Insufficient gain power amp and Pre amp

Hey all,

As im designing a electret condenser mic pre amp using TS974 and connect with power amp TDA2822M,what surprise me is that there is like no amplification occur.When i put audio signal at distance 5cm,the audio signal will amplify but when its out that range,no audio signal present.

As in my calculation ,my pre amp is 45 dB gain and power amp is at minimum 39dB gain.At output of TS974(conversation environment at 0.02Pa,1meter distance),the output voltage as calculated is 0.16mV and at 5cm distance,the output is 3.2mV.

When im using audio generator,i put a 400Hz signal at the input and measure at the output,its look like a perfect amplification(nice sinusoidal).But if i put a real audio signal(my handphone mp3 sound),only at max 5cm it will amplify.


My question : 1)Can i increase the gain for Power amp(TDA2822M)?

2)What is my option?Replace new Power amp with high gain or use transistor as external component?




Thanks in advance!!!

I suggest you post your circuit, you're doing something wrong.

Hi Nigel,here my circuit schematic.

What about coupling capacitors on the output of the opamp, and the input of the power amp?.

Nigel Goodwin said:

What about coupling capacitors on the output of the opamp, and the input of the power amp?.

Click to expand...

I had put 100nF both at the output of the op amo abd input of the power amp,but nothing change.

It supposed to amplify right?

Yes, do you have a scope?.

Nigel Goodwin said:

Yes, do you have a scope?.

Click to expand...

Yes,i have a scope.

Then inject 10mV in the mike input, and check what you get out of the opamp.

Ok.I will do it next morning.Its 2 am in my country right now.

There nothing weird with my circuit right?

I don't dabble much in audio circuits, but i don't see how you're circuit has any gain at all. You have your input tied to the non-inverting input (pin 3) of the TS974 and the inverting input (pin 2) is tied to the output (pin 1) through an 80k resistor. All that will do is yield the same output as is seen on pin 3. I'm also not sure of the purpose in all the RC filtering between the mic input and the amp input, either.

How did you calculate the gain?

The gain of the preamp opamp is 1+ (83k/180)= 462 which is too much for an audio preamp. A gain of 200 is plenty.
The 18k resistor biases the input of the opamp at +4.5V which is half the supply voltage. That is normal for an opamp with a single supply.

But the DC1669 has a +4.5V supply and an input DC voltage that is also +4.5V. Then it won't work as an audio level controller because the input signal will cause its input to exceed its supply voltage which will blow it up. Also the input must never swing below ground.
The max allowed supply voltage and input voltage of the DS1669 is only 8V so the output from the preamp opamp must be attenuated.

The output of the preamp should have a coupling capacitor to the input of the DS1669 and the input of the DS1669 and the bottom of its divider must be biased at half its supply voltage.

A coupling capacitor must be used from the output of the DS1669 which will have a DC voltage then the input of the TDA2822M must have a resistor to ground.

What a mess of errors! You sketch it.

PS: the DS1669 is a linear variable resistor and will make a poor volume control that should be logarithmic. Use a digital volume control instead of this old digital pot.

EDIT:
Hint: get rid of the DS1669 variable resistor and use a mechanical volume control that has its input coupled through a capacitor and its output coupled through a capacitor. Connect a 10k resistor to ground at the input of the TDA2822M power amplifier.

OutToLunch said:

I don't dabble much in audio circuits, but i don't see how you're circuit has any gain at all. You have your input tied to the non-inverting input (pin 3) of the TS974 and the inverting input (pin 2) is tied to the output (pin 1) through an 80k resistor. All that will do is yield the same output as is seen on pin 3. I'm also not sure of the purpose in all the RC filtering between the mic input and the amp input, either.

How did you calculate the gain?

Click to expand...

Hye,the gain calculated as shown by audioguru.The pre amp is a non-inverting(ac coupled) amplifier.The rc filter that you c=mentioned is act as a RF filter,reject signal above 33MHZ.

audioguru said:

The output of the preamp should have a coupling capacitor to the input of the DS1669 and the input of the DS1669 and the bottom of its divider must be biased at half its supply voltage.

A coupling capacitor must be used from the output of the DS1669 which will have a DC voltage then the input of the TDA2822M must have a resistor to ground.

EDIT:
Hint: get rid of the DS1669 variable resistor and use a mechanical volume control that has its input coupled through a capacitor and its output coupled through a capacitor. Connect a 10k resistor to ground at the input of the TDA2822M power amplifier.

Click to expand...

Im already put a 100nF as a coupled capacitor at at the output of DS1669 and no signal pass at all.Really confuse here.

Im replace the DS1669 with DS1666 and my push button doesnt work.Sometime no signal pass through it and suddenly signal can through it.My push button also didnt work well.When i decrease the button,it mute for all time.

Nigel Goodwin said:

Then inject 10mV in the mike input, and check what you get out of the opamp.

