Pardon me if this has been discussed before, but I have a question about the input capacitors on this popular tube unbalanced by Broskie:
The Broskie Cathode Follower circuit shows an input cap on the + side, but not the - side of the balanced input.
I was wondering why that is.
I've seen other circuits that have input caps at both inputs, as this nearly identical cathode follower un-balacing circuit. (sorry I would give credit but I don't know the designer of this)
Can anyone comment on the benefit of the input cap at the -ve input, it's requirement, or lack thereof?
Thanks.
The Broskie Cathode Follower circuit shows an input cap on the + side, but not the - side of the balanced input.
I was wondering why that is.
I've seen other circuits that have input caps at both inputs, as this nearly identical cathode follower un-balacing circuit. (sorry I would give credit but I don't know the designer of this)
Can anyone comment on the benefit of the input cap at the -ve input, it's requirement, or lack thereof?
Thanks.
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The dc voltage at the grid of the upper triode is 1/2 B+. So C1 is needed to block that dc-voltage and only let ac-signal through. Without C1, that input terminal would also sit at 1/2 B+.
The grid of the lower triode doesn't have a dc-voltage on it, and the source will be connected to that grid and ground. So there a capacitor is not needed to block any dc-voltage (provided that the source that will be connected to it doesn't have a dc-voltage on it).
The grid of the lower triode doesn't have a dc-voltage on it, and the source will be connected to that grid and ground. So there a capacitor is not needed to block any dc-voltage (provided that the source that will be connected to it doesn't have a dc-voltage on it).
try to use a beautiful signal trafo.
You have a galvanic isolation and with a proper configuration you have unbal to bal and vice versa.
It is not cheaper but easier
Walter
You have a galvanic isolation and with a proper configuration you have unbal to bal and vice versa.
It is not cheaper but easier
Walter
Caps are often used to block DC voltage.
What is the DC voltage at R4?
Do you NEED a cap here?
Balanced lines should be balanced the whole way. Omitting the cap on one side will upset subsonic balance.
Hobby systems all in one room, one table, do we really need to care?
The DC voltage at R4 is actually undefined since there is no resistor to ground that would establish a DC reference. The DC voltage at R4 is going to be whatever DC voltage appears at the output of the source connected to that input. I would be inclined to use an input blocking capacitor here and add a grid leak resistor to provide an explicit ground reference.
This is not unusual balanced input design. Nelson Pass designed very popular Aleph J also has an input cap only on one side.
R5 R7. (Yes, it is drawn funny.)
And yes, the source.
An awful lot of designs "assume zero DC" comes in. In tube-work, inputs tend to be at zero and outputs tend to be at a hundred volts, so a plate cap is needed but a grid cap "may" not be. Depending on your technical experience, you may decide not to rely on the kindness of straingers, always using your own cap.
That's my personal practice, and it's not hard to set the input LF corner to something in the 5 Hz range or below. But I admit to being ultra-conservative so I don't leave a lot to chance, especially if there is a possibility that a stage operating point could be upset.