Humming toriodal transformer....?

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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.
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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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.
I'm using this schematic here:

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

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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