Click to expand...

Hye Nigel.Im sorry i didnt manage to test what you told me yesterday since im trying to clean all the mess on my circuit.I found that DS1669 give me a little problem and i already replace it with DS1666.However,DS1666 that i used doesnt work properly.

Hi Elnino,
I am glad you replaced the linear digital resistor with the DS1666 pseudo-logarithmic digital volume control.
But these digital pots are designed for a positive and negative supply so that the audio is biased at half their total supply voltage. Before I tried to explain how to bias the input and bottom of the resistor ladder at half of your supply voltage.

The datasheet for the DS1666 does not say if the control inputs have pull-down resistors or if you must add them for switches to work.

The input of the TDA2822M must be connected to 0V with a 10k resistor. A coupling capacitor blocks DC but then the input pin is not at 0V, it is floating without the 10k resistor to 0V.

Try the preamp and power amp without the digital volume control then add it when the amplifier works well.

Hi audioguru,

Here my new post for my schematic.I tested it today.Then,same result occur.I can detect some signal at the output but the amplification is small.After do some re-check at home,i found out that i did some error on calculation.

Suppose max current for polarize the ECM is 0.5mA.However,on my design,it is 4mA(I=Vcc/2.2K+8K).If im replace the 8ohm resistor with 15K,i will get average 0.5mA.Im not test it yet since im not in lab.Does this miscalculation lead to the amplification problem before?

You've ignored every piece of advise, I suggest you read the previous replies.

audioguru said:

Hi Elnino,
I am glad you replaced the linear digital resistor with the DS1666 pseudo-logarithmic digital volume control.
But these digital pots are designed for a positive and negative supply so that the audio is biased at half their total supply voltage. Before I tried to explain how to bias the input and bottom of the resistor ladder at half of your supply voltage.The datasheet for the DS1666 does not say if the control inputs have pull-down resistors or if you must add them for switches to work.

Click to expand...

Hi audioguru,
The pin 12,i ground it as you can see at my new post.I use 10k DS1666.It suppose to work well because i didnt find anything weird on my design(DS1666 part).Maybe i had a biasing problem at pin 13 and 12,am i correct?

I also try to put a 100nF capacitor as a coupling at the TL071 output but no signal can be detect at all.But if im take it out,then i can detect small amplication at the TDA2822M output.

audioguru said:

Try the preamp and power amp without the digital volume control then add it when the amplifier works well.

Click to expand...

Good idea Audioguru.Never thought of it.

elnino86 said:

Hi audioguru,

Here my new post for my schematic.I tested it today.Then,same result occur.I can detect some signal at the output but the amplification is small.After do some re-check at home,i found out that i did some error on calculation.

Click to expand...

Please get rid of the DS1666 until you learn how to use it:
1) The signal level from the preamp is way too high for the DS1666 and might destroy it.
2) The divider resistors of the DS1666 are not biased at half of its supply voltage.
3) The control inputs from the switches do not have pull-up or pull-down resistors.

Suppose max current for polarize the ECM is 0.5mA. However,on my design,it is 4mA(I=Vcc/2.2K+8K).

Click to expand...

No it is not. 9V/2.2k is 4.1mA.
Your ECM is not shorted, it has a fairly high voltage across it. Then the 2.2k+8k resistors have a fairly low voltage across them and have a fairly low current in them.
The load on your ECM is only 2.2k ohms which is too low which will reduce its output level a little. Replace the 2.2k resistor with 10k, replace the 8k resistor with 1k, replace the 18k resistor with 47k and replace the 10uF capacitor with 47uF.

Get rid of the TL071 opamp because it has no power supply and its pin 5 is shorted to ground.

The TDA2822 must have its input pin 7 at 0VDC. Yours floats to some high voltage and stops the amplifier from working.

audioguru said:

Please get rid of the DS1666 until you learn how to use it:
1) The signal level from the preamp is way too high for the DS1666 and might destroy it.
2) The divider resistors of the DS1666 are not biased at half of its supply voltage.
3) The control inputs from the switches do not have pull-up or pull-down resistors.

Click to expand...

Thank for the advice.Really appreciate it as a junior here.I will try to understand it first.I look at Dallas application note 168,regarding to DS1666,it seems it not require any pull-up or pull-down resistors.

audioguru said:

The load on your ECM is only 2.2k ohms which is too low which will reduce its output level a little. Replace the 2.2k resistor with 10k, replace the 8k resistor with 1k, replace the 18k resistor with 47k and replace the 10uF capacitor with 47uF.

Click to expand...

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?

audioguru said:

Get rid of the TL071 opamp because it has no power supply and its pin 5 is shorted to ground.

Click to expand...

My apology to Audioguru.Im really forgot and dont even realize to supply a voltage at pin 7.Regarding to pin 5,since im not using it,can i just simply ground it or capacitively ground?

Thank in advance.