Calculating Zout for Pentode amp with NFB?

[Printer2]

Originally Posted by DF96

At the bass resonance a valve amp is likely to give less damping so you could get greater output. At other frequencies it won't make much difference. A watt is a watt - the issue is whether you are actually delivering a watt to the speaker coil. Low impedance helps, not hinders. Your 'well informed fellow' is probably talking about the bass resonance only.

Sadly, he thinks it occurs throughout the speaker range. And he is not alone in this. Another 'professional amp designer' have been convincing people of this. The one says he did experiments shooting a laser on a speaker cone, and when shot at an angle the reflected light goes through a bigger angle when driven with a tube amp. The other says he has a DI box on the back of his amp and when the amp is off the speaker acts as a microphone. He says it would not do that with a SS amp as the output impedance is low and acts as a brake as with speed drives.

[DF96]

The output impedance of any amp will be quite different when it is switched off. Typically it will be higher than normal.

The damping effect of the output impedance will have little effect away from the bass resonance. As I said, it depends on whether you are actually delivering a watt or not. Anyway, do you want your amp to drive your speaker or do you want your speaker to do its own thing?

I think I am right in saying that actual measurements show that SS watts equal valve watts, provided that no clipping occurs and we really are comparing like with like. Different clipping performance largely explains the persistent perception to the contrary.

I am repeating myself so I will stop.

[Printer2]

Originally Posted by DF96

The output impedance of any amp will be quite different when it is switched off. Typically it will be higher than normal.

The damping effect of the output impedance will have little effect away from the bass resonance. As I said, it depends on whether you are actually delivering a watt or not. Anyway, do you want your amp to drive your speaker or do you want your speaker to do its own thing?

I think I am right in saying that actual measurements show that SS watts equal valve watts, provided that no clipping occurs and we really are comparing like with like. Different clipping performance largely explains the persistent perception to the contrary.

I am repeating myself so I will stop.

In musical creation it is not necessarily bad to have a speaker do its own thing, it adds color to the sound, giving more singing or sustain to the sound. It depends what one is looking for. One day I will do some tests on the perceived loudness between the two kind of amps in clipping. But that will be when I have more time.