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Old 11th Sep 2019, 1:03 pm   #12
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 5,396
Default Re: Cathode follower line out - best practice

I'd be tempted to use a much greater cathode load, nearer 10kΩ and take the grid resistor not to 0V, but to a potential divider from HT+. You should be able to get away with much less than 4mA. You'll have to feed the signal in via a capacitor of course, but that is not a problem.

Ripple on the anode may cause hum, though the PSRR of a cathode follower is pretty good (you can simulate it or there is a formula of course).

Output impedance of a CF is, as GrimJosef says, approx. 1/gm. Tapping down on the cathode resistor may actually give you a source impedance to the line of greater than this, depending on the cathode resistor and where it is tapped.

Originally Posted by GrimJosef View Post
There's a rule of thumb which says the output impedance of the source should be less than 10% of the load impedance.
Then that would be violated within almost every IC; it would be violated in almost every single-ended pentode output stage feeding a loudspeaker; it would be frequently violated between stages in a valve AF amplifier; it would be violated in RF and wideband scenarios where a cable is both-ends-matched (50Ω cable fed from 50Ω source impedance and terminated by 50Ω load, which will absorb reflections either way). It's like most rules-of-thumb, it has areas of applicability, but shouldn't be applied slavishly. Though I do agree with you in this instance!
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