from bottom to top at 2kHz): 2.83V into 8 ohms, 4 ohms, 2 ohms (right channel dashed).
Fig.6 Berning EA-2101, 8 ohm tap, distortion (%) vs 1kHz continuous output power into (from bottom to top at 10W): 8 ohms, 4 ohms, 2 ohms.