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In Reply to: RE: Bypass Cap posted by cpotl on August 28, 2014 at 02:17:21:
"I'm trying to understand if there is in fact any benefit to be had from using an SRPP when driving into a high-impedance load. (Which means it is definitely operating in SE mode, and not push-pull.)"
Morgan Jones in Valve Amplifiers states that the lower half of the SRPP suffers from increased distortion compared to a common cathode stage because it's driving a relatively low impedance load. It's not clear to me if the load he's referring to is the upper tube, the load attached to the circuit, or a combination of both. In other words, his statement may or may not be applicable to an SRPP driving a high Z load. It's very unfortunate that in his description of the circuit, the load Z is never specifically mentioned, nor do his representative schematics include a value. He does however include a graph that shows THD of an SRPP declining rapidly with an increase in applied B+.
The latter might explain why my own SRPP driver with +500V does not inject significant distortion into the amplifier chain. Or, it might be that distortion is low because mine is driving a 200K load. I am unable to find anything in print that really goes into more detail on this point.
Like you, I've noticed that many designers use the SRPP where it doesn't appear useful. I sometimes wonder if they're doing this simply to make use of the other half of a dual triode envelope. It might also be that the audio community in general has so thoroughly bought into the concept of the SRPP as a topology that cancels even-order harmonics, it's employed by many designers in an attempt to reduce THD. When I look at the SRPP from a casual perspective, it also appears that it might be able to swing closer to the rails than a common cathode amplifier, or that the upper half might act somewhat like a triode CCS. In fact, I don't think either is true, but perhaps these concepts are all part of the recipe for the popularity of the circuit.
I think your question about the SRPP being "pointless but harmless" really gets to the heart of the matter. If Morgan Jones had published circuit details for the test he performed regarding THD VS B+, we might have an answer to that. As it stands, I suspect someone will need to revisit this with empirical measurements over various load impedances and voltages before we know.
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Follow Ups
- Good Question! - Triode_Kingdom 07:45:24 08/28/14 (1)
- More (SRPP VS Common Cathode)... - Triode_Kingdom 11:13:54 08/28/14 (0)
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