Hi, i am just testing my power amp which connecting the output of pre-amp(include mic) to a volume control ( before it have 1 CR network) and then connect it to input of power amp which have a earphone connect at the output of power-amp, and i found out that there have alot of noise ( very noisy) can be hear by earphone with the minimum volume, when adjust to maximum, it will be very loud noisy sound come out from earphone.. may i know what is the problem?

I have never used a TDA2822M amplifier so I look at its spec's on the datasheet.
With no input its input noise is rated at 2.5uV when its input has a 10k resistor. It might be 3uV with your volume control turned down to zero.
Its voltage gain is about 95 so its output noise is 285uV.
Earphones produce 90dB when the power is 1mW which is a voltage of 179mV into 32 ohms. 179mV/285uV is 628 times which is about 56dB so the noise level in the earphones is about 90 - 56= 34dB which is a very low sound level.

Maybe you built the circuit on a breadboard and its high stray capacitance is causing the amplifier to oscillate and produce noise.

 

I have never used a TDA2822M amplifier so I look at its spec's on the datasheet.
With no input its input noise is rated at 2.5uV when its input has a 10k resistor. It might be 3uV with your volume control turned down to zero.
Its voltage gain is about 95 so its output noise is 285uV.
Earphones produce 90dB when the power is 1mW which is a voltage of 179mV into 32 ohms. 179mV/285uV is 628 times which is about 56dB so the noise level in the earphones is about 90 - 56= 34dB which is a very low sound level.

Maybe you built the circuit on a breadboard and its high stray capacitance is causing the amplifier to oscillate and produce noise.

When i connect the output of volume control directly to an earphone, there have a clear sound with no much noise can be hear in the earphone. While when i was connected the output of the volume control to the input of the power-amp, and then connect my earphone at the output of power-amp, alot of noise there, while when i am no talking, the earphone can heard a noise with 'pi' sound, when i am talking, there are no 'pi' sound , but my voice from the earphone is totally not clear with alot of noise..


I try to use lm386 as my power-amp, there have also alot of noise can be heard from earphone, but it is less than TDA2822M(don't have the 'pi' sound, may i know is that some connection problem cause the noise?

Thanks!

 

I don't know what you mean when you say "pi" sound.
Is it acoustical feedback howling and squealing because the mic can hear the earphone? Then turn down the volume control.

 

I don't know what you mean when you say "pi" sound.
Is it acoustical feedback howling and squealing because the mic can hear the earphone? Then turn down the volume control.

Yes, there is a howling and squealing sound can hear from earphone, but when i pull the earphone far from the mic, the 'pi' sound still can hear from my earphone. My volume control have been turn down to minimum, but when i am increasing a little bit of volume, the howling and squealing sound appear.

I try to play a music near the mic, but the mic cant really receive the sound, it will just have noise on the earphone .When the mic touch the speaker that playing the music, i only can hear the song with noise and not constanly sound from earphone.

Is that any way to reduce the noise? And the sound from the earphone are not constanly. some time can hear the sound but sometime cant.

Thanks!

 

You did not describe how the "pi" noise sounds like.
Electronic noise from the amplifier circuit is hisssss or shhhhhh.

Hearing aids have an earphone that fits tightly in the ear canal then the mic cannot hear it to cause acoustical feedback howling and squealing. Maybe your earphones are not sealed inside the ear canal. Use sealed over-the-ear headphones instead.

I can't remember how much gain your mic preamp has. Maybe it should be reduced.

 

You did not describe how the "pi" noise sounds like.
Electronic noise from the amplifier circuit is hisssss or shhhhhh.

The 'pi. that i mean was not relate to mathematical that pi, just use to describe the sound that appear in the earphone( can be pi or bi sound).The 'pi" sound just like when we use signal generator generate a high frequency signal to then input of pre-amp, then we can hear a very high frequency sound from earphone. And this case happen when i was using mic with no signal generator as input, when i am no play any sound at mic, the "pi" sound will appear, while i am talking or play some song, the pi sound will dissapear. That mean it will only appear when no sound receive from mic.

Yes, there have hisssss or shhhhhh sound appear in the earphone when i play some music beside the mic. The earphone cant receive a clear sound, and have alot of hissss or shhhh sound.

Hearing aids have an earphone that fits tightly in the ear canal then the mic cannot hear it to cause acoustical feedback howling and squealing. Maybe your earphones are not sealed inside the ear canal. Use sealed over-the-ear headphones instead.

