|
October |
|
Page 14 |
|
Copyright © 2003 John Broskie All Rights Reserved |
|
which when contrasted with the formula for the other four variations, Zo = rp/(2[mu +1]) yields the following ratio Ratio = (mu +2)/(2[mu +1]). Thus, as mu approaches zero, the difference between variations climbs to none (or rather, unity; in other words, there is no difference); as the mu approaches infinity, the difference between variations falls to half (50%). Now, the 6AS7 is an interesting triode, in that its mu is only 2, almost nothing. With a mu of 2, the ratio becomes 2/3 or 66%; with a mu of 100, the ratio becomes 102/202 or 50.5%, which means that the two spilt-output-reference variations are probably not the best topologies to use where output impedance is of prime importance. (Actually, the full cathode follower degeneration can be restored by intertwining the driver stage with the output stage, but at the cost of increased driver stage voltage swing: nothing can be had for free; alas.) Of course, where Zo isn’t important, the equal drive signal amplitudes are truly a blessing, as the unbalanced drive voltages confused the hell of out most tube practitioners and even the makers of Stereophile-Class-A amplifiers and patent examiners. And both center-referenced amplifiers take less driver-stage voltage swing to bring them to full output than the four other variations on this same theme (a result of placing the reference at the midpoint of the output signal; imagine a gain of 100% and this will be more readily apprehended). How true are these formulas to reality? I don’t know, as I don’t have examples of the two amplifiers to test, but I know that it is accurate regarding my own pair of Atma-Sphere M-60s. (What! He owns those amplifiers! I heard he owns an oscilloscope and a distortion analyzer and a FFT, which means that he can test them and evaluate them at his leisure! The horror!) |
|
I do know that it is true to SPICE reality, in that the two variations produces output impedances of 40 and 60 ohms, a ratio of 40/60 (2/3) or 66%. (What I am not too sure of is the SPICE model of the 6080, as I don’t believe the tube is that good in that I don’t believe the rp is as low as the model implies, but that little matters to the results so far obtained, as a different tube could be used, say a 6C33 or EL509 or, even, 12B4. ) By the way, different part values or different part brands or different part technologies do not make a new topology, no more than a flair for interior decoration makes someone an architect. |
|
At least once a month, I receive an email with schematic of a grounded-cathode amplifier, with a request for my viewpoint on the circuit. The last one I received used two resistors in series in place of a single cathode resistor. My take was that two plate resistors would be a better idea, because of resistor voltage induced distortion. |
|
Bold new topology? |