Home Tube DIY Asylum

Do It Yourself (DIY) paradise for tube and SET project builders.

[ Follow Ups ] [ Post Followup ] Thread: [ Display  All  Email ] [ Tube DIY Asylum ] [ Return to Search Results ]
[ Alert Moderator ]

RE: Cap Value

"So I took a 300B biased with a 750Ω resistor (59V bias 78ma 375V B+) and bypassed it with a 135µ cap to get a -3dB point of 2hz. I then fed 50Vac into the grid and the plot below shows the AC across the cap. That "zero value" sure is a tough thing to chase."

Well yes, the physics is what it is. The potential audibility due to the "non-infiniteness" of the capacitor is going to be related to the ratio of the AC voltage across it divided by the voltage swing in the circuit as a whole (most of which will be the maybe hundreds of volts across the output transformer primary). The question then becomes how small is small enough?

In your example, if you took the bypass capacitor to be 25000µ instead of 135µ you would be down to about 100mV at 1Hz.

Practically speaking, probably more of a problem is getting the output impedance of the power supply down low, given the voltages involved.

The calculations to estimate the sizes of the relevant voltages are elementary ones, following basic physical principles. Obviously, as a matter of principle, the AC voltages across the capacitors can be made as small as desired, by increasing the capacitances appropriately.

Again, it comes down to a question of how small is small enough, in order to become an inaudible effect? It seems to me there are (at least) two issues here. One of these is just the simple question of the calculated impedance of the capacitor as a function of frequency, and the extent to which the resulting drop-off of the low-frequency audio output (and associated phase shift) is of concern.

The other issue concerns non-linear distortions that the capacitor may introduce, given that in practice it is still going to have some (possibly) significant AC across it at the very low frequencies. Measurements become important now, if one wants to understand properly what is going on.



This post is made possible by the generous support of people like you and our sponsors:
  Kimber Kable  

Follow Ups Full Thread
Follow Ups
FAQ

Post a Message!

 Forgot Password?
Moniker (Username):
Password (Optional):
  Remember my Moniker & Password  (What's this?)    Eat Me
E-Mail (Optional):
Subject:
Message:   (Posts are subject to Content Rules)
Optional Link URL:
Optional Link Title:
Optional Image URL:
Upload Image:
E-mail Replies:  Automagically notify you when someone responds.

[ Top Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2024, Audio Asylum, all rights reserved ]
[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]