Gold

Formula for Input Coupling Capacitor
« on: March 17, 2020, 03:36:17 PM »
Let's assume an inverting opamp with an input resistor of 10K or anything else I guess. What formula would I use to calculate the -3dB point for an AC input coupling capacitor?


fazer

Re: Formula for Input Coupling Capacitor
« Reply #1 on: March 17, 2020, 03:49:26 PM »
Isn’t it 1/2 pie RC?


PRR

Re: Formula for Input Coupling Capacitor
« Reply #2 on: March 17, 2020, 04:29:46 PM »
For <20Hz:

1Meg == 0.01u
100k == 0.1u
10k == 1u
1k == 10u
100r == 100u
10r == 1000u


Gold

Re: Formula for Input Coupling Capacitor
« Reply #3 on: March 17, 2020, 04:57:10 PM »
For <20Hz:

1Meg == 0.01u
100k == 0.1u
10k == 1u
1k == 10u
100r == 100u
10r == 1000u


Thanks. That's getting thumb tacked to the wall.

JohnRoberts

Re: Formula for Input Coupling Capacitor
« Reply #4 on: March 17, 2020, 05:10:12 PM »
Let's assume an inverting opamp with an input resistor of 10K or anything else I guess. What formula would I use to calculate the -3dB point for an AC input coupling capacitor?
The inverting input is effectively a virtual ground so exact same math as a one pole HPF.

C=1/ (10k X 20 Hz X 2 X 3.1415)
C= 0.8 uF

This is so much easier with RPN calculator.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Re: Formula for Input Coupling Capacitor
« Reply #5 on: March 17, 2020, 05:21:21 PM »
No offense but these types of questions are very theoretical with a lot of modifiers and the best (if not only) option is to try.

Kind of like asking "how many men do I need to lift a load with a 2-point pulley" without measuring the load weight (as the f=m/2)

Very often series input caps in audio circuits are small ceramic ones, in the range of 10-100pF, maybe 10nF max. This is to keep the frequency cutoff within a reasonable range as you don't want the signal attenuated above 30-40Hz but it depends on the input signal strength, ie. whether it's a low-level mic input or a line level signal.

As for the inverting configuration an opamp usually has an internal resistor across the Vin+ and - input terminals so the non-inverting (+) input is directly fed into the amplifier circuitry and the inverting (-) one has a series resistor. I think it introduces a phase shift as well, their use as comparators is based on this feature and many audio applications simply ground the inverting input.

Like in filter theory there quite likely is a formulae for calculating the value but it's application-specific I think as it's very different to use it for audio than for example driving a LED... in a passive HPF fc=1/(2πRC), for your convenience I looked it up from an online calculator, a center frequency of 30Hz yielded a resistor value of 530kOhm with a 10nF cap.

But, you'll most likely end up like an army friend of mine, who was ordered to guard the tent while others were sleeping; the gas light went off and while he was trying to fix it the field stove in turn went cold. In the dark he tripped and burnt his back in the hot chimney and frustrated by all of this hit the sack (although it was -15C outside)
« Last Edit: March 17, 2020, 05:32:33 PM by efinque »

Gold

Re: Formula for Input Coupling Capacitor
« Reply #6 on: March 17, 2020, 05:35:12 PM »
No offense but these types of questions are very theoretical with a lot of modifiers and the best (if not only) option is to try.

Huh? It’s a straightforward question  with a straightforward answer. It can easily be checked with standard test equipment..

Quote
Very often series input caps in audio circuits are small ceramic ones, in the range of 10-100pF, maybe 10nF max. This is to keep the frequency cutoff within a reasonable range as you don't want the signal attenuated above 30-40Hz but it depends on the input signal strength, ie. whether it's a low-level mic input or a line level signal.

Those values sound like  they are for a very high impedance input like a tube.  The level stuff seems irrelevant. 
« Last Edit: March 17, 2020, 05:39:07 PM by Gold »

Gold

Re: Formula for Input Coupling Capacitor
« Reply #7 on: March 17, 2020, 05:36:13 PM »
The inverting input is effectively a virtual ground so exact same math as a one pole HPF.

C=1/ (10k X 20 Hz X 2 X 3.1415)
C= 0.8 uF

This is so much easier with RPN calculator.

