PRR said:For <20Hz:
1Meg == 0.01u
100k == 0.1u
10k == 1u
1k == 10u
100r == 100u
10r == 1000u
The inverting input is effectively a virtual ground so exact same math as a one pole HPF.Gold said: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?
efinque said: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.
JohnRoberts said: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
Gold said:Thanks. That's getting thumb tacked to the wall.
Gold said:Huh? It’s a straightforward question with a straightforward answer. It can easily be checked with standard test equipment..
Those values sound like they are for a very high impedance input like a tube. The level stuff seems irrelevant.
efinque said: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'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.
This is just math and physics... you can't negotiate it to be something else.efinque said: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.
Gold said:I use an RPN calculator.
efinque said: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.
moamps said:Usually -3dB/20Hz isn't targeted point, an octave down or so is much more convenient, so I would multiply all caps by 10.
efinque said: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,
ruairioflaherty said:I didn't know what RPN was. For those like me
https://www.calculator.org/articles/Reverse_Polish_Notation.html
Yes but order of operations matter so it isfazer said:Isn’t it 1/2 pie RC?
PRR said:For <20Hz:
1Meg == 0.01u
100k == 0.1u
10k == 1u
1k == 10u
100r == 100u
10r == 1000u
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