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Old 10th April 2005, 01:11 AM   #1
ped is offline ped  United States
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Default Location of input coupling cap

Hi All,

I've built several Gainclones of differing topologies and up until now haven't had the need to use an input coupling cap. I recently aquired a Behringer DCX 2496 which I believe is direct coupled and can potentially produce a dc offset, so I want to add an input cap to my GC's until I know for sure about the DCX. I'm just not sure of the correct location for this cap. I've seen schematics showing it either before or after the input resistor. Does it make a difference? These are your basic NIGC's with a 50k input resistor to ground, no volume control.

Any help will be greatly appreciated
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Old 10th April 2005, 01:24 AM   #2
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Default Re: Location of input coupling cap

Quote:
Originally posted by ped
I've seen schematics showing it either before or after the input resistor. Does it make a difference?
On an inverting (chip) amp the cap can be after the input resistor to ground, because the inverting input of the op-amp is a virtual ground and what sets the input impedance is the input resistor.
In your case (non-inverting) you have to use the cap before the 50k resistor to ground.
Use a 1uf film cap.
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Old 10th April 2005, 08:54 PM   #3
pjanda1 is offline pjanda1  United States
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I've recently been doing some searches, and it appeared to me that most use a 4.7uf input cap and some a 2.2uf. I have relatively little knowledge, but my understanding was that a higher value has a lower cut off frequency but is worse in terms of sonics, $$ and physical size. Help me if I'm way off on this, as I'm planning on ordering some parts tomorrow.

I'm planning to use caps on only one set of inputs of my NIGC 3875 project (linear pot and 33k to ground). My usual sources don't pass any DC, but I sometimes need to listen to a portable device. In that instance the sonics of that aren't particularly important, so I was planning on using a 4.7 electrolytic to save $$ and space. If 1uf is more appropriate that's good to know.

thanks,

Paul
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Old 10th April 2005, 09:56 PM   #4
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Quote:
Originally posted by pjanda1
I'm planning to use caps on only one set of inputs of my NIGC 3875 project (linear pot and 33k to ground). My usual sources don't pass any DC, but I sometimes need to listen to a portable device. In that instance the sonics of that aren't particularly important, so I was planning on using a 4.7 electrolytic to save $$ and space. If 1uf is more appropriate that's good to know.

thanks,

Paul
I recommended you 1uf because Ped said he has a 50k resistor to ground
I suppose that you use a "passive pre" (a direct pot to the chip), so why not protect the pot from DC too?
Put the cap before the pot.
If the pot is 50k, the 1uf cap suggestion remains.
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Old 10th April 2005, 10:05 PM   #5
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Quote:
Originally posted by pjanda1
so I was planning on using a 4.7 electrolytic to save $$ and space. If 1uf is more appropriate that's good to know.
4.7u electrolytic might be more appropriate choice. I did some comparisons, and it seems that the higher value of electrolytic, the better the sound. It will also depend on type, and my two favourites for coupling are 4.7u BG N or 100/25 Rubycon ZL. Out of the bunch of different Rubycons, for some reason that one sounded the best. It's also cheaper than BG, but presently I returned back to BG N. For coupling, this is the best value (pricewise and sizewise). Smaller values of BG don't sound that good, I also don't like 10u that much, and I don't even mention NX Hi Q, as the higher voltage BG sound better, it's quite opposite with Rubycons though, I was told. Don't even try to bypas those electrolytics with other caps, you will loose all that is natural about them.

The rumour is that 47 Labs were using BC Components electrolytics for coupling application. I never tried them though.
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Old 10th April 2005, 11:44 PM   #6
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Quote:
Originally posted by Peter Daniel
The rumour is that 47 Labs were using BC Components electrolytics for coupling application. I never tried them though.
I use Philips/BC electrolythic caps as PSU bypass on my op-amp circuits.
Quite happy with them.

Peter, if you use a 4.7uf coupling cap and have a 22k resistor to ground, that will help removing DC, but not much.
That is fine for a 10k input impedance.
The results you have depend on the resistor you have from input to ground, 4.7uf can't be considered a "universal" value.
For a 50k resistor the 4.7uf cap will do nothing.
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Old 11th April 2005, 12:13 AM   #7
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Quote:
Originally posted by carlosfm

Peter, if you use a 4.7uf coupling cap and have a 22k resistor to ground, that will help removing DC, but not much.

....For a 50k resistor the 4.7uf cap will do nothing.
Now you got me puzzled, does that mean that some DC will be still going through?
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Old 11th April 2005, 12:19 AM   #8
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Quote:
Originally posted by Peter Daniel
Now you got me puzzled, does that mean that some DC will be still going through?
Yes, it will be attenuated but not much.
Peter, the cap + resistor to ground is a first order high pass filter.
4.7uf with 22k is a high-pass filter at 4.5Hz, not very effective.
4.7uf + 50k is a high-pass filter at 1.99hz, better not use the cap at all.

1uf + 50k = 9.3hz, a value at around 10 to 15hz is fine.

Even lower than 1uf could be used, with 50k.
680nf + 50k = 13.76Hz

This cries out for a quality film cap.
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Old 11th April 2005, 12:21 AM   #9
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Quote:
Originally posted by carlosfm


Yes, it will be attenuated but not much.
Peter, the cap + resistor to ground is a first order high pass filter.
4.7uf with 22k is a high-pass filter at 4.5Hz, not very effective.
4.7uf + 50k is a high-pass filter at 1.99hz, better not use the cap at all.
You know what, now you got me puzzled even more. I don't thoink we are talking about the same thing
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Old 11th April 2005, 12:25 AM   #10
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Aren't we talking about the input coupling cap?
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