Guitar amp kit (problem with noise and humming)

[bzblues]

Hello everyone.

I've built 3 of these kit amps called "Litlle Rebel" from Musikding. So far and I always have the same problem.

The section of the bass potentiometer in the circuit board acts strange. When I turn the pot down to "0", there is a loud hum, like its not grounded, WICH IT IS! I had the exact same problem in the previous two.

Also, when I connect the DC power supply, the humming noise appears very loud, even more when I turn the bass potentiometer. When the amp is using the 9V battery, the noise reduces drastically, but its there in the background. I'll attach a video for you to see what its going on: https://www.youtube.com/watch?v=7n-21zF ... nZditowski

I've tried to do the start point grounding, but nothing, I grounded to the PCB aluminium plate below and to the plate in the control panel and nothing. I built this enclosure of aluminium but NOTHING, absolutely nothing seems to resolve this issue. I've also tried different DC power supplys and NOTHING. I'll attach schemantics and wiring for you to see.

Please, if anyone can help, its driving me crazy not beeing able to fix it. And believe me, I've had checked and re-checked every solder joint, every component, everything is in its right place!

[The Dude]

......The section of the bass potentiometer in the circuit board acts strange. When I turn the pot down to "0", there is a loud hum, like its not grounded, WICH IT IS! I had the exact same problem in the previous two.......

The bass pot controls feedback in the IC2a circuit. I don't see why or where any pot connection would be grounded. The pot is used as a single variable resistor controlling the amount of frequency dependent feedback. Can you figure out where it's getting a ground path? Did you maybe ground the unused lug?

[Jazz P Bass]

^^^^^^^ That.

[bzblues]

I've been getting practically the same answer about the grounding of the pot. I can see that in the bass pot part of the PCB there is no connection to anywhere in the 3rd lug (looking from the back of the pot from left to right). The rest of the pots have a connection for this lug in each one of them. You say that the pots should no be grounded? (sorry, I'm super very much new a this and its all a bit confusing)



Thank you!

[Jazz P Bass]

"You say that the pots should no be grounded? "
The only control that is at ground potential is the Master/ Pin #1.

[g1]

I would try floating your input jack and DC adapter jack (remove from metal panel). The rebelwire.pdf shows them both to be plastic insulated types that would not connect to the chassis.
Post some pictures of your actual build if you can.

[The Dude]

The input jack and DC adapter jack do show that they are both grounded on the schematic. HOWEVER, the speaker output is not. They are running two LM386 for the output using + input for one IC and - input for the other so the IC outputs are out of phase effectively "bridging" the two IC's. Make sure your speaker jack is not chassis grounded. It will need to be an isolation jack if mounted on a conductive panel with other grounded parts. Or, use isolation washers.

[g1]

The input jack and DC adapter jack do show that they are both grounded on the schematic. HOWEVER, the speaker output is not.

They are grounded by wires to the board according to the wiring info. And it shows insulated jacks. In his video it appears the jacks are metal (at least the input jack).
So I was thinking about ground loops with the extra chassis grounding due to non-insulated jacks.

[bzblues]

I not quite following what you guys are sujesting about the jack. It is a metal jack, and its in contact with the aluminium panel, isn't that the way you ground the components? (again, I'm very new in this)

I don't know why am I not alowed to post pictures or files in this forum check the video in the first post, you can see all the details there.

[Jon Snell]

If you do not fault find, as I suggested in the TDPRI forum, (you post verbatim in this forum), you will not find the fault.
Isolate a bit at a time working from the output to the input. Then you will find where the issue is and fix it.
We don't have a magic wand and my crystal ball is fogged up.
Let us know your findings and we will be in a better position to help you.

[bzblues]

I've been told that maybe I should put a Filter but I have no idea what kind of filter. Maybe this one?

https://es.aliexpress.com/item/32949....108c5f70iYCIU B&algo_pvid=c94d55bb-2f45-47c5-b057-6aa77a0a28c0&algo_expid=c94d55bb-2f45-47c5-b057-6aa77a0a28c0-0&btsid=0b0a0ae216046091806486715ebc6b&ws_ab_test=sear ch web0_0,searchweb201602_,searchweb201603_

[The Dude]

I not quite following what you guys are sujesting about the jack. It is a metal jack, and its in contact with the aluminium panel, isn't that the way you ground the components? (again, I'm very new in this)

I don't know why am I not alowed to post pictures or files in this forum check the video in the first post, you can see all the details there.

Yes, a metal (non-isolated) jack installed in a grounded panel will ground the jack. The problem is that the circuit does not always want the jack grounded. The schematic shows you what's supposed to be grounded and what's not.
I believe there is a certain number of posts you have to make in order for the forum to let you post pictures. I'm not sure how many.
I don't see why you'd need a filter. The amp should work as designed if built correctly.

[nickb]

Two things jump out at me looking at the layout that might mislead the unwary.

The input jack wiring should ideally be a screened cable or at least short and tightly twisted. A similar problem exists with the DC power jack wiring as the red and black wires should be twisted all the way from the DC jack to the board. If you have open loops they act like a single turn of a transformer and so can pick up and radiate hum ( and noise in general).