Output Capacitor question

[debk]

I know this has been asked before, but I want to make sure I understand it properly.

This being a cold snowy winter in Northern New York, I find myself with another bout of upgrade-itis.  I would like to upgrade the output capacitors in my Extended Foreplay III.

The Extended Foreplay III has a 3uf auricap for the output capacitor.  Now the amp I just built has an input impedance of about 121K.  The 3uf value would seem like overkill in this situation.  Using the  coupling capacitor calculator on the V-cap web site.  I calculate that the coupling capacitor value of .68uf when using a -3db freq of 2Hz.
So I should have no problem switching out this capacitor for a .68 or 1uf V-cap or other teflon capacitor. ( assuming I keep the interconnect length short)


Was the original value chosen so the preamp would be compatible with a wide range of input impedance amps, or am I missing something in my thinking?

Thanks
Debra

[Grainger49]

The FP III can drive long cables if they are good.  That is a result of the low impedance (700 ohms) cathode follower stage used for the output.  So even the stock 2.2uF would be able to run long cables.  I believe Doc recently stated 50 to 100 feet.  The spec says it is good to drive amps with 10k ohms input and above.

I'll bet the 3uF Auricap in the Extended package is like the 1.0uF 630V Solens that are included in the Eros.  I know that Bottlehead stocked the Solens for another kit and consider it a nice upgrade for the Eros.  I bet that the value for the Extended is another cap that was already stocked.  Nice for the consumer and smart on Bottlehead's part.

[Paul Joppa]

There are two considerations that drive the output capacitor value for preamps:

1) frequency response at the lowest load impedance. 3.3uF gives a 5Hz corner frequency into the minimum rated load impedance of 10K ohms.

2) To minimize the chance of picking up hum, the source impedance should be as low as possible. Longtime poster and Bottlehead VoltSecond introduced me to this idea, and I have adopted his criterion - the output capacitor reactance at 60Hz should be no greater than the output impedance of the preamp. A capacitance of 3.8uF is 700 ohms at 60Hz.

I see no reason not to try a smaller capacitor, if you know the load impedance to be higher than 10K ohms - but be alert to the possibility of hum. You can keep the old capacitor around at first, using it in parallel with the smaller new capacitor to see if the hum is reduced. Like the tube shields on the Seduction tubes, the larger cap is only needed if it is needed in your environment.