DC Blocker

[Cathode Ray]

I have bought them on ebay in lots for about 3-5 dollars each with switches and wired. I will pull one out and take a pic later.

Makes sense.

I was talking about cloning the AudioLabs DC Blocker.

It has more components, and the two low ESR, 10,000µF, 16V caps are about $10 each.

[R.G.]

As I said earlier in this thread, an amp builder in Germany posted a thread on another forum, and pictured a new amp he is selling.
In the photo was an AudioLabs DC Blocker. Never seen one, so I asked him what is was, what it did. The answer was a cryptic, "It makes the amp sound better."

Ah. Well, my technical opinion is that he has found a new advertising approach. "Makes the amp sound better" could apply to everything from magnets clamped around the wires to the color of the knobs.

Started looking around and located a schematic and some hi-res gutshot photos of the DC Blocker. Tried asking this guy some questions about what I had come up with, and his response was, "Never use polarized electrolytic capacitor on AC line!! That's a bomb !"

Showed him a gut-shot image of the AudioLabs DC Blocker, with a couple of polarized electrolytic capacitors inside it.. and he goes quiet.

He's right about not using electros in AC line applications. There are bipolar electro motor start caps, but these are switched out of circuit once the motor starts. If they're left in circuit, they overheat.

In the schematic the schematic there are two anti-parallel electros >protected< by two diodes, also anti-parallel. These limit the voltage across the electros to +/- 1.4V. The diodes also limit how much DC the caps can block and the blocking is problematic as the electros are discharged each AC cycle. I'd have to do some thinking and simulation for a better understanding, but it doesn't look very effective.

I think Sluckey is correct about saying the device I need is a line filter, rather than DC Blocker.. but this all raises a question; Since I use a variable autotransformer (variac) between my amp and the wall outlet, wouldn't that keep any DC (that may or may not be on the mains) from ever reaching the amp ?

In your description, wouldn't the variac (being a transformer) act as an isolation transformer - and therefore, a DC blocker ?

No. As you've already heard, variacs are autotransfromers, and those don't block DC

[Cathode Ray]

He's right about not using electros in AC line applications. There are bipolar electro motor start caps, but these are switched out of circuit once the motor starts. If they're left in circuit, they overheat.

In the schematic the schematic there are two anti-parallel electros >protected< by two diodes, also anti-parallel. These limit the voltage across the electros to +/- 1.4V. The diodes also limit how much DC the caps can block and the blocking is problematic as the electros are discharged each AC cycle. I'd have to do some thinking and simulation for a better understanding, but it doesn't look very effective.

Great info !

Read something similar on another site explaining that schematic(and the interaction between the bridge rectifier and the caps), but your take on it is much more intuitive and easier for me to grasp.

[gktamps]

I re-read your first post, and am trying to figure out what your concerns are about DC on your AC power. Are you actually having mechanical hum from your transformer, or are you just trying to pre-empt such a potential problem? Do you have a large toroidal transformer that is humming from DC on the line? Have you actually checked for DC offset on the line?

There is so much written about various designs of DC blockers that are openly discussed, so you don't need to look into someone's "secret design" - just do a lot of reading to see if you need such a device, and what are the pros/cons of using one. There are plenty of adherents and plenty of opponents, so it's best to only address a problem, and then by using the best solution that doesn't potentially create new problems.

The schematic you requested offline is simple. There is much written about the selection of capacitors you should review, e.g. high ripple, low esr.

fig2.png

Here's Nelson Pass's take on DC blockers: https://www.diyaudio.com/community/thre ... lter.2080/

Another discussion: https://www.audiosciencereview.com/foru ... r-hum.948/

And less than enthusiastic debate: https://www.eevblog.com/forum/beginners ... y-good/25/

[Cathode Ray]

I re-read your first post, and am trying to figure out what your concerns are about DC on your AC power. Are you actually having mechanical hum from your transformer, or are you just trying to pre-empt such a potential problem? Do you have a large toroidal transformer that is humming from DC on the line? Have you actually checked for DC offset on the line?

None of that. As I said before, the builder showed a photo of a new amp he's selling and this device was plugged into it.. I asked him what it was and what it did, and got a non-answer answer.

Upon researching it a little, I understood it removed DC from the mains coming into the amp. Curious about the device (which retails for about $150) I learned that I could build one to try out for about $50.

There is so much written about various designs of DC blockers that are openly discussed, so you don't need to look into someone's "secret design" - just do a lot of reading to see if you need such a device, and what are the pros/cons of using one. There are plenty of adherents and plenty of opponents, so it's best to only address a problem, and then by using the best solution that doesn't potentially create new problems.

The schematic you requested offline is simple. There is much written about the selection of capacitors you should review, e.g. high ripple, low esr.

fig2.png

And that's pretty much the heart of the circuit used in the product the amp builder was using in his photos.

