Fascinating!
I've copied an excerpt from Mr. Atkinson's review of the Ref 150:
"As expected, the Ref 150's output impedance varied according to the transformer tap selected. The 16 ohm tap measured 1.4 ohms at low and middle frequencies, rising to 1.9 ohms at the top of the audioband. The figures for the 8 ohm tap were 1 and 1.4 ohms; for the 4 ohm tap, they were 0.55 and 0.87 ohm. All three taps offer quite a low source impedance for a transformer-coupled design; as a result, the modulation of the amplifier's frequency response, due to the Ohm's Law action between that impedance and that of our standard simulated loudspeaker, was relatively mild. From the 8 ohm tap (fig.1, gray trace), it was ±0.8dB; the 4 ohm tap offered ±0.4dB, the 16 ohm tap ±1dB. Fig.1 indicates that the Ref150 has a wide bandwidth, particularly into loads higher than the nominal tap value, which correlates with a well-defined 10kHz squarewave (fig.2)."
I assume based on the results of these tests that the Ref 150 performed like a Voltage Paradigm amp to a large degree. That is, even though the simulated speaker's source impedance varied with frequency, the amp was able to maintain relatively level amounts of gain notwithstanding. I further assume that gain correlates to power (watts).
In other words, if I have this right, the Ref 150's circuit topology, using NF, was able to appropriately compensate for the changing impedances of the simulated speaker, which change as a function of FR.
Bottom line: the use of judicious amounts of NF in a tube amp enable the amp to perform, to some extent like a tube amp. Hence, the proposition that some speakers are "tube friendly" versus "not tube friendly" should be accepted with circumspection. It may very well be that the tube amp's use of negative feedback is a compensating factor, to some degree, when driving speakers that may otherwise be considered tube unfriendly.
Do I have it??
If I do, I think folks need to take Electronics 101 when mathing speakers and amps. It ain't so simple. ;>')
I've copied an excerpt from Mr. Atkinson's review of the Ref 150:
"As expected, the Ref 150's output impedance varied according to the transformer tap selected. The 16 ohm tap measured 1.4 ohms at low and middle frequencies, rising to 1.9 ohms at the top of the audioband. The figures for the 8 ohm tap were 1 and 1.4 ohms; for the 4 ohm tap, they were 0.55 and 0.87 ohm. All three taps offer quite a low source impedance for a transformer-coupled design; as a result, the modulation of the amplifier's frequency response, due to the Ohm's Law action between that impedance and that of our standard simulated loudspeaker, was relatively mild. From the 8 ohm tap (fig.1, gray trace), it was ±0.8dB; the 4 ohm tap offered ±0.4dB, the 16 ohm tap ±1dB. Fig.1 indicates that the Ref150 has a wide bandwidth, particularly into loads higher than the nominal tap value, which correlates with a well-defined 10kHz squarewave (fig.2)."
I assume based on the results of these tests that the Ref 150 performed like a Voltage Paradigm amp to a large degree. That is, even though the simulated speaker's source impedance varied with frequency, the amp was able to maintain relatively level amounts of gain notwithstanding. I further assume that gain correlates to power (watts).
In other words, if I have this right, the Ref 150's circuit topology, using NF, was able to appropriately compensate for the changing impedances of the simulated speaker, which change as a function of FR.
Bottom line: the use of judicious amounts of NF in a tube amp enable the amp to perform, to some extent like a tube amp. Hence, the proposition that some speakers are "tube friendly" versus "not tube friendly" should be accepted with circumspection. It may very well be that the tube amp's use of negative feedback is a compensating factor, to some degree, when driving speakers that may otherwise be considered tube unfriendly.
Do I have it??
If I do, I think folks need to take Electronics 101 when mathing speakers and amps. It ain't so simple. ;>')