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Old 27th April 2013, 05:23 AM   #11
balerit is offline balerit  South Africa
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Do you have it mounted directly onto a metal casing because this will cause it to hum, I mount mine on wooden blocks lined with rubber, about 20mm off the case. Also don't fit the metal bolt through the centre.
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Old 27th April 2013, 06:31 AM   #12
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Class A amps and power transformers of all types are not good partners for low noise, however they are mounted. Here, there have been a couple of popular 15 & 20W designs which were fine using a traditional source toroidal transformer but when a low cost custom import transformer was supplied in kits, some but not all kits hummed and the case resonated with that noticeably. Other samples of the same type were normally quiet or at least not reported by customers. The bias constant current was less than 2.5 A for both channels.

It was not a an issue with mains DC. It was constant load on a transformer unsuited to low noise requirements. From experience, when you load even a class AB amplifier heavily and listen close, the signal "plays" badly out of the transformer and the whole amplifier seems to make sound without any loudspeaker. If you ever properly load test your amplifiers, you'll be aware of this.

The designers, Silicon Chip Magazine, eventually came up with an effective solution in my example at least, involving a couple of 250 uH powdered iron chokes. Other solutions may work too but you will be barking up the wrong tree simply nailing it as DC on the mains, whether it actually is to a degree or not.
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Old 27th April 2013, 06:59 AM   #13
sajti is offline sajti  Hungary
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Every toroidal transformer has noise. This is depending by:

- quality of the core
- load
- DC on the mains

The first is depending by the manufacturer company, so we have not too much influence on it.
The 2nd is easy to solve: Use oversized transformer (2x of the required power)
The 3rd: Use protector as it was linked in the topic.

Usually I use custom made transformers. My current 500VA type use 630VA core to decrease the core generated noise.

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Old 27th April 2013, 11:56 PM   #14
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I'm using this schematic here:

Click the image to open in full size.

Except I'm using 6800uf caps instead of 4700uf and I am still getting the transformer hum. Any ideas?
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Old 28th April 2013, 02:53 AM   #15
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It's not a mains DC problem so DC breakers don't help. Change the transformer to another, higher rated type as advised or use an EI type or mount it such that the mechanical connection to the chassis is damped and you don't notice the vibration.
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Old 28th April 2013, 10:01 AM   #16
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Fusion916 View Post
Is it "normal" for a toroidal to hum when turned on and drawing high current? Any methods of preventing this humming to couple onto the output?
the maximum continuous DC current from a capacitor input filter is only half (50%) of the rated AC current of the transformer.
For cooler running I recommend a continuous DC current of half of that maximum i.e. one quarter (25%) of the rated AC current.

That will reduce the mechanical noise in the transformer.
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Old 29th April 2013, 07:21 PM   #17
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Originally Posted by Fusion916 View Post
Is it "normal" for a toriodal to hum when turned on and drawing high current? Any methods of preventing this humming to couple onto the output?
I suggest you use batteries. Ac mains has nothing to commend it other than convenience - and even that is illusory. Once you try batteries I doubt you'll want to go back.
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Old 29th April 2013, 07:32 PM   #18
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Quote:
Originally Posted by Fusion916 View Post
I'm using this schematic here:

Click the image to open in full size.

Except I'm using 6800uf caps instead of 4700uf and I am still getting the transformer hum. Any ideas?
If the transformer is humming constantly it's probably not DC on the mains. It could be a case of a bit loose windings, not much to do about this.
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Old 29th April 2013, 08:01 PM   #19
pdul is offline pdul  Denmark
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I suggest you use batteries. Ac mains has nothing to commend it other than convenience - and even that is illusory. Once you try batteries I doubt you'll want to go back
Fuel cells or nuclear power is better than old fasioned (bad)eries
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Old 29th April 2013, 08:14 PM   #20
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Fuel cells or nuclear power is better than old fasioned (bad)eries
Can't argue with that, pdul, pretty much anything is better than ac mains.
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