Stereo Audio power amplifier 2 x 40W - 80W

[faizanbrohi]

Hello Audio Guru , i have almost built the PCB and refined it , for the 80W Stereo That i posted the first time . i am making only a mono version for the university project. so i also need to make a preamplfier . So i had a preamplfier i made some time , but it had very bad distortion , it was made using a AN7116 Power amplfier IC , had awful distortion. I also tested it with the Power amp . didn't work good for my need.
SO here is another schematic . THere is a graph for THD also . I am making a PCB for it . Will this work for my Power Amplfier ?

View attachment 39398

View attachment 39399

 

[audioguru2]

The distortion is quite low, but the voltage gain of the two transistors preamp is also low.
How much preamp gain is needed? What provides the input?
Why not use an audio opamp (TL071) as a preamp?

 

[faizanbrohi]

LM386 is also an audio opamp . well the input is from a electret mic , i toook both diagrams from the PDF . Both the documents agree that each of them might be used together.
I only want simple volume control and less bulky circuitry , how can i impelmet TL071 in it

k17.pdf

kit98.pdf

 

[audioguru2]

The LM386 is not an opamp. It is a low-power speaker amp that is cheap and noisy.
You already have a speaker amp.

If you connect a microphone to an amplifier and speaker, how are you going to stop the acousical feedback? They will howl if the microphone can hear the output from the speaker.

There are no measurements shown for the two-transistors preamp so I think an audio opamp circuit will be much better. Here is one with a volume control:

View attachment 39403

 

[faizanbrohi]

IF i only use the Circuit that you have posted in and connect it to my power amp , it will have volume control . but will it have enough power to drive my power amp , and do i have to use the LM386 for that , or just use your TL071. One more thing i have made the PCB's for the LM386 and two transisitor op amp. Now will make for the TL071.

 

[audioguru2]

The input transistors look like they are mounted backwards in the parts layout of the 1st amplifier you posted. I didn't see its schematic nor spec's so I don't know its voltage gain.

The LM386 is a cheap and noisy little speaker amp that isn't needed to drive your 40W amp. The two transistors and LM386 will work, but the TL071 preamp is much better.

 

[faizanbrohi]

Thanks Audio Guru i think i am making both LM386 one and the TL071 amnd then compare quality .. going to buy parts todays , thanks

 

[faizanbrohi]

I have tested the TL071 opamp preamp it is working great , but the sound when i connect it to the speaker it is really low, i mean it is behaving like a headphone , i think maybe the power amp would increase it .today i will build the Power amp and update you.

 

[audioguru2]

An opamp doesn't have enough output current to drive a speaker. The opamp is designed for a load not less than 2k ohms. Also the output coupling capacitor's value is way too low to pass low frequencies to a low impedance speaker.

Post the schematic of the power amplifier. On your layout drawing it looks like its input transistor is backwards.

 

[faizanbrohi]

The Power amplifier uses bipolar capacitors , and they were hard to find . why are bipolar capaictors , why not use simple polar capacitors when there is an option.
and the input transistors are okay , i have a assembled version of the power amp which i bought as a guide and it is working. no problem with the input tranisitors . Just Hope the amp works .

One more thing i tested the assembled version of the power amp with a computer and it ran fine. don't know if it will work with the TL071 Preamp

 

[audioguru2]

The Power amplifier uses bipolar capacitors

The 80W stereo amp you 1st posted? It uses polarized caps except the 56pF ceramic cap.

and the input transistors are okay

The audio input to a transistor is usually its base. Your pcb layout has the emitter as the input.

 

[faizanbrohi]

Hello Audio Guru , i have built the units together in one PCB they are complete . there is only one problem , the speakers are howling because of the microphone. is there any method i can apply to remove it

One more thing is that the volume when set to zero . i mean 100Kohm , it is still giving a amplified output at the speaker . i want volume at zero to be implemented where at zero no voice is there at the output what should i do , see the figure i think i should should add a resistor.

View attachment 39425

 

[audioguru2]

the speakers are howling because of the microphone. is there any method i can apply to remove it?

Provide distance between the mic and speakers and point them in opposite directions. Turn down the volume.

One more thing is that the volume when set to zero . i mean 100Kohm , it is still giving a amplified output at the speaker . i want volume at zero to be implemented where at zero no voice is there at the output what should i do , see the figure i think i should should add a resistor.

The 100k pot in the preamp circuit is a gain control. It adjusts the gain from 1 to 22. Add a proper volume control (with an audio taper) between the preamp circuit and the power amp circuit.

 

[faizanbrohi]

i have a point where the preamp put out a connection between a preamp and power amp where i can implement it ?

Can i apply it like this , the first circuit shows the actual volume control circuit.

View attachment 39430

View attachment 39431

 

[audioguru2]

Use a 47k audio taper (logarithmic) pot for a volume control. Use a dual pot with a single shaft for a stereo volume control.
The preamp output and power amp input have coupling capacitors so the pot won't mess their DC bias.

View attachment 39432

 

[faizanbrohi]

I have a Pot Rated at 50KA x 2 meaning 50K at both ends and A represents that it is LOg i guess , here is a picture of it.

View attachment 39434

View attachment 39435

 

[audioguru2]

50kA is 50k ohms from input to ground and is audio taper. Here is how to connect it:

View attachment 39436

 

[faizanbrohi]

I think there is one problem i need to disconnect the PCB Track , because from the circuit above input and output is not shorted , therefore it is connected by a log potentiometer and not connected directly.

 

[audioguru2]

i need to disconnect the PCB Track

Yeah, disconnect the track that connects the output of the preamp to the input of the power amp.

 

[faizanbrohi]

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