buffer opamp..

[hamsiii]

I have a small problem while designing my single supply speaker amplifier. It totally works in +12V and -12V but when I exclude -12V from the design and connect the + pin of OPAmp to Vcc/2, the output is only a DC value of 12V rather than 50mV as the input.(it is only a buffer) I cannot find a solution, i wonder someone can help me. Thank you all.

 

[Nigel Goodwin]

You've connected it wrongly - the junction of R4 and R5 is supposed to go to the input of the opamp.

 

[hamsiii]

ok, here is the circuit: 12V to R4 (pin1) , R4 -R5 junction to OPA input and R5 to ground. I still get 10.5V DC (no AC signal) from the output?

 

[audioguru]

Now you have the opamp biased correctly.
But the signal generator is shorting the DC voltage at R1 to ground because an input coupling capacitor is missing. Then the output goes as high as it can.
An output coupling capacitor is also probably missing.

It is not a speaker power amplifier.

 

[hamsiii]

Ok, I will try to add the input coupling capacitor. It is just a part of the speaker amp, buffer part. After the output, circuit has a output C and R connected to TDA7052. Thanks for the tip.

 

[audioguru]

A TDA7052 amplifier IC does not need an input buffer because its input resistance is already high at 100k ohms. The input resistance of a TDA7052A (with built-in DC volume control) is 20k ohms.

 

[hamsiii]

ok, when i add a input coupling cap. the output becomes 50mV-p but at 6V dc. Should I always use another capacitor to eliminate DC offset?

One more thing, if I set this circuit by split supply, there is no need for this input capacitor!?

 

[audioguru]

ok, when i add a input coupling cap. the output becomes 50mV-p but at 6V dc. Should I always use another capacitor to eliminate DC offset?

You want the output to have max voltage swing so you set the (+) input at half the supply voltage with 2 resistors. The feedback will force the DC voltage of the opamp output so that the (-) input is the same voltage as the (+) input. the input coupling capacitor prevented the 0VDC from the signal generator from making R1 at 0V so the feedback caused the (-) input and the opamp output to be at +6V.
This opamp might also need an output coupling capacitor to block its +6VDC.

One more thing, if I set this circuit by split supply, there is no need for this input capacitor!?

Without an input coupling capacitor in this inverting opamp circuit then the (+) input will be at 0VDC and the opamp will amplify its DC input offset voltage.

 

[hamsiii]

https://www.ee.bilkent.edu.tr/~eee211/LectureNotes/LFC.pdf

This loudspeaker amplifier is working very well in deed, with just a slight difference of split power supply. I should make this amp only with single supply.
The changes would be a +6V to + input and connecting the - supply to ground. Also, adding input and output cap. would make my circuit exactly like this?
Because Pspice or Isis does not support TDA7052, I would not have the chance to see the output.

 

[audioguru]

For the mic preamp and buffer you are using a TL082 general purpose dual opamp instead of a TL072 low noise audio opamp. They cost the same.
Your mic preamp will sound without any low frequencies and without any high frequencies worse than a telephone because its filters have cutoffs at 284Hz and at 1800Hz.
Your mic preamp will cause severe distortion when a high level causes the diodes to clip the signal.

You might not find a TL082 or TL072 that works properly from a 6V supply because their minimum supply is 7V.
With a single-polarity positive supply then the opamps must be biased at half the supply voltage.

You are messing up the DC volume control of the TDA7052A with your millions of parts. The datasheet shows only a 1M pot to ground that is bypassed by a 1uf capacitor.

What is the circuit for? The little TDA7052 has an output of only 0.5W at low distortion to 1W when clipping like mad. Like a cheap clock radio.