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LM386N used as differential inputs audio amplifier...

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Externet

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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.
 
I am using the two isolated inputs in the LM386 to sum a music channel to a voice channel in an aircraft headphone amp. I can dig out a schematic if it would help you.
 
Yes, thank you. Am not after noise canceling nor summing, but should be helpful learning details.
Should differential amplification increase gain somewhat for a single 'balanced' crystal microphone ?
 
A "crystal" microphone has not been used in anything for about 50 years. It had a vey high output impedance and fed a preamp that used a vacuum tube. The 50k input resistance of an LM386 amplifier is too low for a crystal microphone.
The sound quality of a crystal microphone was awful.

Maybe your microphone is an "electret" type that is completely different? An electret mic has a Jfet inside that must be powered for it to work. Please post a photo or a datasheet of your crystal microphone.
 
Why are you using such an old poor quality microphone?
Wikipedia says they were supplied with old vacuum tubes equipment that had an input impedance of 10M ohms (200 times higher than the 50k or differential 100k of your LM386 amplifier).
 
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 :happy:

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
 
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Yes, thank you. Am not after noise canceling nor summing, but should be helpful learning details.
Should differential amplification increase gain somewhat for a single 'balanced' crystal microphone ?
A balance output mic may have twice the output of a single ended output mic, but it is not a question worth asking because the output voltages from mics differ so much anyway.
It is bit like asking, does a six cylinder engine produce more power than a four cylinder engine. :D

spec
 
Hi again Externet,

Below is a general schematic for a balanced amplifier. If you would like to peruse this approach, please let me know and I will do a practical circuit for you.
Alternatively, if you would like to go for the single ended approach, let me know and I will do a circuit for that.

ETO_2016_02_15_Iss01_MIC_AMP_DIFFERENTIAL.png
 
Here is a circuit for a high input impedance differential amp:
(1) The input impedance = R3 and R8 respectively. This value can be changed if necessary
(2) The differential gain is set by 2* (R10/R9) and so is 20 in the circuit.
(3) To ensure the differential function, R14 must equal R10, and R22 must equal R9.
(4) R4 plays no part in the primary circuit function. It is only there to isolate the opamp output from any capacitance mainly, but also inductance on the output line, which may cause instability in the frequency domain, and/or peaking in the frequency response.
(5) The differential amp may look complicated but it only amounts to one 14 pin chip and a hand-full of resistors and capacitors.
(6)) A good layout and screening will be important.
(7) By the way, the TL07xx amps are particularly well behaved and sweet-sounding. They are also low cost.

spec


ETO_2016_02_15_Iss01_MIC_AMP_DIFFERENTIAL_SCEMATIC.png

TL07xxx Data Sheet:
https://www.ti.com/lit/ds/symlink/tl074.pdf
 
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Here is an unbalanced high impedance input xtal mic amp:
(1) The voltage gain is 1+ (R23/R26) = 22.36 in the circuit below

ETO_2016_02_15_Iss01_XTAL_MIC_AMP_SCEMATIC.png
 
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