mic preamp opamp working only on a single supply

MikeMl said:

Your problem:

At R1 <1K, you are getting a gain > 101. With 100K source impedance for the input bias current to the non-inverting input of the opamp, and <1K source impedance for the input bias current to the inverting input of the opamp, the bias current causes a differential DC voltage between the inputs, which is amplified by the gain (>101), which causes the output level of the opamp to shift so far that the audio signal is clipped.

Set R1 to get a gain of about 200. With no mic input, measure the DC voltage at the output pin of the opamp. I'll bet that it sits very close to the positive supply voltage.

If you really think you need gains exceeding 100, then use two opamp stages, one with a gain of 10 followed by a second one with a gain of 10 to 50.

What Opamp are you using?

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MikeMl said:

The 558 has rather large input bias currents, so will be greatly affected by unbalanced input resistances...

In the single supply version you posted originally, the resistances at the inputs were almost perfectly balanced. In the split-supply circuit, you have 100K in one input and <1K in the other, which is causing it to rail. Replace the 100K with 10K or less, and that will fix the bias problem, but might load the electret too much...

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audioguru said:

...
Using an ordinary opamp with the single supply circuit then the resistor to ground part of the negative feedback can have a polarized electrolytic capacitor to ground in series with it. Then the input offset current is not amplified...

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MikeMl said:

Just like the circuit in Post #8 of this thread.

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audioguru said:

If you use an opamp with Jfet inputs like a TL071 then the input bias current is extremely low leakage current. The gain of 200 times the extremely low leakage current equals almost no output offset voltage.

Using an ordinary opamp with the single supply circuit then the resistor to ground part of the negative feedback can have a polarized electrolytic capacitor to ground in series with it. Then the input offset current is not amplified. With the dual-polarity supply circuit then the capacitor to ground must be a non-polar film type.

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audioguru said:

A polarized electrolytic capacitor usually has a DC voltage across it. With a dual-polarity supply the DC voltage of the capacitor we are talking about will be small and you do not know its polarity so you might install it backwards.
I have seen equipment with non-polar electrolytic capacitors. These capacitors are smaller and much less expensive than non-polar film capacitors.

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audioguru said:

You used huge 300V capacitors made for old vacuum tube circuits.
I have some 1uF/63V film capacitors with the pins 0.2" (5mm) apart and they are fairly small.

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