|
General Vintage Technology Discussions For general discussions about vintage radio and other vintage electronics etc. |
|
Thread Tools |
18th May 2017, 9:07 am | #1 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
|
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. |
18th May 2017, 9:37 am | #2 |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,061
|
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.
|
18th May 2017, 9:43 am | #3 |
Rest in Peace
Join Date: May 2003
Location: Harlaxton, Lincolnshire, UK.
Posts: 3,944
|
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. |
20th May 2017, 11:26 am | #4 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,953
|
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. |
20th May 2017, 11:55 am | #5 |
Pentode
Join Date: Jan 2013
Location: Powell River, British Columbia, Canada.
Posts: 217
|
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.
__________________
Steve Dow VE7ASO |
20th May 2017, 12:29 pm | #6 |
Rest in Peace
Join Date: May 2003
Location: Harlaxton, Lincolnshire, UK.
Posts: 3,944
|
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.
|
20th May 2017, 12:33 pm | #7 |
Heptode
Join Date: Oct 2009
Location: Melbourne Australia
Posts: 898
|
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).
|
20th May 2017, 12:38 pm | #8 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
|
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. |
20th May 2017, 1:59 pm | #9 | |
Guest
Posts: n/a
|
Re: Hum Cancelling Audio Output Transformer
Quote:
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. |
|