Stereo Audio power amplifier 2 x 40W - 80W

[audioguru2]

Everthing is working fine but there is one problem the Attenuation is still low . i can still hear music when the pot is fully turned off , i think a higher value potentiometer would solve the problem , a 200K or 300K Pot should solve the problem , well i am going to check it in my university because they have big high wattage pots , seems that they should work fine .

Does the microphone pickup music?
Does the amplifier also have an input selector switch and a line-level input for music that needs a shielded audio cable?
When the pot is turned down, the power amp shouldn't have an input. Maybe the pot is defective and doesn't go down to zero.
A higher resistance or high power rating won't make any difference.

and one more thing when not playing any sound or music there is a very annoying hum , how to stop this ??

The cause depends on whether the hum is reduced when the volume control is turned down, then the input wiring or preamp's power supply filtering are to blame.

1)  Use very short wires from the electret mic to the input of the preamp, or use a shielded audio cable.
2) Add a grounded metal enclosure for the preamp.
3) Use better filtering of the power supply for the preamp.
4) Keep high current pcb tracks away from the preamp.

 

[faizanbrohi]

well i have done it , i have placed a B50K Rated Potentiometer and it works exceptionally great and there is amazingly no hum , all Working accurately , thanks . I am a happy man now , now i just have to complete the mechanical details of the enclosure i am putting it in . ok thanks again.

 

[audioguru2]

Good, it works well. ;D

 

[faizanbrohi]

Well Hello again Audio Guru  , how r u . well i have built the power amp successfully , now i am thinking to add flavour in it and found some problems with my flavours . One is the Bass and Treble Control and the other is a Audio VU meter , i want to keep the circuit as simple as possible . here is a illustration for it , as you can see in the first diagram , i not need first opamp as i have the preamp control and i don't even need a mixer after the bass treble control because there is only one input , and my amp is mono.  FOr the second circuit it is using AC , i don't have any ac input , only DC from other Power supply , can i use dc for its VCC and i also need a dual VCC - and + so i will use a ICL7660... So any suggetions ideas .

View attachment 39454

View attachment 39455

 

[audioguru2]

The opamp for the bass and treble control is shown DC biased for positive and negative supplies. You can add a voltage divider and bypass cap to pin 3 then use the same single positive supply as your preamp. Add an input coupling capacitor or use the output coupling capacitor from the preamp.

The LM3915 doesn't need a negative supply. It works from a single positive supply like the rest of your circuits.
The LM3915 circuit has R3 and R4 set for a 9.8V peak input voltage to light the 10th LED. Are you going to use it to display the amplifier's output to the speaker?
The LM3915 dimmly flickers its LEDs with music. You should add the "half-wave peak detector" circuit to its input that is shown in its datasheet where it is powered from a single positive supply.

 

[faizanbrohi]

Well HEllo again , i have modified the schematics and well here is a schematic for the bass and treble control and the Aduio VU LED Meter , i want to keep the circuit as simple as possible. because i don't have enough space.

So , the first circuit as you said i have used the input coupling capacitor of the preamp to the bass treble control and then back to the audio taper / power amp .

The second circuit is a simple LED VU Meter because i just want to use a it after the Preamp , so is there any problem.

View attachment 39459

View attachment 39460

 

[audioguru2]

The bass/treble circuit will work fine.

The diodes at the front of the LM3915 are not needed and will short the output of the preamp.
Use the Half-wave Peak Detector circuit in the datasheet instead.

 

[faizanbrohi]

Hello and i am really really happy today it was raining. i enjoyed the rain very much . after all those warm and humid days , finally the weather is really good now .

Well Here is a look at the LM3915 again , and now it is is using DC 12V with a half wave peak detector circuit . i am just asking if the sensitivity can be adjusted and since it can , will it work good when connecting it to the TL071 preamp .
THere is one dotted line , does it need to be shorted or not??

View attachment 39461

 

[audioguru2]

The output of the peak detector circuit is not audio, it is a fluctuating DC voltage for the display. Therefore the preamp feeds it and feeds the tone controls section, then the volume control takes the output of the tone controls section to feed the power amp.
The peak detector opamp needs positive and a negative supplies.

VR1 can be a trimmer pot to set the sensitivity of the LED display but isn't needed.

The sensitivity of the display is also set with the ratio of R3 and R4.
The total of R3 plus R4 determines the LEDs' brightness. It is a high value so the brightness will be fairly dim.

 

[faizanbrohi]

So this means that the output of the preamp also goes in the peak detector circuit and the peak detector circuit powers the LM3915 IC as well as the LED's , here is the modified schematic , i want a trimpot for the sensitivity for the signal where could i implement it . And one more thing , is there a replacement for LF351 , can i use LM741 or TL071 , TL081 in place of it.

View attachment 39464

 

[audioguru2]

So this means that the output of the preamp also goes in the peak detector circuit

Yes. The output of the preamp also goes to the input of the tone controls section.

the peak detector circuit powers the LM3915 IC

No. The output of the peak detector circuit feeds the SIG input of the LM3915. The LM3915 and LEDs are powered from 12VDC.

i want a trimpot for the sensitivity for the signal

Changing the ratio of R3 and R4 to change the sensitivity also changes the brightness. Use the 50k trimpot in your schematic for a sensitivity control.

 

[faizanbrohi]

Ok i get it the peak detector is just producing fluctuating DC Voltages. so it works like a ADC , with DC Voltages ofcourse . LM3915 is nothing but a bunch of comparators. simple. here is the final Schematic , i think now there is no problem.. thnx

View attachment 39467

 

[audioguru2]

That will work.

 

[faizanbrohi]

HEllo again , everything is working fine , the only problem now is the brightness of the LED how to increase the brightnesss of the LED.

 

[audioguru2]

The brightness of the LEDs is determined by their current. Their current is roughly 10 times the current out of pin 7 of the LM3915 . Your pin 7 is about 9.8V and has a load of 17.2k in parallel with the resistor ladder inside the IC so the total load is about 10.7k and its current is 0.92mA. So the current in each LED is about 9.2mA. Adding a 10k resistor from pin 7 to ground will double the current but the 82 ohm resistor's value is too high and it and the IC might get too hot.