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[SOLVED] Guitar Amplifier noise when a DC adaptor used.

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Removing noise from the power supply should not and must not affect the sound being amplified.

If the capacitor across the DC jack does not help, then the hum is being picked up from air. First of all, you need to figure out whether the hum is 50 or 100 Hz. Compare with a pure tone generator like https://www.szynalski.com/tone-generator/. If the mains frequency is 60Hz, then listen for that.

while running on battery, if you bring the adapter (powered on but output not connected to the amplifier) power connector close to the signal input, do you get any hum pickup? If you touch the signal input with your finger, how much hum it picks up?

The capacitor didn't work. I must say that the noise or hum its much more higher than 100Hz. The sound is similar when you are not conected to ground.
 

The 100Hz sawtooth waveform hum or buzz heard in the video in post #33 (50Hz that is fullwave rectified by the power supply) has all harmonic frequencies.
Before I wrongly said the waveform was a triangle. That kind of sawtooth waveform hum or buzz is heard if you touch only the live audio wire at the input of an amplifier without touching its ground.
 
Removing noise from the power supply should not and must not affect the sound being amplified.

If the capacitor across the DC jack does not help, then the hum is being picked up from air. First of all, you need to figure out whether the hum is 50 or 100 Hz. Compare with a pure tone generator like https://www.szynalski.com/tone-generator/. If the mains frequency is 60Hz, then listen for that.

while running on battery, if you bring the adapter (powered on but output not connected to the amplifier) power connector close to the signal input, do you get any hum pickup? If you touch the signal input with your finger, how much hum it picks up?

Here is a video for you to hear it. Audioguru said that it is 100Hz. See how I get near the "Bass" potentiometer the is a hum increasing. I went to my local electronics supply store and told them about my problem with the DC power supply and they assure me that the problem is the power supply, that I would need a specific power supply that is and "old" one and you can't get them anymore in this part of the world because of european regulations (I'm living in Spain)
 

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I have never seen an audio product with the very cheap and small "modern" power supply you have. I think the power supply grounds your circuit making all the unshielded audio wires to/from all the pots pickup hum from you when you get near it.
 

The capacitor didn't work. I must say that the noise or hum its much more higher than 100Hz. The sound is similar when you are not conected to ground.

You are right. As I am rather tone deaf, I extracted the audio from the video you have given and took a look at the audio using audacity (frequency plot). This is stray noise and large filter capacitor at the DC jack will not work.

The noise you are hearing is getting affected by the presence of your body capacitance. This suggests a ground problem - noise coming via the power line is getting a return path and going into the amplifier.
--- Updated ---

Took another look at the circuit diagram.

The PCB looks compact and nicely laid out. I am not hopeful about the bypass capacitors across the op-amp 1 and 2 (because the offending frequencies are in the audio range). So we need to focus on shielding the whole setup.

You put the whole amplifier in a Faraday cage. Wrap them in small paper box and then wrap 2-3 layers of aluminum foil. Also connect the metal bodies of the potentiometers and the metal plate the connectors are mounted. Use shielded cable to connect the potentiometers and connect the shield to the ground. Or, in a pinch, just wrap the wires connecting to the pots with Al foil.

If that seems too much of a hassle, just put the amplifier in a metal box (with a lid) and see the noise getting reduced.
 
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I also used Audacity to show a frequency plot of the noise in the video and it is mostly 50Hz with with much lower level even and odd harmonics something like a sawtooth waveform. Stray capacitive electricity pickup by all the unshielded wires and metal cases on the pots.

Since the pots are connected with ordinary unshielded wires away from the pcb instead of mounted directly on the pcb with no wires then I agree that
the entire project should be mounted inside a metal box connected to the circuit ground.
 

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    guitar amplifier noise.png
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I also used Audacity to show a frequency plot of the noise in the video and it is mostly 50Hz with with much lower level even and odd harmonics something like a sawtooth waveform. Stray capacitive electricity pickup by all the unshielded wires and metal cases on the pots.

Since the pots are connected with ordinary unshielded wires away from the pcb instead of mounted directly on the pcb with no wires then I agree that
the entire project should be mounted inside a metal box connected to the circuit ground.
Yes, I should try that, but still I think it would be noisy if I use the same power supply that I'd tested with.
 

Hello everyone.

Since I can't find a metal enclosure that fits the circuit board, I was thinking on going the a local aluminium store and ask for a few thin scraps to see if I can bend them into a box for the circuit board as well for the controls. Don't know if that would solve the issue, but I'll try.

I also want to try the 5M resistor with the 47pF capacitor to see if that works out. Should I put it en the DC input? or where is the best place to put it?

Thanks!
 

Interesting that the noise finally reveals as 50 Hz hum (+ harmonics). Review the distiction I tried in post #14. I presume, you did not yet try an earth connection of your circuit ground to cut the power supply common mode voltage (to water pipe or wall outlet protective earth contact)?
 

What do you want a 5M and 47pF filter to do? With your amplifier the 5M resistor value is way too high and the 47pF capacitor value is way too low.
If your amplifier had a 50M input resistance (your amplifier's mic input resistance is probably a few k ohms) then it would gradually cut all high frequencies above 680Hz (lowpass filter) or gradually cut all low frequencies below 680Hz (highpass filter) or block the DC from the power supply.
 
