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
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
ped said:
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.
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
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
pjanda1 said:
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.
Peter Daniel said:
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.
Peter Daniel said:
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.
carlosfm said:
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
You know what, now you got me puzzled even more. I don't thoink we are talking about the same thing
Well, there may be actually something to it. I was using 4.7u and 100u caps in same application ( the output of TDA1543 DAC with 2k7 I/V resistors, the preamp having 80k input resisstance). I was quite surprised when 4.7u caps did not show any DC offset after connecting DAC to a preamp, however, with 100u caps I had some residual offset (below 100mV), which with time was slowly disappearing, and never stayed on in a long run. But I also think that lower value caps of this type showed the same residual offset.
It is quite natural to see 100-200uF value caps used in coupling.
It is quite natural to see 100-200uF value caps used in coupling.
I'm just collecting the parts now, but my situation will be somewhat unusual. I am using a passive pre in the same chassis. My hope is to put the cap on one input in front of the selector for those times when I need to hook up a source of dubious quality. I am also going to try using a 1Meg linear pot with a 33k law faking resistor as per T's recommendation. I may also try a 47k and pick the better. It sounds like a 4.7uf will then be inadaquate. I was planning on getting the BG N series, as the price is certainly friendlier than most films. Even if 1uf doesn't sound as good as 4.7, I'd rather have the extra safety as it's only on one input. So, will 1uf in front of the selector work?
thanks,
Paul
thanks,
Paul
I see no point in using larger values than 4.7u as the larger capacitor is, the slower it reacts to (rapidly changing) DC - more chances of damaging speakers by accident.
I've tried 22u input cap with a portable receiver as a source which creates a large transcient upon shutdown and that transcient felt really bad on my speakers (like a loud boom). And with 1u input cap it was like a click or smth.
So I agree with carlosfm, use the smallest value possible.
I've tried 22u input cap with a portable receiver as a source which creates a large transcient upon shutdown and that transcient felt really bad on my speakers (like a loud boom). And with 1u input cap it was like a click or smth.
So I agree with carlosfm, use the smallest value possible.
carlosfm said:
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
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.
Hmm - so this could be the reason for my dc offset at the output with my LM1875 amp?? - I have a combination of 4,7uF and 22kR ...
This could be reason in your situation stabist only in case that your source is passing through DC. Input coupling cap (I always thought that this is Decoupling cap, and still think!) is here to prevent DC that comes from source. As I recall you've had DC offset with input at the ground, so this had nothing to do with input cap. Am I thinking correctly?
You have read about my problems with DC offset at our local forum stabist.
I always have very big DC offset. Around 150-160mV. Increasing resistor from NI to ground decreases DC offset. With 680R and 22k in NFB I have 150mV. 1k to ground, DC is still over 100mV. With 2K2 i have 45mV. Of course this affects gain in very negative way. I should recalculate NFB resistor, probably input resistor too, but I am always sticking to some recomendations (22k NFB and 22k at input). Well I should not.
but try to calculate another values and find what is apropriate for my situation.
You have read about my problems with DC offset at our local forum stabist.
I always have very big DC offset. Around 150-160mV. Increasing resistor from NI to ground decreases DC offset. With 680R and 22k in NFB I have 150mV. 1k to ground, DC is still over 100mV. With 2K2 i have 45mV. Of course this affects gain in very negative way. I should recalculate NFB resistor, probably input resistor too, but I am always sticking to some recomendations (22k NFB and 22k at input). Well I should not.
but try to calculate another values and find what is apropriate for my situation.- Status
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