2 Wire P.A. (Public Address) System Issue

Thread Starter

Pcflyf

Joined Apr 5, 2020
6
Hello I am currently working on an electronic kit from Chaney Electronics. The issue is that the PA System that was made produces lots of buzzing sound and does not even project my voice when speaking through the "electrec" microphone. I have tried to distance the speaker from the microphone but no difference.
 

LesJones

Joined Jan 8, 2017
2,597
You will need to give some details. The only information I could find on the Chaney Electronics website was a picture. No schematic. The part number of the IC used was not even readable. Are you using it with a battery (PP3 ?) or a mains power supply ?
Does the volume of the buzz change with the setting of the volume control ? Post the schematic if one was supplied. If not trace out the schematic and post it. What is the part number of the IC being used ? (I would guess it might be an LM386)

Les.
 

LesJones

Joined Jan 8, 2017
2,597
My first thought was that the buzz was caused by a mains power supply that did not have enough smoothing but the fact that you ar powering it from a battery rules out that theory. I can't see anything wrong with the schematic that would cause the problem. It is almost identical with the circuit on the data sheet. The data sheet does suggest a zobel network on the output of the amplifier but I don't think the lack of this would cause the problem. (This is just a 10 ohm resistor in series with a 50 nF capacitor connected between pins 5 and 4 of the IC. 50 nF is not a common value so a 47 nF should be OK. if you wanted to try it.) Is the amplifier situated close to any AC wiring or fluorescent lamps ? If so try moving it away. Post pictures of your construction from both the component and etch sides of the board. Try different settings of the volume control and tell us if it changes the volume of the buzz.

Les.
 

Audioguru again

Joined Oct 21, 2019
1,255
The mic and volume control must have very short wires to reduce the buzz.
A compact pcb should replace the solderless breadboard.

Its output is only 0.5W so it will not be loud.
 

AlbertHall

Joined Jun 4, 2014
9,613
The data sheet does suggest a zobel network on the output of the amplifier but I don't think the lack of this would cause the problem. (This is just a 10 ohm resistor in series with a 50 nF capacitor connected between pins 5 and 4 of the IC. 50 nF is not a common value so a 47 nF should be OK. if you wanted to try it.)
The zobel network helps avoid possible instability in the amplifier. Your case may be the amplifier oscillating at some out of band frequency.
https://www.diyaudio.com/forums/multi-way/247002-confuse-zobel-network.html
 

Audioguru again

Joined Oct 21, 2019
1,255
The LM386 is similar to the older LM380. A 48 years old application note for the LM380 explains the oscillation problem that the zobel network prevents.
I think the problem is caused by the inductance of a speaker causesing its impedance to be much higher than 8 ohms at the oscillation frequency.
 

Thread Starter

Pcflyf

Joined Apr 5, 2020
6
Hi Les Here's the aerial view of finished 2 wire PA system...the blue wires connect to the speaker (8 ohms, 0.8 watts)... dont know if that helps I also tried to add the Nobel network and the frequency of the buzz got lower but there was still a loud buzzing noise
I'm guessing that it's the ic that needs replacing as I checked all the other components they seem to be okay...
 

Audioguru again

Joined Oct 21, 2019
1,255
You cannnot make a decent audio amplifier on a solderless breadboard. Each of the very long wires is an antenna that picks up 50Hz or 60Hz sprayed from the electricity wires in walls all around you.
A pcb would easily be as small as 1 square inch in size and produce NO BUZZING.
 

LesJones

Joined Jan 8, 2017
2,597
As AG points out the construction method is probably the cause of the problem. I assumed when you described it as a kit that it was built on the PCB shown on Chaney Electronics website. You have still not answered the question about the effect of different volume control settings has on the buzz.

Les
 

Audioguru again

Joined Oct 21, 2019
1,255
The LM386 works fine with a gain of only 200 using a capacitor between pin 1 and pin 8 as shown in its datasheet, when it has the zobel network also shown in its datasheet, when built properly on a compact pcb.
 

Thread Starter

Pcflyf

Joined Apr 5, 2020
6
Les as you turn the knob on one end the buzz is high then on the other it is low...
Thanks Audioguru gonna try it on compact pcb see if there any difference...
 

Audioguru again

Joined Oct 21, 2019
1,255
Millions of LM386 amplifiers have been made with the circuit shown in the datasheet, with an electret mic and built on a pcb. They produce perfect sounds with no buzzing.
On another website I fixed an Electronic Stethoscope circuit with an LM386 amplifier used to hear heartbeats and car engine noises. It produces no buzzing because it is built on a compact pcb.
 
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