Hi,

Hello everyone.

Since I can't find a metal enclosure that fits the circuit board, I was thinking on going the a local aluminium store and ask for a few thin scraps to see if I can bend them into a box for the circuit board as well for the controls. Don't know if that would solve the issue, but I'll try.

I also want to try the 5M resistor with the 47pF capacitor to see if that works out. Should I put it en the DC input? or where is the best place to put it?

Thanks!


Abandoned modern TVs (and other consumer good, presumably) usually have pre-formed small metal shielding 'boxes' dotted around the PCBs, they (along with other things) are worth salvaging for other projects if they would be too small for this one you are working on.

Where did the 5M, 47pF values come from? That's 1/RC = 4,255 Hz cut-off frequency. Is that the attenuation frequency you wanted? Also, looking at the values used, just as easy to use very common 2,200 Ohms and 100nF for 4.545 kHz, or 4.7KOhms and 47nF for 4.526kHz.
 

Where did the 5M, 47pF values come from? That's 1/RC = 4,255 Hz cut-off frequency. Is that the attenuation frequency you wanted? Also, looking at the values used, just as easy to use very common 2,200 Ohms and 100nF for 4.545 kHz, or 4.7KOhms and 47nF for 4.526kHz.
Your calculations have errors:
1) The cutoff frequency of 5M and 47pF is 1 divided by (2 x pi x 5M x 47(u x u)= 677Hz if you can find an amplifier with an input resistance of 50M.
Stray capacitance will reduce the frequency to about 620Hz.

2) 2.2k and 100nF produce a cutoff frequency of 723Hz.

3) 4.7k and 47nF produce a cutoff frequency of 721Hz.
 
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Hi,

Your calculations have errors:
1) The cutoff frequency of 5M and 47pF is 1 divided by (2 x pi x 5M x 47(u x u)= 677Hz if you can find an amplifier with an input resistance of 50M.
Stray capacitance will reduce the frequency to about 620Hz.

2) 2.2k and 100nF produce a cutoff frequency of 723Hz.

3) 4.7k and 47nF produce a cutoff frequency of 721Hz.

I'm half-asleep and a lot on my mind these days, silly mistake, sorry about that and thanks for pointing it out.
 

Hello everyone!

Here are some updates with the guitar amp:

I tried an aluminium housing for the whole circuit, potentiometer and control plate but it does nothing, maybe just reducing the hum just a tiny bit, but still, nothing. The only time that the hum reduces drastically is when I touch the countrol plate where the plug, the LED and the DC input are mounted.

So, the grounding goes like this:

From the PCB to:
- Aluminium plate below the PCB
- Aluminium plate where the guitar jack, LED and DC jack are mounted
- Aluminium housing that covers all the components

The result: Nothing happens.

I've tried with the cheap power supply and with a higher quality one and the results are the same. I've been offer a DC power supply with noise free that "commutes" at 440kHz (sorry, don't know if in english that term is used) and don't interfere with the signal.

The power supply have a regulated interference filter. So, I'm going to try that one to see if it helps.
 

Hi,

I recommend re-reading post #69 on this page and thinking about why it's possible that only your touching the ground plate reduces the hum... You may not be a great conductor to earth compared to a wire or a metal pipe, but you are grounded to the earth...
 

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.

The hum is still there and at the same level with or without the guitar cable, so it must be the DC adaptor.
 

Hi,

I recommend re-reading post #69 on this page and thinking about why it's possible that only your touching the ground plate reduces the hum... You may not be a great conductor to earth compared to a wire or a metal pipe, but you are grounded to the earth...
Hi!

Interesting that the noise finally reveals as 50 Hz hum (+ harmonics). Review the distiction I tried in post #14. I presume, you did not yet try an earth connection of your circuit ground to cut the power supply common mode voltage (to water pipe or wall outlet protective earth contact)?
Interesting that the noise finally reveals as 50 Hz hum (+ harmonics). Review the distiction I tried in post #14. I presume, you did not yet try an earth connection of your circuit ground to cut the power supply common mode voltage (to water pipe or wall outlet protective earth contact)?


Hello!

About the post #14, the hum is still there with or without the guitar cable plugged in. I don't quite understand what you mean in this post about the earth connection, . I've try star point ground connection, nothing, I've tried groundin every pot, nothing, I've tried put an aluminium housing on top of all the components, nothing.
 

Hi there,

Try POST #69. 69. NOT #14, #69, on this page.

He means connecting a wire from the star ground on your circuit to anything earthed in your house - typically a water pipe (they should be connected to earth ground of the house by law to prevent any risk of electrocution of house-dwellers in case of loose wires, etc.) or if you are comfortable with electrical work, to the earth connection in a wall socket.
 
Hi there,

Try POST #69. 69. NOT #14, #69, on this page.

He means connecting a wire from the star ground on your circuit to anything earthed in your house - typically a water pipe (they should be connected to earth ground of the house by law to prevent any risk of electrocution of house-dwellers in case of loose wires, etc.) or if you are comfortable with electrical work, to the earth connection in a wall socket.

Yes, I know the post 69, but in that post he was also talking about the 14 so I added that answer. I'll try to see if I can connect a cable to a pipe, but I'm not quite confortable with the idea! haha.
 
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