how and why power amps clip

[FullRangeMan]

Power amps clip because the subwoofer is too loud !!

An externally hosted image should be here but it was not working when we last tested it.

 

[Sheldon]

thanks again, but so far i believe no one was able to explain (or i was not able to understand) how you can clip the output stage (let use a SE for simplicity) being that the inductive load with allow it to go higher the PS voltage. Let's use the following example (based on a real fact): if i plot a load line (assuming a fixed load impedance, which i know it is not real) for a EL84 tube and set the bias point midway the load line and plot the signal gain (sin wave, with fixed frequency), one will see the plate voltage can swing to more than the power supply voltage. Everywhere i read, it was said this was due to current collapsing on the inductive load. But it such stage was to clip, where it would clip, since the plate voltage has already surpassed the PS voltage?

Look at your load line. The amp will clip where your load line crosses the zero current line (x axis). It will also clip where the load line crosses the the zero grid volt curve (unless you can supply grid current, in which case you can drive it close to the Y axis). Note that clipping will not be symmetrical.

In order to get the same swing without an inductor, say a current source, you would have to double the B+.

Sheldon

 

[Sheldon]

Just to lead things in a certain direction I will ask a few questions. The magnetic field is collapsing. Why? What is the tube doing to cause the magnetic field to collapse? Does the increase in voltage help or hinder the output tube? Does the collapse cause power to flow out to the load or into the tube?

It's more helpful to think of it this way. The inductor tries to maintain a constant current ( a sort of momentum, if you will). The difference between it and a tube or transistor current source, is that the inductor stores energy. That is the energy stored in the magnetic field. If you try to reduce the current, the stored energy in the inductor tries to maintain it, hence the voltage must raise. With a sine wave (or similar source) within the bandwidth of the transformer, the voltage can rise to about double the input voltage. If you try to completely open the circuit after the inductor, no current can flow and the voltage can rise much higher than double.

Sheldon

 

[quikie22]

btw, because the voltage does go up in an inductor/OPT when it clips, the clipping in a tube amp is so called "gentle"/"graceful"... hence to some inexperienced ears may not be "audible"...

On the other hand the clipping of a solid state amp is ultimately "square-wave" with squaring off of the signal waveform thereby producing a ton of odd harmonics with the typical jarring sound.

 

[hpupo]

I would like to thank everyone here. I just done some thinking last night and understood why my question didnt make a lot of sense. I was looking at the whole thing as a resistive load and not an inductive one. The valve wizard article on SE power amp helped a lot...

thanks again....