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How to fix transformer noise

kjello

kjello

Super Member
Hi

I have a small problem with my subwoofer amplifier.
Its a Zachry DSW-400 mono plate amplifier
The problem is noise, which I belive comes from the transformer.
When I turn the power on, the noise is audible, and when the speaker relay switches on the noise gets
amplified through the woofer, it is a rather loud humming sound.
Now I attached pics of the internals, If someone can see obvious reasons for this hum or problems with the layout or component placement? and last if there are any mods that can be done to get rid of this rather annoying humming? None of my other amps have this problem (other brands)

I kind of like this amp, its powerful and fast, if it only could be more quiet :/
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17916413_10154439502852551_1088175070_o_zpsmfbozmec.jpg
 
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Has it always created this hum or did it develop the hum (typically failing filter capacitors)? Can you measure the frequency of the hum? Does it hum with no input connected, does puting a load across the input change it? The key to getting rid of noise is to figure out what it is and filter it out or eliminate the source.
 
RocketMonkey said:
Has it always created this hum or did it develop the hum (typically failing filter capacitors)? Can you measure the frequency of the hum? Does it hum with no input connected, does puting a load across the input change it? The key to getting rid of noise is to figure out what it is and filter it out or eliminate the source.
Click to expand...

It has always been like this since I bought it new. But I think maybe the hum may vary some in level from day to day.
It hums without input connected, yes.
I will try to meassure the frequency, what load would you suggest across the input ?
Regards
KO
 
i suspect the power supply needs moving outside the box for starters and see if it helps . then if not look at the ac ripple on the supply .
 
Id figure out the cause before modifying it if possible. If it is 120 hz it should be ripple that is not getting filtered out, if its 60 hz its probably magnetic noise from the transformer. Moving the transformer farther away from the filter could make it worse if its a weak filter. Beeing that it is not a developed problem Id guess the filter design is not up to the task (does it sound good under heavy load?) or the transformer placement is not good. You could unbolt the transformer and turn it to change where the field goes or shield it with something. Do you have a schematic? I've seen people short the input with a couple hundred ohm resistor when setting idle or trying to find where noise is coming from.
 
petehall347 said:
i suspect the power supply needs moving outside the box for starters and see if it helps . then if not look at the ac ripple on the supply .
Click to expand...

Yes I guess moving the power supply could be relative easy to do, will check for ripple (not that I know how to do that :D)
 
RocketMonkey said:
Id figure out the cause before modifying it if possible. If it is 120 hz it should be ripple that is not getting filtered out, if its 60 hz its probably magnetic noise from the transformer. Moving the transformer farther away from the filter could make it worse if its a weak filter. Beeing that it is not a developed problem Id guess the filter design is not up to the task (does it sound good under heavy load?) or the transformer placement is not good. You could unbolt the transformer and turn it to change where the field goes or shield it with something. Do you have a schematic? I've seen people short the input with a couple hundred ohm resistor when setting idle or trying to find where noise is coming from.
Click to expand...

I will search for a schematic, I dont have it. What is normally used as shield? Would a steel casing do the trick?
I totally agree on finding the cause before modifying
 
Steel that is grounded, they make specific types of metals for shielding but for your purposes any piece of steel sheet should work. Just be sure it is grounded to the chassis ground. It will become live if its working and not grounded.
 
Is there physical vibration you can feel associated with the hum? Not at the speaker, but something you could feel on the plate.

You might have physical hum from the transformer and noise from the amp. Do you have a power conditioner/filter you can try?
 
Another possible reason is AC coupling from the wiring. Re routing the blue wires from the transformer, twisting them, and re-routing the orange wires from the preamp to the power amp could help too.

I'd move the blue wires far from the preamp section (where all the inputs are), and move that group of orange wires (what seems to be the preamp output going to the power amp) away from the transformer, and twisted.

I see wires coming out of the transformer, carrying AC, tied together with the orange wires (my guess they are unshielded wires carrying audio signal). That's definitely not OK, I'd move the audio wires away from any AC source.

As said, moving the transformer (perhaps rotating it 90º if it fits) could help, or make thing worse. You need to try.

I'd start with the wiring , specially the orange wires.
 
elnaldo said:
Another possible reason is AC coupling from the wiring. Re routing the blue wires from the transformer, twisting them, and re-routing the orange wires from the preamp to the power amp could help too.

I'd move the blue wires far from the preamp section (where all the inputs are), and move that group of orange wires (what seems to be the preamp output going to the power amp) away from the transformer, and twisted.

I see wires coming out of the transformer, carrying AC, tied together with the orange wires (my guess they are unshielded wires carrying audio signal). That's definitely not OK, I'd move the audio wires away from any AC source.

As said, moving the transformer (perhaps rotating it 90º if it fits) could help, or make thing worse. You need to try.

I'd start with the wiring , specially the orange wires.
Click to expand...

Those wires bother me a bit too. looping wires near a transformer just seems like a bad idea.

The biggest transformer hum I've ever heard was from an elevator ramping up. They narrowed it down to the wires running from the xformer to the elevator pit. Unbelievable amount of noise coming from those wires. Enough DB to stop a conversation. (They decided not to upgrade and reroute the 1" copper wires and live with the noise due to costs.)
 
Simple test for mechanical vibration that kinda sorta sounds like a hum would be to apply some pressure to the transformer here and there. I went with rubber mounts on a problem unit with good results.
 
Sorry to beat a dead horse... but how much ripple current is too much? I have the same problem but I don't have any issues with bad/old capacitors. I already replaced all the capacitors that were out of spec.

The (noisy) transformer is feeding a KBR604 bridge rectifier 33V and there is around 100 mV ripple on the DC side. Suggestions on filtering out the hum/noise? The transformer isn't hot but the sub noise does seem to get louder the longer the sub is left on.

Nothing is plugged into the sub and I have it behind an isolation transformer so... that answers those questions.

Thanks!

- Wil
 
Never mind this part... I was measuring the power amp module incorrectly.
 

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