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A DC Blocker to help stop transformer HUM

Colin James Wonfor

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#1
Often the hum is due to DC on the AC causing saturation in the core of the transformer, more often found in toroidal.
This cct works and is cheap.

 

amirm

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#2
Hi Colin. Pretty clever!

Am I right that it takes out 3 times 0.6 volts worth of DC offset in either direction due to diode drop? Is the typical DC offset then below this on mains?

Are D1 and D6 there to limit the voltage across the caps?
 

RayDunzl

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#3
Put it in a nice wooden box and I know where you can sell it for $4,995

Add a couple of Carbon Fiber Outlets and I think $7,995 is not out of the question.
 

RayDunzl

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#5
I understand the diode part of the circuit, but not the capacitor part.

What's going on there?
 

fas42

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#6
Don't want to take too much away from Colin, but a rundown of the mechanism is here: http://sound.whsites.net/articles/xfmr-dc.htm. I actually put one of these together for a systems some years ago, with some cheap electro's; never did an exhaustive test of how beneficial it was, but it didn't appear to hurt the sound.
 

Colin James Wonfor

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#7
Hi Colin. Pretty clever!

Am I right that it takes out 3 times 0.6 volts worth of DC offset in either direction due to diode drop? Is the typical DC offset then below this on mains?

Are D1 and D6 there to limit the voltage across the caps?
Anti reverse diodes that is all.
 

eliash

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#9
Saw this in my T+A old R-series 650W power amp in front of the toroidal transformer as well - 2 x 2 diodes in series (antiparallel) in parallel to 2 x 2200uF/18V in series (reverse connected). Seems to work, no hum audible, despite massive PV installation around here, generating strange current waveforms...
 

777

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#10
The capacitors must to be low ESR with high ripple current. I recomand C=10.000uF/25v
 

sergeauckland

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#11
Don't want to take too much away from Colin, but a rundown of the mechanism is here: http://sound.whsites.net/articles/xfmr-dc.htm. I actually put one of these together for a systems some years ago, with some cheap electro's; never did an exhaustive test of how beneficial it was, but it didn't appear to hurt the sound.
That's the one I built, as having three large toroidal transformers in three power amps, I wanted to avoid potential problems with DC on the mains.

I put the DC blocker in the neutral line rather than the live, just to reduce the voltage difference between the capacitor cases and the well earthed metal box. The circuit works identically whether in the live or neutral line.

S.
 

eliash

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#12
The capacitors must to be low ESR with high ripple current. I recomand C=10.000uF/25v
Even though large capacitance may be better at higher currents (low ESR speaks for itself), the dimensioning in the T+A case is a different one. The amp consumes about 40W in the idle state (some class AB operating point), probably some 200mArms (primary; real part). For this "low" current the cap voltage stays below the 1.3V "clipping voltage" of the diodes, thus avoiding any DC-generated hum. When the amp is cranked up, the voltage at the caps is limited by the flow voltage of the diodes. In this case only a limited DC rejection takes place, but nobody will notice it...
 

g29

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#15
Does anyone know the purpose of the extra R1 and C3 (or the R1 in the high power design) compared to that of the Rod Elliott design which does not have them ?

 

DonH56

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#16
Hard to say without knowing their values but I'd guess a snubber circuit for RFI suppression. The diodes can (will) generate noise spikes as they turn on and off. The resistor may also serve as a bleeder to discharge the caps when the circuit is disconnected.

I used to have one of these, ought to build one again for the media room stuff. IIRC I used big caps with 10-A ripple rating to help support power amps and I had a big 600-V, 25-A bridge. The caps should not ever have to sustain the line voltage but the bridge must. You have to watch peak currents -- if the ESR of the caps is too large and the diodes turn on during normal operation they will generate noise in the line. I had that happen with a power amp and that led to beefing up the components for that application.
 

g29

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#17
Hard to say without knowing their values but I'd guess a snubber circuit for RFI suppression. The diodes can (will) generate noise spikes as they turn on and off. The resistor may also serve as a bleeder to discharge the caps when the circuit is disconnected.
....
I have a couple of toroidals that will periodically grumble, but not all of the time.

Bleeder was my guess. Also thought the extra components might be to distance itself from Rod Elliott's personal use licensing. As for the values ...

– 10W power resistor for the normal power units
- 25w - 22 Ohm (HSA2522RJ ) power resistor for the high power units
– WIMA MKS4 400VAC capacitor (250VDC 0.22uf)



High power resistor:
 
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DonH56

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#18
The 0.22 uF is almost certainly for RFI (it's ~12 k-ohms at 60 Hz). The 22-ohm may be partly bleeder, partly surge (in-rush) current limiter, partly RFI snubber to reduce the noise peaking when this drives an inductive load (e.g. your transformer).

IIRC toroidal transformers are more sensitive to DC because they have no air gap to break up the fields when DC is applied. The core saturates with low levels of DC applied (making the output waveforms ugly). They are also not usually as efficient (or cost-effective) as standard EI transformers, but do have lower radiated field so work well in packed chassis. And of course are audiofile-approved despite their drawbacks.

HTH - Don
 

maty

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#19
I have a very dirty mains. Usually, voltage near to 238-240 Vac, RF/EMI at mains and out and... a lot of DC. For quite some time, I have minimized these inconveniences and I enjoy much more before the excellent recordings that I usually listen to, getting excited much more frequently than before.

https://www.google.com/search?q=dc+blocker+maty :)

and also look at Google Images.
 
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