JR

I use an RPN calculator.

moamps

Re: Formula for Input Coupling Capacitor
« Reply #8 on: March 17, 2020, 06:33:25 PM »
Thanks. That's getting thumb tacked to the wall.

Usually -3dB/20Hz isn't targeted point, an octave down or so is much more convenient, so I would multiply all caps by 10.   

Re: Formula for Input Coupling Capacitor
« Reply #9 on: March 17, 2020, 06:34:42 PM »
Huh? It’s a straightforward question  with a straightforward answer. It can easily be checked with standard test equipment..

You need an oscilloscope/function generator which is far beyond a hobbyist budget. Other than that you could tune it "by ear" but no multimeter will give you a frequency response plot, only coarse voltage/ohm values (someone said you can check the output in watts with a DMM by playing a 50Hz sine wave at unity gain and measuring AC from the terminals but that's about it)

I tried measuring a mixer build frequency response by playing white noise through it and recording the output, then using a spectrum analyzer to plot the curve but unfortunately it also accounts the DAC/ADC and other circuitry used in the measurement which is why I scrapped the test results.

Quote
Those values sound like  they are for a very high impedance input like a tube.  The level stuff seems irrelevant.

I've never designed or repaired/troubleshooted tube equipment. Many preamp designs based around an opamp use input capacitors though and the ones I've seen were in the 10-100pF or 10nF range, however I recall seeing a 100uF/63V electrolytic one (=fairly large in a line level audio/signal path) in a kit I built a few years ago, I can look up the schematic if you wish.

Regarding the guy who burnt his back the moral of the story is that while being able to focus on something is a trait in itself you could be missing something important in the process, ie. the ability to see the big picture.
« Last Edit: March 17, 2020, 07:27:07 PM by efinque »


Gold

Re: Formula for Input Coupling Capacitor
« Reply #10 on: March 17, 2020, 06:51:18 PM »
You need an oscilloscope/function generator which is far beyond a hobbyist budget.
Other than that you could tune it "by ear" but no multimeter will give you a frequency response plot, only coarse voltage/ohm values (someone said you can check the output in watts with a DMM by playing a 50Hz sine wave at unity gain and measuring AC from the terminals but that's about it)

I have an AP Portable one but an oscillator and an AC Voltmeter would do just as well. I think building audio gear without proper test equipment is a fools errand. I try not to tune things by ear that are out of the range of human hearing.

Quote
I've never designed or repaired/troubleshooted tube equipment. Many preamp designs based around an opamp use input capacitors though and the ones I've seen were in the 10-100pF or 10nF range, however I recall seeing a 100uF/63V electrolytic one (=fairly large in a line level audio/signal path) in a kit I built a few years ago, I can look up the schematic if you wish.

I've seen shunt capacitors for RF in the values you listed but not coupling caps.


JohnRoberts

Re: Formula for Input Coupling Capacitor
« Reply #11 on: March 17, 2020, 10:49:19 PM »
You need an oscilloscope/function generator which is far beyond a hobbyist budget. Other than that you could tune it "by ear" but no multimeter will give you a frequency response plot, only coarse voltage/ohm values (someone said you can check the output in watts with a DMM by playing a 50Hz sine wave at unity gain and measuring AC from the terminals but that's about it)

I tried measuring a mixer build frequency response by playing white noise through it and recording the output, then using a spectrum analyzer to plot the curve but unfortunately it also accounts the DAC/ADC and other circuitry used in the measurement which is why I scrapped the test results.

I've never designed or repaired/troubleshooted tube equipment. Many preamp designs based around an opamp use input capacitors though and the ones I've seen were in the 10-100pF or 10nF range, however I recall seeing a 100uF/63V electrolytic one (=fairly large in a line level audio/signal path) in a kit I built a few years ago, I can look up the schematic if you wish.

Regarding the guy who burnt his back the moral of the story is that while being able to focus on something is a trait in itself you could be missing something important in the process, ie. the ability to see the big picture.
This is just math and physics... you can't negotiate it to be something else.

This is why I prefer this stuff to politics.......  There is only one answer to simple questions like this. (But apparently people will still want to debate it.  :o )

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

ruairioflaherty

Re: Formula for Input Coupling Capacitor
« Reply #12 on: March 18, 2020, 03:27:35 AM »

Newmarket

Re: Formula for Input Coupling Capacitor
« Reply #13 on: March 18, 2020, 05:42:37 AM »
No offense but these types of questions are very theoretical with a lot of modifiers and the best (if not only) option is to try.