I have already pull my earphone far from the mic, it still have the sound.

I can't remember how much gain your mic preamp has. Maybe it should be reduced.

My pre-amp have a gain of 101 and an input of 10mV pk then the output will be 1.01V pk. So i need to reduce the R3/R4 value in the pre-amp to reduce my gain?


ps: I try to reduce my gain to 27 ( 300k/11k +1 ) , the "pi" sound was reduce, but the sound that can hear from earphone still contain many noise. I have record down the sound through the earphone. Here is the link.

http://www.megaupload.com/?d=OTHK2BXI

ps2: After that , i was try to connect my pre-amp to the 3band tone control, and then connect the earphone at the output of 3band tone control ( before that have 1 CR network), I found out that there is no sound can hear from earphone, Is that because the signal have been reduce until very weak so when reach the earphone, earphone cant detect the very weak signal?

After that, i try to connect whole circuit which is pre-amp>> 3band tone control >> power amplifier , i can hear the pi sound from earphone, but have to turn my volume to maximum. If i turn down my volume, i cant hear anything from the earphone( I was adjusted the potentiometer for middle band only, bass and treble are remain minimum). When i was turn the earphone to maximum volume and try to play a music, but the earphone cant receive any sound except the "pi" sound.

Thanks!

 

The "pi" sound just like when we use signal generator generate a high frequency signal to then input of pre-amp, then we can hear a very high frequency sound from earphone. And this case happen when i was using mic with no signal generator as input, when i am no play any sound at mic, the "pi" sound will appear, while i am talking or play some song, the pi sound will dissapear. That mean it will only appear when no sound receive from mic.

Then the entire circuit is oscillating.
You posted an "amr" sound file that my computer cannot play. I looked for player software and started to download the software but my computer detected a virus in it so I cannot play your odd file. Why didn't you make the sound file an ordinary WAV or MP3 file?

Yes, there have hisssss or shhhhhh sound appear in the earphone when i play some music beside the mic. The earphone cant receive a clear sound, and have alot of hissss or shhhh sound.

My pre-amp have a gain of 101 and an input of 10mV pk then the output will be 1.01V pk. So i need to reduce the R3/R4 value in the pre-amp to reduce my gain?

The total gain is 9600 which is too much. Maybe the total gain should be reduced to 800 or 400.

i was try to connect my pre-amp to the 3band tone control, and then connect the earphone at the output of 3band tone control ( before that have 1 CR network), I found out that there is no sound can hear from earphone, Is that because the signal have been reduce until very weak so when reach the earphone, earphone cant detect the very weak signal?

The 3 band tone control circuit can drive a load of 10 thousand ohms, not 16 ohms.

Did you make the circuit on a breadboard? With such a high amount of gain a breadboard's capacitance between rows of contacts and wires will cause oscillation. Make the circuit on a compact pcb or stripboard instead.

Do you have a logarithmic volume control? A linear volume control is useless.

 

Then the entire circuit is oscillating.
You posted an "amr" sound file that my computer cannot play. I looked for player software and started to download the software but my computer detected a virus in it so I cannot play your odd file. Why didn't you make the sound file an ordinary WAV or MP3 file?

sorry about that, now i have converted the file into mp3 format.
http://www.megaupload.com/?d=Y07BY2BE

The total gain is 9600 which is too much. Maybe the total gain should be reduced to 800 or 400.

May i know how to calculate the total gain? i know the pre-amp is 101 gain, and power amp have a gain about 90?

Did you make the circuit on a breadboard? With such a high amount of gain a breadboard's capacitance between rows of contacts and wires will cause oscillation. Make the circuit on a compact pcb or stripboard instead.

Yes, i am using breadboard for testing.because i think may have some change in the circuit, so i make my circuit on breadboard.Do i have to make the pre-amp on a pcb?

Do you have a logarithmic volume control? A linear volume control is useless.

May i know the logarithmic volume control is same as logarithmic variable resistor?

Thanks!

 

sorry about that, now i have converted the file into mp3 format.

I heard little high frequency squeaks and severe distortion. All low frequencies below about 4kHz are missing.
Listen to the MP3 sound on your computer and tell me if the earphones sound the same.

May i know how to calculate the total gain? i know the pre-amp is 101 gain, and power amp have a gain about 90?

101 times 90= 9090 which is much too high.

