Noise issue with DIY variable PSU and audio amplifier

[Jolly 7]

I have recently put together a regulated power supply unit along with a low power audio amplifier in the same enclosure and am using the same transformer for both circuits. The PSU works well and so does the audio amplifier, as long as the circuit I'm trying to power is not being powered by the same transformer. If I use the same transformer, I am getting a loud noise, which is not there when I use an external power supply I am not sure if there is a solution for this problem and whether this is an expected problem. Can someone please advise how to get rid of the noise issue. Thanks in advance.

[Tim]

Hello Jolly7.
What kind of noise are you getting? I’m betting it’s a buzz or hum.

We’ll probably need to know how the circuits are connected to the transformers, along with the associated rectification arrangements.
I can see the secondary windings of your transformer are connected in series, but without knowing where the wires leading from them go to we can’t really help.

[Cruisin Marine]

Firstly, what sort of noise?
Is it hum?
What sort of regulator is it and what current is being drawn?
What is the capability of the tfmr?
And what is the amp and current being drawn?
I would guess some isolation on supply lines and extra decoupling at the destination points may help, as long as the tfmr isn't drooping under too much load of course.

Cheers.

[Jolly 7]

Here are some more pictures.

The 240V transformer has two secondaries, each 6v 3vA.

The regulator is an LM317T, which I soldered on to a small PCB myself as per the manufacturer's datasheet. The other two boards are from broken radios. The full wave rectifier board is from a Panasonic and seems to have a noise reduction choke. The audio amplifier board is from a Grundig and has its own full wave rectifier for the C1212 audio chip.

I am not sure of the current consumption of the audio amplifier circuit. The noise is a very loud hum that persists even when I switch off the amplifier power.

I am feeding a mono audio signal through the front. The ferrite ring I fitted to the audio input socket is great for reducing noise when connecting an external audio source powered by an external power supply, but has no beneficial effect if powered by the built-in transformer.

[joebog1]

Sounds to me like its coupling back through your earth and negative of the power supply. I am assuming of course that they are common to the single transfomer and rectifier, audio amp, and power supply.

You could fix that by using a transfomer on the input of the audio amp. It will need to be primary and secondary NOT an auto transfomer.

Joe

[Jolly 7]

Here are some pictures of the front and back panel. The 3.5mm socket on the left hand side of the front panel is for amplifier input.

[Silicon]

Are you familiar with the concept of 'star grounding' and 'ground loops'?

The website valvewizard.co.uk has a good explanation.

[Chrispy57]

Just an observation - in spite of the jolly-green wiring inside the unit I noticed that there is no external earth connection, just a figure of 8 mains input socket. Perhaps therein lies your problem?

Cheers
Chris

[Jolly 7]

Hi Chris
As I built this in a plastic enclosure, I didn't think about using an earthed mains socket.
However, thanks for the suggestion. After going through the ideas kindly suggested by other members, if I am still unable to cure the problem, I might try replacing the mains socket.
Cheers

[Chrispy57]

To test this theory you could just try connecting up an earth lead, perhaps from another mains plug solely wired with an earth lead and plugged into the adjacent double-socket outlet, connected to the 0V of your PSU rather than hack the case unnecessarily?
Good luck
Chris

[Al (astral highway)]

It would help if you could represent the circuits even by quickly-sketch block diagram and show how they are connected to eachother, how the input jack is connected to the amp. board and how you are taking the output to wherever it’s going. If there’s a socket how is it connected to the amp board ? Diagrams , not words, please.

Lack of a mains safety earth isn’t deterministic for this problem. (That doesn’t mean that having one couldn’t be part of the solution, but we need to know more).

[G6ONEDave]

To me it looks like the transformer secondary wires are running parallel with the DC from the bridge rectifier and all tied together with a pink wire. Firstly I would remove the pink wire and keep the DC output as far away from the transformer and it's wiring as possible. Likewise what is that yellow wire doing that appears to be wrapped around the mains input cable? If you want to hold internal wiring in place it is better use an insulated tie of some sort.

