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Old 8th March 2012, 08:50 AM   #1
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Default active hum cancellation. PCB idea?

With DHT hum can be a problem and by searching fro good, efficient solutions I found ttis site

PLL Hum Cancellation Circuit

th author claims the cancellation is obtained by injecting a AC to cancel hum out.

The project seems interesting. Is he a member here? can anyone provide some feedback on the schematic?

If more than one person is interested there may be an opportunity for PCB creation and a group buy.

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Old 9th March 2012, 07:40 PM   #2
tomchr is offline tomchr  Canada
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Honestly, I think a PLL would inject more spurious content into the system than it'll cancel. The VCO in the circuit runs at 800~960 Hz (16x the mains frequency) so you'll have a spur there for sure. Smack in the middle of the audio band.

If you want to cancel the hum, why not just use regulated DC on the heaters?! Guaranteed quiet. You can see my website for some options on how to do that.

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Old 9th March 2012, 07:51 PM   #3
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Thanks tomchr. Only problem with my project isthe total current which (tested) reaches 45A continuous @ 12.6
The cost, weight and overall requirements for such an output put me off from pursuing DC. The transformer I had made for this application was designed to achieve the best possibile symmetry between channels. The CT on the tubes will be grounded and a 1w pot will be in place between filament leads and with the wiper cap coupled with ground.

However I was researching the matter and active systems would appear to be something of an upgrade from passive devices.

I do have a 2 channel oscilloscope so testing can be done...

I do not have the experience to judge if this road is a wise one to walk.. I love experiemnting and learning. I'd rather not do anything DIY at all if it meant not trying new routes.
(I also arced my first MV rectifier just a couple of hours. Being nickel protected the cathode is perfect but I definetely learnt how to approach things there! )

In conclusion do you think the circuit is worth a try?
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Old 9th March 2012, 07:52 PM   #4
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Far better to get rid of the problem than to hide it with technology.
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Old 9th March 2012, 07:58 PM   #5
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I am all for it but how can I improve over the hum pot method? Is there a better/more reliable way?
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Old 10th March 2012, 09:06 AM   #6
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The source of the hum is right there in your amp, so that should also be the source of the anti-hum IMO.

Assuming balancing the DHT heater will not work (it should), then inject some of the hum source into a negative bit of the amp. Job done.
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Old 10th March 2012, 09:13 AM   #7
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interesting idea. Will certainly try it!
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Old 11th March 2012, 06:09 AM   #8
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this is a neat circuit. but if you build the tube circuit correctly, you would not have to resort to over engineering
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