A DC Blocker to help stop transformer HUM

[NTK]

Reading their brief "GND Defender'" description it seems doubtful that it has either a 'UL' stamp or is safe.
Maybe bits & pieces have a UL stamp.

UL 94 is a flammability rating of the plastic (or rubber or PCB or other) material. It doesn't say anything else, and definitely not on the safety of the circuitry.

 

[egellings]

Why are there 2 diodes in series in one direction, but just one diode in the other?

 

[tmtomh]

I don't think I got used to the hum, but it was more noticeable at certain times; presumably the DC offset varied according to what else was in use nearby.




That's rather pricey; given how small it is, there can't be much in it: https://www.amazon.co.uk/dp/B08XWHJRZ1

I built a some DC traps using ATL Audio PCBs: https://www.atlhifi.com/shop/bare-p...ker-filter-for-toroidal-transformers-toroids/

It cost me around £90.00 to build two, including the PCBs, the cables, boxes and all the components.


View attachment 121390

In my experience, DC blockers are always overpriced given the cost of parts and the simplicity of the design. But labor is labor, and DC blockers are a niche product, which I assume means there aren't great economies of scale in production.

But yes, if one is handy with a soldering iron, it's definitely cheaper to make one yourself.

That said, if the ifi is a $150US product (which I'm guessing given its 129gpb price on Amazon UK), that puts it in line with the Van Alstine Humdinger, which is the least-expensive professionally produced unit I'm aware of. Assuming the ifi works properly and is built decently, its compact form factor, enabling it to be used inline at the power jack, is quite handy and could be worth money for some folks.

 

[Count Arthur]

ATL also supplies the DC filters, and a few other things, ready assembled: https://www.atlhifi.com/product-category/fully-assembled-devices/

Given the cost of the parts, I think the prices are very reasonable and the DC blocker definitely works; it completely silenced the grumbling from the large toroidal transformers in the power amps I had.

 

[Harmonie]

iFi released two new products to tackle DC offset and HUM:

http://ifi-audio.com/products/dc-blocker/



https://ifi-audio.com/products/gnd-defender/



Both plugged in:



Would be nice to see the internals of these.

I wonder if it's not a cheap Schaffner filter that is inside

 

[Count Arthur]

I wonder if it's not a cheap Schaffner filter that is inside

The Schaffner filters are a different thing, they are EMC/EMI filters, to reduce mains noise: https://www.soselectronic.com/articles/schurter/how-does-the-emc-filter-work-2164, they won't do anything to help with DC offset.

 

[Harmonie]

The Schaffner filters are a different thing, they are EMC/EMI filters, to reduce mains noise: https://www.soselectronic.com/articles/schurter/how-does-the-emc-filter-work-2164, they won't do anything to help with DC offset.

Sorry, I posted too quickly.
But please teach me:
Isn't DC supposed to be in place after the transformer and not in the AC ?

 

[somebodyelse]

Sorry, I posted too quickly.
But please teach me:
Isn't DC supposed to be in place after the transformer and not in the AC ?

It's not supposed to be on the AC, but as mentioned earlier in the thread various forms of cheap rectification in other mains powered devices can lead to a small DC offset on the mains AC, at least locally. This can be enough to cause adverse effects like audible hum/buzz from transformers, hence people wanting to get rid of it.

 

[tmtomh]

It's not supposed to be on the AC, but as mentioned earlier in the thread various forms of cheap rectification in other mains powered devices can lead to a small DC offset on the mains AC, at least locally. This can be enough to cause adverse effects like audible hum/buzz from transformers, hence people wanting to get rid of it.

Exactly.

 

[Observer]

Tomanek from Poland (EU) make DC blockers in few versions, from 300PLN ... 67€.
I am using one with my integrated amp, except reducing transformer hum it lowers amp working temperature for a few *C.
You may also notice a slight change in amp sound.

https://sklep.tomanek.net.pl/tomanek-dcb-1-reduktor-skladowej-stalej-sieci-zasilajacej-230v-44.html

 

[egellings]

A DC component can occur in the AC power line.

