[Audio Preamp] - Capacitor question?

[ElectricGuy]

Hi all;

I understand most of this schematic, but i don't know the purpose of the C7?
What is the difference between using C7 or a direct short between R2 and R8?

Thank you!

[grumpydoc]

It limits the DC gain of the stage to 1

[w2aew]

C7 makes the DC gain unity, such that the DC operating point (and output quiescent point) is established by the R6-R7 votlage divider.  For AC signals, C7 looks like a near short, so the AC gain is determined by resistors.

[ElectricGuy]

HI«i;

Thank you both for the answers. So if i understand, C7 don't amplifie the DC voltage, only the AC voltage by 6 in this case, right?
How can i calculate the value of that capacitor?

[Richard Crowley]

C7 does not "amplify" anything.  The gain comes from the op-amp, IC1B.
As others have observed, the addition of C7 into that feedback voltage divider means that AC signals will be attenuated by the voltage divider (R2/R8), and thus make the op-amp work harder to equalize the differential inputs.  i.e. the opamp will AMPLIFY the AC signal.
However, at DC, there is no path to ground, so the opamp is limited to unity gain for DC signals.  That is exactly what we want in an AC (audio) amplifier. We don't want any extra DC coming out of the output.

But note that there is ANOTHER function of C7 in this particular case.
Since this circuit has only a unipolar power supply, they are using that other voltage divider, R6/R7 to create an artificial ground for the opamp.
But since the voltage divider R2/R8 is connected to "real ground", that would create quite an offset in the output.
So C7 is also used to compensate for this difference in ground references.

You can calculate the value of C7 from the requirement of what the low frequency response of amplifier should be.
Roughly the 3dB down point is where the "reactance" of the capacitor equals 12K (R2 and R8 in series)
For example, a 0.5 uF capactor would make the gain of the amplifier roll off by 3dB at 26 Hz.
http://www.calculatoredge.com/enggcalc/Resistance-Frequency-Capacitance.htm

[ElectricGuy]

Ok, i understand.
I know that C7 doesn't amplify... i write wrong.

Tank you for the link too.

[Syntax_Error]

Yes, thank you for the detailed technical responses. I just wanted to point out that there are many lurkers here who benefit and learn from the more advanced members' conversations here, and I thank you for that.

[David Hess]

You can calculate the value of C7 from the requirement of what the low frequency response of amplifier should be.
Roughly the 3dB down point is where the "reactance" of the capacitor equals 12K (R2 and R8 in series)
For example, a 0.5 uF capactor would make the gain of the amplifier roll off by 3dB at 26 Hz.
http://www.calculatoredge.com/enggcalc/Resistance-Frequency-Capacitance.htm

That is not the number I end up with.  The capacitor and R8 operate into the operational amplifier's virtual ground (which follows the non-inverting input) so they cannot see R2.  Just calculating the non-inverting gain yields a 3 dB point with a 0.5 uF capacitor of 290 Hz instead of 26 Hz although because the amplifier is non-inverting and has a gain of 1 + R2/R8, the phase is not 45 degrees at that point.

Or maybe I have too much blood circulating in my caffeine.