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Guitar Amplifier noise when a DC adaptor used.

bzblues

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Hello!

I've build this guitar amplifier kit. You can run it on a 9V battery or plug it with a DC adaptor. I'm using a 9V 1A (or 2A) adaptor but the problem is that when used, it makes a humming noise, like its not grounded, wich it is! and its very loud, but when I use tha bettery the hum dessapears of course.

Should I use another DC adaptor value? I'll attach the schemantis of the amp to see if its ussefull.

Thanks!
 

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Carry for cents bazar

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I will look through the schematic if I have time. Normally this happens because there is a noise which is amplified with the audio signal. The noise is present even when the audio signal is not. You have to see what kind of noise it is and remove it. Your router should be with 9V too, it safe to plug it in the amplifier if it fits.
 

bzblues

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By "Router" what do you mean?

Yes, the noise is present even when the guitar is not plugged in. The hum is like when something its not connected to ground, but the circuit is connected to an aluminium plate.
 

KlausST

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Hi,

what´s the root cause?
* The battery? No.
* the circuit? No, because it is working well with the battery.
* the DC adapter? Yes, because only when this is involved it causes the problem.

--> thus it´s urgent to show what exact type of DC adapter to use.
--> and for sure it´s the first thing that shloud be replaced... a try won´t hurt.

*****
Hum may have various reasons:
* Hum on the power supply voltage ... when DC voltage is not regulated or not properly decoupled
* or hum generated by a Ground loop.
* capacitive or inductive coupling
* bad circuit
* bad PCB layout

The schematc doesn´t look bad. A hum critical path is V1/2 which is generated by R29, R30 and C23.
But it seems to be properly bypassed and should be clean.

Klaus
 

bzblues

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Hi,

what´s the root cause?
* The battery? No.
* the circuit? No, because it is working well with the battery.
* the DC adapter? Yes, because only when this is involved it causes the problem.

--> thus it´s urgent to show what exact type of DC adapter to use.
--> and for sure it´s the first thing that shloud be replaced... a try won´t hurt.

*****
Hum may have various reasons:
* Hum on the power supply voltage ... when DC voltage is not regulated or not properly decoupled
* or hum generated by a Ground loop.
* capacitive or inductive coupling
* bad circuit
* bad PCB layout

The schematc doesn´t look bad. A hum critical path is V1/2 which is generated by R29, R30 and C23.
But it seems to be properly bypassed and should be clean.

Klaus
Hi Klaus! very clear answer! thank you! Yes, I think it could be the DC adaptor. How can I know what kind of adaptor should I use with this kind of amp? I'm using this adaptor (picture attached)
 

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KlausST

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Added:
From what you write, it seems the DC power supply causes the noise, but the circuit does not suppress it sufficently.
* I doubt there is a true GND plane at the PCB.
* I miss fast decoupling capacitors and power input filters

Your picture shows a switch mode power supply without Earth ground.
Either try a high quality one. (medical power supply compliant)
Or use a filter: common mode and differential mode at the 9V DC.
Maybe even a base load (try 50mA --> 180..220 Ohm, at least rated with 1W, better 3W to keep temperature low) to prevent the SMPS to operate in burst mode.

Klaus
 
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bzblues

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And try connecting the DC adapter to a different wall connector, preferably in another room.
Yes, I've plugged into different plugs into my house but its all the same. The noise is there. Surely is the DC adaptor.
 

Carry for cents bazar

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Yes, I've plugged into different plugs into my house but its all the same. The noise is there. Surely is the DC adaptor.
Surely no, likely yes. If possible try to filter the noise from the source with a middle device. A noise filter with the same jack on both sides(don't forget to put the male and female on the correct ends). This way you should be able to use all adapters. Of course we are guessing and until you solve it, we will not be certain.
 

bzblues

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Added:
From what you write, it seems the DC power supply causes the noise, but the circuit does not suppress it sufficently.
* I doubt there is a true GND plane at the PCB.
* I miss fast decoupling capacitors and power input filters

Your picture shows a switch mode power supply without Earth ground.
Either try a high quality one. (medical power supply compliant)
Or use a filter: common mode and differential mode at the 9V DC.
Maybe even a base load (try 50mA --> 180..220 Ohm, at least rated with 1W, better 3W to keep temperature low) to prevent the SMPS to operate in burst mode.

Klaus
I must tell that I'm very new in electronics, still don't know how to read schemantics and I'm learning the basics so I got very lost... I'll try another DC adaptor, this one is cheap so I was expecting some kind of trouble.
 

Carry for cents bazar

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I don't want to create doubt or mix the knowledge in your head, do as you intended before my last post.
 

bzblues

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No problem! I'm learning so much from this forum! so its cool!

I've would have to make a noise filter but I have a few doubts. Don't know how to do it (I'm doing my research) and don't know where to puit (I believe that when I find how to do it, te question will answer itself)

If you can help with that, I'll be gratefull!

Thanks!
 

FvM

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There are essentially two ways how hum can be injected to your circuit
- the adaptor doesn't produce clean DC but has some 100 Hz ripple superimposed
- an adaptor without earthing couples 50 Hz mains voltage to the floating circuit ground. Due to insufficient shielding of circuit, cable or guitar, the sensitive input picks up hum. A clear indicator for this problem would be that the hum reduces or even disappears when you unplug the guitar cable.

