Since the input impedance of the (-) opamp input is R9 (1k) but the input impedance at the (+) opamp input is R8 + R12= 11k and the coupling capacitors pass different low frequency levels then isn't the microphone not properly balanced like a transformer or instrumentation amplifier would do?
No.
The DC input impedances seen at the inputs are: 10k5 at the +input and 10k at the -input, a mismatch of only 5%, which won't matter much, especially as this is a JFET input op-amp, with tiny bias currents.
C6 (it should be bigger than 10µF) bypasses R11 to 0V, at AC, so the AC impedance of the mic. inputs should fairly closely match, giving roughtly 1k.
[EDIT]
Sorry, for my snappy response. I see what you mean, after reading this post, years later, after searing for the attachment to use. Someone might refer back to this thread, so I through I'd edit it. The inputs will have different cut-off frequencies, when they're left floating. As you correctly stated, there's 11k on the +input and 1k on the -input, but that's no longer an issue, with the microphone connected. If the circuit is designed properly, the microphone will have a much lower impedance than the input resistors, thus connecting them together at a relatively low impedance, so any differences in cut-off frequency won't matter.
[/EDIT]
EDIT: Why are the values of R7 and R11 so low that they draw more current than the entire circuit and get almost smoking hot? They bias the Jfet inputs of the opamps with almost no current.
Yes, they should be higher, but the power dissipation is only 288mW, hardly smoking hot, unless tiny resistors are used.
Sorry Audioguru, I'm trying to follow the schema posted by Hero999. I'm in single supply differential mode so R7 and R11 should be 1/10 of R12 and R10. Moveover R8 and R9 also should be 1/10 of R12 and R10 to get 10 of gain amplification, what am I missing?
I raised the R7 R11 to 1M and the others to respect the schema balance (I think), I also changed the C8 C9 to cut under ~80Hz.
thank you
The schematic I posted shows best general practise. It's good to follow it but is far more important to understand why, then you can know when it's important and when it doesn't apply.
The reason for recommending that the DC impedances seen by the op-amp inputs is equal, is because the voltages generated by the bias currents will equal one another and get cancelled out by the op-amp. This is the reason for the
1/
10R
F rule of thumb: it ensures the DC impedance mismatch is no more than 5%. The question is, whether the bias currents are high enough to cause a significant offset? In the case of a bipolar op-amp, such as the NE5532, the input bias currents are typically 200nA, so with an amplifier with gain of 100 and a 600k input impedance at one input and only 100k at the other input, a difference of 500k, as in the
second circuit, the input offset voltage will be 200×10
-9×500×10
3 = 0.1V. This will be multiplied by the gain of the amplifier, 100 in this case, 0.1×100 = 10V on the output which will cause it to saturate, near one of the power rails.
http://www.ti.com/lit/ds/symlink/ne5532.pdfNow look at the above situation, but using a J-FET input, such as the TL072. The input bias currents are only 65pA. If the above calculation is repeated 65×10
-12×500×10
3×100 = 3.25mV, which won't make any difference, especially as the output has a capacitor in series with it, which blocks the DC voltage.
Dave made a video about this awhile ago. Please make time to watch it, as it explains everything.
EDIT:
I've just realised, this isn't as bad as I first thought, even with a bipolar op-amp, the DC gain of the circuit is only 1, so the offset voltages will be
1/
100 of the previously calculated values, which shouldn't be an issue.
Sorry Audioguru, I'm trying to follow the schema posted by Hero999. I'm in single supply differential mode so R7 and R11 should be 1/10 of R12 and R10. Moveover R8 and R9 also should be 1/10 of R12 and R10 to get 10 of gain amplification, what am I missing?
I raised the R7 R11 to 1M and the others to respect the schema balance (I think), I also changed the C8 C9 to cut under ~80Hz.
thank you
Should be fine. You need to decide what gain you need. This circuit has a gain of 10. Some of the other circuits had a gain of 100.