.47uF output cap to 1uF? - diyAudio
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Old 18th February 2011, 10:31 PM   #1
phrarod is offline phrarod  United States
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Default .47uF output cap to 1uF?

I have a tubed dac I built with tube output stage. My friend originally designed it to have a .39uF output cap but I had to go to a .47uF as that was the best option for a high quality cap. Everything works fine, no complaints.

But I want to try out the new flat caps from Jupiter which are like a poor man's Dueland. I head these in a dac (no tube buffer/output) and they sounded great. However the lowest they make is 1uF.

I've read other things here about it depends on the grid leak resistor which I don't have. I have an I/V transformer into the tube grid and the second half of the tube is wired to be a dynamic cathode bias.

What things might I run into if I essentially double the capacitance of the output cap?
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Old 18th February 2011, 11:55 PM   #2
kevinkr is offline kevinkr  United States
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A schematic even partial would help.. I'm clueless as to whether you're talking cathode follower, anode follower and whether this is the output cap to your RCA jacks.

In most cases all this is going to do given an excellent quality cap to start with is to move the LF corner down one octave which should be fine or might potentially be an improvement. Caps this large are sometimes shunted with a much smaller cap in order to improve high frequency performance. In some cases a small amount of resistance may be inserted in series with one or both of the caps to reduce the Q of the resonant circuit created with the ESL of the larger cap. I generally choose caps that sound good without the smaller shunt capacitor.
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Old 19th February 2011, 12:12 AM   #3
phrarod is offline phrarod  United States
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Understood. Here's a schematic
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Old 19th February 2011, 12:27 AM   #4
kevinkr is offline kevinkr  United States
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Very odd cathode bias scheme, lower triode is saturated, but I wonder what the voltage drop is across it, and the internal resistance I think would increase significantly as the tube ages.. Linearity? Know you weren't looking for a design critique.

I can't see any particular issue with the 1uF cap..
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