Yes, i am using breadboard for testing.because i think may have some change in the circuit, so i make my circuit on breadboard.Do i have to make the pre-amp on a pcb?

Everybody who says their circuit doesn't work built it on a breadboard.
But everybody who builds a circuit on a compact pcb or stripboard says their circuit works perfectly.
But maybe you have a wrong parts value or wrong connection. Post your entire circuit's schematic.

May i know the logarithmic volume control is same as logarithmic variable resistor?

They are the same.
When a linear pot is turned down to halfway then its resistance is half and the signal is reduced to half which is still fairly loud.
But when a logarithmic pot is turned down to halfway the voltage is about 1/10th and the loudness is reduced noticeably.

 

I heard little high frequency squeaks and severe distortion. All low frequencies below about 4kHz are missing.
Listen to the MP3 sound on your computer and tell me if the earphones sound the same.

Is that the circuit cut the frequencies below 4kHz?
Do you mean that listen to the mp3 sound that i was recorder and the sound from earphone then compare whether they was same or not? I test it again, the sound from earphone is worst than the mp3 sound. Alot of noise can be heard from earphone.Beside that, my earphone can only heard the sound with noise when my speaker put very close to the mic(even touch the mic) just like the recorder file, while when i just put the speaker a little bit away from the mic, I can hear very unclear sound with many noise from the earphone( ect when some one say hello far from the mic, i can only hear "bizz bizz" sound for two times from earphone which mean hello ,which I cant understand what is the person talking about when hear from earhone)

101 times 90= 9090 which is much too high.

So i have to reduce to gain to the range of 400-800? ( etc for pre-amp with 21k/3k +1 = 8 then 8 times 90 = 720 gain)

Everybody who says their circuit doesn't work built it on a breadboard.
But everybody who builds a circuit on a compact pcb or stripboard says their circuit works perfectly.
But maybe you have a wrong parts value or wrong connection. Post your entire circuit's schematic.

Thanks, i will build it on the stripboard.
The attachment below are my circuit's schematic which i haven't modify the gain yet.

 

Your schematic drawing software is a nightmare of extra wires all over the place, many parts upside-down and dots for connections that are very difficult to see. None of your electrolytic capacitors have their polarity shown.

You have C3 in the tone controls with a value that is 1000 times wrong and your volume control is wired so it short-circuits the output of the tone controls circuit (cutting all low frequencies and causing severe distortion) instead of being a voltage divider. Here is how a volume control is supposed to be connected:

 

Your schematic drawing software is a nightmare of extra wires all over the place, many parts upside-down and dots for connections that are very difficult to see. None of your electrolytic capacitors have their polarity shown.

Sorry, because that software is user friendly compare to pspice which i feel difficulty when search certain component. I will change other tool to draw the shematic.

You have C3 in the tone controls with a value that is 1000 times wrong and your volume control is wired so it short-circuits the output of the tone controls circuit (cutting all low frequencies and causing severe distortion) instead of being a voltage divider. Here is how a volume control is supposed to be connected:

May i know what wrong is that?The C3 value in the schematic is 0.05uF which is 50pF, I cant find 50pF then i use 47pF to replace it.


About the volume control, i have a bit confuse, coz i am using the attacment below as my volume control.Is that the pin 2(refer as input pin) should connect to output of 3band tone control as input to it and then the pin 3 (refer to output pin ) should connect to the input of power-amp?

Ps: About the volume control, is that instead of logarithmic volume control, is there have any other volume control can replace it? Because my project need to consider the cost (low cost hearing aid), but the logarithmic volume control that selling at my place are quite expensive , the price is about 25 times greather than a linear potentiometer. Can i replace it with bigger value of linear pot such as 1M ohm?

I found a cheaper log potentiomer via internet, but i am not sure whether it can be use or not, can you help me take a look?

http://malaysia.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=2633062

or

http://my.element14.com/tyco-electronics-citec/23esb103mmf50nf/potentiometer-log-10k/dp/350175


Thanks!

 

I have build my power-amp into the stripboard, and i found out that the resistor with 4.7 ohm will short circuit, Then it will make some of my pin which connected with the resistor short crucuit too. Is that correct?(ect pin3 and pin 6 will short circuit(same ground); pin 3 and pin 4 will short cicuit due to the resistor was short circuit from pin 3 to ground )

When i connect my output of pre-amp to input of power-amp( share a same power supply and ground for both pre-amp and power-amp), i found out that the voltage at the output of voltage regulator will drop to about 2.75V when both are connected. When i disconnect the pre-amp and power-amp (power supply and ground) then measure the output of voltage regulator again, the output voltage of voltage regulator will back to 5V, may i know why this will happen? Is that because of short circuit in the power-amp? My pin 3,pin 4 and pin 5 of power amp will short circuit.

