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A DC Blocker to help stop transformer HUM

DonH56

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DonH56

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In the new amps I am designing the above circuit is added to them all plus a few refinements.
I was going to use a transformer and a cap to set the transformer into resonant, but the 1500 V on the cap worried me as it would burn finger a bit.
The prototype rejects below 40Hz and above 100Hz, we used the same technique at Harmer and Simmons for battery charger 55VDC at 100,000Amps,
The then drove a series regulator using chop thyristors (3 phase) followed by 1000s of transistors to regulate the last few volts but we manged at full load to only have 5mV ripple.
A wicked beast to cut your teeth on when first starting out at a real job of work. I recommend it.

Thanks, but no... :) My career has been analog/mixed-signal IC design, using nice low voltages, for the most part...
 

DonH56

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I did a bit of high-voltage design but have mercifully forgotten most of it, and worked as a TV/stereo repair tech from junior high through college so have a few scars from unexpected CRT anode contacts. Which sometimes result in a bang when I broke the #$@! tube. Inadvertent contact with flyback transformers caused me to smash a few other (smaller) tubes as well. Ham radio caused me a few lessons, and working with broadcast equipment (reportedly as a gofer since I was not allowed to touch the equipment without a first-class license, but I helped install and debug a few big transmitters and towers). As for IC design, it has not been boring, but yeah big bangs are fewer. It does remind me of when I was a lab teaching assistant and the first project was to build a power supply. At least once a semester someone would connect the big filter cap backwards and we'd get a bang followed by bits of metal, paper, and plastic raining across the lab.

Fun stuff!
 

vco1

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Excuse my ignorance, but would there be any benefit from having 2 DC blockers in the same chain?

I'm asking because I have a DC blocker that works well about 90% of the time. The other 10% audible hum from a toroidal transformer in my integrated amp can still be noticed. Even at some distance. Unfortunately, this 10% occurs mostly in the weekends when I have time to listen to music. ;-)
As a noob I'm thinking this is because the DC component is higher than the threshold that the DC blocker can handle. However, what if I'd put a second DC blocker behind it? Wouldn't that filter out the remainder?
 

Geert

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If you know what your doing you could install 2 extra diodes in the box I guess.
 

egellings

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I agree with Geert: add extra diodes in series with those already in the unit. Be sure that the large capacitors are rated for the little extra voltage drop from the added diodes, which should have at least the current rating of the originals. Most likely, the rated capacitor voltage will be be more than adequate.
 

vco1

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I was glad the ATL unit came fully assembled. ;-) Adding extra components in a power device without knowing what I'm doing doesn't look like a smart experiment. Thanks for the suggestions though. It's appreciated.

As a sidenote: the ATL unit already seems to be the most powerful on the market. That is, the one that should be able to remove the most DC.
 

egellings

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If it's cap-coupled and the diodes are not forward biased, it's removing all of the DC.
 

Geert

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The ATL removes only a maximum of the total diode voltage drop, 2V in this case. Do you have an example of the design you're referring to?
 
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DonH56

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More than 2 V is a rather high level of DC... I suppose there could be that much, but it could also be something else, like some sort of noise on the line, or the transformer mechanical mounting or isolation (like rubber donuts to shock-mount it, if any), or simply a bad transformer. Maybe another noise source (dimmer, fridge, HVAC, etc.) coupled into the amp, though from the description I assume you have moved your ear around to confirm it is mechanical noise (buzz, hum) coming from the transformer and not somewhere else. Or even some sort of ground loop or other component failing. If it is mostly on the weekends, and assuming it is on for a longer then when you have time to listen, there could be some sort of thermal problem in the amp.

It is a little strange that it is time-dependent, but it could depend upon what else is on the power line.

Is the hum there when you turn on the amp, or does it build up? Does it vary over time as you have the amp on?

This may be a case for the local dealer's tech to help.
 

vco1

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I'm pretty sure it's (partly) DC. I had this issue for years. Sent back the amp for inspection twice shortly after I bought it, and both times the tech and the dealer returned it to me with the message that they couldn't hear any hum at all. At that time I didn't know about toroidal hum. And I doubt they were aware of it either.

It's definitely a mechanical hum coming from the rather large toroidal transformer. There is no hum in the signal.

After I bought the ATL DC Blocker the hum was gone. Which felt like a miracle, tbh. And still is fully (!) gone 90% of the time. The amp is dead silent, which is quite remarkable as it always had a hum. At some random times it retunes though. Sometimes barely noticeable, sometimes a bit louder (like noticeable at 1-2m). And when it's back, it's usually from the moment I switch on my amp.

I am already very happy that I could get rid of the hum most if the time. That last 10% would be nice, but is probably something I have to deal with. Or buy a new amp, with a different transformer. ;-)
 

audio2design

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Yes -- I am a little surprised. From their website:


High-performance glass passivated semiconductor. High-case dielectric strength of 1500VRMS. UL listed. Flame resistant IEC socket/connecter/chassis (Fireproof polymers UL94-V0).


Too bad that electrical enclosures for a UL listed product need to be 5VA, not 94V0. 94V0 rating requires a secondary enclosure.
 

audio2design

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Why the oversized resistors? Maximum sustained voltage is the diode drop so worst case with 3 diodes about 3V. 22R resistor in the examples, 3^2/22 = 0.41W.
 

Geert

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Safety in case a diode fails???
 

Anterantz

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Hello, I have taken a look at the dc block that you have recommended, it would cost me about € 110 since I am from Spain. Do you think it could solve my problem?

Or better try the ifi dc blocker? Which do you think will be better?

I have read that these
devices degrade the sound and some say that the bass becomes flaccid, what do you think?

Greetings
 
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vco1

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Hello, I have taken a look at the dc block that you have recommended, it would cost me about € 110 since I am from Spain. Do you think it could solve my problem?

Or better try the ifi dc blocker? Which do you think will be better?

I have read that these
devices degrade the sound and some say that the bass becomes flaccid, what do you think?

Greetings
I have used both the iFi and ATL (2V version) DC Blockers. Had the best result with the ATL. But not constantly, unfortunately. Eventually I decided to replace my amp by 2 Purifi monoblocks. I couldn't stand the hum anymore.

I didn't hear any audible difference in the sound coming from my speakers and headphones when any of the DC blockers were connected. Same bass, same mids, same highs.

Both the iFi and ATL are for sale now (from The Netherlands).
 

Geert

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For most people it solves the problem, unless there's something else going on than DC of course. Note that when the problem is realy bad you need a model that capable of blocking a higher DC voltage than the standard model. How do you know which one you need, you don't until you try it (unless you're expirienced enough to measure DC levels in advance). The device does not deteriorate sound, and the ATL is far better than the iFI toy.
 
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