Insufficient gain power amp and Pre amp

[Nigel Goodwin]

Ok Audioguru.I will replace them and see the result.Regarding to 18K resistor,i put it 18k because TS974 had an input impedance about 18k.If im goin to replace it with 47K,im afraid it will collapse the signal due to mismatch input impedance.Am i correct?

What makes you think it has an input impedance of 18K?.

 

[audioguru]

I look at Dallas application note 168,regarding to DS1666,it seems it not require any pull-up or pull-down resistors.

The application note talks about different digital pots, not the DS1666.
The DS1666 is controlled by logic voltages, not switches.

Regarding to 18K resistor, i put it 18k because TS974 had an input impedance about 18k. If im goin to replace it with 47K, im afraid it will collapse the signal due to mismatch input impedance.Am i correct?

No.
The TS974 datasheet does not spec its input impedance but since it is an opamp then it is extremely high, a few Megohms. Its max input bias current is 0.75uA at room temperature which will cause a DC offset voltage of slightly more than nothing.

It is wrong to match impedances because then half the signal is thrown away.
A low impedance load on the mic will collapse its signal, not a high impedance load.

Regarding to pin 5 (of the TL071 opamp), since im not using it, can i just simply ground it or capacitively ground?

No.
The datasheet for most single opamps show that pin 1 and pin 5 are for an input offset voltage adjustment potentiometer, not a short to ground nor a capacitor to ground. Do not connect anything to pin 1 nor to pin 5 if you don't have an offset adjustment pot.

 

[elnino86]

The application note talks about different digital pots, not the DS1666.
The DS1666 is controlled by logic voltages, not switches.

Hye,Audioguru.Do you have any good online article or application note for my reference and study?Im done on googling some of it but didnt manage to search any good article.Do you have it,mind to share?

I do a troubleshoot on my circuit today.Im implement the pre amp and power amp only with 10k variable resistance.I replace the 8 Ω with 15kΩ to get 0.5mA through the mic.Im also replace all the capacitor as Audioguru said.I leave the 18kΩ(Sorry Audioguru..Im also put the RC network at the input of TDA2822M as Audioguru propose to me.The signal that i get at the output was perfect.

Again,i cant run away from problem.My 16 Ω earphone slowly to get heat,then it was blast and 'burn'. Huh.

I wonder why it happen?

 

[Nigel Goodwin]

Probably because it's unstable and oscillating? - presumably above the audible range. Stick your scope on the output.

 

[OutToLunch]

The gain of the preamp opamp is 1+ (83k/180)= 462 which is too much for an audio preamp.

I still don't see how the gain is anything but 1 - either AC or DC. Pin 1 is the OUTPUT of the amplifier. The 180 ohm resistor and 4.7uF cap are just loads to the amplifier - they don't contribute to gain in the least. To get gain, he needs to connect the series RC from Pin 2 to ground.

 

[elnino86]

Here is my circuitry for today.Hi Nigel,my 16Ω earphone heat too fast then blast!!Anything wrong with my battery power supply?

 

[elnino86]

I still don't see how the gain is anything but 1 - either AC or DC. Pin 1 is the OUTPUT of the amplifier. The 180 ohm resistor and 4.7uF cap are just loads to the amplifier - they don't contribute to gain in the least. To get gain, he needs to connect the series RC from Pin 2 to ground.

My apology bro.The diagram suppose to be like this.Thank for remind me.

 

[audioguru]

Hi El Nino,
Your preamp has a gain of 462 and your power amp has a gain of 100. The total gain is extremely high at 46,200 and could easily oscillate at a high frequency unless it is carfully made on a pcb. for certain it will oscillate on a breadboard.

Your "volume control" is not correct.

EDIT: My volume control was also wrong and is corrected in my next couple of posts.

 

[audioguru]

I still don't see how the gain is anything but 1 - either AC or DC. Pin 1 is the OUTPUT of the amplifier. The 180 ohm resistor and 4.7uF cap are just loads to the amplifier - they don't contribute to gain in the least. To get gain, he needs to connect the series RC from Pin 2 to ground.

The DC gain is 1 because of the capacitor in series with the 180 ohm resistor.

The 83k and the 180 ohm form a voltage divider from the output to ground and the attenuated signal is fed to the inverting input of the opamp.
Opamps have such a high internal gain that they keep the inverting input at the same level as the non-inverting input. Multiply the signal level at the input by the 83k to 180 ratio of the resistors to see that the gain is 462 times the input.

 

[elnino86]

Hi El Nino,
Your preamp has a gain of 462 and your power amp has a gain of 100. The total gain is extremely high at 46,200 and could easily oscillate at a high frequency unless it is carfully made on a pcb. for certain it will oscillate on a breadboard.

Your "volume control" is not correct.

Hi audioguru,

Thanks for "volume control" schematic.