Thanks!

 

The C3 value in the schematic is 0.05uF which is 50pF, I cant find 50pF then i use 47pF to replace it.

No.
0.05uF is 50nF, which is 1000 times more than 50pF (0.00005uF). The capacitor should be 0.047uF or 47nF. An Oriental "greencap" will be labelled 473 and a European film capacitor is labelled 47n.

About the volume control, i have a bit confuse, coz i am using the attacment below as my volume control.Is that the pin 2(refer as input pin) should connect to output of 3band tone control as input to it and then the pin 3 (refer to output pin ) should connect to the input of power-amp?

No.
Pin 1 is ground, pin 2 is the slider which is the output of the volume control and pin 3 is the input of the volume control. You showed it backwards which short-circuits the output of the tone contols circuit which caused severe distortion and passes only very high frequencies.

About the volume control, is that instead of logarithmic volume control, is there have any other volume control can replace it? Because my project need to consider the cost (low cost hearing aid), but the logarithmic volume control that selling at my place are quite expensive , the price is about 25 times greather than a linear potentiometer. Can i replace it with bigger value of linear pot such as 1M ohm?

You show the brand ALPS which is very high quality and very expensive. Get a very cheap volume control made in the Orient.

I found a cheaper log potentiomer via internet, but i am not sure whether it can be use or not, can you help me take a look?

There are much cheaper volume controls made but the TYCO one will also work fine.

I have build my power-amp into the stripboard, and i found out that the resistor with 4.7 ohm will short circuit, Then it will make some of my pin which connected with the resistor short crucuit too. Is that correct?(ect pin3 and pin 6 will short circuit(same ground); pin 3 and pin 4 will short cicuit due to the resistor was short circuit from pin 3 to ground )

When i connect my output of pre-amp to input of power-amp( share a same power supply and ground for both pre-amp and power-amp), i found out that the voltage at the output of voltage regulator will drop to about 2.75V when both are connected. When i disconnect the pre-amp and power-amp (power supply and ground) then measure the output of voltage regulator again, the output voltage of voltage regulator will back to 5V, may i know why this will happen? Is that because of short circuit in the power-amp? My pin 3,pin 4 and pin 5 of power amp will short circuit.

The strips of stripboard are supposed to be cut on the copper side below an IC or resistor. A strip-cutting tool is a drill bit in a handle. Twist it a couple of times at a hole and it cuts the copper strip. I cut the strips many times so one strip is separated into many parts of a circuit.

Cut the strips under the TDA2822M IC so that pin 1 and pin 8 are separated, pin 2 and pin 7 are separated, pin 3 and pin 6 are separated and pin 4 and pin 5 are separated.
I plan a stripboard layout on gridpaper and make a red X on each hole that needs the strip to be cut. There is no wire and no IC pin in a hole that has the strip cut.
Usually resistors and capacitors are connected between two different strips, not along one strip.

 

No.
Pin 1 is ground, pin 2 is the slider which is the output of the volume control and pin 3 is the input of the volume control. You showed it backwards which short-circuits the output of the tone contols circuit which caused severe distortion and passes only very high frequencies.

So for the potentiomer in the tone control, I need to connect them as show in the attachement picture?

You show the brand ALPS which is very high quality and very expensive. Get a very cheap volume control made in the Orient.
There are much cheaper volume controls made but the TYCO one will also work fine.

I found this log potentiometer that make by ALPS which is more cheaper than the TYCO, would this can be use?

http://malaysia.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=2633062

The strips of stripboard are supposed to be cut on the copper side below an IC or resistor. A strip-cutting tool is a drill bit in a handle. Twist it a couple of times at a hole and it cuts the copper strip. I cut the strips many times so one strip is separated into many parts of a circuit.

Cut the strips under the TDA2822M IC so that pin 1 and pin 8 are separated, pin 2 and pin 7 are separated, pin 3 and pin 6 are separated and pin 4 and pin 5 are separated.
I plan a stripboard layout on gridpaper and make a red X on each hole that needs the strip to be cut. There is no wire and no IC pin in a hole that has the strip cut.
Usually resistors and capacitors are connected between two different strips, not along one strip.[/QUOTE]

Thanks, i have solve this problem, now my power-amp was not short circuit in these pin.