 

[Speedskater]

A DC component can occur in the AC power line.

Yes, but it was more common some decades ago. Residential problems were appliances like old tea pots and hair dryers, Large date centers had 3 phase problems from old computer power supplies.

 

[tmtomh]

Yes, but it was more common some decades ago. Residential problems were appliances like old tea pots and hair dryers, Large date centers had 3 phase problems from old computer power supplies.

However common it was decades ago, it remains common today, at least in denser urban areas with electrical infrastructure, home appliances, and home wiring of many different vintages and states of repair on the local grid.

 

[mSpot]

Here's a member's comparison of several DC blocker products that he tried:
https://www.audiosciencereview.com/forum/index.php?threads/my-toroids-buzz.22587/post-758401
(read the entire thread for the full story)

 

[Speedskater]

However common it was decades ago, it remains common today, at least in denser urban areas with electrical infrastructure, home appliances, and home wiring of many different vintages and states of repair on the local grid.

In old commercial/industrial areas, yes. But in residential areas, no. Home appliances are intermittent in operation. 'home wiring of many different vintages and states of repair on the local grid' can not cause a DC offset problem.

 

[tmtomh]

In old commercial/industrial areas, yes. But in residential areas, no. Home appliances are intermittent in operation. 'home wiring of many different vintages and states of repair on the local grid' can not cause a DC offset problem.

I don't want this to devolve into a protracted argument. I will only say that my experience does not match your broad claim.

 

[mSpot]

I don't want this to devolve into a protracted argument. I will only say that my experience does not match your broad claim.

It's my experience as well, and the iFi DC Blocker eliminated the transformer hum from my amp.

 

[Geert]

OK just for fun, no fun, who has worked out what the rest of the circuit do?
I am using a version of this in amp I make for Holland.

Automatically switch on the amp when it senses an input signal???

 

[DonH56]

OK just for fun, no fun, who has worked out what the rest of the circuit do?
I am using a version of this in amp I make for Holland.

Thermistor for soft-start is a nice idea, as is all the noise filtering (I'll pass on the usual "why/how much is needed?" debate). Curious: why two bridges (BR1 and BR2) in parallel? R1/C1 is something a lot of hobbyists forget and get to hear loud snaps as a result...

The upper circuit looks like an input signal sensing circuit to turn on the relay (and thus the amp) through the timer circuit (I assume that's what TM1 is). Are the "OFF" and "START" headers used in your amp design or are they there for test/debug? I have a vague memory of adding a clamp across Q1 when I built something similar, long ago, after the relay snubber didn't prevent Q1 from getting reverse biased and destroying itself.

Only thing I would change would likely be to use a DPST relay so I could use the second pole to provide a 12 V trigger output for the amp or other devices in the chain, probably after a regulator (or just zener circuit) off the RAW voltage. Of course then I'd probably have to use a bit bigger transformer (TX1).

Neat work, Colin!

 

[DonH56]

Hi Don,
Two bridges because I ran out of leaded and went SMD, so assumed others would do the same.
R1/C1 Snubber many new designers forget that, and on a amp it a Zobel.
No OFF/ON is to allow you to override the sense circuit.
Using DPST relay present a safety issue if used on a low voltage and mains, so I would not recommend it.
But using the drive to run two SSR's with optical isolation would be good.
I will put up a SSR design I did that was very low noise and no filters than lets say the Crydom ones. Oh is 100A @ 400VAC ok?

Got it. Agree on the DPST, brain fart on my part, I know better. Or should. Duh! Mea culpa...

I like the filters.

Not sure where the 100 A @ 400 VAC applies? The TX1 comment was about the 1 VA little guy for the low-voltage rail; to drive a trigger circuit I'd want at least a couple of hundred mA instead of 83 mA. No argument about it being big enough for your design.

If you mean the SSR is 100 A @ 400 VAC, then yah, that should do...

I noticed the common-mode chokes are 1 A but presumed knowing you that they are fine for your application.

Good seeing you contributing again, always interesting stuff!

Thanks,
Don