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
Yes, I've plugged into different plugs into my house but its all the same. The noise is there. Surely is the DC adaptor.And try connecting the DC adapter to a different wall connector, preferably in another room.
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.Yes, I've plugged into different plugs into my house but its all the same. The noise is there. Surely is the DC adaptor.
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.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
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.
I think your power supply is defective or has very poor quality. Use a better 12VDC/2A that is not a switching type.Replace it with what? what type of power adapter, what value, etc?
No. The two bridged LM386 amplifiers are overloaded so their output power is low and their heating is much too high.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. 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.2) So, you say that there is a better way to take advantage of the two LM386? What is the way?[/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.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]
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.
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