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What's the best DIY DC blocker circuit?

Too late to be of any use, but I've used the simple design to block DC for many years, ever since buying a buzz-tastic Cairn 4808 in about 2005. I'm still using the exact same one on my telly. It survived use in my previous home where the voltage never dropped below 250VAC and peaked at 265VAC fairly often. It's survived where I live now, which suffers from a very unstable electrical supply - regular sags, surges, cuts, even a lightning strike on a pole-mounted substation nearby. I tested it with a 3kW electric fire for several hours when I first made it - didn't even get warm, so I put a second fire on the circuit ... and the outlet on the wall started to melt and smell bad... It's robust.

People complained these blockers squash dynamics, so I used bigger caps - by far - as they are the chief limiting factor for current. I started with 33,000 uF / 15VDC and later 100,000uF / 35VDC bypassed with X or Y-class films for added peace of mind. Wholly over-the-top, but there ya go.

Whether or not transformers are buzzing, I find the subjective effect on SQ is always positive.

I've read a lot over the years about theoretical problems with them, just never encountered those problems in the real world. I understand why the more complicated versions exist, and if I was building to sell to other people, and needed a CE rating, I'd probably do things differently. Otherwise, simple is good, so long as you don't skimp on parts.
 
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That's really good to know. I have some spare components on order, so I might give the simple circuit a go too.
 
Hi Sjöström,
Just bought a new amp, amazing sound, but I've some toroids buzz (since ever, but this time I can clearly hear it).
Any feedback from PFMer ?
Is Sjöström the way to go?
Is there a power limit to it? Can I plug it between wall plug and conditioner (who obviously doesn't have DC Killer)
Thanks
 
naca5 said:
... Is there a power limit to it? Can I plug it between wall plug and conditioner (who obviously doesn't have DC Killer)
Click to expand...

Your limiting factor is the ripple current rating of the capacitors you choose to use and the conditions under which you use them; when I've built this kind of blocker I've always overspecified the caps far beyond theoretical limits to give me a Brunel-level of overengineering! 100,000uF upwards. Plenty of people call that overkill. I take that as a compliment.

And yes, between outlet and conditioner is the ideal place: ALL of your equipment (inc. the conditioner) will benefit from blocking DC offset - not all toroidals audibly magnetostrict, but the efficiency of all of them is negatively affected by it.
 
Thanks !
I'm not skilled enough to built it, so cap will be 68000 uF.
I hope it will not limit the set-up dynamic...
 
What is a recommended case/does anyone supply ready made?
My power supplies for phono amp have started buzzing recently and it varies day to day, so i guess it may be DC on mains. Not sure that i would trust myself making anything that involved mains electricity.
 
I'm going to mention this here chiefly because it's an instructive example of just one of the many reasons for transformer buzz which don't involve DC offset on the mains:

The Jay's Audio CDT-2 that I bought recently had one minor fault - a discrete mechanical buzz when powered-up. It wasn't audible from my listening position, but it's such a well designed and constructed thing I wanted to sort it. The problem proved to be one of the two toroidal transformers.

My entire system is fed by a high-current DC blocker I made years ago, and since that still works fine, it had to be something else. Only one transformer was doing it, and a clue lay in the fact that both TX are tightly potted into almost conformal metal screening cans. The cans are lifted off the metal of the chassis with thin washers of EVA foam and held in place with steel screws - the foam had done what EVA always does and pancaked, and in any case, duh, steel screws...

One TX had developed a 'hot spot' - some kind of eddy loop had developed, perhaps partly due to an irregularity in the winding, but with the can grounded to the common ground plane it was enough to set-up a localised area of magnetostriction, and thus unwanted noise. Removing the connection to ground stopped the noise, so I replaced the steel screws with nylon, and placed some 1mm silicone washers between can and casework: silent TX, problem solved.
 
earlofsodbury said:
I'm going to mention this here chiefly because it's an instructive example of just one of the many reasons for transformer buzz which don't involve DC offset on the mains:

The Jay's Audio CDT-2 that I bought recently had one minor fault - a discrete mechanical buzz when powered-up. It wasn't audible from my listening position, but it's such a well designed and constructed thing I wanted to sort it. The problem proved to be one of the two toroidal transformers.

My entire system is fed by a high-current DC blocker I made years ago, and since that still works fine, it had to be something else. Only one transformer was doing it, and a clue lay in the fact that both TX are tightly potted into almost conformal metal screening cans. The cans are lifted off the metal of the chassis with thin washers of EVA foam and held in place with steel screws - the foam had done what EVA always does and pancaked, and in any case, duh, steel screws...

One TX had developed a 'hot spot' - some kind of eddy loop had developed, perhaps partly due to an irregularity in the winding, but with the can grounded to the common ground plane it was enough to set-up a localised area of magnetostriction, and thus unwanted noise. Removing the connection to ground stopped the noise, so I replaced the steel screws with nylon, and placed some 1mm silicone washers between can and casework: silent TX, problem solved.
Click to expand...
I have a Rotel with a similar TX noise issue. Like yours, it is potted into a can. I noticed the buzzing was greatly diminished with the biscuit tin amplifier cover off. I ended up bracing the TX against the cover with folded paper. Problem solved. I think your solution is more elegant, though.
 
In the example above, the transformer cover had become a shorted turn on the toroidal transformer, something that should never be allowed to happen. Too close to the transformer and it will destroy the primary winding.
 
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