SRPP question
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| Posted on March 1, 2015 at 10:53:51 | ||
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Posts: 1002
Location: Texas Joined: December 6, 2009 |
 I became curious about the use of an SRPP as an input stage. Although the name conveys the idea of push-pull operation, it only actually operates in push-pull mode with balanced signal currents in antiphase in the two tubes when driving into a load with one very specific impedance. In the schematic appended above, I was finding in LTSpice that the signal currents are balanced in antiphase when the load R6 is about 23 Kohms. If the load is below that, the antiphase current in the upper tube is bigger than the one in the lower tube. As R6 is increased, a point is reached, at about 94.4 Kohms, where the signal current in the upper tube is very nearly zero. In fact it is then roughly like a 2nd harmonic "residual," i.e. at twice the frequency of the input signal. (The 1 ohm resistors in the schematic are for convenience for measuring the currents in the two tubes.) When R6 is increased beyond 94.4 Kohms the signal current in the upper tube rises again, but now it is in phase with the signal current in the lower tube. Of course, in the limit of a very high impedance load the signal currents in the upper and lower tubes are very nearly equal (and in phase). All this is well known. There is, for example, a nice discussion in an article by John Broskie (link below). By the way, I found the output impedance (measured in the standard way by driving a signal into the output of the amplifier, and using Ohm's law) came out to be about 11.5 Kohms. The thing that I, at least, wasn't aware of is that even when driving into the critical load where the SRPP is working in balanced push-pull mode (about 23 Kohms in my simulation), there seems to be no evidence of any cancellation of even-order harmonics. For all the load impedances I tried, the dominant distortion was second harmonic, and in all cases it was around 40dB down from the fundamental. The 3rd harmonic distortion seems to improve considerably when the load impedance is increased to be much larger than the "balanced PP" value. I was getting 3rd harmonic distortion at about 58dB below fundamental when the load was 23 Kohms, and about 75dB down by the time the load was 500 Kohms. So I guess my question is what really are the advantages of an SRPP over any other input stage topology? It certainly doesn't appear to shine particularly when it is actually operating in balanced PP mode. And although the 2nd harmonic distortion seems to be more or less uniformly unimpressive over a wide range of load impedances, the 3rd harmonic distortion seems to be very considerably reduced if one runs it into a high-impedance load for which there is really nothing "push-pull" at all about its operation. Of course, the results for distortion that I was getting must be quite dependent on the accuracy of the tube modelling for the Spice simulation, but I suppose they would be likely to be reasonably indicative of what one might find in practice. The absence of 2nd-harmonic cancellation when operating at the "balanced PP" load is unlikely to be just an artefact of the model, I imagine. Chris
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