Electret microphone capsules are cheap and ideal for measuring your latest loudspeakers, but you need a pre-amplifier to take it into your sound card. Traditionally, people have fitted a resistive load to the capsule's FET as per the manufacturer's recommendation, but this isn't the only way it can be done. I've successfully turned the FET into the lower element of a cascode, with the advantage that the low load resistance means that cable capacitance from the capsule to the pre-amplifier is lesss of a problem. I power the whole shebang from a pair of 9V batteries in series. It all fitted in a 4" x 2" x 1" cast aluminium box...
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Yet Another Mic Amp
Here is a variation on the theme. The transimpedance amplifier stage also provides a low impedance input, similar to the cascode transistor, reducing the cable capacitance problem. The 2nd opamp forms a offset correction amp (DC-servo) with a 3Hz roll-off. Hence the total amp as a 3Hz high-pass characteristic. 220R at the output to keep the Opamp stable. When using 220kOhm for both input of the 2nd opamp, the DC offset at the output <5mV. Whole circuit is supplied with two 9V batteries. If more gain is required the 2k2 resistor can be increased. Should fit in the same box.
Here is a variation on the theme. The transimpedance amplifier stage also provides a low impedance input, similar to the cascode transistor, reducing the cable capacitance problem. The 2nd opamp forms a offset correction amp (DC-servo) with a 3Hz roll-off. Hence the total amp as a 3Hz high-pass characteristic. 220R at the output to keep the Opamp stable. When using 220kOhm for both input of the 2nd opamp, the DC offset at the output <5mV. Whole circuit is supplied with two 9V batteries. If more gain is required the 2k2 resistor can be increased. Should fit in the same box.
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jeepee,
your preamp looks simple enough that I can build one. The main problem is that the NE5533AN is hard to find or very expensive, while I can have a couple of NE5534AP at the rs-components warehouse which is only a little walk from home for small money. It is possible to use 2 x NE5534AP instead of the NE5533AN?
If the answer is yes, I have other questions about the connection/build - can I contact you about that?
TIA, Ralf
your preamp looks simple enough that I can build one. The main problem is that the NE5533AN is hard to find or very expensive, while I can have a couple of NE5534AP at the rs-components warehouse which is only a little walk from home for small money. It is possible to use 2 x NE5534AP instead of the NE5533AN?
If the answer is yes, I have other questions about the connection/build - can I contact you about that?
TIA, Ralf
In a word, no. I hadn't any answer for my doubts. I quickly built the TL071 version (first circuit on this thread), using a different opamp that I had lying around (a 4560DD). It worked and was pretty easy to build, but eventually I built the Wallin preamp for the clipping circuit. It was a lot more work, because of the greater number of components (and me not having a PCB).
Ralf
Ralf
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