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Old 19th May 2011, 03:14 PM   #1
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Default how and why power amps clip

Hello...

I am in need of some explanation. I understand that guitar tube amps may have their power amps designed to clip. I do understand that it will clip if one require the power amp to put out more power than the power supply can provide. But what exactly does "more power than the power supply can supply" means? Does it mean that current will continue to grow and voltage will drop (P = V*I)? Or does it mean it will reach a point which neither current or voltage can be increased?
I have more questions about this issue, but one thing at a time...

thanks
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Old 19th May 2011, 03:34 PM   #2
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maybe im wrong... but i thought they distorted not clipped. distortion from clipping is diffrent, right?
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Old 19th May 2011, 03:35 PM   #3
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Consider this simplified example: Say you have an amp that is powered by 12 volts. And it is fed a signal of 2 volts that will be amplified 8 times. That is 16 volts. The output will go to 12 volts and remain there for a time (determined by the wavelength of the signal) because it can't go any higher. Neither current nor voltage can be increased above that point. The power out cannot exceed the power in. So it clips.

All clipping is distortion, but not all distortion is clipping.
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Old 19th May 2011, 03:40 PM   #4
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If you look at the final output stage in a push pull amp, there is a center tapped transformer connected to two output tubes (or multiples thereof), and the center tap to B+.

For simplification assume that the output voltage is 400V, and the voltage gain of the output stage is 40. Therefore an input signal of 10V peak sinewave (20V p-p) will cause one of the output tubes to shut off as it approaches 400V and the opposite tube to saturate as it apporaches 0V. This is as much as the stage can put out. Any increase in the input voltage will casue the output to reach it's limit earlier in the waveform phase and come out of max later in the waveform phase.

This is clipping. The top and bottom of the waveform as viewed on an oscilloscope looks like they were cut off or clipped.
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Old 19th May 2011, 03:43 PM   #5
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Clipping produces distortion as the clipped waveform is a series of frequencies combined together (fourier series). Second harmonic is produced first, then third, fourth, fifth, etc. The percent of each harmonic is dependent on the tube, circuit topology, and other factors. These are harmonic distortion.

Clipping also introduces intermodulation distortion (TIM) where the different frequencies modulate each other.

Phase distortion may be introduced as well.

As the tube clips, it will not shut off/saturate instantaniously, but rather gradually transition. This gradual transition can be represented by a series of even harmonics, as oposed to a square wave which is a series of odd harmonics.

In addition, the non-linearity (output current vs input voltage) of the tube increases as one approaches cutoff/saturation. This non linearity introduces predominantly second order harmonics.

Last edited by TheGimp; 19th May 2011 at 03:46 PM.
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Old 19th May 2011, 03:43 PM   #6
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Thanks for the replies. So correct if im wrong, but can i still think of a power amp as a voltage amplifier???
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Old 19th May 2011, 03:54 PM   #7
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Not necessarily. A power amp is a power amp. P=I*V, so you can have a voltage amplifier, a current amplifier, or both. With audio, I'd say it's usually both. But all three amplify the power.
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Old 19th May 2011, 03:54 PM   #8
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In order to make power, one has to have both current and voltage. The input stages of the tube amplifier are predoiminantly voltage amplifier stages. There may be current gain in latter stages, but early stages are typically 12AX7s which are used to progressivly increase the voltage of the guitar output (up to 400mV?) up to the drive level needed by the grid of the output tubes (20Vp-p to 70vp-p typically).

The output tubes because of their greater input capacitance will require more grid drive current than smaller voltage gain tubes so current gain is required in the tubes driving the grids of the output tubes.

Furthermore, the output tubes are used for current gain as they drive inductive transformers. Voltage without current could not transfer power to drive the speaker (and would imply very high internal impedance of the output tubes) .
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Old 19th May 2011, 04:00 PM   #9
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i think i got. Now to something else i do not understand. I learned from my studies on power amps, that because of the transformer (i still do not fully comprehend this effect) the voltage will rise to more than the supply voltage (something to do with collapsing of the magnetic field in the transformer). My questions is, if the transformer allows the voltage to go higher, how can it clip? Or is plate current that is being clipped in this case?

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Old 19th May 2011, 04:19 PM   #10
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A transformer has limits for voltage, current, and frequency same as (virtually, if not literally) all other components. It can step up the power supply voltage, true, but it is still vulnerable to the limits described in the previous responses. The transformer voltage output is directly proportional to the voltage input.
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