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Author Topic: ac capacitors and high frequency  (Read 4067 times)

Taylor Birkey

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ac capacitors and high frequency
« on: May 15, 2015, 12:16:59 am »

I understand how DC capacitors work. But I can't find anything that easily explains ac ones. I especially would like to understand why ac caps allow high frequencies. Relating to shielded cables and tone controlls?.
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Scott Holtzman

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Re: ac capacitors and high frequency
« Reply #1 on: May 15, 2015, 01:45:28 am »

Well there are hundreds of books on basic electronics.  You can't think of the cap by itself but how it operates in tandem with coil of the speaker, an inductor with a stated impedance.  An inductor and a capacitor form LC.  These circuits have a resonance based on the component value and resistance inherent in the physical components.

Just like a cabinet has a natural resonance and frequencies below it won't pass. You will hear old timers call these tank circuits. 

Hope this helps.
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Scott AKA "Skyking" Holtzman

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John Rutirasiri

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Re: ac capacitors and high frequency
« Reply #2 on: May 15, 2015, 03:37:54 am »

I understand how DC capacitors work. But I can't find anything that easily explains ac ones. I especially would like to understand why ac caps allow high frequencies. Relating to shielded cables and tone controlls?.

These are types of electrolytic caps.  They both work the same way, and both block DC while letting AC go through.  DC-rated caps (such as those use in amp power supplies) have polarity that must be observed.  They are generally used as energy storage after the AC is rectified (converted into DC).  They can withstand little AC input (a rectified AC still has some AC component), but too much and the dielectric breaks down and it's dead. 

AC-rated caps are constructed differently -- the dielectric doesn't break down.  They can handle direct AC current and are often used to get additional starting torque so a motor can going without pulling so much current.  It's also used to run the motor by generating an out of phase current for one of the windings.

John R.
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TJ (Tom) Cornish

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Re: ac capacitors and high frequency
« Reply #3 on: May 15, 2015, 03:29:57 pm »

I understand how DC capacitors work. But I can't find anything that easily explains ac ones. I especially would like to understand why ac caps allow high frequencies. Relating to shielded cables and tone controlls?.
At the risk of opening several cans of worms here (how far down does the rabbit hole go, Alice?)

- There is no such thing as an "AC capacitor" in the sense that it can store AC power.  A capacitor is an energy storage device, and a voltage bias will charge it up, and reversing that voltage bias - as in the downward part of the AC waveform - will discharge the capacitor.  Capacitors are made from different materials, and some of these can handle both positive and negative voltages, some can't.  Some are very energy dense, some are less so.  Some can handle high-frequency charging and discharging, some types can handle higher or lower charge/discharge current.

- As Scott mentioned,using a capacitor and an inductor one can create a resonant circuit that sort of stores AC power, but that isn't just a capacitor, and I'm pretty sure isn't what you're talking about.

Back to your original question, take a gander at the Wiki pages for a high-pass filter and a low-pass filter:
http://en.wikipedia.org/wiki/High-pass
http://en.wikipedia.org/wiki/Low-pass

A typical cable acts as a low-pass filter - long lengths reduce high-frequencies.  The cable capacitance is to ground (the shield is grounded), and so creates a low-pass circuit.
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TJ (Tom) Cornish

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Re: ac capacitors and high frequency
« Reply #4 on: May 15, 2015, 03:35:09 pm »

These are types of electrolytic caps.  They both work the same way, and both block DC while letting AC go through. 
This depends on where the cap is in the circuit.  A cap in series with a circuit will block DC (eventually) while letting AC through; a cap with one end to ground - such as a power supply filter cap - actually dampens the AC while letting the DC pass.
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Mike Sokol

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Re: ac capacitors and high frequency
« Reply #5 on: May 15, 2015, 05:23:19 pm »

This depends on where the cap is in the circuit.  A cap in series with a circuit will block DC (eventually) while letting AC through; a cap with one end to ground - such as a power supply filter cap - actually dampens the AC while letting the DC pass.

I like to think of a capacitor as a water tank you can fill up with electricity or a big spring you push on to store up energy. In a DC system, you're filling the capacitor up to capacity, sort of like a beer glass at a tap. In an AC system, it passes pulses of electrons which changes with how quickly the pulses are reversed. That's why AC capacitor circuits are frequency dependent. If you're a guitar player you know that changing the value of the capacitor in the tone circuit of your guitar completely changes who the tone circuit sounds.

One of the really cool things about resonant circuits is that you can consider a speaker cabinet as a combination of a capacitor (the spring of the cone plus the spongy air in the cabinet) and an inductor (the mass of the cone which has inertia). In fact, that's how the proper volume and port size of a bass reflex cabinet is designed, by modeling the actual cabinet inertia and compression factors into equations that calculates the various resonant frequencies, just like the aforementioned "tank circuits".   
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Steve M Smith

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Re: ac capacitors and high frequency
« Reply #6 on: May 16, 2015, 01:47:42 am »

It sometimes helps to think of the capacitor as a small rechargeable battery when used on dc circuits.


Steve.
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David Buckley

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Re: ac capacitors and high frequency
« Reply #7 on: June 21, 2015, 05:53:49 pm »

In an AC circuit, simplified, a capacitor is the same as a resistor, with the weird property that the resistance depends upon the frequency.  For a given value of a capacitor, the higher the frequency, the lower the resistance.

Only in AC we don't call it "resistance", we call it "impedance", just because.  But its still measured on Ohms. 

One can use maths to determine the impedance of a capacitor given its capacitance and the frequency, and the most fun way to do this is to use on online calculator like this one, there are many others.  The formula is on that page, obviously one can put it into Excel and draw graphs and stuff.
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Dennis Wiggins

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Re: ac capacitors and high frequency
« Reply #8 on: June 21, 2015, 06:46:41 pm »

I understand how DC capacitors work. But I can't find anything that easily explains ac ones. I especially would like to understand why ac caps allow high frequencies. Relating to shielded cables and tone controlls?.

DC will charge a capacitor ~ So will AC.

Capacitors are an amazing device.  As was said, they are like a battery that charges immediately; IMMEDIATELY! ... and wait for ANY opportunity to discharge into something; anything (sounds like Todd Rundgren, eh?).

Do not underestimate the speed that they can discharge.

<edit>

You should look at an oscilloscope display of the output of an amplifier playing music. 

An amplifier can only output a single voltage level at any point in time, regardless of the frequencies involved. 

The question is, how can an amp output all these frequencies at the same time? 

Read that again. 

It doesn't.  The high frequencies ride on top of lower frequencies by minutely
changing direction (polarity, + or -) of the LF wave and THAT is what the little (HF) capacitors use to charge themselves.  They then discharge into the tweeters.

Perhaps someone smarter that me  can explain "more better".

-Dennis
« Last Edit: June 21, 2015, 08:09:13 pm by Dennis Wiggins »
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Dennis Wiggins

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Re: ac capacitors and high frequency
« Reply #9 on: June 21, 2015, 07:25:20 pm »

.. bad keyboard skills

Further (there's always a further), this is why the High Frequencies suffer first, when an amplifier is pushed beyond it's limit (aka: hitting the rails -> the rails are the + and - voltages of the amp's power supply.). The HF's are the first to get clipped because their + and - levels are just a little beyond (higher +, or lower -) the LF voltages.

Thank you for listening.

-Again, Dennis

« Last Edit: June 21, 2015, 08:08:55 pm by Dennis Wiggins »
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ProSoundWeb Community

Re: ac capacitors and high frequency
« Reply #9 on: June 21, 2015, 07:25:20 pm »


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