I just had a little thought whilst I'm having a beer listening to the muppets waffle on about the 'brexit' lol. The one thing that is missing in this rig is the little bypass capacitors that you often see on the bridge rectifier terminals. As far as I am aware, they are usually fit on to surpress the switching noise created by the diodes inside the bridge. However Albert recently mentioned on another post that they can be helpful for removing rf from the supply rails? Maybe that could cure this problem...

Make connection between any part of box and ground on your wall socket. Will it solve hum i cannot be sure, but it will be more safe for you.


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Thanks guys. I have now earthed the box but it hasn't made any real improvement. S2000 I'm really unsure how to do that sounds a little too technical for me lol

Uksoundz wrote:Thanks guys. I have now earthed the box but it hasn't made any real improvement. S2000 I'm really unsure how to do that sounds a little too technical for me lol

Can you solder?

You just need 4 capacitors - 10n 100V disc ceramic will be ideal. The bridge rectifier will have four terminals, two labelled ~, one labelled - and one labelled +. You just have to wire the capacitors each ~ to - and each ~ to +

Thanks does one leg go to ~ and the other leg to +, then one leg of another to ~ and the other to -?

Yeah. Basically it's a cap across each diode in the bridge rectifier. There are two ~'s so it's one cap from each ~ to + and -. I'm not convinced it will help much though (sorry Albert) that box looks very similar to another I saw on this site recently with, amongst other mistakes, a stereo coder screwed to the side of the case right next to the PA output and no shielding. I can't see it that well in the above pic but if it's the same, you need to seriously have a re-think about the layout.

shuffy wrote:Yeah. Basically it's a cap across each diode in the bridge rectifier. There are two ~'s so it's one cap from each ~ to + and -. I'm not convinced it will help much though (sorry Albert) that box looks very similar to another I saw on this site recently with, amongst other mistakes, a stereo coder screwed to the side of the case right next to the PA output and no shielding. I can't see it that well in the above pic but if it's the same, you need to seriously have a re-think about the layout.

Agreed, I doubt it will help much. It is always good practice to fit these though, and it won't do any harm.

1. Try turning down the volume on the driver board (small black and white pot nearest metal can on the board screwed to the bottom of the case). If the hum reduces/ goes away then suspect inadequate shielding of the stereo encoder. Make a note of the pot position before you do this as the level is important.
If this works, try putting a piece of metal plate in between the transformer and stereo encoder. You can fit it diagonally and touch it against the sides of the box and the PA heatsink to ground it at both sides. Be careful not to short any wires to ground whilst trying this.

2. If the above doesn't make any difference then suspect ripple on the power supply. RF head who built this rig said that was the cause. It can be confimed by turning down the output power on the PA board, then hum should reduce as you turn the power down.
This can be fixed properly by adding voltage regulation to the main supply or lessened by adding more smoothing capacitance (it'll only help a little).

Thanks analyser. I'll try all those bits you mentioned this week and let you know what I find.

What do you mean by adding voltage regulation to the main supply though?

I know a very limited amount about Electronics, but could Ferrites be used anywhere to reduce this problem - should it be RF induced related, that is?

Uksoundz wrote:Thanks analyser. I'll try all those bits you mentioned this week and let you know what I find.

What do you mean by adding voltage regulation to the main supply though?

A simple transformer-based power supply has a bridge rectifier and smoothing capacitors to produce a DC output voltage from the AC generated by the transformer, which is a simple circuit but can have significant mains ripple riding on top of the DC voltage. This ripple can produce hum in some type of circuits so you can add other components to make this voltage smoother, which is called regulation. In your case it might just be a case of adding an LM350 voltage regulator and a few external components, but I wouldn't go down that route until you've found the cause of the problem.
The reason I say this is that the driver board in your rig is particularly immune to this type of noise, and in most cases will only pick up a very small hum.
Try some experiments when you get some time and report back.

MiXiN wrote:I know a very limited amount about Electronics, but could Ferrites be used anywhere to reduce this problem - should it be RF induced related, that is?

In short - yes, but that rig format is quite popular and shouldn't need ferrites everywhere.

There is nothing wrong with the rig from the topic starter
yes there is a little hum when you put your amplifier to max
The vid that the topic starter made shows the rig working and when he puts his phone next to the rig the hum is louder because of RF next to his phone.
The driver in this box is a proved disign and used by most builders in the Uk because the driver is
immune for hum and noise
There is a 10000uf capacitor in the power supply so that must be good (for 25-30W rig)
He can try to disconnect the stereo-encoder and see what happend
but the stereo-encoder is also placed in higher power rigs without screening

Looking at the vid and pictures? i believe this is a lot of fuss over nothing!
The main unregulated PSU is more than adequate for its purpose!
Noise from that and the 19khz stereo pilot is normal, especially with no modulation!
You will most likely find the signal to noise ratio is excellent when kickin some tunes!!

Albert H wrote: ↑Wed May 11, 2016 11:50 pm You just need 4 capacitors - 10n 100V disc ceramic will be ideal. The bridge rectifier will have four terminals, two labelled ~, one labelled - and one labelled +. You just have to wire the capacitors each ~ to - and each ~ to +

Hi All
Hi Albert
Question for All i need good source od ceramic disc capacitor CTC 100V or 200v?.. I see in UK is factory? tru
Can I buy directly
Thank You ina Advance
Best Regards

You can get suitable capacitors from:

https://uk.farnell.com

https://cpc.farnell.com/

https://www.rapidonline.com/

..... and many more.