Trying to fix a humming practice guitar amplifier

[Yamin]

Hi guys,
I have got a small guitar practice amp. Its powered from a typical 19V power brick. Its humming so bad. There are some known issues with these units and its has got something to do with grounding issues. There are couple of suggestions that people have made and have worked for them:
1. Connect a usb cable to the unit and the other end to a properly grounded laptop/computer.
2. Attach a separate cable from the chassis of the unit to the ground pin of the mains.
3. Change the power supply to a ‘three pronged’ power brick.
I didn’t have any issues with the amp out of the box and with in a weeks time the humming began. I used option 1 and connected the USB port to a well grounded computer. It did seem to help at first but then the hum came back. Right now the usb cable remedy is not working. I tried option 2 as well by alligator clipping the chassis to the mains earth and there is no improvement/change on the hum whats so ever. My mains out are properly grounded and I have confirmed this.
I am starting to think that maybe my unit has got a failing component somewhere as the problem kept progressively deteriorating to the point where it is now.
So I was hoping to get suggestions on what to look for.
I measured the resistances between the AGND, PGND and DGND (marked on the pcb). All seems to be well connected. However I did notice that the chassis of the USB port to PGND showed resistance value jumping around before it settled to 0ohms, I am assuming that its error on my side with my probes.
Also I have got a question about the 3rd option (‘three pronged’ power brick) - how does grounding help with noise etc with an SMPS type power brick. I see with SMPS schematics that the ground is not directly connected to anything on the SMPS but sometimes through a Y type cap to com. Is that connection doing the magic?

Anyway I was hoping for more suggestions on what to look for. I have got the unit opened.

Thanks in advance.

[Audiorepair]

Does it hum with no guitar connected?

[Yamin]

Yes it does. I did check the input connector and the tip does ground properly if no jack is inserted.

Also I just came across a hackaday.io post about the internals of the amplifier
https://hackaday.io/project/174163/logs

[Audiorepair]

What is this "hum"?

Is it a typical 50/60 Hz hum, or is it actually some kind of digital hash plus maybe some hum?


One mans hum is another mans switching noise.

[Yamin]

Umm, its not the typical 50Hz hum I would say, more harsh. One more interesting thing I would remark is that I heard this high pitch whistle sound which comes and goes now and then (this is on top of the constant hum). I have heard this before in a vox tube amp - haven't figured out what that noise is or come across anyone else describing this.

[bob91343]

This could be a case of oscillation of the amplifier.  You would need an oscilloscope to see.

You could also try a different power source to see if the problem lies therein.

[Audiorepair]

Run the amp from batteries if you can. 
2 x 9 volt PP3 will do for testing purposes.

See what the noise difference is.

[Yamin]

Right guys, so I have discovered something while having look at the pre amp section. For the preamp TP2582  opamp is used. I am measuring 12V at pin 1 which is output A - no guitar plug inserted and no input signal. +VS is 12V!. Differential voltage between the input pins are around 0V.  At first I thought that the ‘A’ section of the opamp was bypassed but it looks like the input signal does in fact go through here (would need to double check though). Also I’ll remark that when I touch this pin 1 with the DMM positive probe I can hear a buzzing sound from the main speakers.
Another thing I sort of discovered was that when I checked the point marked with pink with a poor man’s spectrum analyser that I made (basically a pocket radio with probes connected to the amplifier section an idea I borrowed from one of a tech here) I get the same ‘noise profile’ heard at the main speakers.

Could I be onto something? Any more ideas what to check for.

[Audiorepair]

If, as  you say, there is full supply voltage on the output of an op-amp that is supposed to amplify audio, then that op-amp is almost certainly faulty, or something around it is.

Op amps pretty much always fail with the output being either + Vpower supply or - Vpower supply.

I don't know why you are further speculating and pontificating, when there is clearly a problem that needs fixing first, and might be the cause of the symptoms.

[Yamin]

Yes true @Audiorepair. However I need to make sure that this bit of op amp is used. I have seen on another part of the board an op amp after the DAC one half of it is in use and the other half is bypassed like a voltage follower and its output is also ramped at the supply voltage which is 5V.
Plus the main reason is I don't have any spare for these types of smd op amps.

[Audiorepair]

Ah, should have said this before.

If a dual op-amp goes faulty, almost invariably both outputs will go the same full DC.
You won't get just one faulty side.

[Yamin]

Really? Both of the outputs should be ramped right? In this case only one output is ramped at VS.
But I do remember having seen one side of op amps failing(if I remember correctly ), this was sometime back though working on a fender bass amp.

[Audiorepair]

I doubt that.

Bye.