In the former case, the power supply needs additional filtering. In the latter case, try an additional earth connection of the circuit ground.
 

Audioguru

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Of course the power supply is making the hum. Replace it.

The output stage of the circuit is completely wrong.
1) One LM386 works well when it has a 9V supply and is driving an 8 ohm speaker. Then its output power is 0.56W and its heating is also 0.56W.
Its datasheet shows that with a 12V supply and driving a 4 ohm speaker its output power is only 0.38W but its heating is its max allowed of 1.2W!
With a 12V supply and a 16 ohm speaker its output power is 0.78W and its heating is 0.85W.

2) Here the output stage uses two LM386 bridged amplifiers then its output current is trying to be doubled and its heating is trying to be 4 times, but with less output power than with only one LM386 amplifier.

The value of coupling capacitor C21 is so low at only 10nF that it cuts all low frequencies below 640Hz.
 

Easy peasy

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switch mode type wall warts make a lot of common mode noise - this often affects things they are powering - either a wall wart with a 60Hz transformer - or a very large CM inductance on the output
 

bzblues

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Of course the power supply is making the hum. Replace it.

The output stage of the circuit is completely wrong.
1) One LM386 works well when it has a 9V supply and is driving an 8 ohm speaker. Then its output power is 0.56W and its heating is also 0.56W.
Its datasheet shows that with a 12V supply and driving a 4 ohm speaker its output power is only 0.38W but its heating is its max allowed of 1.2W!
With a 12V supply and a 16 ohm speaker its output power is 0.78W and its heating is 0.85W.

2) Here the output stage uses two LM386 bridged amplifiers then its output current is trying to be doubled and its heating is trying to be 4 times, but with less output power than with only one LM386 amplifier.

The value of coupling capacitor C21 is so low at only 10nF that it cuts all low frequencies below 640Hz.
Hello!

Ok. I'll ask you a few things about this post:

"Of course the power supply is making the hum. Replace it." Replace it with what? what type of power adapter, what value, etc?

1) So, if I understood correctly, the power and heating must match? or one must be higher than the other? I'm using a 6ohm speaker. What should I do to get the best of it?

2) So, you say that there is a better way to take advantage of the two LM386? What is the way?

About the C21 capacitor. Should I change it so the cutting freq. is better and more suitable for the amp? Wich capacitor should I use?

THANKS.
 

FvM

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I agree with Audioguru that LM386 bridge circuit is useless to drive a 4 ohm speaker. I'm not sure if the low frequency cut-off is a design flaw or rather intentional to get a certain loudness with the low power amp. E string fundamental frequency is 82 Hz, by the way. I see other circuit parts with similar cut-off frequency, e.g. R4/C5 creating a similar high-pass edge at higher gain settings.

Altough discussion of output power and low frequency cut-off is interesting, it's mostly unrelated to the hum problem.
 

Audioguru

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Replace it with what? what type of power adapter, what value, etc?
I think your power supply is defective or has very poor quality. Use a better 12VDC/2A that is not a switching type.

1) So, if I understood correctly, the power and heating must match? or one must be higher than the other? I'm using a 6ohm speaker. What should I do to get the best of it?
No. The two bridged LM386 amplifiers are overloaded so their output power is low and their heating is much too high.
Instead, use one channel of a TDA2004 amplifier. With a small heatsink its output power into your 6 ohm speaker will be about 1.5W to 2W.

2) So, you say that there is a better way to take advantage of the two LM386? What is the way?[/B]
No. An LM386 is used by itself with a 9V supply (close to the minimum allowed voltage for the TL081 opamps used). It is never bridged with two of them, because bridging them overloads them much more.

About the C21 capacitor. Should I change it so the cutting freq. is better and more suitable for the amp? Which capacitor should I use?[/B]
Select the cutoff frequency you want (maybe 80Hz for a guitar) then calculate its value for the input impedance of the amplifier it feeds.

I agree that the value of C5 is also too low and cuts low frequencies. Also, it is silly to have C6 and C7 in series.
 

bzblues

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I think your power supply is defective or has very poor quality. Use a better 12VDC/2A that is not a switching type.


No. The two bridged LM386 amplifiers are overloaded so their output power is low and their heating is much too high.
Instead, use one channel of a TDA2004 amplifier. With a small heatsink its output power into your 6 ohm speaker will be about 1.5W to 2W.


No. An LM386 is used by itself with a 9V supply (close to the minimum allowed voltage for the TL081 opamps used). It is never bridged with two of them, because bridging them overloads them much more.


Select the cutoff frequency you want (maybe 80Hz for a guitar) then calculate its value for the input impedance of the amplifier it feeds.

I agree that the value of C5 is also too low and cuts low frequencies. Also, it is silly to have C6 and C7 in series.
Ok! I'll try to find another kind of adapter that is not a switching one. But still, I've been told there is a noise filter circuir for a switching power adapter that is fairly simple (so I've been told) any ideas?

Thanks!
 

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