When i am connecting my pre-amp( still haven't build at stripboard) with the power-amp, I still can heard alot of noise from the earphone. I have already lower down my gain in pre-amp which is 7.33 times a gain of 90 (as you told me in previous) will have a gain of 659.7.Is that because my pre-amp are build in breadboard which cause alot of noise?

Thanks!

 

You have the pot in the tone controls connected correctly.

Whenever you say the word "noise" you should describe the noise which might be hiss, hum or a high pitched squealing sound.

 

You have the pot in the tone controls connected correctly.

Whenever you say the word "noise" you should describe the noise which might be hiss, hum or a high pitched squealing sound.

I am still using the linear potentiometer as my temporary volume control due to waiting the log pot arrive.
When i connect my pre-amp with power amp and adjusted the volume control to minimum, i can hear the sound from earphone with low noise, when i adjusted the volume control to middle level, the sound that can be heard from earphone will increase with noise.Sorry, i dnno how to discribe the noise,what i heard is like electrical sound which is similar to a external speaker ,when you open it to maximum volume without playing song, there have some noise that can be heard from speaker. I have record down the sound and upload it as below.

Beside that,When i turned the volume control to maximum, the sound that can be heard from earphone have decrease again.I have draw out the volume level as show in the attachement picture.

This is the sound clip that volume increased from minimum to maximum (speaker is 5cm away from the mic)
http://www.megaupload.com/?d=KYES6LAP

This is the sound clip that speaker is within 10cm away from the mic. The sound that can be heard from earphone was reduce and with increased of noise.
http://www.megaupload.com/?d=U7VOVEY3

Thanks!

 

Your sound clip #26 sounds horrible. There are no frequencies below about 4kHz and none above about 6kHz.

You said your circuit is playing on a speaker? It sounds like a tiny cheap speaker without an enclosure. Is it a cheap earphone that is playing?

Replace the tiny speaker or earphone with a high quality 8 ohms real speaker that is in an enclosure designed for it so it produces a wide flat frequency response then record it and post it here. To avoid acoustical feedback then the speaker and your recording mic must be far away from the original hearing aid mic. Keep the volume setting low enough to avoid clipping.

 

Your sound clip #26 sounds horrible. There are no frequencies below about 4kHz and none above about 6kHz.

You said your circuit is playing on a speaker? It sounds like a tiny cheap speaker without an enclosure. Is it a cheap earphone that is playing?

Replace the tiny speaker or earphone with a high quality 8 ohms real speaker that is in an enclosure designed for it so it produces a wide flat frequency response then record it and post it here. To avoid acoustical feedback then the speaker and your recording mic must be far away from the original hearing aid mic. Keep the volume setting low enough to avoid clipping.

No, my circuit is playing on an earphone, the song was play from a handphone .The speaker i mean was an handphone's speaker which near to the mic to play a song. Now i have change a better quality of earphone which have 32 ohm resistance and then record the sound clip as below.I was set my volume setting to minimum.

http://www.megaupload.com/?d=225MTJFA

I have some question about the pre-amp and 3 band tone control. The attachement below are my circuit, i plan to use a 4 build-in op-amp, so pre-amp and 3 band tone control will share a same IC.
For the pre-amp, there have a bias voltage(C10,R17 and R18) ,a capacitor and resistors(C11,C12 and R14) connect with 5V power supply. For the 3 band tone control, there have a bias voltage(C14,R21 and R22), capacitors (C7 and C8) connected with 5V power supply.
since only have 1 IC(4 build-in op-amp) is used, so do i need to combine them(ect using 1 bias voltage for both circuit and for the capacitor that use to block the high frequency in power supply for both circuit, using either 1 of capcitor? ect if i choose C7 and C8, then the C12 no need to use while C11 and R14 remain the same in pre-amp? )?

For the IC, i have used only 3 op-amp out of 4, so the 1 that no use , do i need to ground them? or just leave it?
Thanks!

 

You did not couple the earphone to the mic on your computer so it sounds awful.
It sounds severely distorted and has no low frequencies. Can't you hear what I hear?

With one quad opamp, use C7 and C8 to bypass the 5V supply. C12 is not needed.
C11 and R14 are used in the preamp because it has gain. C11 and R14 should not be used for the tone controls circuit because it has C14.