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- Thread starter cl1238
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The feedback value should be (for 20dB negative feedback) 1200 X the square root of the output impedance (in your case 6) about 3K.

How can you decide the value of one resistor in a potential divider without knowing the value of the other resistor? Are you psychic, or merely guessing?JonSnell Electronic said:The feedback value should be (for 20dB negative feedback) 1200 X the square root of the output impedance (in your case 6) about 3K.

And 6, of course, is not the output impedance but the load impedance.

How can you decide the value of one resistor in a potential divider without knowing the value of the other resistor? Are you psychic, or merely guessing?

And 6, of course, is not the output impedance but the load impedance.

The formula shown is a standard formula for all transformer coupled valve amplifiers. The dB level changes with the cathode resistor value but most driver cathode resistors tend to be around 470R. Some are as low as 100 after the cathode bias resistor and they have less negative feedback but the formulae stays the same, the cathode resistor is raised to increase the feedback and reduce distortion/noise. In the Williamson design, the output winding is universal between 3R and 15R and the feedback is worked out using that formulae, not the impedance of the winding. Making it universal for loudspeaker loads.

So to put it in one sentence: the formula shown is a "standard formula" for all transformer coupled (?) valve amplifiers with negative feedback returned to a 470R cathode resistor in the first stage. For all other amplifiers you need a different formula, the correct formula, the general formula. And, of course, whether it gives you 20dB of feedback depends on the open loop gain of the amplifier - which also varies quite a bit from one design to another.JonSnell Electronic said:The formula shown is a standard formula for all transformer coupled valve amplifiers. The dB level changes with the cathode resistor value but most driver cathode resistors tend to be around 470R.

So perhaps we should say that it is a "standard formula" which applies to just a small subset of all valve amplifiers, and should otherwise be ignored?

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