Today i got a little problem with the circuit again..This time,very weird.I didnt get the output from the TDA2822M.There no signal detect on the oscliscope.I checked my vertical and horizontal scale.Everything is on the range.

But there is a signal detected at the output of TS974.I just implement the same design as i posted.

Im also replace the 330nF capacitor(non polar) with 10uF(polar) capacitor.Nothing happen also.

Now,the problem is with TDA2822M.The TS974 part is already perfect i guess.

What do you think Audioguru?Is the problem relate to the gain at the pre amp as yo said earlier-"462 gain is too much for the pre amp".Do i need to reduce it?

 

[elnino86]

Hi Audioguru and all of you,

Why in this diagram,resistor pot is put before 10k resistor?

**broken link removed**

 

[OutToLunch]

The DC gain is 1 because of the capacitor in series with the 180 ohm resistor.

The 83k and the 180 ohm form a voltage divider from the output to ground and the attenuated signal is fed to the inverting input of the opamp.
Opamps have such a high internal gain that they keep the inverting input at the same level as the non-inverting input. Multiply the signal level at the input by the 83k to 180 ratio of the resistors to see that the gain is 462 times the input.

thanks for the lesson, BUT...

the way his circuit was drawn did not have the 180 ohm resistor and cap tied to the inverting input of the amplifier - it was tied to the OUTPUT of the amplifier - there was no divider to ground from the output to the inverting input - thus the gain was 1 whether it was AC or DC.

take a look at the pinout of the part and then look at his original wiring. If that is how he constructed his circuit, then his gain was 1 and he needs to fix the circuit - not just the drawing.

 

[elnino86]

thanks for the lesson, BUT...

the way his circuit was drawn did not have the 180 ohm resistor and cap tied to the inverting input of the amplifier - it was tied to the OUTPUT of the amplifier - there was no divider to ground from the output to the inverting input - thus the gain was 1 whether it was AC or DC.

take a look at the pinout of the part and then look at his original wiring. If that is how he constructed his circuit, then his gain was 1 and he needs to fix the circuit - not just the drawing.

Hye Outtolunch,yup.I admitt it there is an error in my drawing but i had changed it.It actually like as you said.When i re draw the circuit,im not realize that i connect the output pin to the RC network.Sorry for any misunderstanding.I hope i clear it already.

 

[audioguru]

Here is the corrected TDA2822M volume control.

 

[audioguru]

Hi OutToLunch,
Fixing kids' school work is a nightmare, isn't it?
I am sorry, I didn't notice the mistakes with the negative feedback on the first couple of schematics.

Hi El Nino,
If the polarized 10uF capacitor has its polarity backwards then it might be destroyed or it might couple a DC voltage to the input of the TDA2822M.
A 10uF capacitor will have a cutoff frequency of only 1.6Hz and take 6 seconds to charge. Use a 330nF film capacitor instead for a cutoff frequency of 49Hz.

 

[elnino86]

Hi OutToLunch,
Fixing kids' school work is a nightmare, isn't it?
I am sorry, I didn't notice the mistakes with the negative feedback on the first couple of schematics.

Hi El Nino,
If the polarized 10uF capacitor has its polarity backwards then it might be destroyed or it might couple a DC voltage to the input of the TDA2822M.
A 10uF capacitor will have a cutoff frequency of only 1.6Hz and take 6 seconds to charge. Use a 330nF film capacitor instead for a cutoff frequency of 49Hz.

Hye Audioguru,

Sorry for my mistake in drawing a circuit.

Today my pre amp and power amp working well,i guess.Im only analyze them using oscilloscope since my canal earphone(16Ω) was 'destroyed' by TDA2822M last few days.

What i can observe is that at the output of the TDA2822,the sinusoidal graph had been 'clip' at certain gain.If im increase the resistance,it will continue clipping at that level.Im afraid, if im connect it to the 16Ω load again,it will destroy my earphone(canal earphone cost me $20).

 

[Nigel Goodwin]

Add a series resistor to the headphone - the amplifier is for feeding speakers, not headphones.

 

[elnino86]

Add a series resistor to the headphone - the amplifier is for feeding speakers, not headphones.

What is the different between speaker 16Ω and headphone 16Ω?Doesnt they have a same function but different in size?

 

[adrian sphank]

What is the different between speaker 16Ω and headphone 16Ω?Doesnt they have a same function but different in size?

Yup,they have a similar function and differ in size.I guess your pre amp gain is too high.I think you should reduce it.

 

[elnino86]

Yup,they have a similar function and differ in size.I guess your pre amp gain is too high.I think you should reduce it.

Yes,obviously they differ in size paw.

Im already reduce it to 200(im not post my schematic yet),i guess that the reason why im able to read the output,Only 'clipping' problem give me some trouble right now.