Very often series input caps in audio circuits are small ceramic ones, in the range of 10-100pF, maybe 10nF max. This is to keep the frequency cutoff within a reasonable range as you don't want the signal attenuated above 30-40Hz but it depends on the input signal strength, ie. whether it's a low-level mic input or a line level signal.

As for the inverting configuration an opamp usually has an internal resistor across the Vin+ and - input terminals so the non-inverting (+) input is directly fed into the amplifier circuitry and the inverting (-) one has a series resistor. I think it introduces a phase shift as well, their use as comparators is based on this feature and many audio applications simply ground the inverting input.

Like in filter theory there quite likely is a formulae for calculating the value but it's application-specific I think as it's very different to use it for audio than for example driving a LED... in a passive HPF fc=1/(2πRC), for your convenience I looked it up from an online calculator, a center frequency of 30Hz yielded a resistor value of 530kOhm with a 10nF cap.


It was a very concise and simple enquiry with a simple answer.

Input caps in the pF range are unusual (to say the least) in audio applications.
With an input cap it's generally about just removing the DC component so it's generally a lot larger.
Smaller values are usable with high impedance eg jfet input opamps but more like 10n rather than 10p.
As has been said caps in the pF range are commonly seen for rf and stability reasons.

The 'signal strength' is not relevant (assuming it doesn't exceed the dynamic range of the circuit but that's not a filter issue)
It is simply maths and formula - there's no need to overcomplicate it.
It's not "like filter theory" - It is filter theory (well established and demonstrable)

As for the test equipment side - you really just need a soundcard setup and a test tone generated from your software and some analysis function as you outlined. wrt the DAC / ADC influence - well if ther's a concern about that just measure with a "loop through" connection then with the filter / input circuit in line. The difference is your filter characteristic.
Bear in mind that it's only frequency response that is being looked at here. Not THD etc.
tbh you could simply simulate it using LTSpice or online tools from,say, Texas Instruments.

Not sure where driving leds comes into it...

Newmarket

Re: Formula for Input Coupling Capacitor
« Reply #14 on: March 18, 2020, 06:40:24 AM »
Usually -3dB/20Hz isn't targeted point, an octave down or so is much more convenient, so I would multiply all caps by 10.

True - but I think you might mean a decade down hence x10 caps ?
so -3dB @ 2Hz.
Reducing phase shift in the audible band.
Additionally , assuming electrolytics used then larger value gives less distortion (all else being equal).

ruffrecords

Re: Formula for Input Coupling Capacitor
« Reply #15 on: March 18, 2020, 07:15:59 AM »

 Many preamp designs based around an opamp use input capacitors though and the ones I've seen were in the 10-100pF or 10nF range,

This would be extremely unusual in my experience. Can you post an example schematic?

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

Re: Formula for Input Coupling Capacitor
« Reply #16 on: March 18, 2020, 07:16:36 AM »
I posted a link to this page previously ,but wasnt able to find it after , its dedicated mainly to tube design but some will be  relevant
to solid state too .

https://www.ampbooks.com/mobile/amplifier-calculators/coupling-capacitor/calculator/

Graphs everything up beautifully , theres a whole range of usefull tools here for all kinds of calculations , very interested to hear what others think of it.

Gold

Re: Formula for Input Coupling Capacitor
« Reply #17 on: March 18, 2020, 09:45:17 AM »
I didn't know what RPN was.  For those like me

https://www.calculator.org/articles/Reverse_Polish_Notation.html

I use P Calc on iOS. It has an RPN mode..

JohnRoberts

Re: Formula for Input Coupling Capacitor
« Reply #18 on: March 18, 2020, 12:02:39 PM »
Isn’t it 1/2 pie RC?
Yes but order of operations matter so it is

1/(2 X pi X R X C )

with RPN you can just enter 2 X pi X R X C, then reform 1/x

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

EmRR

Re: Formula for Input Coupling Capacitor
« Reply #19 on: March 18, 2020, 12:11:51 PM »
For <20Hz:

1Meg == 0.01u
100k == 0.1u
10k == 1u
1k == 10u
100r == 100u
10r == 1000u

Jives with my mental benchmark, 100K/0.1.  I do the math in my head relative to that reference.  Usually in tube world here. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde


 

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