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Old 4th November 2013, 07:43 PM   #11
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Good deal, thanks. I do realize that clipping, distortion and other termsd are relative and rather subjective............just asking in the general sense. I am understanding load lines much better, thanks.......
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Old 7th November 2013, 08:24 PM   #12
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Actually, you can get substantially more gain out of anything; distortion starts to bite into performance, however.

By applying positive feedback, gain can be bootstrapped to infinity if you want. However, once it "passes" infinity, the transfer curve opens up into a hysteresis loop (it becomes bistable for part of the range).

Typically, a non-ideal amplifier's transfer curve has the highest gain in the middle (where it's biased), and lower gain to either side. This tends to compress and distort large signals, which is normal. The difference may not be much over the intended operating range (a typical stage might produce an output voltage only a few percent from ideal, when producing a signal level of half the maximum output), but just as negative feedback reduces gain, and errors in gain, positive feedback enhances gain and its error. A few percent error in the transfer curve might grow to a whopping 50% error (i.e., gain drops by half) when the gain has been increased by, say, a factor of 20 (perhaps from 100x to 2000x), and obviously, the difference is infinite when gain is pushed to infinity (which will only happen in the middle, where gain, and therefore positive feedback, are highest). If you require that your signal is still amplified with less than a couple percent error, its amplitude must be reduced by the same factor, so that instead of a 50% voltage swing, you might get only 2.5% out of the above example.

Is positive feedback therefore bad? Absolutely not. When small signal amplification is desired, a 200V swing is not at all required, or even desirable! It would actually be useful to apply positive feedback in a phono amp, where the signals are small, and therefore the amount of distortion will be small. (Distortion will be higher than the same stage without PFB, but when the difference is 0.1 versus 0.01%, no one will know; indeed, this is desirable as it produces more of Our Beloved Tube Sound.)

You also have to pay more attention to dynamic behavior, since gain and oscillation go hand in hand. Excessive positive feedback may yield a statically bistable circuit, or it may oscillate (guaranteed if the positive feedback is strong enough to cause hysteresis and is AC coupled).

Now, the gain-bandwidth of a given stage remains fairly constant, so that a triode that's capable of, say, 90x gain at 20kHz (dropping to, say, 70x at 40kHz) could be boosted to a gain of 900, but it will only do so up to a frequency of around 2-4kHz. (Again, the gain error is amplified, so that essentially nothing comes through at frequencies where, normally, only moderate attenuation would be seen.)

Bandwidth is the main motivation for cascading stages.

Cascaded stages can deliver unlimited gain-bandwidth, but the phase shift or delay through the chain may be intolerable for other reasons (e.g., an external error must be corrected, such as using the amplifier to control the voltage across a speaker), or the signal-to-noise ratio (SNR) becomes too terrible (if the noise factor of a given stage is greater than its gain, it's only making things worse!). For very large gains, shielding is also required (more an issue in radio receivers than audio amplifiers, but the exact same situation applies): if you have 100dB of gain through a chain of stages, but only 90dB of shielding (which isn't bad to begin with!), you will get an oscillator.

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Old 7th November 2013, 08:31 PM   #13
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Talking re gain

I am beginning to realize there are few absolutes, but a host of trade offs in design. Very fascinating study. Your in depth answer will take some digestion, but on the surface it makes sense.
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Old 8th November 2013, 09:54 AM   #14
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Quote:
Originally Posted by Sch3mat1c
Distortion will be higher than the same stage without PFB, but when the difference is 0.1 versus 0.01%, no one will know; indeed, this is desirable as it produces more of Our Beloved Tube Sound.
I find distortion undesirable. I don't use valves because I like distortion, but because I like valves.

Positive feedback is rarely a good idea, unless you want to make an oscillator or are using careful bootstrapping.
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Old 8th November 2013, 10:09 AM   #15
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one quick way is to look at the resistance coupled charts at the back of say an RCA tube manual, tables there will give you an idea of gains using predetermined values of resistors per tube type....http://www.tubebooks.org/tubedata/RC17.pdf
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Old 8th November 2013, 02:30 PM   #16
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Default Tube Manual.

Thanks, good info. I think there are tube manuals online until I can acquire one
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