Audio amplifier using a preamp and an lm386 audio amplifier

Thread Starter

Arnab Kumar Mondal

Joined May 19, 2016
8
I had tried to build an audio amplifier using a preamp build out of a transistor and thereafter using lm386 for the audio amplifier. I had received a suitable response using an earphone. But when I use an 8 ohm 0.5 speaker the audio output gets distorted and also the sensitivity reduces. Also I am unable to eliminate the mains noise which distorts the output of the speaker. Along with it when I increase the volume control or the gain control too high i hear a sharp beep sound of 2.7 kHz. I would love to hear what improvments i could make in the circuit to make it sensitive as well as clear
 

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Alec_t

Joined Sep 17, 2013
10,584
Try wiring the 386 the conventional way, as shown in the datasheet :-
LM386Amp.PNG
Change the 100k input pot to a lower value (e.g. 10k) and remove the 10nF cap on pin 3 of the 386, to reduce signal attenuation.
Re-think the pre-amp. The way it is biased will probably result in clipping of positive-going parts of the signal.
 

atferrari

Joined Jan 6, 2004
3,570
Start by getting the LM386 working on its own and use the schematic from the datasheet!
Try wiring the 386 the conventional way, as shown in the datasheet :-
The most consistent piece of advise ever. Been there...

Wondering if the OP actually needs a preamp... Been there...

In my last version, (schematic from the datasheet built in Veroboard) not at hand now, I recall adding a .1uF cap somewhere which killed some awful oscillation.
 

AnalogKid

Joined Aug 1, 2013
8,212
Normally I wouldn't pile on, but your 386 schematic will not work. Audio power amplifier chips, even small ones, are notoriously unstable. Usually the circuits on the datasheet are the only ones that will work, and even then they need exceptional decoupling and careful attention to ground paths. The LM386 is *not* an opamp, and traditional feedback methods will not work. Also, why do you have one volume control and two gain controls all built into a single gain stage? I recommend that you keep the volume control and make the 386 gain fixed. As mentioned above, with a fixed gain of 200 you might not need the preamp stage.

Separate from that, the preamp circuit has no negative feedback to stabilize its gain. It is a single transistor running wide open, which will have significant harmonic distortion in the center of the wave and clipping at the peaks.

ak
 

crutschow

Joined Mar 14, 2008
23,724
A preamp should be needed only if you have a very low (mV) source such as from a magnetic vinyl-record cartridge.
And if that's the source, then you also need RIAA frequency equalization.
 

Thread Starter

Arnab Kumar Mondal

Joined May 19, 2016
8
These are some of the modifications that I did in the circuit . But i am still getting acoustic howling. What can I do to prevent it dynamically. Also I want to increase the sensitivity to the extent that when I rub my fingers in front of the electret mic the speaker should be able to play it .

 

Thread Starter

Arnab Kumar Mondal

Joined May 19, 2016
8
The most consistent piece of advise ever. Been there...

Wondering if the OP actually needs a preamp... Been there...

In my last version, (schematic from the datasheet built in Veroboard) not at hand now, I recall adding a .1uF cap somewhere which killed some awful oscillation.
Can you show the datasheet
 

Alec_t

Joined Sep 17, 2013
10,584
Is there any way I can reduce it as well as increase the sensitivity
Build the circuit on a pcb (or small stripboad), use screened short wiring, use a directional mic aimed away from the speaker, use a star (single point) ground system, enclose the ciruit in a screening box, use a better pre-amp design.
 

Willen

Joined Nov 13, 2015
287
Or keep condenser mic's audio signal wire and base wire of the pre-amp transistor as short as possible! Experiment once, 2 inch naked wire as mic leads has horrible noise but 1cm has no. Use long wire for output speaker instead so the Mic will not hear the speakers output audio, thus you can remove howling.

Placing a supply isolator resistor like 470 ohms and decoupling it with 0.1uF capacitor and feeding the supply to the condenser Mic's load resistor helps to filter hum noises too.

VERY high gain preamp for the condenser Mic is not good to the device like you are making. Condenser Mic with high gain can catch audio from hundreds of meters and catching its own output makes howling. Decrease down the gain so the Mic catch audio just within 1meter.
 
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