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Old 29th October 2015, 09:53 AM   #1
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Default Coupling capacitors

Hello everybody

I'm just curious about the values in this preamp : C4-C6 .22uF.
Why not 10uF like the output one C7 ?
Are they acting like low frequencies cut-off ?
There are such value in ARC SP6, for exemple.
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Old 29th October 2015, 10:14 AM   #2
AndrewT is online now AndrewT  Scotland
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the coupling capacitor acts as a High pass filter when combined with the following load.
The load is 1Meg in parallel to the grid input impedance.

I don't know how to calculate that input impedance.

If I assume it is 0.5M , then the total load after the coupling capacitor is:
500k*1000k/(500k+1000k) = 333k

The turn over frequency of this single pole filter is given by
F-3dB = 1 / (2 PiRC) = 1/(2*3.14*333k*220*10^-9) = 1/0.46 = 2.2Hz
The response will be down by ~1dB @ 4.4Hz

You need an estimate of the input impedance to allow you to check the F-3dB.
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Old 29th October 2015, 10:16 AM   #3
AndrewT is online now AndrewT  Scotland
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Your profile says you joined in July 2001, but under your nickname it states Jan 2009
What up?
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Old 29th October 2015, 10:39 AM   #4
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Quote:
Originally Posted by AndrewT View Post
Your profile says you joined in July 2001, but under your nickname it states Jan 2009
What up?
Weird indeed If one clicks on profile, the name Stan 33 something appears, with last activity in 2006.
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Old 29th October 2015, 12:12 PM   #5
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Quote:
Originally Posted by AndrewT View Post
the coupling capacitor acts as a High pass filter when combined with the following load.
The load is 1Meg in parallel to the grid input impedance.

I don't know how to calculate that input impedance.

If I assume it is 0.5M , then the total load after the coupling capacitor is:
500k*1000k/(500k+1000k) = 333k

The turn over frequency of this single pole filter is given by
F-3dB = 1 / (2 PiRC) = 1/(2*3.14*333k*220*10^-9) = 1/0.46 = 2.2Hz
The response will be down by ~1dB @ 4.4Hz

You need an estimate of the input impedance to allow you to check the F-3dB.
OK, Andrew.
What about C6 and C7 ?
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Old 29th October 2015, 12:17 PM   #6
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Your profile says you joined in July 2001, but under your nickname it states Jan 2009
What up?
what nickname ? stan_33 ? I don't know who is it. I never entered that name.
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Old 29th October 2015, 12:24 PM   #7
DF96 is online now DF96  England
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I seem to recall other people finding that the site software can confuse people with what it considers to be similar IDs. stan_33 and sb****33 both start with 's' and finish with '33', so easy for a dumb computer to think they are the same person!

Raise a query in the web problems forum.
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Old 29th October 2015, 12:31 PM   #8
AndrewT is online now AndrewT  Scotland
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is "****" appearing because he typed ****, or because the Forum recognises that as a banned word and has overwritten his entry?
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Old 29th October 2015, 12:36 PM   #9
AndrewT is online now AndrewT  Scotland
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C7 is coupling capacitor, but the output looks wrong.
C6 is a bootstrap that creates one of those totem pole thingies that I don't understand.
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Old 29th October 2015, 01:05 PM   #10
Merlinb is offline Merlinb  United Kingdom
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R2 is incorrect. It should be much larger, say 100k to 1Meg.
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