Buzz generated by the PSU

[Tazio]

Hello everyone.
I'm working on my first DYI hifi amp.
A "simple" assembly of pre-assembled boards...let's say a school prototype with no pretensions other than to sound decent. The result isn't top, but I learned many lessons that I hope to apply at best way in V2.
However, there is a very annoying problem for which I would like the valuable opinion: the PSU, I know it is not the best, has two 12v auxiliary outputs. I needed one to power the LEDs on the front panel. Well, if I connect even a single LED to one of the outputs, a constant buzz is generated regardless of the volume setting. With the 12v contacts empty, nothing. I also noticed that if I touch one of the two contacts (under load) with a multimeter tip, the buzz fades out by half or so. The case is made out of wood with no grounding.
Can you help me remove the problem?

 

[Doodski]

That appears to be a great learning project. I would be very careful with that wood case though. Fire hazard.

 

[Tazio]

Thank you mate!
I opted for a DIY wooden box
not to spoil and wreck an expensive aluminum one as I knew I'd make mistakes and reworkings: in fact the V2 will be pre + power amp, alu cases, with all the controls into the pre case, which I learned is way easier.
On the other hand the ampli board is a quite powerful 3e Audio 260-2-29A based on TPA3255 capable of 180 W over 8 ohm and so far its heat sink plate never passed 45°C ...

 

[Doodski]

Thank you mate!
I opted for a DIY wooden box
not to spoil and wreck an expensive aluminum one as I knew I'd make mistakes and reworkings: in fact the V2 will be pre + power amp, alu cases, with all the controls into the pre case, which I learned is way easier.
On the other hand the ampli board is a quite powerful 3e Audio 260-2-29A based on TPA3255 capable of 180 W over 8 ohm and so far its heat sink plate never passed 45°C ...

It's not the operating temp one needs to be concerned about. It is a catastrophe that one needs to be aware of. A blown amp/power supply causing the case to ignite. As long as you are aware of this... Get a metal case...

 

[ta240]

I've built a few amps in wood cases. I treated them with a fireproofing spray just to be on the safe side. I've hit the treated wood with a mapp gas torch years later and it charred the wood but no flame.

 

[Tazio]

Guys, I appreciate your suggestions and I'll treasure them although, as already mentioned, the wooden case is temporary.
Anyhow, the real point is the buzz jumping out as I put any load onto one of the PSU's auxiliary extits ...

 

[somebodyelse]

On the other hand the ampli board is a quite powerful 3e Audio 260-2-29A based on TPA3255 capable of 180 W over 8 ohm and so far its heat sink plate never passed 45°C ...

It's meant to be a heat spreader not a heatsink. You're meant to use something with more surface area as the heatsink if you want to sustain more than a fraction of the full output. Having said that, if it's not getting too hot then it may be enough for your application. It should shut down if it gets too hot.

The buzz makes me suspect an issue with your ground connections. It's not clear from the photo exactly what is connected to what, let alone whether the PSU 12V and main outputs share a ground or are separate floating outputs. Could you at least provide a block diagram of exactly what connects to what?

 

[Tazio]

It's meant to be a heat spreader not a heatsink. You're meant to use something with more surface area as the heatsink if you want to sustain more than a fraction of the full output. Having said that, if it's not getting too hot then it may be enough for your application. It should shut down if it gets too hot.

The buzz makes me suspect an issue with your ground connections. It's not clear from the photo exactly what is connected to what, let alone whether the PSU 12V and main outputs share a ground or are separate floating outputs. Could you at least provide a block diagram of exactly what connects to what?

That's correct, it's a heat spreader, and I'm not pushing the ampli a lot. As mentioned this project is meant to be a training and as such with limited budget. I have already identified a casing with finned heat sink for the next version.
About the PCB, unfortunately I have no diagram. This is the link where I procured it, with a summary description, unsure whether it may help. https://www.aliexpress.us/item/3256...533a36966we1fz&gatewayAdapt=glo2usa4itemAdapt

 

[somebodyelse]

We don't need a full schematic for the PCBs, just a block diagram showing what all the major bits are and how you've connected them with wires. I'm talking about amp board, PSU, VU meter, inputs, volume pot, power switch, LEDs etc. Links to datasheets might be useful too - I guess it's http://www.3e-audio.com/wp-content/uploads/2024/09/DS_EAUMT-0xx0-x-FB_Rev1.3.pdf for the amp board, but no idea for the PSU.

