Starting point

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mediatechnology
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Re: Starting point

Post by mediatechnology »

lofi - In that circuit C1-C3 provide radio frequency "RFI" protection the effectiveness of which is something we can debate later.

For more background take a look in your texts at RC circuits. While you might find some references to an RC circuit in terms of transient analysis the way you want to look at this is as a filter with the "R," or some complex impedance, external and not shown. An example of this external impedance would be the microphone feeding the preamp.

A simple RC circuit's cutoff (-3dB point) is calculated fc = 1/[(2pi)RC].

For a 100 ohm resistor and a 470 pF cap this is about what frequency?

This circuit is a little different because C1 and C2 have C3 in the middle connected to ground. They do this so that the capacitance in common mode (i.e. the capacitance from -In and +In) is much larger than the capacitance differentially from either -In or +In to ground. More on that later.

For a simple analysis neglect everything to the right of C1-C3.

When capacitors are in series they "add" this way. 1/Ct = 1/C1 + 1/C2. Where Ct is the total capacitance. If C3 were not connected to ground, then the capacitance from -In to +In is = 1/470 + 1/470 = 1/Ct. This comes out to 235 pF. In other words, connecting to equal value caps in series reduces their value by 1/2.

If +In is open (and C3 connected to ground) the capacitance from -In to ground is what?

Get back with me on an answer and I'll explain why they are connected this way.
lofi
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Re: Starting point

Post by lofi »

I would first like to thank you unreservedly for all your help and wisdom!!

but (funny how a compliment alway precedes a caveat :D
mediatechnology wrote:
A simple RC circuit's cutoff (-3dB point) is calculated fc = 1/[(2pi)RC].

For a 100 ohm resistor and a 470 pF cap this is about what frequency?
this is where i am falling down, i should convert pF to F?

at the moment fc is 0.339~ and on the very very slim chance thats correct is it kHz? i am thinking out loud here, if its along the wrong lines just say wrong please. I have been at work till 10 pm rebuilding an engine, try as i might its going to take over a day to get my hands anywhere near clean enough to rifle through my tombs. im gonna crack this sport if it kills me :D

and again thanks for all the help,

Iain
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mediatechnology
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Re: Starting point

Post by mediatechnology »

Yes, convert pF to Farad and then you will be within the right order of magnitude. In other words 470 pF = 470 * 10^-12th.

And the answer is?
lofi
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Re: Starting point

Post by lofi »

gobledygook!! back tomorrow, and it R in Kohms?

seen it put as 1/(2pi)*C(uF)*R(Kohms)*0.001

been working way too long for this today, my name is forrest and life is a box of chocolates

Iain
lofi
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Re: Starting point

Post by lofi »

just going back to work, but had a eureka moment in the shower ....

this is for RADIO frequency interference!! thats going to be a very big number!! Duh!!

3386375 Hz ish?
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mediatechnology
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Re: Starting point

Post by mediatechnology »

Yes.
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JR.
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Re: Starting point

Post by JR. »

I always wondered if there is a shock hazard from the light bulb over your head lighting up in the shower. :lol:

JR
Cancel the "cancel culture", do not support mob hatred.
lofi
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Re: Starting point

Post by lofi »

mediatechnology wrote:When capacitors are in series they "add" this way. 1/Ct = 1/C1 + 1/C2. Where Ct is the total capacitance. If C3 were not connected to ground, then the capacitance from -In to +In is = 1/470 + 1/470 = 1/Ct. This comes out to 235 pF. In other words, connecting to equal value caps in series reduces their value by 1/2.

If +In is open (and C3 connected to ground) the capacitance from -In to ground is what?
1/CT = 1/470 + 1/47 or about 43pF

Iain
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Re: Starting point

Post by lofi »

JR. wrote:I always wondered if there is a shock hazard from the light bulb over your head lighting up in the shower. :lol:

JR
read my posts, the light bulbs solar powered and i live in an eclipse, my biggest danger would be slipping in the shower as its so dark :lol:
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Re: Starting point

Post by mediatechnology »

1/CT = 1/470 + 1/47 or about 43pF
OK, very good.

Now why do you suppose that they would want a higher capacitance from the +/- inputs than either the + or - input to ground? From "tip to ring" (from telephony) it is 235 pF. From "tip" (or ring) to ground its 43 pF. (That would be with the other input floating.) Why is that 47 pF shared between the two?
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