any R/C filter charts?

[daz]

Does anyone know of a chart that shows what different combinations of paralleled cap and resistor combos yield in terms of what frequency that various cap values will pass and how the resistor value will affect how big a difference there is between the body of the tone and the highs that are passed?
In other words, the frequency that each cap value allows, and the Db value of the full signal thru the resistor compared to what it was before the resistor, or it in other words the Db diff between the passed treble and the post resistor full tone. Hope i described that right.

[Earl]

Not sure exactly what application you're after, but generally, the RC filter is a high pass, or low pass filter with a "cut-off" frequency value. It's a 1st order filter with a 20dB/decade response.

Not sure of a chart for this, but it's not too difficult to make a little program (like with Excell) to plot the various combinations.

The lower 3dB cut-off ponit is represented by - Fc= 1/(2piRC), in hZ.

If you want to plot the response of a particular filter combo, you just take the Xc||R at various frequencies - Xc being 1/(2piFC). You can add a third axis with the dB conversion.

[martin manning]

As it happens, I made an Excel sheet that does these calculations and makes a dB-log(f) plot like the one below, which is for 20nf and 1 meg. I don't think I can upload the file (can I?), but I'd be glad to send it... PM me.

MPM

[Steve Conner]

You don't need a chart. Just remember, 1uF, 1k Ohm, 159Hz.

It all scales linearly, so if you see a 1M volume control fed by a 0.022uF coupling cap, you can guess it has a 3dB rolloff at 8Hz quicker than you can get the calculator out.

With this, and the potential divider equation, you can reason your way round most any RC circuit.

The potential what equation? The dB of "boost" you get from shunting a cap across a resistor depends on the other impedances in the circuit. If you put 100pF across a 1M Ohm resistor in series with the signal path, and the other impedances add up to 100k, you will get a 10x boost. The boost will be +3dB at 1590Hz and nearly its full 20dB (+17?) at 15900Hz.

[daz]

Ok, well this is all a bit more than i though. i thought there would be someone who'd have made up a chart, having seen similar charts. But i guess theres more to it than just the 2 components. My brain is allergic to anything dealing with math, so i think i'll just let my ears do the work. Thanks.

[Earl]

Steve's right about the other factors in the circuit affecting the RC "outcome".

For instance, the bypass cap on a common cathode pre-amp stage...typically 25uF across a 1500 ohm cathode resistor wil net you some numbers.

In fact, that resistor is in parallel with the tube's internal cathode resistance....~1600 ohms. In that case, using the cut-off equation, you'd need to use ~700 ohms as R, rather than 1500 ohms.

There's really not a lot that you could consider 'straigh forward' in AC analysis....

[martin manning]

You don't need a chart. Just remember, 1uF, 1k Ohm, 159Hz.

Good shortcut, thanks! I see now that 1/(2*Pi), the part of the expression for turnover frequency besides R and C, is 0.159.

It was useful for me (being at about the "Circuits 101" level) to work through the math to attenuation in dB, and to see the shape of the roll-off. I note, for example, that it's not 6 dB per octave until the third one after the turnover, and in fact you only get 4 dB for the first.

MPM