output cap sizing

[Ritchie]

Thinking of changing my output caps on extended foreplay III to a smaller value.
My amps have an input impedance of 470K, so a 3Hz -3db point would be approx. 0.1uf.
I am using a 25 foot low capacitance audio interconnect. With this length of cable what value would be safe to go with.
I know the capacitance formula is a good guideline but how do you factor in the cable length?

Thanks,
Ritchie

 

[Grainger49]

The amplifier input and the coupling cap establish a low frequency roll off as you have already calculated.  I bet it will sound just fine unless you listen to a pair of Wilson Audio speakers in a large room (the room limits bass more than most speakers do). 

The cable capacitance shunts high frequencies to signal common.  So it rolls off the highs.

You can calculate anything but the best way to determine if it affects the sound is to try it.  Output capacitors are easy to try. 

 

[Paul Joppa]

In order to reduce capacitive pickup of electrical fields, you want wiring to be at a low impedance. Foreplay III has an output impedance of about 500 ohms, and an output capacitor of 2.2uF which add 1200 ohms reactance at 60Hz, 500 ohms at 120Hz. With a 0.1uF capacitor, you would add 27,000 ohms, increasing the susceptibility to 60-Hz hum fields by about 26dB.

If there are no significant electric fields inside your preamp or inside your power amp, and if the interconnect shield is sufficiently complete, then this will not be a problem. Given the long cable run, I would expect that the cable shielding would be the wek point.

At high frequencies the output capacitor has an impedance near zero and is not a factor. The ability to drive the cable capacitance is the limiting factor. There are two considerations:

1) frequency response determined by the output impedance (500 ohms) and cable capacitance. If your cable is 30pF per foot and 25 feet long then it would be 750pF, which gives a -3dB frequency of 400kHz - not a problem.

2) Ability to drive current into the capacitance. 750pF is 10K ohms at 20kHz, 2K at 100kHz. Foreplay can drive a 10K load, so you would probably be fine but with little margin.

A low-capacitance cable probably has less capacitance than what I used as an example.

 

[Ritchie]

Thanks for the excellent info. guys.
I think I'm going to go with a 0.47uf cap.
Would this be ok or should I go to 1uf?
I guess I can try both, but it's nice to know technically first.

Ritchie