 

[Tazio]

We don't need a full schematic for the PCBs, just a block diagram showing what all the major bits are and how you've connected them with wires. I'm talking about amp board, PSU, VU meter, inputs, volume pot, power switch, LEDs etc. Links to datasheets might be useful too - I guess it's http://www.3e-audio.com/wp-content/uploads/2024/09/DS_EAUMT-0xx0-x-FB_Rev1.3.pdf for the amp board, but no idea for the PSU.

Hope the sketch attached may do. I haven't used any ground terminal because of the wooden case, and I'm unsure whether connecting the PSU outputs to the power supply grounding is permitted ...

 

[Tazio]

Well, I tried connecting the two 0 points of both the output rails to the power supply grounding.
The buzzing has dampened dramatically, didn't fade out completely though.
Perhaps, this residual noise is caused by some spurious frequency captured from the power supply? Do I need a filter?

 

[somebodyelse]

On the face of it there doesn't seem much wrong with the block diagram. It's odd that connecting the LEDs makes a difference as they're nominally separate. Are you sure there is no inadvertent connection, maybe in the VU meter? Maybe try one LED at a time to narrow down where the problem is?

Buzzing is usually a multiple of mains frequency, so 50Hz, 100Hz etc. or 60Hz, 120Hz etc. depending in where you are. It's usually indicative of a grounding problem where line level inputs are concerned. SMPS frequencies are typically outside the audible range, but odd interactions are possible although rare. Are you testing with an audio source connected, with the input grounded, or just with the input floating disconnected? I'm guessing it's single ended RCA inputs as I'd expect to see the back of the connector clearly for XLR or TRS.

 

[antcollinet]

The case is made out of wood with no grounding.

Well that is never going to help.

Well, if I connect even a single LED to one of the outputs, a constant buzz is generated regardless of the volume setting

What are you using to limit the current in the LED?

 

[Tazio]

On the face of it there doesn't seem much wrong with the block diagram. It's odd that connecting the LEDs makes a difference as they're nominally separate. Are you sure there is no inadvertent connection, maybe in the VU meter? Maybe try one LED at a time to narrow down where the problem is?

Buzzing is usually a multiple of mains frequency, so 50Hz, 100Hz etc. or 60Hz, 120Hz etc. depending in where you are. It's usually indicative of a grounding problem where line level inputs are concerned. SMPS frequencies are typically outside the audible range, but odd interactions are possible although rare. Are you testing with an audio source connected, with the input grounded, or just with the input floating disconnected? I'm guessing it's single ended RCA inputs as I'd expect to see the back of the connector clearly for XLR or TRS.

Thank you for your hints. I checked the connections several times and also tried to connect every LED one at a time with no effect.
The source is a LDAC. Signal connections are SE.
The noise is present even with source disconnected.
Tried also a filtered mains input IEC connector with no improvement.

 

[Tazio]

Well that is never going to help.



What are you using to limit the current in the LED?

1k resistor. From an online calculator I found a resistor value of 680 ohm which I haven't. Also 1k gives dampened LED brightness which I like better. The Vu-meter LED has own 2k trimmer to adjust brightness.

 

[Tazio]

Do you guys think I'd rather try a better quality PSU?
The ampli board accepts max 48 Vdc mono, any hint on what make I may look for?

 

[somebodyelse]

Hard to say - we still know nothing about the existing supply.

 

[Tazio]

Hard to say - we still know nothing about the existing supply.

I'm afraid I've no details of the PSU ...it's cheap chinese stuff ...

 

[Mojo Warrior]

I had a similar problem that was solved by wrapping the power supply in aluminum foil. YMMV

 

[Punter]

Firstly, switchmode power supplies are noisy by nature as they operate by using a transistor to "chop" the mains frequency in such a way that the resultant square wave can have a mark/space ratio that allows the waveform to be filtered and rectified to a DC output that is a lower voltage than the input. The first thing that I would check is that the auxiliary DC output shares a ground with the main output. If this is not the case and they are separate, this might account for the buzz. I see the supply also has an "independent" 12V supply which might be preferable to use. Alternatively, run the LED's off the main supply with appropriate current limiting resistors.