calculating roll-off point

[brrt]

Hi! I'm having a problem with calculating the roll-off point in situations where (in my opinion) only a cap is used.
I can imaging it's possible to calculate a -3dB point for filters (e.g. high-pass) made from an opamp and some resistors and caps.
But looking at the schematic of, for instance, the Boss SD-1 (at fuzzcentral), there's also a highpass filter around C1, C2, C4, C8, etc. Although they are used as coupling-caps, they are filtering right?
So what about the formulas? Which formulas can be used to calculate their roll-off point?

Greetings, Brrt

[niftydog]

to calculate any filter -3dB point you need values for the capacitance and the impedance of the following circuitry.

You can calulate the reactance of a capacitor at a particular frequency, but this figure is meaningless (virtually) without being able to relate it to the impedance of the next section of the circuit. (ultimately, it all comes down to the voltage divider equation using input/output impedances... or impedance matching)

As for active filters (filters using op amps) you might want to check out FilterLab from microchip. You might be able to "simulate" the values and topography of the filter you're trying to examine.

Æ'-3dB = 1 / (2pi.R.C)

[Reactance] Xc = 1 / (2pi.Æ'.C)

[Steben]

Hi! I'm having a problem with calculating the roll-off point in situations where (in my opinion) only a cap is used.
I can imaging it's possible to calculate a -3dB point for filters (e.g. high-pass) made from an opamp and some resistors and caps.
But looking at the schematic of, for instance, the Boss SD-1 (at fuzzcentral), there's also a highpass filter around C1, C2, C4, C8, etc. Although they are used as coupling-caps, they are filtering right?
So what about the formulas? Which formulas can be used to calculate their roll-off point?

Greetings, Brrt

Well the input impedance of the Q1 is 470k (input resistor).
So you have f= 1/(2*pi*0.000000047F*470000ohms)= 7.2Hz
This means there is practically no audible filtering!

Always look at the input resistor before a Q or OpAmp.

[Steben]

Just for the record: C2 with 100k resistor=> 88Hz roll-off
C3 and R6 in non-inverting opamp=> 720Hz roll-off as in Tube screamers
C7 with R18 and volume pot (14,7k in total)=> 11Hz roll-off

Lowest string on Guitar is 88Hz as you may know.

[R.G.]

I'm having a problem with calculating the roll-off point in situations where (in my opinion) only a cap is used.

Yes, you are! That's because
(a) it's impossible to calculate a rolloff where only capacitors are used
but more importantly
(b) there are really resistors there but you can't easily see them

The resistors that are hiding are the ones associated with the biasing or loading on each side of the coupling capacitors and the "resistance" effects of the active devices themselves that the capacitors connect to.

For a live how-it's-done example of how to calculate the rolloffs invoved, see the "Technology of" series at GEO. Particularly the tech of the Tube Screamer, Fuzz Face, and the Rangemaster clone article.

[Ansil]

hmm i got tired of doing the math so i decided to make extra "calculators"

basically if i know the freq but dont' know the cap i would like to be able to plug it in and make it work

so

1/C = F(2pi*R)
1/R = F(2pi*C)
i think that was what i wrote down anyway it was turned into a calculator like over at jacks site.