Should I expect hum from TDA7294 power amp?

[Steve Blackdog]

Hi Folks

I have built a Chip-amp using a Tda 7294. If built properly should I expect hum at full volume?

I am getting loud hum when I plug the speaker in, with or without the pre-amp plugged in. Everything else is great, but I need to sort out the hum.

I have no volume pot on the chip-amp (just the pre) so it is always on full volume. Is that a problem?

If the answer is that it shouldn’t hum, then I will go in and check and test everything, but before I do, I want to know whether at full volume chip amps should be hum free.

I don’t have a schematic, but could draw one if necessary. I can also post some photos (as long as you promise not to laugh).

cheers

steve

[xtian]

No, it should not hum!

[Steve Blackdog]

No, it should not hum!

I thought not!

thank you. I will go back and see if I can find where I went wrong!

all the best

steve

[J M Fahey]

Does it hum with the input shorted?

[Steve Blackdog]

Does it hum with the input shorted?

Hi JM

Yes, it continues to hum, but the tone of the hum is higher.

Does it sound like an earthing issue to you?

I should add that the tranny is a toroid. You can hear it humming with nothing connected at input or output.

[Steve Blackdog]

This is my power supply. Is there anything here that could be causing hum?

[nosaj]

This is my power supply. Is there anything here that could be causing hum?

Doulblecheck your grounds reflow them if necessary, Do you have a scope?

nosaj

[Steve Blackdog]

Yes, I have a basic scope. Not sure what I am looking for. Will I be able to find where the hum is introduced?

[Mick Bailey]

Are you using a published design (say, from the application notes)? Also, it appears that you have two sources of hum - one that's coupled into the signal input and another that could be oscillation. Do you have the supply rails decoupled close to the pins?

AC leads need to be twisted and routed away from signal paths and you need to examine your grounding scheme layout - you could have a ground loop. I would scope the +/- supply pins at the IC to see if there's any noise there.

[Steve Blackdog]

Are you using a published design (say, from the application notes)? Also, it appears that you have two sources of hum - one that's coupled into the signal input and another that could be oscillation. Do you have the supply rails decoupled close to the pins?

AC leads need to be twisted and routed away from signal paths and you need to examine your grounding scheme layout - you could have a ground loop. I would scope the +/- supply pins at the IC to see if there's any noise there.

Hi Mick

I will had not twisted the leads from the tranny, which I will do and report back.

I am wondering if my chassis might be a contributing factor. For a bit of something to do, I used an old aluminium kettle as the chassis. I thought it shouldn't be a problem. But the design means the toroidal transformer sits pretty central with the pcbs on the walls, so to speak. It looks rather groovy, but if necessary, I could take the transformer outside the chassis.


I used this generic Pcb.
https://www.ebay.co.uk/itm/293967890...MAAOSw0CdgCu76

I think there is only one source of the hum. With a pre-amp plugged into the input the only buzz/hum is the same one that is there with nothing plugged in.

[Mick Bailey]

When you said the tone of the hum was higher, I took this to mean the frequency was higher - in which case there could be two sources.

[Steve Blackdog]

When you said the tone of the hum was higher, I took this to mean the frequency was higher - in which case there could be two sources.

Yes it was higher when I shorted the input. No change when I add pre-amp.

[Enzo]

Don't just guess. COuld it be this? Could it be that? Those don't get you there.

Define th problem, then isolate the problem. First and foremost: is the hum 60Hz or 120Hz? They sound similar to the ear. Your scope will help you there. MY shop tip is to touch the scope probe with your finger to put a 60Hz noise signal into the scope. Adjust the sweep rate for ONE complete cycle on the screen. Now leave the sweep where it is and scope the hum signal at the speaker. Is there one cycle or two across your screen?

120Hz is from power supply ripple. It could be insufficient filtering - use the scope to see the ripple on each supply. But it could also be improper ground wiring for those filter caps. We have no idea how you ran your grounds.

60Hz hum comes from other sources. Many sources. It could be lack of shielding, could be improper grounding, could be picking up radiated hum, and on and on. Isolate the problem. Is the hum in the preamp signal? Is it only in the power amp? DO ANY of the controls affect the sound of the hum in ANY way?

You think the transformer might be involved? Then move the board around to see if position changes anything.

[Steve Blackdog]

Thanks Enzo. Excellent advice.

Tomorrow I will get the scope out.

in the meantime I have done sme experiments with the power supply. I disconnected a/c supply / the transformer and rectifier stage. In place I connected two 9v batteries as follows: battery 1 +ve to v+ on pcb; -ve to battery 2 +ve and pcb ground. Battery 2 -ve to v. I still got hum. Shorting input stopped the hum.

I connected a short patch lead to input and found that the hum increased if I moved the other end Jack plug close tho wall sockets, the boiler programmer etc.

So it looks like the source of the hum/buzz is external inference - at least in part.

For completeness I attach the schematic for the pcb. I also attach a picture or two of the power amp/repurposed kettle.

Schematic_TDA7294-Russ-T2_2021-11-12.pdf


feel free to laugh at my kettle!

[Enzo]

Hah, I have a couple tin cans from Hubs Peanuts I have been itching to build a little Champ type amp in. Mine are the 32oz size.