Little is shown on its data sheet for using inputs differentially. What has to be considered ?
Input being a crystal microphone, gain set to near 200. Should the microphone be preferably fed trough a series capacitor ? Clipper diodes are implemented at the inputs. No bass boosting.
Hi Externet,
Here is a link to an overview of single ended and balanced audio signals:
http://www.aviom.com/blog/balanced-vs-unbalanced/
Basically, the wanted signals in a balanced system are in antiphase, but noise and interference are normally in phase so that, if you put the two balanced signals into a differential amplifier, the wanted signals add and the unwanted signals cancel, in theory anyway.
Another advantage is that, as the two signals can have different signal paths, amplifier noise can be 0.707 compared to 1 for a single ended amp.
Here is a link about xtal mics:
http://hyperphysics.phy-astr.gsu.edu/hbase/audio/mic3.html
At a first order, the equivalent circuit of piezoelectric (xtal of ceramic) device is a voltage generator in series with a very small capacitor, of only a few picro farads. The voltage generator in an xtal mic typically produces a few milivolts to around 200mV, depending on the mic.
Because the impedance of a capacitor is, 1 /(6.28 * f * C), you can see that at low frequencies the xtal mic output impedance will be higher so, to ensure a good base response, you need an amplifier with a high input impedance, typically, 1M Ohm to 10M Ohm. There are other approaches to ensuring a good base response too.
In the meantime, can you give the model number of your xtal mic? What is its recommended loading impedance? What is the output voltage of one of the lines (or even the differential output voltage)? What is its frequency response?
What voltage supply line are you using on your LM386 audio power amp? I will assume 12V for now.
Are you using the mic for music or just voice? If just voice, the differential amp input impedance need not be so high, because the low frequency requirements for voice are less than music.
It would be asking a lot to make a differential amp with a high input impedance and a voltage gain of 200, with a single LM386 audio power amplifier, as audioguru says. You would really need three additional amplifiers like the FET input TL074, for example. But possibly it could be done with a single TL074. Note that the minimum supply line for a TL074 is 10V, so if you are using a supply line of less than 10V it would be necessary to find a different opamp- no big deal!
Is your requirement to just drive the LM386 with the mic, because if so, depending on the mic output impedance at low frequencies, you could consider connecting just one of the balanced mic signal lines to to LM386, via one single ended TL074 amp.
I will give your requirement some thought- it is years since I messed with xtal mics
Cheers
spec
LM386 Data Sheet
http://www.ti.com/lit/ds/symlink/lm386.pdf
TL07xx Data Sheet
http://www.ti.com/lit/ds/symlink/tl074.pdf