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Old 18th May 2017, 9:07 am   #1
ms660
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Default Hum Cancelling Audio Output Transformer

I scanned the following page for another forum but thought I'd post it on here as well.

It gives an explanation as to how it works, these types of transformers are found in many radio designs and will probably be familiar to many folks.

Ref: Radio and Television Engineers Reference Book, Hawker & Pannett, 4th Edition.

Lawrence.
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Old 18th May 2017, 9:37 am   #2
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Default Re: Hum Cancelling Audio Output Transformer

It's a Philips technique, and although Philips were renowned for all sorts of weird and wacky ideas, this one deserves a solid round of applause.
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Old 18th May 2017, 9:43 am   #3
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Default Re: Hum Cancelling Audio Output Transformer

Something that has always made me wonder about this circuit is this:

If we assume the impedance of C1 & C2 is near to zero at audio frequencies then the resistor R1 is effectively in parallel with winding B of the transformer; so the question arises, how much audio power is wasted in R1 and is it a significant proportion of the total audio output power? I appreciate that this is likely to vary between designs but this variation is likely to be limited because of the need to maintain the hum cancellation function of the circuit.
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Old 20th May 2017, 11:26 am   #4
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Default Re: Hum Cancelling Audio Output Transformer

The number of turns on winding "B" is quite small compared with the number of turns in "A". making the audio impedance-match of winding B low too.

The resistor R being usually a KOhm or greater, the amount of audio-power that the low-impedance audio-winding B can push into it will be rather small due to the mismatch. I guess you might only get a few volts of audio across "B" even at full volume.

Interestingly, in times-past I experimented with one of these transformers as anode-and-screen modulator in a low-power transmitter (with a transistor amplifier feeding audio into the "speaker" winding).

Feeding the screen-grid of the modulated stage from the tapping rather than from the full winding - a sort of 'single-ended ultra-linear' - made no noticeable difference.

And yes I tried swapping the ends round.
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Old 20th May 2017, 11:55 am   #5
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Default Re: Hum Cancelling Audio Output Transformer

These types of transformers were listed with either the hum winding
being 3 or 6 % of the main section.

This is how Hammond listed them in 1962.

I was a radio tech in 1962.
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Old 20th May 2017, 12:29 pm   #6
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Default Re: Hum Cancelling Audio Output Transformer

I am wondering no longer, thanks for the explanation. I never thought that winding B would have so few turns to achieve the hum cancellation.
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Old 20th May 2017, 12:33 pm   #7
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Default Re: Hum Cancelling Audio Output Transformer

Thanks for the '3%' details, as that clarifies the extent of DC current balancing going on for the core (ie. near to zilch).
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Old 20th May 2017, 12:38 pm   #8
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Default Re: Hum Cancelling Audio Output Transformer

As an example the one used in the Bush VHF70 gives the primary winding resistances as 600 ohms and 5.5 ohms.

Lawrence.
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Old 20th May 2017, 1:59 pm   #9
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Default Re: Hum Cancelling Audio Output Transformer

Quote:
I never thought that winding B would have so few turns to achieve the hum cancellation.
The ear is logarithmic so a weeny amount of hum which sounds quite loud to us mere humans is cancelled by a weeny amount of winding B.

An aside, I was tasked to do a screen brightness thing, I went for logarithmic steps, it looked super. I have yet to find a screen brightness control that does that, they all suddenly become bright and then creep up after half way or so.
 
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