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Primaluna's AC Offset Killer
Primaluna's AC Offset Killer
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Old 1st June 2018, 05:22 PM   #11
RobSoet is offline RobSoet
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I had a terribly humming NAP 250 DR (as in very audible @ 7m). When measured (with primitive tools), I measured slightly more than 500mV offset. I've built the schematic as shown by Tom here above and the humming went away (and while I was at it, I put a 1F cap in parrallel as well. Not sure what difference that made).

This brought peace and quite in the music. Difficult to say whether this is the result of what in the end comes out of the speakers, or because the absence of the hum.


In the meantime the nap is replaced with NC400's, and if I put these after this circuit, they'll cause my preamp to hum...
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Old 1st June 2018, 09:29 PM   #12
martin clark is offline martin clark  Europe
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Quote:
Originally Posted by basreflex View Post
ech line has 2 series 2 antiparell diodes, the rod elliot circuit in each line, except 0.1uf instead of 2200uF to save money.more a 10$ circuit
This, as the Rod Elliott article elegantly describes, is what is wrong with such cheap versions: the AC current is (should be!)a passed entirely by the capacitance. The only purpose of the diodes is only to limit the maximum reverese DC voltage offset the large electrolytic capacitors are exposed to. The idea that two diodes is better than one (for '1.4v dc protection') is completely false.

In any case, so is calling the problem 'DC' ; it is not; it is low-harmonic noise, principally 2nd HD. Build a filter to try to measure 'DC' on the mains (requires a little finesse*, and since here I'm on 240VAC I'm not posting it in interests of ..general safety/not breaching the DIYA 'no mains-direct connection' discussion reqt: it could kill) - but I found there's little but c. <10mV at worst.


*sticking your cheap DVM probes into the wall socket and selecting 'dc' measurement range is no measure at all.

Last edited by martin clark; 1st June 2018 at 09:33 PM.
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Old 1st June 2018, 10:11 PM   #13
freddi is offline freddi  United States
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Primaluna's AC Offset Killer
what voltage rating is proper for the electrolytic caps with US 120vac mains?
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Old 2nd June 2018, 06:29 AM   #14
Alexandru is offline Alexandru  United States
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Quote:
Originally Posted by basreflex View Post
ech line has 2 series 2 antiparell diodes, the rod elliot circuit in each line, except 0.1uf instead of 2200uF to save money.more a 10$ circuit
Especially after looking at the 2nd photo, it doesn't look like Elliot's circuit. Also, what's the purpose of the whole in the middle of the circuit board? The brown and blue wire come from the power plug or from the power button top left?

More info on their blog, they seem pretty pretty proud of this little circuit:

Click the image to open in full size.

Welcome to the Machine… PrimaLuna DiaLogue Premium Integrated Amplifier 2012 (part 2) | PrimaLuna's Blog

===
"Functionality:

The filter was designed by us many years ago to eliminate mains pollution, especially DC components coming from the mains. A small amount of DC on the mains, occurs a lot of hum in Toroidal transformers of High End Audio equipment. Specially in case of power amplifiers which use larger toroid transformers. The bigger the amp (and the transformer) the worse the hum might become.

(PS For your information and better understanding: The first time we used this filter was when we were dealing with a very annoying humming integrated amplifier. We also offered the board to be built in other products suffering from humming toroid transformers. Most recently we built a higher powered version of this circuit to make some top-of-the-bill humming mono amplifiers dead silent and best sounding).

Why add it to a product that is already known to be dead silent in every possible way? Just because we know AC is getting more polluted every day. So what we do now, might benefit you in the future: our desire to give you a “best level possible” sound quality, reliability, value. Now and in the years to come.

DC Killer:

The DC Killer is designed around twelve fast switching diodes with soft recovery time, like the STTH8L06FP. This diode can handle 10Amps, with a recovery time of 105ns! They are also hermetically sealed in a plastic TO220FP case. Next to that the diodes are filtered with good quality capacitors. The full circuit is built on a dedicate pc board positioned at the AC entre for maximum effect."

Last edited by Alexandru; 2nd June 2018 at 06:59 AM.
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Old 2nd June 2018, 07:01 AM   #15
Alexandru is offline Alexandru  United States
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I've found an older version of this circuit that was apparently used in Krell KAV 300i?!

