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Output cap calculation for Nakamichi CD-4 tube/valve output stage

Posted by valveophile (A) on January 22, 2004 at 04:15:53

All

I've just had a valve output stage fitted to my Nakamichi CD-4 (true output stage which bypasses the op-amps entirely as opposed to a buffer). SRPP 6N1P per channel.

The existing caps across the outputs are 2.7uF polyprop, but I'd like to fit some paper in oils.

The guy who built it suggested a "rule of thumb" that the capacitor should be 50 / Load impedance (in KOhm). In the case of my 300B amp (this stage is not really suitable for solid state low impedance inputs!), it will be 150KOhm giving a value of 0.33uF. To err a little bit on the high side I'm going to start with a 0.47uF, but am not sure whether the LF rolloff (which could be caused if it's too low) will be perceptible enough to know if I've hit the right value.

Any thoughts on this, or advice on a more "exact" equation to calculate with?

Regards

Valveophile

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