Extended Foreplay Coupling Caps

[J Leng]

Am I correct in thinking that if my power amp has an input impedence of 100k then I only need the coupling capacitor value to be 0.47uF and any large capacitor will not make any difference to the sound quality?

[Paul Joppa]

For frequency response you are correct.

However, there is a hum-reduction argument that says to keep the source impedance of the capacitor no greater than the preamp's output impedance at 60Hz. That works out to be 3.8uF for a Foreplay III.

If it were me, I'd go ahead and try it. If there is any hum or interference problem, just replace the original cap leaving the new one in place as a bypass. (I'm assuming you ask because you have a higher-quality 0.47uF cap in mind as an upgrade ... )

[J Leng]

Thanks for that, would the hum be present on all sources, as there is hum present on my phono stage?

[Paul Joppa]

It's an output thing, so would be present identically on all sources.

For what it's worth, I had the same thing - hum on phono only - which disappeared when I changed phono preamps.

[J Leng]

I thought so, I've just changed op amps in my phono stage to a discreate GD Audio op amp and I think that's where the problem is.

J