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Cap Rolloff Frequency

From: mgarvi–(at)–anix.com (Mark Garvin)
Date: 2 Nov 1995 11:29:54 -0500

>>coupling cap from .02 to .2 mF? According to Weber, this is supposed to
>>let more mids and lows through. Perhaps this cap value could be decreased
>>to reduce bass response.

>In <473tpe$jo--(at)--anix2.panix.com> mgarvi–(at)–anix.com (Mark Garvin) writes:
>
>That’s correct. You could compute the corner frequency for rolloff by:
>
> Freq = 160000 / (cap_in_uf * resistor)
>
>where ‘resistor’ is the total resistance to ground on the ‘right’
>side of the cap. Don’t remember which cap you’re referring to, but
>the resistor is probably the grid resistor of the next stage.
>
>Ex: Low E on the guitar is 82.4 hz, so a .002 cap and 1meg grid
>resistor would roll of your low E string about 3db.
>
> Freq = 160000 / (.002 * 1meg)
>
>You can see that if the next resistor is 1 meg, changing from .02
>to .2 is overkill.

I got some email about my previous posting about bass cutoff frequencies,
so I guess I wasn’t clear enough. I’ll try again:

|| C
—————–||——–+———-
||/
/ R
/
/
|
—–

In this type of circuit, the bass will start rolling off at
a frequency determined by the cap and resistor values. The
formula for computing the 3db-down point (corner frequency) is:

159155
F = ——–
R * C

F is the frequency in hertz
R is ohms
C is microfarads

The ‘magic number’ 159155 is derived from 1000000 / (2 * pi).
The 1000000 eliminates the need to state capacitance in farads.
The 2 * pi has to do with rotational velocity of a sine wave at the
frequency in question. You don’t really have to know about this.
Components are just not that precise, so I generally just use 160k.

The three variables can be interchanged freely to derive two other
similar equations–for situations where you want to compute C or R:

159155
C = ——–
R * F

or…

159155
R = ——–
C * F

So if you had a 1 meg grid resistor and wanted to find the
value of cap that would reduce your low E string volume by 3db,
use:

160000
C = —————-
80hz * 1000000

80hz is the approx freq of the low E string (actually 82.4hz)
1000000 is the value of the grid resistor in ohms.
This tells you that you need a .002 uf cap to do it.

——————————————————————

For high-freq rolloff, reverse the positions of the cap and the
resistor in the ascii schematic. Don’t forget that you still
need a DC-blocking cap if you are working with a coupling circuit.

Hope this is a bit more clear this time.

Mark Garvin

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