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Old 10th Sep 2019, 8:53 am   #5
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 2,922
Default Re: Cathode follower line out - best practice

Originally Posted by unitelex View Post
... I was considering increasing the cathode resistor, but mindful of the good practice of a low output impedance to drive line levels ...
The output impedance of a cathode follower stage is not usually dominated by the value of the cathode resistor. The action of the valve is that it can source or sink a good deal of current out of or into its cathode for a relatively small change in the cathode voltage. So the valve itself behaves as if it has a cathode 'impedance' which is in parallel with the cathode resistor, and that impedance tends to be low and therefore, in a parallel circuit, to dominate.

The formula for the cathode impedance is a little complicated, but in the approximation that it does dominate and that the external resistance in the anode circuit (including the HT rail's effective impedance) is small, it is approximately 1/gm, where gm is the transconductance of the valve. I don't know what operating point your ECC82 is running at, but the valve's typical gm is 2-3mA/V in which case your circuit's output impedance would be 500-330ohms in parallel with the cathode resistor. Changing the cathode resistor will affect this, but the major change might come from its effect on gm rather than its parallel contribution.

EDIT: You might think this gets you off the hook and that you're free to lower the standing current in the valve to make life easier for the HT supply. But as others have said it sounds like your HT rail is not very 'stiff' to DC load. If it's also not very stiff to AC load (because there isn't a substantial smoothing capacitor) then your cathode follower may induce an AC signal on the HT rail as it tries to drive significant current into the output cable. This would be A Bad Thing. So beefing up the HT supply as far as you can would still be a good idea.



Last edited by GrimJosef; 10th Sep 2019 at 9:21 am.
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