*To*: Harp-L@xxxxxxxxxxxxxx*Subject*: Re: Blues Blaster Mic - Capacitor*From*: Bruce Steinberg <bruces@xxxxxxx>*Date*: Mon, 24 Oct 94 17:05:58 PDT*Archive-date*: Mon, 24 Oct 1994 19:08:03 CDT*Reply-to*: Harp-L@xxxxxxxxxxxxxx*Sender*: owner-harp-l@xxxxxxxxxxxxxx

re: <...> Pretty close. Had I seen the original post, I would have responded. Actually there are only a couple of thing that need clarification: :The cap is there as a filter. A cap appears as an open circuit to low :frequency signals and a closed (or short) circuit to high frequencies. Actually a capacitor blocks DC and passes AC which is essentially what you said. Depending on any given resistance that is coupled with the capacitance will determine what if any frequencies are passed. :Not a whole lot of complex filtering you can do with one capacitor... :either pass treble or lose treble. To an extent this is also true but in reality all one capacitor can do is pass ALL AC. It takes resistence to create a filter and that filter can either be a high pass, low pass or band pass. In other words it can filter out the highs, leaving lows and mids, filter out the lows, leaving highs and mids or filter out the lows and highs, leaving mids(band pass). Most mics end up with the cap and a volume pot wired as a band pass circuit. The <...> //////////////// Actually, while Rs and Cs can be used to make simple high- or low-pass filters with specific frequency responses, a C by itself ~can~ be used to create basic high- or low-pass filtering in the widest sense (e.g., audio vs. radio frequencies). In its most basic terms, what we call "impedance" of a given circuit is the vector sum of its DC "resistance," plus something called "reactance" (think of it as frequency-specific "AC resistance") provided independently by any C (or inductance, L) in the circuit. Both resistance and reactance are expressed and measured in ohms (although they are always out of phase to some degree, which is another discussion :). In the case of capacitors, capacitive reactance is inverse to frequency (1/2[pi]fC), so a given C has an infinite reactance to DC, decreasing as the frequency increases, all the way up to effectively a dead short for very high frequencies -- even with with no appreciable R in the circuit at all. Therefore, capacitors don't "pass all AC"; in fact, they are ~lousy~ conductors at very low frequencies, but get "better" as the frequency impressed across them goes up. So it's possible for a simple C by itself to be used as the simplest and grossest possible low- or high-pass filter, either in parallel or in series as required. Adding resistance can only help to shape a more specific frequency response (as in "db/octave"). BTW, you need an *L* and a C to make a tuned *band*-pass filter, at least out of strictly passive components (i.e., no active amp to provide balancing phase shift at the "tuned" frequency). Rs and Cs can only be combined passively (even in complex configurations) to provide high- and low-pass responses. Hope this helps (even if it still doesn't explain what the heck that cap is really doing in the JT-30 :). B*

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