Low frequency cutoff calculation for Noninverted.

[5150ed]

Can anybody explain how to calculate the lowfrequency cutoff of input capasitor for inverted GC?

I commonly found the 4.7mf input cap for noninverted schematic but I already a pair of 1mf of very good cap, is the value is too low?

 

[jaudio]

159155/(resistance * frequency)

 

[5150ed]

hmmmm..... I do not quite get it.

Do u mean? 159155/(resistance * frequency)= capasitor value

 

[gengcard]

The low frequency cuttoff is 4Hz, so you can calculate the DC blocking capacitor by below formula

Ci = 1/(2*PI*f*Ri)

If your input impedance is 22K Ohm Ci should be 2.2uF by the formula.

 

[gengcard]

159155 = 1000000/(2*pi)

The formula unit is microfarad.

 

[5150ed]

Thank a lot.

But what is PI???

 

[Greg Erskine]

22/7

 

[jaudio]

I tried both formulas or formuli. I got 1.80uf.

 

[ppcblaster]

Units of measurement

Thank a lot.

But what is PI???

 

[Sherman]

Thank a lot.

But what is PI???

Technically it is the ratio of the circumference of a circle to its diameter. It is an 'irrational number', when the division is carried out it never ends and never repeats.

For all practical purposes in these formulas just use 3.1416.

 

[AndrewT]

Hi ppc,
that pdf was even more confusing than the philosophical argument on why I need to pay my taxes to provide a state education system.

 

[jean-paul]

Ci = 1/(2*PI*f*Ri) where Pi is equivalent to 3.1416

so:

Ci = 1/(2*3.1416*f*Ri)

f=frequency in Herz
Ri = inputresistance in Ohms
Ci = inputcapacitance, it will be shown in Farads

With this formula you calculate the right capacitor for the -3 dB point i.e. with calculated capacitor you will have 3 db attenuation at given frequency. It is wise to slightly overdimension the cap or to calculate with realistic - 3dB points After all, you will like to have your amp amplifying 20 Hz, the caps main purpose is to block DC in most cases anyway. So I would calculate with a -3 dB point of 10 Hz.

 

[ppcblaster]

Perhaps this will clear it up?

Perhaps this is more understandable for you.

 

[ppcblaster]

Second part

schmetic from datasheet

 

[jackinnj]

Technically it is the ratio of the circumference of a circle to its diameter. It is an 'irrational number', when the division is carried out it never ends and never repeats.

For all practical purposes in these formulas just use 3.1416.

The square root of 2 is also an irrational number --

Now here's a toughie -- "What is pie day?" (Hint, pie day does not exist in the EU).

 

[5150ed]

Thank you very much.

Rightnow I know that using 1mf input cap has -3db rolloff around 7.3 hz, which is over than enough for the number.

But can we know that which frequency is -1, -2 db roll off or start to roll off for 1mf input cap?

 

[AndrewT]

Hi,
I recommend the DC blocking input cap be dimensioned to roll off (-3db) a full decade below the frequency you require to be played flat.
i.e. below 2Hz. I use 1Hz(160ms) to 2Hz(80ms) for mine.

 

[gengcard]

I want the amplitude is flat at 20Hz so when I calculated the capacitor by input impedance 22k Ohm, I would get 2.2uF from the solution.

 

[jackinnj]

I want the amplitude is flat at 20Hz so when I calculated the capacitor by input impedance 22k Ohm, I would get 2.2uF from the solution.

Optimally, the f3 point about a decade lower. the phase is going to continue to shift if you target 20Hz -- is it hugely problematic -- no -- take a look at the attached bode plots:

 

[AndrewT]

Hi Jack,
I have already told him, but Geng doesn't want to listen.