My issue is almost certainly RFI, rather than DC on the mains.. however, I have a pre-amp that uses a toroidal transformer, so a DC blocker my be useful to me with that device.

I live on a hillside in central Texas, and at the top of the hill is a 400' tall radio repeater antenna. I have built a couple of small devices and played with CL values and been somewhat successful in filtering it out by running this between my guitar and amp.



The trade-off is; When I get the component values tuned such that it completely eliminates the RFI, it also turns the guitar into a bass.

[TUBEDUDE]

As I said earlier in this thread, an amp builder in Germany posted a thread on another forum, and pictured a new amp he is selling.
In the photo was an AudioLabs DC Blocker. Never seen one, so I asked him what is was, what it did. The answer was a cryptic, "It makes the amp sound better."

Ah. Well, my technical opinion is that he has found a new advertising approach. "Makes the amp sound better" could apply to everything from magnets clamped around the wires to the color of the knobs.

Started looking around and located a schematic and some hi-res gutshot photos of the DC Blocker. Tried asking this guy some questions about what I had come up with, and his response was, "Never use polarized electrolytic capacitor on AC line!! That's a bomb !"

Showed him a gut-shot image of the AudioLabs DC Blocker, with a couple of polarized electrolytic capacitors inside it.. and he goes quiet.

He's right about not using electros in AC line applications. There are bipolar electro motor start caps, but these are switched out of circuit once the motor starts. If they're left in circuit, they overheat.

In the schematic the schematic there are two anti-parallel electros >protected< by two diodes, also anti-parallel. These limit the voltage across the electros to +/- 1.4V. The diodes also limit how much DC the caps can block and the blocking is problematic as the electros are discharged each AC cycle. I'd have to do some thinking and simulation for a better understanding, but it doesn't look very effective.

I think Sluckey is correct about saying the device I need is a line filter, rather than DC Blocker.. but this all raises a question; Since I use a variable autotransformer (variac) between my amp and the wall outlet, wouldn't that keep any DC (that may or may not be on the mains) from ever reaching the amp ?

In your description, wouldn't the variac (being a transformer) act as an isolation transformer - and therefore, a DC blocker ?

No. As you've already heard, variacs are autotransfromers, and those don't block DC

Some variacs are apparently mislabeled isolation xfmrs. Look for the term "isolated", because you can trust the Chinese.

20230425_134943.jpg

[Cathode Ray]

Some variacs are apparently mislabeled isolation xfmrs. Look for the term "isolated", because you can trust the Chinese.
20230425_134943.jpg

I use a Tenma 72-110, made in the USA.



It is not an isolation transformer.

[sluckey]

Some variacs are apparently mislabeled isolation xfmrs. Look for the term "isolated", because you can trust the Chinese.
20230425_134943.jpg

That SC-10T really is isolated. Not only is the line input isolated from the variable output, but the entire unit is also isolated from earth ground! I guess you call that double isolated.

[gktamps]

Yes, the two-prong plug was an indication.

[Cathode Ray]

Some variacs are apparently mislabeled isolation xfmrs. Look for the term "isolated", because you can trust the Chinese.
20230425_134943.jpg

That SC-10T really is isolated. Not only is the line input isolated from the variable output, but the entire unit is also isolated from earth ground! I guess you call that double isolated.

[R.G.]

I did some quick scouting for parts for a DC blocker that's really a blocker.

My quick earlier guess was that you need about 1300 uF of blocking capacitor. You can buy 50uF 275VAC or 330VAC motor run capacitors for about $5 to $10. Surplus houses will be in the $5 range, discount suppliers in the $10 range, and of course the sky is the limit from a prime retail outlet.
1300uF put together 50uF at a time is 26 of them in parallel. That's $130, plus any mounting and enclosure issues. A blocker made of motor run caps will effectively block DC offsets up to the peak of the AC line voltage if the cap is rated for 2X the AC line voltage. They're rugged, as they're intended to pass AC mains currents. This idea is not as crazy as it sounds if the problem being solved is really DC offset on the AC line.

My intuition says that if you have DC offsets of anywhere near the peak voltage of your AC line, you have other problems than just your amp sounds funny. Like dead appliances and fires.

Some more thinking about the DC "blocker" schematic. I think it actually works by the diodes not conducting until the AC line gets more than two diode drops positive or negative, with the electros conducting some of the voltage during the crossover. I would not expect it to be effective for DC offsets of more than two diodes - or four, or eight, depending on how you set it up. The load it feeds and any phase shift between current and voltage will probably determine how hard the electrolytics get worked. Probably more thought needed.

[gktamps]

I would not expect it to be effective for DC offsets of more than two diodes

RG: from everything I've read about these, they are intended to be effective for up to two diode drops, not more. Many use only 16V caps.