Practice guitar amplifier humming.

Thread Starter

Yami

Joined Jan 18, 2016
332
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.
Photo1.JPG
Thanks in advance.
 

Thread Starter

Yami

Joined Jan 18, 2016
332
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.

Photo2.JPG
 

Thread Starter

Yami

Joined Jan 18, 2016
332
@Dodgydave thanks, I haven't got access to an oscilloscope at the moment. Its at work. Yes I do have a 12V power supply - is it to power the op amp circuitry? The main power brick is 19V haven't got spare one of those.
I'd like to ask what would I see (scope) on the supply pin if there was a fault present?
Thanks
 

Thread Starter

Yami

Joined Jan 18, 2016
332
Also I want to draw your attention to the buzzing I got in the speakers when touching pin 1 with the DMM probe. What would cause that?
 

Thread Starter

Yami

Joined Jan 18, 2016
332
So just to give an update, I have gotten further from actually solving it I guess :/
I was able to borrow a 19V power brick from a friend - didn't help with the humming, it looked very similar to the one I had - meaning two prongs and no ferrite bead near the jack. I don't know why but users have been reporting that three prongs power bricks were getting rid of this hum. Its still a SMPS.
With regards to the pre-amp section I removed R40 to take the first half of the op-amp out of circuit. No difference to the humming.
The 12V power brick I tried didn't help.

Can't think of what else to look for.
 

ci139

Joined Jul 11, 2016
1,696
it's not that expensive ... the bare chance of destroying a delicate device without the precaution is 50% ...100% ... the idea is not to populate yet unknown fault to more alike ... just about the time

? did it hum from the beginning or did it appeared ... say after plugging the device in with max. vol. or after sudden turn-up of the vol. button ? - - - would indicate the mal-designed soft-start circuitry or miss-estimation of the tolerance ranges for some critical components

a trivia -- ? does the hum gets significantly more audible starting from certain volume level --- would indicate a non-linear pos.-fbk chain (? a possible design flaw of)

the first tings last : linear reg.-ed supply from 24v ? automotive PbSo4 (-s) to verify it's not the relict of the SMPS
 
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