45 Type Push/Pull

Thanks Phil for your reasoned response. I am basing my comments on the simple observation of the amps themselves on the bench. I've seen crossover and notch distortion in transistor amps (which in some cases it was possible to correct by adjusting the bias) but in a good number of tube amps its just not something that shows up on the 'scope like it does with transistors.

I *have* seen it at higher power levels in amps that were malfunctioning. But never in a functional amp- an example is the HK Citation 2, which I have always held in high esteem. You can run it to full output and no sign of crossover/notch artifacts.

I have to admit to having far less experience with class AB2 amplifiers. They seem to be uncommon in home audio. However in AB2 we have the problem of the driver circuit which has to be able to maintain linearity when the tubes are in the grid current region. If this circuit is not up to the task you will indeed see artifacts- but in my comments earlier I was careful to use the caveats of proper design and proper functionality.

It's also possible that something about the wave hand-off in push-pull is audible, that may not be measurable notch distortion and being something else doesn't show up on a scope. I've discussed that possibility with some amp designers and EEs as well, but for lack of another way to reference it the phenomenon inherits the notch handle.

Phil

Phil, if you can't see it on the scope, that does not always mean it does not exist. But it does mean that its pretty slight- if you have a really high bandwidth scope (200MHz and higher) that stuff usually shows up pretty well if its there.

However P-P is prone to harmonic distortion that SETs are not. A lot depends on the topology of the amp. For example if the amp is P-P at the output but combines single-ended circuits in the front end, the result will be that is has a 2nd harmonic that predominates, with a 3rd and a 5th also showing up. This is because the Single-ended distortion is compounded by the later stages and even though P-P can cancel even orders in the load, it can only do that with distortion that is also generated by the output section itself, not the driver or voltage amps.

If on the other hand if the circuit is fully balanced the only harmonic that shows up is the 3rd. This is because even orders are cancelled throughout the circuit. A lot also depends on the inherent linearity of any of the circuits; if things are bit off other harmonics and IM (inharmonic) distortions can show up pretty fast.

Its my opinion that P-P has gotten a bad rap in this regard, mostly due to some sloppy design and execution. To me this is very much a baby and the bath sort of thing.