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Glowbug 11th March 2006 11:30 PM

Question about DIY 'Humbuster' schematic
Hey all...

I found this schematic while doing a search here:

This seems like just the ticket to getting rid of my transformer lamination hum that's coming out of my preamp & monoblocks.

However, I have some questions...I've got a big, honking half-farad electrolytic cap left over from a friend's car audio installation. Could I use that instead of C1 & C2? I'm pretty sure it's 450V rated...would have to check, though.

I also noticed it's floating off of ground, does it matter which side hot and neutral go to?

I'd love to be able to assemble this thing for $100, even if I threw in a nice aluminum case.

ilimzn 12th March 2006 12:42 AM

You really are not too sure what you are doing, are you?

0.5F = 1/2F = 500000uF = 5 million times larger than in the schematic. A 1/2F 450V capacitor would be roughly the size of a small refridgerator.
Also, it is electrolyits - which means it is POLARIZED, i.e. intedned for DC ONLY. AC mains is AC - I thought that would be quite obvious. What would a 450uF cap be doing in a car where the power is 12V DC, aside...
Oh, and BTW - it will not get rid of transformer lamination hum. That is a part mechanical, part electrical (due to rectification) problem.

Glowbug 12th March 2006 12:56 AM

You're right, I'm not too sure what I'm doing, hence my question here :)

But the caps *are* polarized, unless I'm seeing those plus signs out of nowhere.

AndrewT 12th March 2006 09:10 AM

the schematic you posted showed the balanced version.

I would prefer the unbalanced version with both caps in EITHER the live feed OR in the neutral feed.

There is some discussion in previous threads on which is safer. But your version has both unsafe arguments incorporated.
Try searching but be selective in what you read because unfortunately some contributors did not understand the operation of the circuit.
However I do recommend a good (and well designed) version of it to cure the problem.

The caps are shorted by side by side diodes. These ensure each of the caps sees only a low voltage during each half AC cycle. So 10Vdc or 16Vdc caps can be used to block the small amount of DC sometimes (often) found on the mains supply. It is this DC that can cause mechanical humming and buzzing in toroidal transformers. EI types are more resistant but not immune.

The diodes allow peak currents to bypass the caps eg. during start up or fault conditions.

The caps should be scaled to suit the load you need to serve. The peak volts drop across each cap should be about 50% to 75% of diode voltage when drawing maximum operating current.
1/2PiFC allows you to calculate.

A small improvement can be extracted by adding line to ground caps. These must be Y rated types and only about 10nF to 47nF. They can be attached either side and/or both sides of the inductors.
Total cap to ground determines your earth leakage so, if you protect a number of pieces this way, you need to scale the grounding caps to ensure you stay within your local regulations.
Here in the UK the limit is about 95nF to ground to stay under the 25% of rated trip current when using 30mA breakers.

AndrewT 12th March 2006 09:23 AM

just noticed you are in the USA.
Your low supply voltage ensures that your current draw is double most other countries. This forces you to adopt doubled value of caps to keep the volts drop down to your chosen design level.

lndm 13th March 2006 10:22 PM

Half a farad is huge. It will no doubt create new problems, not to mention the cap itself may not sound too wonderful.

I believe some DIYers sometimes wish they could go to extremes whenever there is a problem. Maybe this thinking is a means to an end. In the end, they often need an elegant/clever compromise.

EC8010 13th March 2006 10:37 PM

Glowbug: Leave this idea alone. I'm not saying it's a bad idea, but it needs far more experience than you have to do it safely. I can see the problem it's addressing but I'd be very cautious about implementing its solution. No, I'll be firmer than that. I'm about to use a toroid in a high performance application and for various reasons I don't dare use this solution.

Nordic 14th March 2006 03:50 AM

Allthough I don't have the technical knowhow to back it up I read from enough reliable sources that loud humming, or even buzzing from traffos are not because of a broblem in the traffo, but because of a problem in the rest of the circuit's design, unless of course you are trying to pull more power from the transformer than its rated for.

lndm 14th March 2006 03:57 AM

No. It could be because of the load _or_ the xfmr itself. BTW i agree with EC8010 here. Not dissing your abilities, just that the mains has relentless amounts of juice.

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