Capacitor question

[Kipper4]

So after a recent dyna comp type build I see there is a 200pf cap to ground on the input. Literally from input to ground. Now some have reported that they prefer a 100pf to Gnd reporting that it lets more highs through.
So naturally I didn't want to miss the opportunity of a quick and easy mod so I tried it and I can't hear it. Goddam you ears.........

It's late. Beware dumb question coming up.
So if I use a series capacitor. How do I know where the corner freqauncy is and what it's doing.
We are not talking R/C filter.
Or as in the dyna comp a cap to ground. What then how do I know where the corner freqauncy is etc.
I realise the cap to ground is gonna bleed of some freqauncy but which ones.

Time I hit the hay. Night all.

[wilrecar77]

All RC filters function as a "dynamic" voltage dividers. Since the capacitor impedance (reactance?) changes with frequency, its as if you have a volume pot and the knob turns as the signal frequency changes.

So, looking at the dynacomp, the 220pf cap and the output impedance of whatever comes before the dynacomp forms an RC lowpass (right?). A cap by itself is not a filter, so you have to look around a bit to find the other parts that make up the filter.

[PBE6]

Very true. In this case, the "top" part of the voltage divider will be whatever came before - either your guitar, or a pedal output. If it's your guitar, you can expect something on the order of 10 kohms, if it's a pedal with an output buffer you can expect something on the order of 0.1-1 kohms. In either case, the capacitor cutoff is so high (~70 kHz best case) that it won't affect your guitar sound. It's most likely there to shunt radio frequencies to ground.

[ubersam]

Most DynaComp schems I've seen has a 10K resistor in series with the input. If the pF cap to ground is after that it would affect the cutoff too. Say a 10KΩ pickup in series with the 10KΩ input resistor, 220pF to ground after the resistor, cutoff = 36KHz; 100pF = 79KHz; both still pretty high.

[PRR]

> 200pf cap to ground on the input. Literally from input to ground.

So is your guitar cable. That's 30pFd a foot. 200pFd is just like another 6 feet (2 meters) of guitar cable.

If you insist on 3-foot cable for maximum highs, 6 feet more will suck.

If you run a 25 foot curly-cord so you can lap-dance the hotties in the front row, another 6 feet is nothing.

Calculating the effect may not be trivial. If the axe Vol is near full-up there is 5H or 10H of inductance on top of the several series and shunt resistances. 10H with only 200pFd is 3.7KHz. Double/half the pFd causes a half-octave shift (400pFd is 2.6KHz). The response at this point may be peaked-up, slumped-down, depending on resistances.

> some have reported that they prefer a 100pf

And you believed them?

I find the cable color has more effect. (Not totally joking...)

Are pFd caps expensive? Ah, no. You might find a dozen 100pFd for less than a buck, maybe free shipping from Peking. Use your "regular" cable, hot-up the soldering iron, tack more or less in until happy or bored.

[Kipper4]

Oh yer I forgot to factor the guitar

70khz you say. I doubt I'd hear that.

Thanks guys told you it was a dumb question. I'm not sorry I asked.

Paul it's 6foot and grey ,co axial (Don't ask I botched it from a cable a friend gave me) No coils, no hotties. Except Mrs Kipper. Oddly enough I only use it for testing pedals.

Sometime you just need reminding and yer I believed it enough to want to throw a few pence of componants at it for a try.

Thanks all