Variations of DC Main Filter against buzzing Toroid Transformers - what is the right? - Page 14 - diyAudio
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Old 17th July 2013, 05:52 PM   #131
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Quote:
Originally Posted by sextaafondo View Post
AFAIU the circuit, I can not agree with you on the above statement.

The capacitors are actually doing the whole job, especially under no load or light loads until the event in which the current value through them raises enough so that the diodes will take over, preventing capacitors from overloading.

.... With larger transformers (500VA and above), the DC resistance is usually so low that even a very small offset will cause mechanical noise due to saturation.....

.....From the DC perspective, the most critical region is at no-load. The saturation effects are greatly reduced when we are drawing significant current, so we will hopefully be able to simplify the circuit......[/COLOR]
Yes, the diodes are the protectors for the electrolytics against voltages above 700mV.

A question: what are online concerning reports and papers to this topic in your language (Spanish language)? Thank you for advices.
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Old 17th July 2013, 11:57 PM   #132
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Quote:
Originally Posted by tiefbassuebertr View Post
Yes, the diodes are the protectors for the electrolytics against voltages above 700mV.

A question: what are online concerning reports and papers to this topic in your language (Spanish language)? Thank you for advices.
No articles in spanish regarding this topic that I'm aware of.

I admit I prefer english spoken forums as they bring together a wider spectrum of knowleadge.
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Old 29th November 2014, 04:50 PM   #133
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Are there interesting news concerning commercial available DC blockers against buzzing toroidal transformer cores ?
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Old 23rd April 2015, 12:01 AM   #134
wlowes is online now wlowes  Canada
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Default Hum Curred

Thanks for all the pointers. I have been putting up with a lot of transformer hum. I built the circuit referenced in this site. It is in my power distribution box serving 2 large OTL tube amps and a music server.
Mains DC and Transformers
I used 2 big electrolytic caps.. 30v 12,000 uf. They have 8A ripple current.

The initial results were terrible. Much more humming from all the torroids. It sounded like they were being overloaded. I felt it possible they were starving for sufficient current. I added 2 more caps. MUCH better but still some hum.

Added 2 more caps. Now 3 pairs of series caps with a total of 72mf. All hum from all transformers gone. The biggest change was much improved dynamics and sound quality. Clearly ridding the transformer saturation was a good thing for the sound.
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Old 23rd April 2015, 02:33 AM   #135
mlloyd1 is offline mlloyd1  United States
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hmmmmm,
(no pun intended )
that sounds like an awful lot of capacitance for this application.
are you sure it was powerline DC-caused toroid hum you were trying to eliminate?

mlloyd1
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Old 23rd April 2015, 11:23 AM   #136
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Power line DC slows down the electricity meter. This is why the companies are so much interested in preventing DC entering your house.
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Old 23rd April 2015, 12:15 PM   #137
wlowes is online now wlowes  Canada
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mlloyd1
I cannot be certain... given the first set had a negative effect, the second set improved the hum over the starting point and the 3rd set completely eliminated it, I would say it was required and successful. There are a lot of big transformers being fed. One other key was that after the first set, a 10A isolation transformer would present no hum, but by switching on amps and adding a fair bit of load it would hum like crazy. After the second set of caps it would strain for 2-3 seconds then settle down to a dull hum. With third set, all stable.. no hum, better dynamics. The amps each have 2 torrids for high load filament supply to 6c33c tubes plus a large pair of torroids for driving the power tube b+.
I put this post out there in case anyone follows the path and finds this approach degrades the situation with heavier load. You may just need to work out the math and add more ripple current capacity.
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Old 23rd April 2015, 01:44 PM   #138
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Quote:
Originally Posted by mlloyd1 View Post
hmmmmm,
(no pun intended )
that sounds like an awful lot of capacitance for this application.
are you sure it was powerline DC-caused toroid hum you were trying to eliminate?

mlloyd1
Yes, I am confused as well. I would think that as long as the reactance of the capacitor is less than, say. 0.25 ohms at 60Hz, that should be enough. That would be 10,100uF if my calculation is right. Using an NP cap would be nice, but I'm not sure they easily come that big. Low ESR is important, but high voltage rating is less important given that the diodes are there and we rarely want the drop across the capacitors to get up to 0.7V anyway. If the voltage across the cap is limited to less than 1 volt, does anyone know if we can get away with a polarized cap without a reliability or other problem? Anyone tried?

Cheers,
Bob
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Old 23rd April 2015, 03:34 PM   #139
AndrewT is offline AndrewT  Scotland
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Quote:
total of 72mf.
does that equate to 72mF or something else?
6 caps in series parallel coming to 72mF, requires each capacitor to be 12mF.
Is that what you used?

The capacitance of six series/parallel connected 12mF is 18mF

The impedance of these 6 caps in series parallel is predicted to be
Xc=1 / {2PiFC} = 1/{2*3.14*50*0.018} = 0.18ohms for a 50Hz supply.
The peak voltage across the diodes must not exceed ~400mVpk for each diode not to pass and for the DC blocker to remain effective.
The maximum peak current across the capacitors will therefore be ~0.8Vpk/0.18ohms ~ 4.5Apk

That is the peak voltage of the primary current pulses that must not be exceeded for the DC blocker to work.

For the Canadian mains frequency the allowable peak current is 6/5ths of that figure, i.e. 5.4Apk for 60Hz.
Quote:
mlloyd1
I cannot be certain... given the first set had a negative effect, the second set improved the hum over the starting point and the 3rd set completely eliminated it, I would say it was required and successful.
you are right and you could, with a little bit of arithmetic, have modeled the effect.
Mlloyd1 is wrong. He does not understand.

BTW, I use 3m3F 16Vdc polarised caps. 10 in series parallel works effectively for my 50Hz 240Vac supply.
I suspect that both 6Vdc and 10Vdc would be equally effective. But the diodes must protect the caps even during a fault condition through the blocker. I have used 1n5404, but not tested it for fault current.
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Last edited by AndrewT; 23rd April 2015 at 03:42 PM.
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Old 23rd April 2015, 05:50 PM   #140
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The Bryston 4BSST uses two 33,000uF, 6.3V polarized capacitors connected in parallel in opposing directions across the blocking diodes. They use a high-current bridge rectifier to implement the blocking diodes.

Most of the time the capacitors will be conducting high-current spikes of relatively low duty cycle, as is normal for rectifier circuits, as opposed to sinusoidal current.

Inrush current at turn-on may be stressful to these circuits, so it might be a good idea to employ a soft-start circuit.

Cheers,
Bob
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