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Old 1st August 2003, 11:01 AM   #1
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Default VERY loud hum issue

I have built and active xover and ran it off batteries as ive posted before. However upon upgrade of the xover the batteries lasted a max of ten hours so a power supply was built (just finished about thirty mins ago). Anyway under battery operation there is ZERO noise added by the xover maybe a bit of hiss but thats because its the preamp to and everything has a bit of hiss. Anyway I wired in the power supply and MAD MAD hum issue ensued. This is loud.

The powersupply is nothing outrageous a 30VA 12*2 trannie with duel rectifier and 1100uf of smoothing (i plan to add regulators at a later date to see what diff it makes). The input from the mains goes thru a simple shop bought IEC filter stage such as the ones by belden.

More on the hum. This hum is louder then ive ever encountered its at 100hz and 50hz. The 100hz dissapears when u break the ground to the wall socket when you unplug the unit. As the caps drain the 50hz remains however, still very loud. Then the caps drain and the infernal racket ceases.

This hum is clearly audible thru bass heavey material when played at a level which is normally too loud for comfortable listening.

The xover is constructed on vero strip board about 30cm long. Now I dedicated a single track to the earth and everthing wires back to this from the opamp circuits.

Having searched the forum I found a couple of things to try -

One would be a 10ohm resistor from the signal ground to case or to power supply in my situation. (The power supply is wires thru to the case too tho)

and the other was dissconnect the unit from mains ground. This Im not to plussed about, however the unit would still be grounded thru the signal ground form the dac and the power amp so technically there would be a link to the mains.

As the whole xover is on a large piece of vero and none of it has sheilding, (where possible its sheilded like on the input and output wires) its possible that the circuit it picking up the mains hum. I have no objection to sticking the power supply in a seperate box as my poweramps is seperate too and that doesny hum at all, well a little bit but well within acceptable limits.

Im off to try a few things now but would appreciate anyone else input into solving this.

Would seperate wires from each ground location thruout the xover to a common star, not running down a whole track on the vero be worth trying??? if not and the aforementioned doesnt help what else could I try??

Cheers Matt
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Old 1st August 2003, 11:26 AM   #2
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Ive now moved the power supply to outside the case, about one meter away. This doesnt do a thing to solve the problem
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Old 1st August 2003, 12:59 PM   #3
bocka is offline bocka  Germany
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seems to be a ground problem or ground loop problem. Try to connect only the power supply ground to your xover. Open the earth wire from the power supply and look what happens.
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Old 1st August 2003, 01:40 PM   #4
moamps is offline moamps  Croatia
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Default Re: VERY loud hum issue

Quote:
Originally posted by 5th element
[B]
The powersupply is nothing outrageous a 30VA 12*2 trannie with duel rectifier and 1100uf of smoothing (i plan to add regulators at a later date to see what diff it makes).

First step...add regs.

Regards
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Old 1st August 2003, 01:58 PM   #5
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5th element I think you should check the ground in your Mains wiring as there might be some corruption there.
I dont know your housing situation but if you own the house try the equipment on another circuit and see if it persists
I typically install a ground rod close to the mains panel and wire a separate circuit for high end installations
Most houses are wired up that the plumbing pipes have a ground attached that has been done for years and over time it becomes corroded


DIRT®
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Old 1st August 2003, 11:20 PM   #6
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cheers for your responses ppl much appreciated. This seems the most daft thing to help solve the prob but I guess it makes sense kinda to which my brain says add regs and the prob will vanish.

Here goes.

I added an extra 6800uf of smoothing to each rail this dramitically lowered the hum but not enough for satisfaction. The caps before we bog standard 220uf 5 per rail I dont know abou their smoothing capabilities but to me it wasnt enough. Anyway the next step will be to add regulators im going to do one of those pretracking things (if thats the right name) using lm317's and thier complementaries, hope this helps.

Is it possible that the 5*220uf on each rail has a detrimental effect on the supply? and if so would removing them be a good idea. 6800uf is what 100watt commercials sometimes have in 'em and this xover draws 60mA.

Cheers Matt

Oh and BTW I know ive mentioned my power amp (slones 11.4 design in HPAACM) but with no pics or anything just problems with it. Anyway thats because the case sucks. Anyway Im awaiting the arrival of heatsinks so when the whole shebang is finished ill be sure to share it all with the DIY community. I could even take a pic of its crappyness and then do a before and after makeover of the amp lol.
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Old 1st August 2003, 11:29 PM   #7
SY is offline SY  United States
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"When you hear hoofbeats, expect to see horses, not zebras." I'll wager a nice cold beer that what you have there is indeed a ground loop. Chase that down before worrying about caps or regulators.
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Old 1st August 2003, 11:57 PM   #8
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I have tried removing the ground from the mains plug and all this did was remove the 100hz buzz and leave the 50hz. Initially all I had connected was the power supply earth to the xover earth and that was it with the power supply earth connected to the wall outlet. I then removed the wall outlet earth as recommended as a try it and see. This removed the 100hz buzz but was either replaced of just revealed a 50hz buzz. Having the power supply in different locations didnt help matters one little bit.

So I added the 6800uf and the noise reduced dramatically but not to within acceptable limits.

The power supply design I have build twice for power amps and that has no hum issues at all. I've never encountered a ground loop before. If this is one I dont know what im suppossed to do to combat it. Im not having a go but saying solve this before adding regs is all very well but I dont know where to start to solve it.

Could you please list a few things to try to break the ground loop as ive never had to deal with one before.

Cheers again Matt.
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Old 2nd August 2003, 12:02 AM   #9
SY is offline SY  United States
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Can you sketch out and post your grounding scheme? That would help us give some specific suggestions. I have a suspicion that it's how you're tying the PS caps' ground and the analog ground together, but it's a suspicion based on very little data.

Some potential band-aids: run as many grounds separately as possible, all connected at one common point on the chassis. Insulate input and output jacks from the chassis and connect them to the chassis via 10 ohm resistors. Keep power grounds and signal grounds as isolated as possible, connecting together only at the one common point.
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Old 2nd August 2003, 12:46 AM   #10
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Its difficult for me to sketch show as I cant d/l images to net my brother has the know how, how to as he does regularly I just havnt benn bothered to sort anything like that out but I'll have a go at describing what ive got regarding grouding.

Ok PSU 1st. This is done on vero board, duel rectifier, with the centre two tracks soldered together with the caps running the length of the board.

Rec1--------------------------------


==================

Rec2++++++++++++++++++

Kinda like that with caps along the length. So nothing special.

The xover/preamp (alps pot before input) is about 30cm*10cm on vero board again one 30cm track I dedicated to ground/earth with all other points nescessary wired back to this. The power supply board joins this ground via a flying lead. The same point on the PSU joins via another flying lead to the earth pin on the input plug to the mains. And then another wire from the earth pin connects to the case. The input wires are all shielded and earthed to the case. The output are shielded but not earthed.

The input/output was like this on battery operation and I got no noise what-so-ever.

Should I take the ground from the PSU to the case and the case only, then attach the xover ground via the case thru 10 ohm resistors?

The only other thing that I could try but would require some considerable effort is to take individual grounds from the xover to a common star ground on the case. I would only want to do this as a last resort tho.

If you need anymore info I can try and post a pic but that will be tomorrow, I will offer more written help for maybe another hour as Im on the phone too which is gonna last a while.

Anyway cheers for all this I can imagine it could get quite annoying.

Matt
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