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Old 27th December 2003, 09:05 PM   #1
brsanko is offline brsanko  United States
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Default AC coupling

how do I know what size caps to use for AC coupling
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Old 27th December 2003, 09:31 PM   #2
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Hi

It would depend on the impedance of the following stage, and your lowest frequency of interest.

An example, if you have a DC input impedance of say 100k, and Flow of 10 Hz, you need a capacitor of:

1 / (100000 * 10 * 6.28) F = 0.000000159 F = 159 nF

Recalculate according to your particular values
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Old 27th December 2003, 09:34 PM   #3
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thank you that was just the formula I needed I've been reading Bruce Rosenblitz's book and while it's a great book there were a few details that were just left out
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Old 27th December 2003, 09:40 PM   #4
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I can give you a little bit of additional advice, calculate for the lowest acceptable frequency, not the highest.

So if your speakers fall off at say 30 Hz, but your input DC spike you want to remove has a frequency of 2 Hz, you should settle for 5 Hz rather than 25 Hz. Just to preserve as much of the low frequency phase accuracy as possible...
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Old 27th December 2003, 09:42 PM   #5
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ok I'm really new to this so what would cause a DC spike and how would I know about it
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Old 27th December 2003, 09:55 PM   #6
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It could be from switching on the preceding apparatus, usually this could result in a DC spike, in other cases it could be normal thermal drift, in the latter case the frequencies would usually be very low, like 0.2 Hz.
In the switching on case, the f requency ddepends on the time it takes to charge the PS capacitors, and that in turn depends on the size of caps, and the power capability of the charging transformer.
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Old 28th December 2003, 04:52 AM   #7
SHiFTY is offline SHiFTY  New Zealand
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I ususally just go with old circuits- they used plenty of 0.22uF and 0.47uF in the mullard/RCA manuals, so they must be pretty decent values...

DC thumps are pretty rare, unless your power supply is on the edge of oscillation. Not much of a worry IMHO
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Old 12th October 2011, 07:30 PM   #8
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Default Ac coupling capacitor value

Quote:
Originally Posted by Lars Clausen View Post
Hi
you need a capacitor of:

1 / (100000 * 10 * 6.28) F = 0.000000159 F = 159 nF

In my simple understanding of electronics, I recall a rule of thumb about RC time constants:

1 Farad : 1 Ohm : 1 second
or
1uF : 1MegOhm : 1second

which would mean , for 100K input impedance

10Hz response would require 1uF cap.

My situation has 10K input impedance, si I would be looking at 10uF film cap. (ouch)

Frankly, I like the 2pi (6.28) term in there, because it would give me a smaller capacitor, but then, what is wrong with my understanding of this time constant rule of thumb?
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Old 12th October 2011, 07:40 PM   #9
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Why is your input impedance SO low...?

Can not the grid-resistor of the input stage be changed...?
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Old 12th October 2011, 09:01 PM   #10
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by telewatt View Post
In my simple understanding of electronics, I recall a rule of thumb about RC time constants:

1 Farad : 1 Ohm : 1 second
or
1uF : 1MegOhm : 1second

which would mean , for 100K input impedance

10Hz response would require 1uF cap.

My situation has 10K input impedance, si I would be looking at 10uF film cap. (ouch)

Frankly, I like the 2pi (6.28) term in there, because it would give me a smaller capacitor, but then, what is wrong with my understanding of this time constant rule of thumb?
Try this: RC time constant - Wikipedia, the free encyclopedia

And this if you are brave: Time constant - Wikipedia, the free encyclopedia

Avoid the issue (my choice ): http://www.sengpielaudio.com/calcula...meconstant.htm
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