eBay/Aliexpress DC Blockers

[pilau]

I am not an expert when it comes to electronics, and this post is a curious question directed to the experienced engineers in the crowd:

I want to know if they are snake oil, or if anyone has an experience, or can tell by the circuits/construction, and confirm that they deliver on their promise?

Here are some examples:
High performance EMI filter high frequency filter module AC purification | eBay
Assembled 25A power supply DC component filter | eBay
Filter DC Component Secondary EMI High Frequency Power Filter Purification Board | eBay
AIYIMA 2000W 15A EMI Power Filter Module AC110V 265V High Efficiency DC Power Supply Filter DIY Audio Sound Home Amplifiers|Amplifier| - AliExpress
12A 110 250V Filter Power Supply Purification HiFi Audio purification power board improves audio quality for Preamp CD DAC|Amplifier| - AliExpress
https://www.aliexpress.com/item/1005002327351402.html

Some look similar, or share some components, and others have completely different circuits.

Is there any merit to their claims of blocking mains DC?

Another question: I have an unrelated mains filter that I bought on eBay, with a completely different circuit to the ones above. A better Chinese translation of the description reads: "The components form a circuit that absorbs and neutralizes spikes and clutters, in order to shape, filter and absorb 220V AC sine waves. Its effect is different from that of inductive filters".
It does not claim to be a mains DC blocker.

Just for fun, I hooked it up to a 9VDC power supply, and measured appx. 4.5VDC on the output - so does it mean it's DC blocking?

Thanks

 

[nigelwright7557]

I have sold a few mains DC blockers and had good feedback.
Seem to work well with toroidal transformers.

 

[pcan]

The first listings looks to be a trivial combination of two generic textbook classic circuits for DC blocker and common mode EMI filter functions. A DC blocker is almost mandatory to avoid mechanical buzzing from the amplifier power supply toroidal transformer. Both circuits are often already installed in commercial power amplifiers, tailored for the job at hand, toghether with additional protections such as a overvoltage protector and a surge suppression thermistor. The boards may be useful for lower tier products and DIY builds, but the naked board does pose a shock hazard and needs to be properly connected. The low price on the listings is deceptive. A nice box with good quality sockets and a fuse holder will easily cost more than the board itself. No specification are given on the EMI filter section; a good quality one from a reputable manufacturer does cost only marginally more than some of the listings, will give a better confidence on the results, and is already shielded in a metal box easing the installatin and removing a shock hazard. The last board looks scary (no safety marking on the X capacitors, insufficient clearance) and it probably don't work as advertised. I would not use it.

 

[kouiky]

Is there any merit to their claims of blocking mains DC?

Toroids can physically hum due a little DC on the primary side, the mains AC. EI cores can hum, too, but toroids are more likely to.

Emotiva makes a two-outlet power bar to remove the DC.

...Just for fun, I hooked it up to a 9VDC power supply, and measured appx. 4.5VDC on the output - so does it mean it's DC blocking?
Thanks

I don’t think that one is, no. Based on the description it’s a noise suppressor.

 

[knauf1919]

For low power it is not necessary. Over 100W you can use DC blocking.

The EMI filter is another option besides this.