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Old 29th May 2020, 11:22 pm   #21
Alan_G3XAQ
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Default Re: Hum pickup on inter-stage coupling transformer

Hi G6TAN, Yes, all the transformer wiring goes through a single half inch hole (plus grommet) in the chassis. I've examined your suggestion about optimising the decoupling (and coupling) capacitors for LF rolloff to attenuate the 100Hz mains hum without affecting the wanted CW signal at 550Hz too much. I don't have a lot of margin but LTspice suggests I can get maybe 15dB improvement, which will be worthwhile.

Chris, I think my post #5 shows the hum is picked up directly by the interstage audio transformer so I don't see how your suggestion would work. Also I am using a 12AX7 so I don't have the luxury of a suppressor grid.

73, Alan G3XAQ
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Old 29th May 2020, 11:37 pm   #22
Radio Wrangler
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Default Re: Hum pickup on inter-stage coupling transformer

Hum strap around the transformer?

Quick to do if you have some copper foil.

David
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Old 30th May 2020, 12:07 am   #23
joebog1
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Default Re: Hum pickup on inter-stage coupling transformer

What David said, but I would use a "slice" of baked bean tin or similar. Lot easier to find than copper foil around the house. Plus being steel it may even be more effective

Joe
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Old 30th May 2020, 9:56 am   #24
Alan_G3XAQ
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Default Re: Hum pickup on inter-stage coupling transformer

I'm unsure how to do this with a toroidal transformer. What shape do I use to get the equivalent effect to a "belly band" on an EI core? One book reference that a Google search brought up spoke of grain orientated silicon steel (GOSS) in what sounded like a totally enclosing box. Somewhat pessimistically that author said about 6dB hum reduction is as much as you can get with this bandaid, although 6dB for each of my two transformers would be worthwhile if the effect is additive.

I do have copper foil on the shelf (doesn't everyone, Joe?) but we don't use many food cans in our house. These days our baked beans come in a plastic pot. I've ordered some tinplate to try from a model shop on eBay, although at 0.25mm it sounds a bit thin to be useful at 50 and 100Hz skin depths.

Alan
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Old 30th May 2020, 7:29 pm   #25
Chris55000
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Default Re: Hum pickup on inter-stage coupling transformer

Hi!

Quote:
Chris, I think my post #5 shows the hum is picked up directly by the interstage audio transformer so I don't see how your suggestion would work. Also I am using a 12AX7 so I don't have the luxury of a suppressor grid.
Ah!

Even if the a.c. magnetic field was picked up by the turns of the transformer directly, the 50Hz alternating field would still be cutting the turns of the transformer, therefore a hum–voltage that can be seen with a sensitive c.r.o. amplifier would still be added into the grid–circuit, therefore it should (theoretically!) be possible to attempt to cancel it out!

Obviously I appreciate you cannot use supressor–grid cancellation in a triode stage – your initial query didn't make clear what valve was in use, and in most cases microphone preamplifier stages are assumed to be pentode stages for their high gain!

Chris Williams
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Old 13th Jun 2020, 1:25 pm   #26
Alan_G3XAQ
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Default Re: Hum pickup on inter-stage coupling transformer

To close off this thread, I have the principal signal handling stages on my lockdown receiver project working as I want now. In the end I moved the power transformer into a separate box with flying leads to get rid of the final traces of hum. As I move the PSU box around it is clear that 6in spacing from the audio transformer is too close but 1ft is enough. The drop in induced hum with distance is very steep.

Maybe the physicists on here can give us the theoretical induced voltage versus distance rule.

73, Alan G3XAQ
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