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Old 11th December 2012, 04:58 PM   #1
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Default Question on input dc blocking capacitors

I try to replace the DC blocking capacitor of my Revel B15a internal amp. and found that it is with 330uf , of course , no film possible , I have good 220uf in hand , is it OK to replace 330uf with 220uf in this occasion ? Will it impair its performance ? I heard this big value is important for low end response .

I have replaced my BAT amp. before and it is only with 3.3uf film capacitor only , why that big difference ? Is BAT not intended as bass amp. It is almost flat to Zero Hz .

Yours kind reply to this is appreciated .

Larry
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Old 11th December 2012, 06:43 PM   #2
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It is suggested to replace a cap always by another equal or larger than the original, in voltage and in capacitance value. In your case I suggest to go to 470F in place of going down to 220F.
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Old 11th December 2012, 07:08 PM   #3
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Very odd to have that large a value for a low lvl high impedance input since most line level sources wouldn't drive an impedance as low as what that size would imply. Are you sure these aren't maybe rail filters that happen to be close to the input traces? Are there smaller ones very close to them that may be them instead?
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Old 11th December 2012, 07:10 PM   #4
RJM1 is offline RJM1  United States
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Determine the input impedance and calculate your desired 3db low frequency cutoff point 1/(2*3.14159*F*C)
F in Hz
C in Farads.
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Old 11th December 2012, 08:12 PM   #5
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Thanks all for reply .
jerluwoo - Not power rail capacitors , there are 2 of them for balance input , connected directly to Hot & Cold pins , 3rd one connect to tap of hot pin - for RCA input .
RJM1 - thanks for suggestion , but I am not that good on this , just DIY with general knowledge .
Osvaldo - I have some Panasonic Premium Cap , 220uf - 25v , can I pair 2 to 440 uf , will it be as good as one single one .

Thanks all again .

Larry
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Old 12th December 2012, 05:53 AM   #6
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Maybe someone replaced them caps in the past.. To be honest Even 47uF is on the high side..
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Old 12th December 2012, 07:45 AM   #7
gootee is offline gootee  United States
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Quote:
Originally Posted by Larry Ng View Post
Thanks all for reply .
jerluwoo - Not power rail capacitors , there are 2 of them for balance input , connected directly to Hot & Cold pins , 3rd one connect to tap of hot pin - for RCA input .
RJM1 - thanks for suggestion , but I am not that good on this , just DIY with general knowledge .
Osvaldo - I have some Panasonic Premium Cap , 220uf - 25v , can I pair 2 to 440 uf , will it be as good as one single one .

Thanks all again .

Larry
The capacitance does not determine the frequency response, by itself. It is part of a passive RC high-pass filter, which is formed whenever a series capacitor is followed by a "shunt" resistance to ground.

A high-pass filter attenuates low frequencies, not letting them pass as well as it does higher frequencies.

It's just like when two resistors are used to make a voltage divider, except here the series one is replaced by a capacitor, which then makes it a "frequency-dependent voltage divider".

By making the R be the series component and putting the C after it, to ground, you could have a low-pass filter (not that you want that, here).

The roll-off in amplitude as the frequency gets lower is somewhat gradual, but people often specify RC filters by what they call their "cutoff frequency" or corner frequency, which is the freequency where the output voltage is 3 dB lower than the input voltage (-3dB is a factor of 0.7071 if we're talking voltage, or a factor of 0.5 if we're talking power in Watts. The cutoff frequency is given by

f = 1 / ( 2 π R C )

Looking at that equation, where the 2, Pi, R, and C are all multiplied together in the denominator, you can see that any C could make any cutoff frequency, because the R could be changed so that R times C was whatever you wanted.

So a very large C with a very small R could still not give good bass, for example.

Usually there is a resistor to ground, just before the first actual amplifying component (like a transistor or opamp). You would have to determine the resistance between the downstream lead of the capacitor and the signal ground, and then use that, AND the C value, to calculate the filter's cutoff frequency.

Typically, the cutoff frequency is set to be very low, often to less than 1 Hertz (one cycle per second). Since the attenuation by the passive RC high-pass filter gradually increases as the frequency gets lower, you typically should pick a cutoff frequency that is at MOST 1/10th of the lowest frequency that you don't want the filter to affect, although many people go 10x further down than that.

Example: If the input-to-ground resistor in your amp is 10,000 Ohms, i.e. 10kOhms, and you want a 0.1 Hz cutoff frquency, then solve the equation for C and plug in the values:

C = 1 / ( 2 π R f ) = 1 / ( 2 x 3.14 x 10000 x 0.1) = 0.001590 Farads

That's 1590 microFarads.

But if the input R was 100,000 Ohms (100k), i.e. ten times more than before, then C = 159 microFarads would give exactly the same cutoff frequency of 0.1 Hz.

There are usually other reasons for picking the value of either the R or the C, which then dictates the value of the other one, for any given desired cutoff frequency.

For example, you "could" use ten million Ohms (10 Meg) and then you'd only need 1.59 uF to get the same 0.1 Hz cutoff frequency. That capacitor would either be a lot cheaper or you could get a really good-quality cap for the same price as a crappy much-larger one.

But we don't use 10 Meg because it would pick up a lot of noise, and would also generate a lot more noise of its own. Lower resistor values are less noisy. But we can't go TOO low or else the source device might have trouble supplying enough current to keep the voltage up at the right level. So R usually gets picked and then C is calculated based on what f we want.

Cheers,

Tom

Last edited by gootee; 12th December 2012 at 07:49 AM.
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Old 12th December 2012, 11:09 AM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Larry Ng View Post
T.................. there are 2 of them for balance input , connected directly to Hot & Cold pins , .................... I have some Panasonic Premium Cap , 220uf - 25v , can I pair 2 to 440 uf , will it be as good as one single one ..............
this is what Rane suggest. a 220uF panasonic non-polar on each signal line with a film in parallel.
Find the Rane note.

Although the more recent Bateman cap report strongly suggests that the reduced distortion of using back to back non-polar would be worth investigating.
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Old 12th December 2012, 11:09 AM   #9
AndrewT is offline AndrewT  Scotland
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Goottee,
this is another Thread where you are using a non standard font !
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Old 12th December 2012, 11:30 AM   #10
Bonsai is online now Bonsai  Taiwan
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Andrew, why do you hassle about these things? Picture resolution, picture size, fonts etc. Ease up man - enjoy life a bit! What was it some wise guy said: I hope I have the wisdom to know the things I can change and the patience to accept the things I cannot.
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