Click the image to open in full size.

Last edited by Alexandru; 2nd June 2018 at 07:16 AM.
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Old 2nd June 2018, 07:32 AM   #16
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by Alexandru View Post
Interesting, I wonder why they are claiming it improves sound?!
Because it sells amps. "Our amp is special! It has our amazing patented WizzBang2000 in it. You should buy our amp." It's a pretty common snake oil sales technique.

Quote:
Originally Posted by Alexandru View Post
Ok, so the circuit costs aprox $20 in parts at Mouser.
Are these DC blocks in series or in parallel?
Shouldn't be that much. The caps won't see more than 2*Vd = 1.4 V, so a 4 V or 6.3 V type is plenty. The diode bridge needs to be able to handle the primary current. Something like a GBPC2510 is overkill, but would make for a nice unit that you can bolt to the chassis.

It looks like Primaluna is using a DC blocking circuit in series with both the live and the neutral mains wire. That's misguided at best. You only need one DC block. They also use 0.1 uF, which is rather small. Basically, they're adding resistance in series with the transformer primary. That may change the sound of the amp, but I highly doubt it changes it for the better.

Tom
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Old 2nd June 2018, 08:00 AM   #17
analog_sa is offline analog_sa  Europe
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Primaluna's AC Offset Killer
This last board is not a Krell product. It has been designed as an "upgrade board" for Krell by AH! and in the middle there is also a Schaffner filter - a bad, bad idea. The board also provides overvoltage protection.

Could anyone enlighten me how is dc blocking provided without caps?
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Old 2nd June 2018, 08:13 AM   #18
analog_sa is offline analog_sa  Europe
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Primaluna's AC Offset Killer
Quote:
Originally Posted by Alexandru View Post
Interesting, I wonder why they are claiming it improves sound?!
Anyone who has built a dc blocker and is not entirely deaf knows that it significantly changes the sound. My experience has been limited to SS amps where the bass using a blocker suffered to a degree, but the blocking elements were capacitors which can significantly colour the sound. A capacitor free dc blocker, as the one from Prima Luna, may not have this problem.
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Old 2nd June 2018, 09:03 AM   #19
RobSoet is offline RobSoet
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Quote:
Originally Posted by martin clark View Post
....

In any case, so is calling the problem 'DC' ; it is not; it is low-harmonic noise, principally 2nd HD. Build a filter to try to measure 'DC' on the mains (requires a little finesse*, and since here I'm on 240VAC I'm not posting it in interests of ..general safety/not breaching the DIYA 'no mains-direct connection' discussion reqt: it could kill) - but I found there's little but c. <10mV at worst.

If you have less than 10mv offset, you're probably not interested in a DC blocker. It won't do much. I have more around 4-500mV (with a distinct 24h and 7d period). A DC blocker will not block a 2nd harmonic (neither do I believe 2nd harmonics are problematic, they look uggly on scope, but should be well filtered by your power supply).
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Old 2nd June 2018, 04:59 PM   #20
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by RobSoet View Post
I had a terribly humming NAP 250 DR (as in very audible @ 7m). When measured (with primitive tools), I measured slightly more than 500mV offset. I've built the schematic as shown by Tom here above and the humming went away (and while I was at it, I put a 1F cap in parrallel as well. Not sure what difference that made).

This brought peace and quite in the music. Difficult to say whether this is the result of what in the end comes out of the speakers, or because the absence of the hum.
I can see one path where mechanical hum can make it into the signal path in a tube amp: Microphonics. Some tubes are pretty microphonic, i.e. they will pick up mechanical vibration and translate it to a change in current or voltage. If the hum from the transformer is enough to cause vibrations in the chassis, you could end up with IMD components at the mains frequency and those can certainly be audible. I generally experience those IMD products as "fuzz" or "haze" or "lacking clarity".

Quote:
Originally Posted by analog_sa View Post
Anyone who has built a dc blocker and is not entirely deaf knows that it significantly changes the sound.
With bombastic language like that, would you happen to have a link to the research that backs this up? Preferably a controlled study with a significant sample size (N ≥ 30).

Tom
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