Gainclone HUM, THUMP problems - The solution

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chylld

Member
Joined 2005
jw.id.au
pacificblue said:
Could it be that the trick does not only lie in the distance between transformer and heatsink, but also in the fact that the transformer is removed from the amp case that looks very ferromagnetic.

Does it reduce the hum, when you lift the transformer up even without separating it further from the heatsink? I. e. put a sufficiently thick piece of wood or plastic below it, and so separate the transformer from the bottom and the sides? That would indicate that your case is the electromagnetic conductor.

Solutions could then be to move the transformer to a separate housing outside of the amplifier. Or to build a small shielding housing around the transformer, and mount that with as much separation as possible on all sides into your amplifier case.
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the bottom half of the case (what you see in the photos) is aluminum which is non-ferromagnetic, and the top is steel which is ferromagnetic.

i tried standing the transformer up on 1cm tall rubber feet, didn't help the noise at all.

when i tried my ferromagnetic shield, i moved it around up and down side to side (as much as possible) and the noise character didn't change at all. the shield was made from an arrangement of flexible bracket steel strips, you can see here in this pic of it that it reaches around pretty much all the way (right third covered with electrical tape since it was close to the power supply PCB):

An externally hosted image should be here but it was not working when we last tested it.


i verified several times that it was electrically connected to the chassis, but alas, no cake. i really expected this one to work :confused:

AndrewT said:
Hi,
or use an all aluminium casing.

Try assembling the whole amp without any casing, but safely, and see/hear if the noise goes away.
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i tried with the cover on and off, no difference to the noise.

Nordic said:
arew your RCA sockets isolated from the case?
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yes, rca sockets as well as power ground and output ground are all isolated from the chassis using a ground loop breaker.
 
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pacificblue

Member
Joined 2008
But you do hear that buzz from your speakers, right? Or are we talking about a resonating chassis?

originally posted by chylld
i'm starting to hear a buzzing/humming from my gainclone
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You can check that best with the speakers disconnected, because the human ear is not so good in locating sources at low frequencies.

If it is not a resonating chassis, do you have that noise also with all inputs disconnected?
 
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chylld

Member
Joined 2005
jw.id.au
AndrewT said:
and shorten that transformer bolt. Then insulate over the washer and bolt to prevent electrical contact with the lid.
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the bolt in the pic is actually about an inch away from the lid already. if it was near/touching the lid there would be a chance of shorted turn phenomena however this was not the case.

pacificblue said:
But you do hear that buzz from your speakers, right? Or are we talking about a resonating chassis?
...
If it is not a resonating chassis, do you have that noise also with all inputs disconnected?
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the buzzing was from the speakers, not the transformer itself. (i.e. electrical noise, not mechanical noise.) the noise presented itself with inputs connected or disconnected (which made it easy to test, just plug in the speakers and the power and turn it on: bzzzzzzzzzzzzzzzzzzzzzzzz.

however...



PROBLEM SOLVED!!!!!!11

after a whole day's work i am glad to contribute the following solution to gainclone hum. i can now put my ear literally within ear-hair's distance away from the speaker and i cannot hear ANYTHING. (unless i'm playing music of course!)

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.


i assume the pictures are self-explanatory. the transformer box (i call it "chibi") sits on the floor about 3 feet underneath the amp.

can i just say: transformers suck.

thanks to all who helped. your collective perseverance kept me motivated enough to reach this point.
 
rabbitz

rabbitz

Member
Joined 2003
That looks like an Altronics transformer and I've never had a problem with induced noise from them. I've had 1 fry and another was noisy, but generally fine.

I'm sure there was more to your problem as I've used these right next to the amp PCB without issues. The main thing is that it's working well.

Once thing you might look at for future projects is to twist your secondary wire pairs or any other AC line.

If you are looking for good transformers in Australia, get a Harbuch (in Sydney). They make Antrim under licence.
 
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chylld

Member
Joined 2005
jw.id.au
rabbitz said:
That looks like an Altronics transformer and I've never had a problem with induced noise from them. I've had 1 fry and another was noisy, but generally fine.

I'm sure there was more to your problem as I've used these right next to the amp PCB without issues. The main thing is that it's working well.

Once thing you might look at for future projects is to twist your secondary wire pairs or any other AC line.

If you are looking for good transformers in Australia, get a Harbuch (in Sydney). They make Antrim under licence.
Click to expand...

good eyes! that is indeed an Altronics 160VA 2x25V M5325 transformer.

i tried twisting primary, secondary and rectified output lines, none of those changes had any effect.

the most reasonable explanation is owen's theory that the particular heatsink i bought was acting as an inductive coupler, transmitting the noise to the amp PCBs.

i had eliminated ground and emi problems, so there are very few (if any) other explanations. i even went so far as to try a dc blocker, which no matter how you look at it, is not the safest thing you can do with an amp.

so i could either try a new heatsink with a 50% chance of working, or i could move the transformer out of the main case which had a 99% chance of working. not the cheapest solution, but my goal was to fix the problem and it is now fixed :)
 
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Sparky OR

Member
Joined 2003
I've been lurking, slowly building DIY Kit 50 LM1875 amps. The first unit I assembled after getting the PS working. It sounded fine, plenty of power for my 4 ohm T&B 4" TL spkrs. Second unit, lots of hum. Tried different grounding rountines, the AC gnd is still not hooked up, running through a Variac. Well, using a star ground point, and feeding EVERYTHING to it resulted in getting rid of hum. Thanks for all the info!:) Now I will have to work on reincorporating the Alps 50k pot back in, finallizing a chassis, and putting in a proper AC (safety) ground.
The only changes I did to the kit50 was lower R4 from 10l to 5.6k, it uses 180k fb resistor. It was interesting that the hum only occurred when I had both inputs hooked up to my little Panasonic CD player. Only one connected, no hum.
 
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chylld

Member
Joined 2005
jw.id.au
one thing i've been curious about with my transformer box... there are 2 cables going from the transformer box to the main amp box, each one carries 25v ac. inside the amp box, this gets turned to V+/PG+ and V-/PG-.

so my question: what happens if i leave one of the cables unattached? (i.e. giving the chips only V+ or V-)
 
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chylld

Member
Joined 2005
jw.id.au
AndrewT said:
again
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thanks for the clarification. reason i asked is because i was afraid it would blow my speakers up:

Decibel Dungeon wrote:
When I reconnected the amp, I found that I had an offset of 34 volts! After assuming that I must have a faulty chip, and ordering some replacements, further investigating found that the negative supply rail measured 0 volts at the amp but -36 volts in the PSU. A wire had come loose in the umbilical from PSU to amp and reconnecting it solved the problem. It was lucky that I had decided to check the DC offset that morning of I would have destroyed a speaker.
Click to expand...
 
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chylld

Member
Joined 2005
jw.id.au
AndrewT said:
it might.
measure the output offsets with the various disconnections that you think are possible.
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ok these are the offsets i've measured (all in +/- volts):

both V+/V- connected: 0.002
only one connected: 1.94
only the other one connected: 0.49

so max of about 2V. i assume that's not a harmful voltage given that the AA (1.5V) battery polarity test is quite common and harmless?
 
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AndriyOL

Member
Joined 2008
I have build a parallel configuration amp, based on LM4780 chips but have some nasty hiss at low frequency response, but the bass is not gone at all.
My unconventional solution is a use of non-polarized 47uF Ci cap in parallel to 100nF MKT cap.
Gould it be from improper grounding?

Thanks in advance for answers.
 
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