Hum noise problem - solved

Felipe · « **on:** 24 Dec 2005, 05:55 pm »

Hi guys,

Merry Xmas to all !!!!!

I am testing bias and seting up the 1st module, after upgrading to N version and i am having a damned problem that i dont know how to solve :

First settings for DC output voltage, and all chekings were OK.
I started up the AKSA with a speaker connected and a CD player. Music was playing nicely through it, when i bumped in to the CD player and a BIG noise (HUM type) started coming from the speaker.
I disconnected everyting and removed the CD player.
Turned on the AKSA....same HUM. Big one.

So i dismantled everything and started BIAS setting again....all was normal to my surprise

Connected the loudspeaker again....SAME HUM !!???

I removed the cable that was connecting the input/output pins directly to the CD player.....HUM gone !! (just a smalish hum stayed...nothing like the previous)
I must REMEMBER that the cable was not attached to anything. It was just a wire hanging on there.

So....i had the cable working as an antenna there....i tried other types of cable with no success (the CAT5 was the lesser noise).

Does anyone have

? How does this happen AFTER bumping in to the CD player ? What changed ?

Here is a pic of my desktop :

http://www.audiocircle.com/image.php?id=3446

Hope someone can help
Thanks a lot

Filipe

avahifi · « **Reply #1 on:** 24 Dec 2005, 08:32 pm »

The most common reason for hum from an amplifier is that it does not know the words.

Merry Christmas

Frank Van Alstine

AKSA · « **Reply #2 on:** 25 Dec 2005, 12:10 am »

Felipe,

Frank, Bless him, is dead right. The amps don't know the words, and are therefore forced to hum to avoid being accused of antisocial attitudes......

For the life of me I can't think why this has happened. Perhaps by knocking the CD player you disturbed a connection inside. It's possible.

But with long input leads on the module, you will attract hum intrusion from transformers, radio frequencies, etc.

Does the amp amplifier OK? NO other problems with bias or offset?

Keep looking; it is probably just hum pickup.

Cheers,

Hugh

Felipe · « **Reply #3 on:** 25 Dec 2005, 09:00 am »

My amp is Humming like hell....it really should have learned the words by now....

Hugh, the stragest thing is that with no input connected to the module, it has no hum. when i connect a simple 30cm cat5 wire to the inputs....it HUMS like crazy. And this with a simple wire....no CD attached yet !!

If any component was damaged when the CD was knocked....wouldnt it show during bias and offset setting ?? Because the hum started with the knock.....and i cant seem to be able to remove it now....

CORRECTION :

The voltage across R25 is 1.8V when the BIAS is not set yet.
If P1 is rotated clockwise to adjust the 55mv in the R22/23 the voltage across R25 is increased to 4.6V.....i assume its normal ?
Because in the STEP 7 manual, it says not to rotate P1 unless its a newley constructed amp.

Can the T1/T2/T3 be damaged ?

Greg Erskine · « **Reply #4 on:** 25 Dec 2005, 07:45 pm »

Hi Felipe,

Sorry I can't help but....

....a few weeks ago I had the same experience with an amp on my test bench. It was not an AKSA but with just the RCA able plugged into the amp is hummed badly. Different cables simply had different levels of hum. No cable, no hum. Just realised this amp had no zobel, but that probably is of no help. If my memory serves me right, when I connected a 100K pot onto the input cable I found the hum bad a low volume settings but suddenly disappearing at higher volume settings. I think that means something but I don't know what.

After a few days of thought, I when back and repeated the test and guess what - no hum. I've got no explaination. When I was testing this time, I tried hard to induce the hum by running the input cables in all the wrong locations, like accross and around the toriod, in parallel with the 240v power cable and even run the input cable over a switchmode PSU but still no hum. All I can "guess" is there was a bad connection somewhere or a small possibility of outside EMI or radio interference or maybe something coming from the house earth.

regards
Greg

Felipe · « **Reply #5 on:** 25 Dec 2005, 08:22 pm »

Hello aksaphiles !!

PROBLEM SOLVED !!

I was so far away, you will not believe the cause of the damned thing !!

Greg,

You were so close....the zobel is very very needed. I'll explain why :

When i was playing music through it, the amp was fine, after the bump the HUM went crazy. Thats because when i bumped...i didnt just bumped the CD player...i bumped the entire workbench. What was on the workbench you might ask ? My working light !

A horrible fluorescent lamp, that introduces HUM like hell into the power line. This was the source of the HUM. Lights off....no hum !!
What a HUGE difference....

So Greg...my amp didnt have the zobel yet...i am gonna put it now. I'll bet this will make a difference in my case. Did you have a lamp like this in your line when you were testing it ?? My guess is that you did.

1 thing Engineering school has taught me....everything happens for a reason...i couldt settle with just a "divine chance" guess.

Cheers !!!

Here is a pic of the damned thing !! I'll never turn it on again !!

http://www.audiocircle.com/image.php?id=3447

AKSA · « **Reply #6 on:** 26 Dec 2005, 01:34 am »

Felipe,

I'm so relieved

....... I had thought there was an amp problem!!

Fluoros can be a major source of hum. They have UV emissions at audio frequencies. And with the plastic covers, there is no EM screening around the ballast or electronic limiter, either.

Thanks for letting us know, Felipe, Happy New Year!!

Cheers,

Hugh

Greg Erskine · « **Reply #7 on:** 26 Dec 2005, 04:37 am »

Hi Felipe,

It's great when you solve a problem and know the cause because it won't happen again.

I don't have a desk light but I do have 4 x 20 watt fluoros at roof height. I don't think these were the problem because I always have them on when working in th garage. I'll have to pay closer attention next time the problem surfaces.

I learnt a lesson when I was an IT manager. One day virtually every computer screen in the building where I worked started wobbling, like when you deguass the screen. Some were very bad, while others were almost OK. The difference turned out to be what frequency the screen refresh rate was set at, the closer to 50Hz, the bigger the problem.

One of the electronics engineers wound a small coil and hooked it up to a CRO and you could see the 50 Hz interference. Well after looking at all electrical devices and circuits in our building, we started looking next door at the new building they were contructing. It turned out to be a problem with the switchboard and they couldn't turn it off. The electricity authority came out but the crew wasn't suitabliably qualified for the voltage in the substation that was in front of the new complex, so the next crew was summoned. They arrived on the scene and got a "big wooden stick" out of the truck and turned off some circuit breaker. Problem solved. This was no ordinary big stick, it had insulated handle at one end.

It still scares me a bit to think of the strength of the field that it could interfer with computer screens in a two storey building next door. Some of the screens were probably over a 100 metres away and separated by many concrete walls.

regards
Greg

Geoff-AU · « **Reply #8 on:** 27 Dec 2005, 04:31 am »

Yeah you probably don't want to think too hard about what's whizzing around in the air.. everything from radiated mains fields to AM/FM radio waves to hundreds or even thousands of watts of amateur radio broadcasts from that crazy guy down the street with the huge antenna, to massively powerful TV broadcast towers and then you've got all the mobile phone traffic and WLAN devices around a gigahertz and higher.

And people wonder why we've got cancer

When we had an electric hot water system my CRT would wobble violently whenever the booster element turned on (frequently, during winter evenings) as the cabling ran past my room. I upgraded to an LCD to "fix" the problem

but we have gas now anyway. Hopefully I'm not sterile

I've seen monitors shake on the second storey when there are power poles running past the window but to have a whole building affected by EMI like you describe is kinda scary!