Ideally the mains input socket should be wired to the power on/off switch, then go to the transformer. The transformer secondry then goes to the rectifier and then to the smoothing resevoir capacitors. After that the wiring goes to the regulator if one is needed. The output of the power supply would normally then go to the audio output stage. The pre-amp section if fitted would usually get a reduced supply derived from the audio output stage supply rail/s.

There are a lot of different ways to wire up circuitry and quite a few will work even though they might not be best practice. Try to keep the inter stage wiring as short as is reasonably possible and try to get a layout where non of the stages can interfere with others by running parallel wiring.

These are just my thoughts but hopefully something might help.

Dave

[Cruisin Marine]

A Scope is the quickest way to find out what is happening, on the supply rails to start with.
If you have one (hopefully).
Adding some 1nF or even 10 nF caps across the rectifier diode's may well help too.
I can't figure out the wiring and grounding/earthing at all from the piccies.

[Terry_VK5TM]

It would help if we knew where the "hum" was coming from.

Does it come out of the speaker, is something else "humming" - the transformer perhaps.

[Jolly 7]

Thanks everyone for all the replies. I shortened, unbundled and rerouted the wires as suggested, but the issue persists. As I have some other spare DIY amplifier circuit boards lying around I think I'll try one of them instead to see if that gets rid of the noise problem. This is probably the easiest way for me. If it doesn't, it's back to the drawing board 😄

[Jolly 7]

Originally Posted by Terry_VK5TM

It would help if we knew where the "hum" was coming from.

Does it come out of the speaker, is something else "humming" - the transformer perhaps.

Hi Terry
The hum is definitely from the speaker and very loud with a grating like sound.

[Jolly 7]

Here are some pictures of the amplifier board. The preamp is a C1815 transistor.

[G6ONEDave]

Looking at the pics in thread 17, the amplifier pcb appears to have a bridge rectifier on it, so why is there also what appears to be a bridge rectifier circuit on the pcb seen below the transformer in your first pic?

You only need one bridge rectifier for this build unless there is something else attached somewhere that is not showing in the pics. I would suggest the removal of the pcb below the transformer and the voltage regulator. Then take the transformer output to the bridge rectifier that is on the amplifier pcb.

Something else to check is if the amp chip is stereo or mono. If the amp chip is a stereo type then both inputs will need a signal and both outputs will need speakers to be attached. If there is only one input of a stereo amplifier circuit being given a signal then the other channel could be picking up interference that would produce noise in that channels output.

Dave

[Al (astral highway)]

Originally Posted by Jolly 7

I think I'll try one of them instead to see if that gets rid of the noise problem. This is probably the easiest way for me. If it doesn't, it's back to the drawing board 😄

Jolly , it’s so unlikely to be a fault with a board.

It’s likely to do with layout and grounding. You will probably just replicate the same outcome.

As suggested earlier, please will you annotate your photos so we can see which wires are carrying a signal and which are connecting the boards’ power supplies snd how. It looks to me like you have one board with a bridge rectifier one the left , an amp to picture right and a linear voltage regulator on a small board behind it , with a heatsink ? Is that right ?

And you identified the yellow wire clamp as being on one of the signal leads , which is connected to the wire from the ferrite suppressor with the brown lead? Is that right?

[Jolly 7]

Originally Posted by Al (astral highway)

Jolly , it’s so unlikely to be a fault with a board.

It’s likely to do with layout and grounding. You will probably just replicate the same outcome.

As suggested earlier, please will you annotate your photos so we can see which wires are carrying a signal and which are connecting the boards’ power supplies snd how. It looks to me like you have one board with a bridge rectifier one the left , an amp to picture right and a linear voltage regulator on a small board behind it , with a heatsink ? Is that right ?

And you identified the yellow wire clamp as being on one of the signal leads , which is connected to the wire from the ferrite suppressor with the brown lead? Is that right?

Sure... I'll try to do that as soon as I'm able to. It might take me a day or two. I would like to preserve the Grundig board, as the amplifier sounds great when feeding it with an audio source not powered by the same transformer e.g. an android tablet or battery operated radio.